Covering 70% of Earth’s surface, oceans exert a major control on climate. They absorb large amounts of solar energy. Heat and water vapor are then redistributed globally through ocean currents and the circulation of the atmosphere. Jump to The Oceans
Living Landscapes: Northeast Region
Culture, Climate Science & Education
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Principle Two: Climate is Complex
The Cultural Value is Humility
Episode Two: Rivers are Complex
Episode 2: The River is Complex
Transcript with Description of Visuals
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Audio |
Visual |
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Soft Pleasant Music |
Rylee and Alyssa Pretty On Top are walking with their grandfather on a trail through a field of tall grass to a point that overlooks the river. |
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Voice Over continues: |
Close up of the three on the overlook, looking out across the river, which lies in the canyon below. |
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My name is Rylee and I want to tell you about the lower Flathead River, the river that winds through the heart of our reservation and the heart of our culture. |
Aerial view of the lower Flathead River, a large blue-green river winding through a canyon that snakes across a prairie. Flathead Lake is in the background. |
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Salish Wake-up Song |
Teepees on the banks of the river. |
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a place to hunt and fish, a place to harvest roots and berries, |
Old black and white photo of a Pend d’Oreille man paddling a canoe on the river. |
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a place to find quiet and peace, |
Old black and white photo of a Salish woman in a beaded dress scooping water from the river into a buffalo horn. |
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and a place for song and prayer. |
Old black and white photo of a Salish woman doing beadwork at the base of a large pine tree near the river. |
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Salish Wake-up Song |
Old black and white photo of a Salish or Pend d’Oreille man fishing on the bank of the river. |
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Soft Instrumental Music |
Flying above rapids on the river, the water a clear blue-green. |
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In the 1930's, a large dam was built on tribal land. |
Old black and white photo of the dam site during construction. |
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The tribes opposed construction of the dam, which changed the habitat for our fish and wildlife on the river and damaged an area sacred to our people. |
Old black and white photo of six Indian men, some with full headdress, just below the dam. In front of them, great torrents of water flow through the gates of the dam. Then a photo giving an aerial view of the dam from the same time period. Then photo of the same Indian men standing on a boardwalk overlooking the rush of water coming out of the dam. |
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After a long battle, we were able to buy the dam. Now we manage the river's flows to reduce the impact on plants and animals. We are working hard to restore areas that the dam has harmed. |
Flying above rapids on the river, large boulders in the water. |
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Rippling Water Now we face other challenges. |
Almost eye-level with the river, which is at high flow, just below flood stage. A leafless rose bush is inundated by the flow. |
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As the climate continues to change, so does the river. Hotter temperatures in the spring lead to faster snow melt, and as a result, high levels of runoff. |
Flying just above the rushing waters of the river, which is flowing near flood stage. |
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We've come down to fish today, but the water is still too high and full of debris. In years past, the fishing was good at this time of the year. |
Alyssa and Rylee and their grandfather walking on the bank of the river. All three hold fishing poles. |
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Rylee’s Grandfather |
Close up of the three of them on the bank of the river, watching the water rushing by. |
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Rylee |
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Rylee's Grandfather Water. |
Close up of the river at eye-level. |
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This is precious stuff. You've got to respect the water. Treat it good, and it'll treat you good. So maybe the fish will wait another day for us. |
Close up of the three of them on the bank of the river, watching the water rushing by. |
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Soft Instrumental Music |
They turn to leave, walking on the trail that leads away from the river and passing by purple larkspur flowers. |
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Soft Instrumental Music |
Aerial view of the river at high flow, peaks of the snow-covered Mission Range in the background. |
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The following credits in white text over a black background: |
Principle 2
What You Need to Know About Principle 2: Climate is Complex
This principle relates to the complex interactions that drive climate processes. Interactions between land, snow and ice, living things, oceans, and the atmosphere generate things like the greenhouse effect and a number of other climate processes. The interactions are complex and require different scientific disciplines working together to understand them. Click the tabs below to learn more about the factors that regulate climate.
- Climate Varies by Region
Earth’s climate is influenced by interactions involving the Sun, ocean, atmosphere, clouds, ice, land, and life. Climate varies by region because of local differences in these interactions. Jump to Climate Varies by Region
- The Oceans
- The Atmosphere
The amount of solar energy absorbed or radiated by the Earth is moderated by the atmosphere. Greenhouse gases like water vapor, carbon dioxide, and methane occur naturally in small amounts and absorb and release heat energy. Jump to The Atmosphere
- Carbon Cycling
The abundance of greenhouse gases in the atmosphere is controlled by cycles that move carbon between the ocean, land, life, and atmosphere. Jump to Carbon Cycling
- Aerosols
Airborne particulates, called “aerosols,” have a complex effect on Earth’s energy balance: they can cause both cooling, by reflecting incoming sunlight back out to space, and warming, by absorbing and releasing heat energy into the atmosphere. Jump to Aerosols
- Feedback Loops
The interconnectedness of the Earth’s systems means that a significant change in any one part of the climate system can influence the equilibrium of the entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system. Jump to Feedback Loops
Principle 2a
Climate Varies by Region
click the image to enlarge the graphic
Earth’s climate is influenced by interactions involving the Sun, ocean, atmosphere, clouds, ice, land, and life. In other words, all elements of the Earth as well as important variables from space affect climate. 
Because of local differences in these interactions, climate varies by region, and that explains for example, why regions of New York receive up to 150 inches of snow a year, while areas just to the south receive just 40 inches. Similarly, parts of northeastern Ohio receive 80 to 100 inches of snow, while farther south, Ohio receives less than 30 inches.
The climate of Northeast Ohio is influenced by Lake Erie, the big body of water to the north. By warming and cooling more slowly than the land, the lake delays the coming of spring and fall. But the most dramatic impact is the phenomenon of lake effect snow, which produces much higher average snowfall in a concentrated area southeast of the lake. Read More…
High in the sky, the jet stream is changing course. The stream of hurricane-force winds races around the globe, crossing North America from west to east, following the boundary between the warm and cold air masses. Now it loops farther south, deep into the midwestern United States. It pulls the polar air behind it. The leading edge forms a cold front, advancing, picking up speed. Dry, cold air rolling out of the north, and then crossing the warm waters of Lake Erie.
In the fall and early winter, the lake is warmer than surrounding land. The land is a spendthrift that rapidly fritters away its summer warmth. But the lake is a Scrooge. It hoards a vast fortune of heat during the summer and parts with it penny by penny during September, October, November, December.
As the surface water cools, it becomes more dense and sinks. Warmer water stored in the depths then rises to take its place. Until it reaches about 39 degrees F., the lake is a circulating reservoir of heat.
The cold air from the north passes over this warm, open body. The air warms, takes on moisture, rises up in clouds. The clouds pile up on the cold southern shore of the lake and keep rising and cooling until they can no longer hold the water they took from the lake.
Then it snows. The faster the clouds rise and cool, the faster it snows. To the west of Cleveland are low lake plains. The moisture-laden air meets no sudden elevation. It can slip inland and deposit snow gradually over some distance. But to the east of the city is the Portage Escarpment, the abrupt climb to the Heights, the last gasp of the Allegheny Plateau. While the lake is about 570 feet above sea level and the city is 600 to 700 feet, the Heights are 1000 feet and, beyond, parts of Geauga County are over 1250.
As the air is driven up these elevations, it dumps snow suddenly, thickly. As a result of this lake effect snow, the snowbelt near Chardon gets two to three times the snowfall of the rest of northern Ohio. The location of any one of these snowfalls is hard to predict. The squalls often appear in narrow bands, like curtains of snow which extend inland 25 to 30 miles before depleting themselves. The bands may stay in one place or move with the shifting winds. One spot may be buried under two feet of snow, while a town just a few miles away may escape with light flurries.
The chances for lake effect snow depend largely on the temperature difference between lake and air, the elevation of the land, and the amount of open water the cold air crosses (the fetch). A northerly wind may travel only 50 or 60 miles over the narrow width of the lake before reaching Northeast Ohio. Much more snow can be generated by a southwesterly wind blowing up the length of the lake to Buffalo and then climbing the mountains farther east.
Source: http://www.gcbl.org/explore/climate/lake-effects
Temperature and Precipitation Distribution in the Northeast
Left: Average (1981-2010) annual temperature (°F) for the Northeast region. Notice the north-south and west-east gradient. Average (1981-2010) annual precipitation (inches) for the Northeast region.
REGIONAL CLIMATE VARIATION AND WEATHER
Source:http://discoveringantarctica.org.uk/oceans-atmosphere-landscape/atmosphere-weather-and-climate/regional-climate-variation-and-weather/
In what ways does the climate vary across Antarctica and why? What weather phenomena occur in different parts of the continent?
Warm up
There are large variations in climate across the continent owing mainly to differences in latitude, altitude, and distance from the Southern Ocean. The Antarctic climate as a whole can be discussed in terms of three different climate areas: the interior climate, the coastal climate and the climate of the Antarctic Peninsula.
The coldest and driest areas are inland, where the ice sheets form high plateaux (exceeding 4000m altitude on the East Antarctic Ice Sheet). In these areas, extremely cold air descends to create persistent high pressure that brings settled conditions with relatively low wind speeds. Temperatures during the austral summer rarely exceed -20°C and during the winter months temperatures are often around -60°C. Precipitation is usually snow in the form of ‘diamond dust’ – tiny ice crystals formed from the sublimation of water vapour in a clear, but intensely cold atmosphere – averaging less than 50mm (water equivalent) per year.
Winter at Halley research station where temperatures can drop below -50°C: a bulldozer covered in drifted snow © British Antarctic Survey, Martin Bell
Near the coast there is greater seasonal variation in temperature, and in the austral summer, temperatures can warm to around 0°C. Due to the lower altitude and latitude the air is warmer than inland, so enabling it to hold a lot more water vapour; and in addition, storm systems from the Southern Ocean have more influence. Hence, annual precipitation is much higher than inland, reaching levels of around 400mm in some coastal locations, and skies are often cloudy. Precipitation is almost always in the form of snow, with rain only falling extremely rarely in some areas in the summer months. Winds can reach high velocities in areas near the coast due to topographic factors and the phenomenon of katabatic winds discussed further below.
The Antarctic Peninsula extends into the Southern Ocean and is therefore more influenced by the sea than other parts of the continent. Annual precipitation varies greatly across the Peninsula, with some areas receiving as little as 250mm water equivalent, and other areas as much as 5000mm. Near the tip of the peninsula (which lies outside the Antarctic Circle) summer temperatures often rise a little above freezing.
Cold Facts
Regional measures of temperature and precipitation
Compared with other continents, places in Antarctica where the weather is measured regularly are few and far between (although the placement of automatic weather stations and satellite-based weather measurements are improving coverage of the continent). Of the weather stations that exist, most have relatively short records. As noted in Key factors behind Antarctica’s climate, strict conditions need to be met for weather recordings to be classed as ‘official’ for statistical purposes, and climatologists like to base any statements about the climate of an area (the ‘average weather’ experienced at a locality) on at least 30 years of data (the ’30 year mean’). There are, however, some stations in Antarctica that meet these stringent requirements and allow scientists to build up a picture of how temperature, precipitation, and other climate measures vary across the continent. A few stations have records extending back well beyond 30 years, and these stations are crucial for studies of climate change as described in Climate change: past and future.
Important stations where weather is recorded in the interior of the continent include the Amundsen-Scott station (USA) at the South Pole and the Vostok station (Russia) in East Antarctica. Both stations were established in 1957 as part of the International Geophysical Year. The Amundsen-Scott station is at an elevation of 2900m above sea level and the Vostok station is at 3500m. In addition to being very cold, these stations also represent the extreme of the polar desert climate with very low levels of humidity and precipitation. Weather conditions characteristic of coastal areas of the continent are represented at Halley station (UK) bordering the Weddell Sea, which has been in operation since 1956 and McMurdo station (USA) bordering the Ross Sea, also started in 1956. Also on the coast, the Mawson station (Australia) was set up in 1954 and is the oldest continuously occupied station inside the Antarctic Circle. This station is also notable for the force of the katabatic winds (explained below) that come down off of the high ice sheet: gusts of wind here can sometimes be over 100mph. The Rothera station (UK) provides data on the climate of the Antarctic Peninsula. It is located on the west side of the peninsula at Rothera Point on Adelaide Island and has been in operation since 1975. Peninsula weather recordings have been taken over a longer period of time at Vernadsky station (Ukraine) which was previously a British station called Faraday (the climate data are sometimes referred to as being from Faraday/Vernadsky). There are also other stations along the Peninsula, notably the Palmer station (USA) and the San Martin station (Argentina). There are stations on several islands off the coast of Antarctica that provide data on the weather and climate of the Southern Ocean, for example Signy station (UK) on Signy Island in the South Orkney Islands, first occupied in 1947. Data from a selection of stations are contained in the spreadsheet attached to Student activity 2 below.
Find out the current weather conditions at various stations at the British Antarctic Survey
Winds
The dominant pattern of atmospheric circulation over Antarctica involves cold air sinking in the interior and then moving away from this area of high pressure towards the edges of the continent. The air that sinks down onto the interior is replaced by air from aloft that has travelled southwards at a higher altitude. This large-scale circulation of air is often referred to as the polar cell. (There is also a polar cell in the Arctic.) As air from the interior moves outwards across Antarctica, it doesn’t travel directly from south to north. Instead, the direction of the air is deflected by the Earth’s rotation to form polar easterlies, meaning that the prevailing wind direction is from east to west across much of the continent. This occurs because the Coriolis Force causes air to be deflected to the left of its ‘pressure gradient path’ (the most direct route from high to low pressure) in the Southern Hemisphere.
Where these easterly winds approach the coast they can reach high velocities. A katabatic wind refers to cold and relatively dense air near the surface moving downhill due to gravity. Across much of the gently sloping ice sheet katabatic winds average about 10 miles per hour; but towards the edges of the continent, slope gradients become steep and air can be funnelled down into coastal valleys causing much higher average velocities. In some places the average annual wind velocity is nearly 50 miles per hour!
High winds make life difficult at a field camp, Rutford Ice Stream © British Antarctic Survey, David Vaughan
Moving northwards, away from the continental interior, the polar easterlies eventually give way to the zone of westerly winds affecting the Southern Ocean. The westerly winds are associated with the low pressure area (circumpolar trough) that surrounds the continent owing to the large temperature difference between cold polar air and milder air from the southern mid-latitudes. The circumpolar trough exists because where warmer air meets the cold polar air (along the polar front) the warmer air is forced upwards, creating lower pressure at sea level and the frequent storm systems that affect the Southern Ocean. Air moving from the mid-latitudes towards the circumpolar trough is deflected to the left by the Coriolis force thereby producing the westerly prevailing winds. The westerly winds and Southern Ocean storm systems affect parts of the Antarctic coastline, the Antarctic Peninsula, and the islands off the coast of Antarctica.
Other weather phenomena
Antarctica is also known for several intriguing types of atmosphere and weather phenomena.
Aurora Australis
Known as the Aurora borealis in the Northern Hemisphere, this spectacular phenomenon is often described as waving or shimmering curtains of light across the night sky. It is caused by charged particles from the Sun being pulled towards the Earth’s magnetic poles and interacting with gases high in the atmosphere to illuminate the night sky.
Aurora Australis at Halley Research Station © British Antarctic Survey, Thomas Speiss
Diamond dust
This occurs when water vapour sublimates directly out of the cold atmosphere to form ice crystals, creating a form of precipitation that occurs under clear skies.
Halo
This optical phenomenon occurs frequently in Antarctica because of ice crystals in the atmosphere that reflect and refract light. Halos are typically rings of light circling the Sun or Moon, but they can also appear as pillars and arcs of light.
A sun halo above the Laws Platform at Halley V Research Station © British Antarctic Survey, Agnieszka Fryckowska
Whiteout
Under whiteout conditions, it is difficult to judge distances, perceive gradients or make out the horizon. Whiteouts can occur under different weather conditions (for example blizzards or calm, overcast conditions) where light is diffuse and it is hard to distinguish the surface from the sky.
Q
Principle 2b
The Oceans
Covering 70% of the Earth's surface, the oceans exert a major control on climate by dominating Earth's energy and water cycles.
Oceans have the capacity to absorb large amounts of solar energy. Heat and water vapor are redistributed globally through ocean currents and atmospheric circulation.
Changes in ocean circulation caused by tectonic movements or large influxes of fresh water from melting polar ice can lead to large and even abrupt changes in climate, both locally and on global scales.
click the graphic to enlarge it
Annual global sea-surface temperature anomalies from 1880 to 2015.
This graph shows how much ocean temperatures have risen since 1880. Each bar represents how much the temperature in a given year was above or below the average of the base period (1951 to 1980). Red is positive (how much the temperature was above the average), blue is negative (how much the temperatures was below the average). From: http://www.ncdc.noaa.gov/cag/time-series/global/globe/ocean/ytd/12/1880-2016.
click the graphic to enlarge it
The global ocean conveyor belt.
The global ocean conveyor belt is a constantly moving system of deep-ocean circulation driven by temperature and salinity. The great ocean conveyor moves water around the globe. Cold, salty water is dense and sinks to the bottom of the ocean while warm water is less dense and remains on the surface. The conveyor belt plays an enormous role in regulating Earth's climate.
Climate Change, and Cod, Are Causing One Heck of a Lobster Boom in Maine
Maine has had a commercial lobster industry since the 1700s, and the lobster's place in food has changed a lot during that time.
Today, Maine is faced with an unprecedented glut of lobsters–so many that the price of lobster is on the way down. But it wasn’t always so. And it may be different tomorrow.
In the 1600s and 1700s, writes Daniel Luzer for Pacific Standard, there were so many lobsters around Massachusetts Bay Colony, for instance, that they washed up on the beach in piles two feet high. “People thought of them as trash food,” Luzer writes. The ocean bugs were regarded as food for lower-class people and convicts, and used as fertilizer at times.
That began changing in the 1800s. Lobster prices–and interest in eating lobster–began to go up and down according to price, culinary innovations (like cooking lobster alive rather than dead) and availability. A century and a half later, he writes, “lobster was firmly established as a delicacy; lobster was something movie stars ate when they went out to dinner.” Read more…
Climate Change, and Cod, Are Causing One Heck of a Lobster Boom in Maine
The complex relationships between humans, lobster, and cod are creating boom times—for now
The American Lobster, 'Homarus americanus,' found on the northern area of the Atlantic coast of America. (Pixabay)
By Kat Eschner
SMITHSONIAN.COM
JUNE 15, 2017
Maine has had a commercial lobster industry since the 1700s, and the lobster's place in food has changed a lot during that time.
Today, Maine is faced with an unprecedented glut of lobsters–so many that the price of lobster is on the way down. But it wasn’t always so. And it may be different tomorrow.
In the 1600s and 1700s, writes Daniel Luzer for Pacific Standard, there were so many lobsters around Massachusetts Bay Colony, for instance, that they washed up on the beach in piles two feet high. “People thought of them as trash food,” Luzer writes. The ocean bugs were regarded as food for lower-class people and convicts, and used as fertilizer at times.
That began changing in the 1800s. Lobster prices–and interest in eating lobster–began to go up and down according to price, culinary innovations (like cooking lobster alive rather than dead) and availability. A century and a half later, he writes, “lobster was firmly established as a delicacy; lobster was something movie stars ate when they went out to dinner.”
On the coast of Maine, lobster culture became a way of life. But all was not well. .In the early twentieth century, once-abundant lobster had become rare, writes the University of Maine’s Lobster Institute, and “there were plenty of rumors about lobstermen turning to rum-running along the Maine coast during Prohibition days.”
According to Gwynn Guilford for Quartz, lobster stocks dwindled and the number of boats fishing lobster went up–a pattern, she writes, that looked like that of “other fisheries on the eve of collapse.” But today, Maine is in the midst of a lobster boom.
Maine now produces 80 percent of American-caught lobster, writes Justin Fox for Bloomberg View, and more than seven times the average take in a pre-2000 year.
The reason is anthropogenic change, although the exact factors are hard to assess. For one thing, we're eating more cod, one of the lobster's main predators. As cod stocks fall because of overfishing and other factors, more lobsters are reaching catchable age, and they wander around the seabed with impunity, making them easier for humans to catch. For another, the ocean is warming, which has moved the lobstering "sweet spot" away from the state's southern coast to the northeast, Fox writes.
Fishery practices, like not catching lobsters under or over a certain size and not catching egg-bearing females, are also part of the equation–though not, Guilford writes, to the degree lobstermen would like to think: "As the Maine’s lobster industry’s improbable rise reveals, no single species exists in a vacuum. Unfortunately, conservation efforts don’t either. Two decades of lobster abundance isn’t thanks to human mastery of “sustainability.” The ecosystem extremes that seem likely to have produced it—how we’ve pulled apart the food web, heated up the sea, re-rigged the lobster population structure—are volatile. Inevitably, nature warps again."
When Fox mentioned to Mainers that lobster stocks might again crash, he writes, he “was greeted with something of a shrug.” For now, the industry's bigger problem is preventing the oversaturation of the lobster market from tanking prices. Trade with China–as well as the growing popularity of the lobster roll–have helped.
Ocean circulation
Published: June 7, 2007, 1:14 am
Updated: March 26, 2013, 3:33 pm
Author: Michael Pidwirny
Source: http://editors.eol.org/eoearth/wiki/Ocean_currents
Ocean Circulation Conveyor Belt. The ocean plays a major role in the distribution of the planet's heat through deep sea circulation. This simplified illustration shows this "conveyor belt" circulation which is driven by the difference in heat and salinity
Surface Ocean Currents
An ocean current can be defined as a horizontal movement of seawater in the ocean. Ocean currents are driven by the circulation of wind above surface waters, interacting with evaporation, sinking of water at high latitudes, and the Coriolis force generated by the earth's rotation. Frictional stress at the interface between the ocean and the wind causes the water to move in the direction of the wind. Large surface ocean currents are a response of the atmosphere and ocean to the flow of energy from the tropics to polar regions. In some cases, currents are transient features and affect only a small area. Other ocean currents are essentially permanent and extend over large horizontal distances.
On a global scale, large ocean currents are constrained by the continental masses found bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre. Each ocean basin has a large gyre located at approximately 30° North and one at 30° South latitude in the subtropical regions. The currents in these gyres are driven by the atmospheric flow produced by the subtropical high pressure systems. Smaller gyres occur in the North Atlantic and Pacific Oceans centered at 50° North. Currents in these systems are propelled by the circulation produced by polar low pressure centers. In the Southern Hemisphere, these gyre systems do not develop because of the lack of constraining land masses.
A typical gyre displays four types of joined currents: two east-west aligned currents found respectively at the top and bottom ends of the gyre; and two boundary currents oriented north-south and flowing parallel to the continental margins. Direction of flow within these currents is determined by the direction of the macro-scale wind circulation interacting with the Coriolis force. Boundary currents play a role in redistributing global heat latitudinally.
Surface Currents of the Subtropical Gyres
On either side of the equator, in all ocean basins, there are two west-flowing currents: the North and South Equatorial (Figure 1). These currents flow between 3 and 6 kilometers per day and usually extend 100 to 200 meters in depth below the ocean surface. The Equatorial Counter Current, which flows towards the east, is a partial return of water carried westward by the North and South Equatorial currents. In El Niño years, this current intensifies in the Pacific Ocean.
Flowing from the equator to high latitudes are the western boundary currents. These warm water currents have specific names associated with their location: North Atlantic - Gulf Stream; North Pacific - Kuroshio; South Atlantic - Brazil; South Pacific - East Australia; and Indian Ocean - Agulhas. All of these currents are generally narrow, jet-like flows that travel at speeds between 40 and 120 kilometers per day. Western boundary currents are the deepest ocean surface flows, usually extending 1,000 meters below the ocean surface.
Flowing from high latitudes to the equator are the eastern boundary currents. These cold water currents also have specific names associated with their location: North Atlantic - Canary; North Pacific - California; South Atlantic - Benguela; South Pacific - Peru; and Indian Ocean - West Australia. All of these currents are generally broad, shallow moving flows that travel at speeds between 3 and 7 kilometers per day.
In the Northern Hemisphere, the east-flowing North Pacific Current and North Atlantic Drift move the waters of western boundary currents to the starting points of the eastern boundary currents. The South Pacific Current, South Indian Current and South Atlantic Current provide the same function in the Southern Hemisphere. These currents are associated with the Antarctic Circumpolar Current (West Wind Drift). Because of the absence of landmass at this latitude zone, the Antarctic Circumpolar flows in continuous fashion around Antarctica and only provides a partial return of water to the three Southern Hemispheric ocean basins.
Surface Currents of the Polar Gyres
The polar gyres exist only in the Atlantic and Pacific basins in the Northern Hemisphere. They are propelled by the counterclockwise winds associated with the development of permanent low pressure centers at 50° of latitude over the ocean basins. Note that the west-flowing current forming the southern margin of the polar gyres is also the eastward-flowing flowing current forming the northen margin of the subtropical gyres. Other currents associated with these gyres are shown on Figure 1.
Subsurface Currents
Figure 2: The following illustration describes the flow pattern of the major subsurface ocean currents. Near surface warm currents are drawn in red. Blue depicts the deep cold currents. Note how this system is continuously moving water from the surface to deep within the oceans and back to the top of the ocean. (Source: Arctic Climate Impact Assessment (ACIA)).
The world's oceans also have significant currents that flow beneath the surface (Figure 2). Subsurface currents generally travel at a much slower speed when compared to surface flows. The subsurface currents are driven by differences in the density of seawater. The density of seawater deviates in the oceans because of variations in temperature and salinity. Near-surface seawater begins its travel deep into the ocean in the North Atlantic. The downwelling of this water is caused by high levels of evaporation that cool and increase the salinity of the seawater as it flows poleward. The downwelling (sinking) of this cold, dense, saline water takes place between Northern Europe and Greenland and just north of of Labrador, Canada. This seawater then moves south at depth along the coast of North and South America until it reaches Antarctica. At Antarctica, the cold and dense seawater then travels eastward joining another deep current that is created by evaporation and sinking occuring between Antarctica and the southern tip of South America. Slightly into its eastward voyage, the deep cold flow splits off into two currents, one of which moves northward. In the North Pacific and in the northern Indian Ocean, these two currents are drawn up from the ocean floor to its surface by wind-induced upwelling. The water warms at the surface and forms a current that flows at the surface eventually back to the starting point in the North Atlantic, or creating a shallow flow that circles around Antarctica. One complete circuit of this flow of seawater is estimated to take about 1,000 years.
Citation
Pidwirny, M. (2013). Ocean circulation. Retrieved from http://www.eoearth.org/view/article/154990
For another, more extensive article on the oceans and climate, click here.
Q
Soaring ocean temperature is 'greatest hidden challenge of our generation'
IUCN report warns that ‘truly staggering’ rate of warming is changing the behaviour of marine species, reducing fishing zones and spreading disease
Source: https://www.theguardian.com/environment/2016/sep/05/soaring-ocean-temperature-is-greatest-hidden-challenge-of-our-generation
The scale of warming in the ocean is ‘truly staggering’, the report warns. Photograph: Ralph Lee Hopkins/Alamy
Oliver Milman in Honolulu
The soaring temperature of the oceans is the “greatest hidden challenge of our generation” that is altering the make-up of marine species, shrinking fishing areas and starting to spread disease to humans, according to the most comprehensive analysis yet of ocean warming.
The oceans have already sucked up an enormous amount of heat due to escalating greenhouse gas emissions, affecting marine species from microbes to whales, according to an International Union for Conservation of Nature (IUCN) report involving the work of 80 scientists from a dozen countries.
The profound changes underway in the oceans are starting to impact people, the report states. “Due to a domino effect, key human sectors are at threat, especially fisheries, aquaculture, coastal risk management, health and coastal tourism.”
Dan Laffoley, IUCN marine adviser and one of the report’s lead authors, said: “What we are seeing now is running well ahead of what we can cope with. The overall outlook is pretty gloomy.
“We perhaps haven’t realised the gross effect we are having on the oceans, we don’t appreciate what they do for us. We are locking ourselves into a future where a lot of the poorer people in the world will miss out.”
The scale of warming in the ocean, which covers around 70% of the planet, is “truly staggering”, the report states. The upper few metres of ocean have warmed by around 0.13C a decade since the start of the 20th century, with a 1-4C increase in global ocean warming by the end of this century.The ocean has absorbed more than 90% of the extra heat created by human activity. If the same amount of heat that has been buried in the upper 2km of the ocean had gone into the atmosphere, the surface of the Earth would have warmed by a devastating 36C, rather than 1C, over the past century.
Global sea surface temperatures are currently at their highest level since records began
Annual global sea surface temperature anomalies in degrees celsius
-0.500.5188019031926194919721995
At some point, the report says, warming waters could unlock billions of tonnes of frozen methane, a powerful greenhouse gas, from the seabed and cook the surface of the planet. This could occur even if emissions are drastically cut, due to the lag time between emitting greenhouse gases and their visible consequences.
Warming is already causing fish, seabirds, sea turtles, jellyfish and other species to change their behaviour and habitat, it says. Species are fleeing to the cooler poles, away from the equator, at a rate that is up to five times faster than the shifts seen by species on land.
Even in the north Atlantic, fish will move northwards by nearly 30km per decade until 2050 in search of suitable temperatures, with shifts already documented for pilchard, anchovy, mackerel and herring.
The warming is having its greatest impact upon the building blocks of life in the seas, such as phytoplankton, zooplankton and krill. Changes in abundance and reproduction are, in turn, feeding their way up the food chain, with some fish pushed out of their preferred range and others diminished by invasive arrivals.
With more than 550 types of marine fishes and invertebrates already considered threatened, ocean warming will exacerbate the declines of some species, the report also found.
The movement of fish will create winners and losers among the 4.3 billion people in the world who rely heavily upon fish for sustenance. In south-east Asia, harvests from fisheries could drop by nearly a third by 2050 if emissions are not severely curtailed. Global production from capture fisheries has already levelled off at 90m tonnes a year, mainly due to overfishing, at a time when millions more tonnes will need to be caught to feed a human population expected to grow to 9 billion by 2050.
Humans are also set to suffer from the spread of disease as the ocean continues to heat up. The IUCN report found there is growing evidence of vibrio bacterial disease, which can cause cholera, and harmful algal bloom species that can cause food poisoning. People are also being affected by more severe, if not more numerous, hurricanes due to the extra energy in the ocean and atmosphere.
Coral reefs, which support around a quarter of all marine species, are suffering from episodes of bleaching that have increased three-fold over the past 30 years. This bleaching occurs when prolonged high temperatures cause coral to expel its symbiotic algae, causing it to whiten and ultimately die, such as the mass mortality that has gripped the Great Barrier Reef.
Ocean acidification, where rising carbon dioxide absorption increases the acidity of the water, is making it harder for animals such as crabs, shrimps and clams to form their calcium carbonate shells.
The IUCN report recommends expanding protected areas of the ocean and, above all, reduce the amount of heat-trapping gases pumped into the atmosphere.
“The only way to preserve the rich diversity of marine life, and to safeguard the protection and resources the ocean provides us with, is to cut greenhouse gas emissions rapidly and substantially,” said Inger Andersen, director general of the IUCN.
Q
Principle 2c
The Atmosphere
Our atmosphere effects the amount of solar energy absorbed by the Earth and the amount of that energy that the Earth radiates back out into space. 
Greenhouse gases — water vapor, carbon dioxide, and methane — occur naturally in small amounts in our atmosphere. They absorb and release heat energy more efficiently than other gases like nitrogen and oxygen, which are actually more abundant.
Indeed, the greenhouse gases are so efficient that even a small increase in their concentration can have an enormous effect on our climate. Human released CO2 and methane are two greenhouse gases that are dramatically changing Earth’s climate. Read more…
The Atmosphere
Our atmosphere effects the amount of solar energy absorbed by the Earth and the amount of that energy that the Earth radiates back out into space. In the 1800s, scientist John Tyndall was studying the energy balance of the Earth, looking at incoming solar energy, primarily short-wave ultraviolet and visible light, and measuring that against outgoing infrared heat from the Earth after it has absorbed the energy from the sun. Tyndall set up an atmospheric laboratory in the basement of the Royal Institute in London and began experimenting with various trace gases in the atmosphere to determine their radiant properties. The primary gases in air— nitrogen (79%), oxygen (21%), and argon (~1%) — make up 99.9% of the gas in the atmosphere. But none of them absorb light and re-radiate infrared heat. Through his experiments, Tyndall determined that two gases in particular, water vapor (H2O) and carbon dioxide (CO2) do absorb light and re-radiate heat. He published his findings in 1872. From this research the concept of the “Greenhouse Effect” as an important component of the Earth’s energy balance was developed—so we have known about the greenhouse effect at least since then. Tyndall’s laboratory experiments contribute to today’s detailed understanding of how solar energy drives the Earth’s climate system.
Greenhouse gases — water vapor, carbon dioxide, and methane — occur naturally in small amounts in our atmosphere. They absorb and release heat energy more efficiently than other gases like nitrogen and oxygen, which are actually more abundant.
Indeed, the greenhouse gases are so efficient that even a small increase in their concentration can have an enormous effect on our climate. Human released CO2 and methane are two greenhouse gases that are dramatically changing Earth’s climate.With the setup below, Tyndall observed new chemical reactions produced by high frequency light waves acting on certain vapors. This work help lead to his discovery of the greenhouse effect.
Atmospheric composition
Published: July 14, 2010, 12:11 am
Updated: April 4, 2013, 5:45 pm
Author: Michael Pidwirny
Table 1 lists the eleven most abundant gases found in the Earth's lower atmosphere by volume. Of the most abundant, nitrogen, oxygen, water vapor, carbon dioxide, methane, nitrous oxide, and ozone are extremely important to the health of the Earth's biosphere.
The table indicates that nitrogen and oxygen are the main components of the atmosphere by volume. Together these two gases make up approximately 99% of the dry atmosphere. Both of these gases have very important associations with life. Nitrogen is removed from the atmosphere and deposited at the Earth's surface mainly by specialized nitrogen fixing bacteria, and by way of lightning through precipitation. The addition of this nitrogen to the Earth's surface soils and various water bodies supplies much needed nutrition for plant growth. Nitrogen returns to the atmosphere primarily through biomass combustion and denitrification.
Oxygen is exchanged between the atmosphere and life through the processes of photosynthesis and respiration. Photosynthesis produces oxygen when carbon dioxide and water are chemically converted into glucose with the help of sunlight. Respiration is the opposite process of photosynthesis. In respiration, oxygen is combined with glucose to chemically release energy for metabolism. The products of this reaction are water and carbon dioxide.
Figure 1:Global distribution of water vapor for January 2003 from the Earth's surface to the top of the atmosphere as measured in millimeters of precipitable water. During January, the atmosphere contains low amounts of water vapor over the continents of North America and Eurasia which are experiencing winter. The highest concentrations of water vapor are found over the equator, Brazil, Indonesia, northern Australia, the Indian Ocean, and the western side of South Pacific Ocean at the tropics and subtropics.(Source: NASA's Atmospheric Infrared Sounder)
The next most abundant gas on the table is water vapor. Water vapor varies in concentration in the atmosphere both spatially and temporally (see Figures 1 and 2). The highest concentrations of water vapor are found near the equator over the oceans and tropical rain forests. Cold polar areas and subtropical continental deserts are locations where the volume of water vapor can approach zero percent. Water vapor has several very important functional roles on our planet:
The fifth most abundant gas in the atmosphere is carbon dioxide. The volume of this gas has increased by over 35% in the last three hundred years (see Figure 3). This increase is primarily due to human activities such as combustion of fossil fuels, deforestation, and other forms of land-use change. Some scientists believe that this increase is causing global warming through an enhancement of the greenhouse effect. Carbon dioxide is also exchanged between the atmosphere and life through the processes of photosynthesis and respiration.
Figure 2:Global distribution of water vapor for July 2003 from the Earth's surface to the top of the atmosphere as measured in millimeters of precipitable water. Compared to the month of January, the continents of North America and Eurasia see a significant increase in atmospheric water vapor. Areas with the highest concentrations of water vapor are found over the equator, India, southeast Asia, the Indian Ocean, the Gulf of Mexico, central Africa, western side of the North Pacific Ocean at the tropics and subtropics, and along the east coast of the USA. (Source: NASA's Atmospheric Infrared Sounder)
Methane is a very strong greenhouse gas. Since 1750, methane concentrations in the atmosphere have increased by more than 150%. The primary sources for the additional methane added to the atmosphere (in order of importance) are: rice cultivation; domestic grazing animals; termites; landfills; coal mining; and oil and gas extraction. Anaerobic conditions associated with rice paddy flooding results in the formation of methane gas. However, an accurate estimate of how much methane is being produced from rice paddies has been difficult to ascertain. More than 60% of all rice paddies are found in India and China where scientific data concerning emission rates are unavailable. Nevertheless, scientists believe that the contribution of rice paddies is large because this form of crop production has more than doubled since 1950. Grazing animals release methane to the environment as a result of herbaceous digestion. Some researchers believe the addition of methane from this source has more than quadrupled over the last century. Termites also release methane through similar processes. Land-use change in the tropics, due to deforestation, ranching, and farming, may be causing termite numbers to expand. If this assumption is correct, the contribution from these insects may be important. Methane is also released from landfills, coal mines, and gas and oil drilling. Landfills produce methane as organic wastes decompose over time. Coal, oil, and natural gas deposits release methane to the atmosphere when these deposits are excavated or drilled.
The average concentration of the greenhouse gas nitrous oxide is now increasing at a rate of 0.2 to 0.3% per year. Its part in the enhancement of the greenhouse effect is minor relative to the other greenhouse gases already mentioned. However, it does have an important role in the artificial fertilization of ecosystems. In extreme cases, this fertilization can lead to the death of forests, eutrophication of aquatic habitats, and species exclusion. Sources for the increase of nitrous oxide in the atmosphere include: land-use conversion; fossil fuel combustion; biomass burning; and soil fertilization. Most of the nitrous oxide added to the atmosphere each year comes from deforestation and the conversion of forest, savanna and grassland ecosystems into agricultural fields and rangeland. Both of these processes reduce the amount of nitrogen stored in living vegetation and soil through the decomposition of organic matter. Nitrous oxide is also released into the atmosphere when fossil fuels and biomass are burned. However, the combined contribution to the increase of this gas in the atmosphere is thought to be minor. The use of nitrate and ammonium fertilizers to enhance plant growth is another source of nitrous oxide, yet how much is released from this process has been difficult to quantify. Estimates suggest that the contribution from this source represents from 50% to 0.2% of nitrous oxide added to the atmosphere annually.
Figure 3: Changing Content of Carbon Dioxide in the Earth's Atmosphere. The following graph illustrates the rise in atmospheric carbon dioxide from 1744 to 2005. Note that the increase in carbon dioxide's concentration in the atmosphere has been exponential during the period examined. An extrapolation into the immediate future would suggest continued increases. (Source: PhysicalGeography.net)
Ozone's role in the enhancement of the greenhouse effect has been difficult to determine. Accurate measurements of past long-term (more than 25 years in the past) levels of this gas in the atmosphere are currently unavailable. Moreover, concentrations of ozone gas are found in two different regions of the Earth's atmosphere. The majority of the ozone (about 97%) found in the atmosphere is concentrated in the stratosphere at an altitude of 15 to 55 kilometers above the Earth's surface. This stratospheric ozone provides an important service to life on the Earth as it absorbs harmful ultraviolet radiation. In recent years, levels of stratospheric ozone have been decreasing due to the buildup of human-created chlorofluorocarbons in the atmosphere. Since the late 1970s, scientists have noticed the development of severe holes in the ozone layer over Antarctica. Satellite measurements have indicated that the zone from 65° North to 65° South latitude has had a 3% decrease in stratospheric ozone since 1978.
Ozone is also highly concentrated at the Earth's surface in and around cities. Most of this ozone is created as a byproduct of human-created photochemical smog. This buildup of ozone is toxic to organisms living at the Earth's surface.
Citation
Pidwirny, M. (2013). Atmospheric composition. Retrieved from http://www.eoearth.org/view/article/150296
- See more at: http://www.eoearth.org/view/article/150296/#sthash.gMGIJDaL.dpuf
Q
Greenhouse effect
Published: September 17, 2008, 4:48 pm
Updated: July 17, 2012, 8:52 pm
Author: Michael Pidwirny
Figure 1: As illustrated in the diagram above, of all the sunlight that passes through the atmosphere annually, only 51% is available at the Earth's surface to do work. This energy is used to heat the Earth's surface and lower atmosphere, melt and evaporate water, and run photosynthesis in plants. Of the other 49%, 4% is reflected back to space by the Earth's surface, 26% is scattered or reflected to space by clouds and atmospheric particles, and 19% is absorbed by atmospheric gases and clouds. (Source: PhysicalGeography.net)
The greenhouse effect is a naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the Earth's surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18° Celsius, rather than the present 15° Celsius.
As energy from the sun passes through the atmosphere a number of things take place (see Figure 1). A portion of the energy (26% globally) is reflected or scattered back to space by clouds and other atmospheric particles. About 19% of the energy available is absorbed by clouds, gases (like ozone), and particles in the atmosphere. Of the remaining 55% of the solar energy passing through the Earth's atmosphere, 4% is reflected from the surface back to space. On average, about 51% of the sun's radiation reaches the surface. This energy is then used in a number of processes, including the heating of the ground surface; the melting of ice and snow and the evaporation of water; and plant photosynthesis.
Figure 2: The diagram above illustrates the greenhouse effect. This process begins with the absorption of shortwave radiation from the sun. Absorption causes the solar energy to be converted into sensible heat at the Earth's surface. Some of this heat is transferred to the lower atmosphere by conduction and convection. After the heating of the ground and the lower atmosphere, these surfaces become radiators of infrared or longwave radiation and they begin to cool. This emission of energy is directed to space. However, only a portion of this energy actually makes it through the atmosphere. About 70% of the longwave radiation emitted from the Earth's surface is absorbed by the atmosphere's greenhouse gases. (Source: PhysicalGeography.net)
The heating of the ground by sunlight causes the Earth's surface to become a radiator of energy in the longwave band (sometimes called infrared radiation). This emission of energy is generally directed to space (see Figure 2). However, only a small portion of this energy actually makes it back to space. The majority of the outgoing infrared radiation is absorbed by the greenhouse gases (see Figure 3).
Absorption of longwave radiation by the atmosphere causes additional heat energy to be added to the Earth's atmospheric system. The now warmer atmospheric greenhouse gas molecules begin radiating longwave energy in all directions. Over 90% of this emission of longwave energy is directed back to the Earth's surface where it once again is absorbed by the surface. The heating of the ground by the longwave radiation causes the ground surface to once again radiate, repeating the cycle described above, again and again, until no more longwave is available for absorption.
Figure 3: Annual (1987) quantity of outgoing longwave radiation absorbed in the atmosphere. Image created by the CoVis Greenhouse Effect Visualizer. (Source: PhysicalGeography.net)
The amount of heat energy added to the atmosphere by the greenhouse effect is controlled by the concentration of greenhouse gases in the Earth's atmosphere. All of the major greenhouse gases have increased in concentration since the beginning of the Industrial Revolution (about 1700 AD). As a result of these higher concentrations, scientists predict that the greenhouse effect will be enhanced and the Earth's climate will become warmer. Predicting the amount of warming is accomplished by computer modeling. Computer models suggest that a doubling of the concentration of the main greenhouse gas, carbon dioxide, may raise the average global temperature between 1 and 3° Celsius. However, the numeric equations of computer models do not accurately simulate the effects of a number of possible negative feedbacks. For example, many of the models cannot properly simulate the negative effects that increased cloud cover would have on the radiation balance of a warmer Earth. Increasing the Earth's temperature would cause the oceans to evaporate greater amounts of water, causing the atmosphere to become cloudier. These extra clouds would then reflect a greater proportion of the sun's energy back to space reducing the amount of solar radiation absorbed by the atmosphere and the Earth's surface. With less solar energy being absorbed at the surface, the effects of an enhanced greenhouse effect may be counteracted.
A number of gases are involved in the human-caused enhancement of the greenhouse effect (see Table 1). These gases include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFXClX); and tropospheric ozone (O3). Of these gases, the single most important gas is carbon dioxide which accounts for about 63% of the change in the intensity of the Earth's greenhouse effect as measured in 2005 (Foster et al., 2007). The contributions of the other gases are 18% for methane, 10% for chlorofluorocarbons, and 6% for nitrous oxide. Ozone's contribution to the enhancement of greenhouse effect is still yet to be quantified.
In summary, the greenhouse effect causes the atmosphere to trap more heat energy at the Earth's surface and within the atmosphere by absorbing and re-emitting longwave energy. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. In the last few centuries, the activities of humans have directly or indirectly caused the concentration of the major greenhouse gases to increase. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that the Earth's average global temperature has already increased by 0.3 to 0.6° Celsius, since the beginning of this century, because of this enhancement. Predictions of future climates indicate that by the middle of the next century the Earth's global temperature may be 1 to 3° Celsius higher than today.
Further Reading
- PhysicalGeography.net
- Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland. 2007. Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.). Cambridge University Press, Cambridge, United Kingdom.
Citation
Pidwirny, M. (2012). Greenhouse effect. Retrieved from http://www.eoearth.org/view/article/153146
Q
Greenhouse gas
Published: May 9, 2010, 5:02 pm
Updated: September 22, 2012, 11:56 am
Author: Dave Reay
Author: C Michael Hogan
Methane clathrate deep ocean deposit. Source: Wikimedia Commons
A greenhouse gas is one of several gases that can absorb and emit longwave (infrared) radiation in a planetary atmosphere. This phenomenon is often termed the greenhouse effect. Of the sunlight that falls on the Earth's surface, approximately 40% of that energy is reradiated upward into the atmosphere in the form of longwave radiation. Approximately 75% of that upward radiated longwave energy is absorbed by water vapor, carbon dioxide, methane and other greenhouse gases. Since this absorption process is molecular in nature, the subsequent re-radiation of energy by these gases is multidirectional. As a result, about 50% of the longwave emission is reradiated back toward the Earth where it is once again turned into heat energy. Through this process, greenhouse gases contribute to the amount of heat energy released at the Earth's surface and in the lower atmosphere.
Since the beginning of Industrial Revolution, concentrations of carbon dioxide, methane, and nitrous oxide have all risen dramatically because of human activities. Fossil fuel combustion, land-use change, increasingly intensive agriculture, and an expanding global human population are the primary causes for these increases. Other greenhouse gases found in our planet's atmosphere include water vapor, ozone, sulfur hexafluoride and chlorofluorocarbons.
Carbon Dioxide
Before 1700, levels of carbon dioxide were about 280 ppm (parts per million). Concentrations of carbon dioxide in the atmosphere are now about 390 ppm (Figure 1). This increase in carbon dioxide in the atmosphere is mainly due to activities associated with the Industrial Revolution. Emissions from the combustion of fossil fuels account for about 65% of the carbon dioxide added to the atmosphere. The remaining 35% is derived from deforestation and the conversion of prairie, woodland, and forested ecosystems primarily into less productive agricultural systems. Natural ecosystems can store 20 to 100 times more carbon dioxide per unit area than agricultural systems. Both deforestation and natural land-use change reduce the amount of standing plant mass or biomass found on the Earth’s surface. This reduction causes a net export of carbon stored in biomass into the atmosphere through decomposition and burning.
Figure 1. The following graph illustrates the rise in atmospheric carbon dioxide from 1744 to 2006. Note that the increase in carbon dioxide's concentration in the atmosphere is exponential in nature. An extrapolation into the immediate future would suggest continued annual increases. (Image Copyright: Michael Pidwirny, Data Source: Neftel, A., H. Friedli, E. Moore, H. Lotscher, H. Oeschger, U. Siegenthaler, and B. Stauffer. 1994. Historical carbon dioxide record from the Siple Station ice core. pp. 11-14. In T.A. Boden, D.P. Kaiser, R.J. Sepanski, and F.W. Stoss (eds.) Trends'93: A Compendium of Data on Global Change. ORNL/CDIAC-65. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tenn. USA and C.D. Keeling and T.P. Whorf. 2001. Carbon Dioxide Research Group, Scripps Institution of Oceanography, University of California, La Jolla, California 92093-0444, USA.).
Fossil Fuel Combustion
Fossil fuel combustion currently contributes about 8.7 gigatonnes of carbon as carbon dioxide to the atmosphere each year. Coal combustion for electricity generation represents the primary source (Figure 2), with oil and gas combustion also being significant contributors.
Figure 2. The Lakeview coal-fired power plant in Mississauga, Ontario, Canada. (Source: Wikimedia Commons, Photographer Hmvh1. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license).
Deforestation Emissions
Deforestation includes tree removal, charcoal production, slash-and-burn practices and forest degradation. In total, these activities contribute approximately 25% of the carbon added to the atmosphere due to human activities, largely as carbon dioxide (Figure 3). Deforestation and land-use change can have a double impact on greenhouse gas fluxes: carbon is released when forests are burned or logged, and the land-based 'sink' of carbon dioxide (the long term uptake and storage of carbon by plants and soils) is reduced when forest is replaced by cropland or rangeland. The majority of carbon dioxide emissions due to deforestation arise from activities in Africa, Asia and South America according to UN FAO sources. It is worth noting that the terrestrial carbon sink - primarily the forests and soils - currently absorbs around 1 billion tonnes of carbon each year from the atmosphere.
Figure 3. Deforestation in Brazil for the purpose of expanding rangeland for livestock..
Source: Antonio Cruz, da Abr. licensed under the Creative Commons Attribution 2.5 Brazil
Carbon Dioxide Sinks
Terrestrial ecosystems emit approximately 119 billion tonnes of carbon each year via the process of respiration and absorb approximately 120 billion tonnes of carbon each year via photosynthesis - a net sink of 1 billion tonnes of carbon. The oceans emit approximately 88 billion tonnes of carbon each year and absorb about 90 billion tonnes of carbon each year - a net sink of about 2 billion tonnes of carbon. Our planet's oceans and terrestrial ecosystems represent net sinks for carbon dioxide, together absorbing approximately 3 billion tonnes of carbon more from the atmosphere each year than they emit. Since the Industrial Revolution these sinks have absorbed about 40% of the carbon dioxide emissions released by human activities.
The response of these land and ocean carbon sinks to climate change in the 21st century remains a key area of uncertainty. Elevated temperatures, changes in rainfall patterns, and acidification of the oceans may serve to reduce the size of the land and ocean sinks - further increasing the concentration of carbon dioxide in the atmosphere.
Methane
Since 1750, atmospheric concentrations of the greenhouse gas methane have increased more than 150% (Figure 4). The primary sources of the man-made methane added to the atmosphere are rice cultivation, ruminant livestock, landfill out-gassing, oil and gas extraction, and coal mining. Key natural sources include wetlands, termites and geological sources such as the thawing of methane clathrates.
Figure 4. The following graph illustrates the rise in atmospheric methane from 1008 to 2001. Note that the increase in methane's concentration in the atmosphere is exponential in nature. An extrapolation into the immediate future would suggest continued annual increases. (Image Copyright: Michael Pidwirny, Data Source: D.M. Etheridge, L.P. Steele, R.J. Francey, and R.L. Langenfelds. 2002. Historical CH4 Records Since About 1000 A.D. From Ice Core Data. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., USA and Steele, L. P., P. B. Krummel and R. L. Langenfelds. 2002. Atmospheric CH4 concentrations from sites in the CSIRO Atmospheric Research GASLAB air sampling network (October 2002 version). In Trends: A Compendium of Data on Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN, USA.).
Worldwide the countries responsible for generating significant amounts of methane include China, India, Brazil, Mexico and Russia. These same nations are also projected to still be the greatest generators of methane in the year 2050. The USA and combined European Union countries provide lesser emissions, and the trends in those two advanced regions are flat to down.
Rice Cultivation
Rice cultivation presently accounts for about 20% of worldwide methane emissions, including both natural and human-made sources. Anaerobic conditions associated with rice paddy flooding results in the formation of methane gas. An accurate estimate of how much methane is being produced from rice paddies has been difficult to determine. More than 60% of all rice paddies are found in India and China where scientific data concerning emission rates are hard to obtain. Nevertheless, scientists believe that the contribution of rice paddies is large because this type of crop production has more than doubled since 1950. Much of the recent expansion of rice paddy methane emissions is attributed to the need to feed an expanding human population by increasing crop yields through the cultivation of more land (Figure 5). 
Figure 5. Rice paddy terraces in Sapa, Vietnam. Source: A.J. Oswald/Wikimedia Commons
Ruminant Livestock Emissions
Ruminant grazing is another important source of methane emissions (Figure 6). Grazing animals release methane to the environment as a result of herbaceous digestion. For example, one cow typically produces approximately 150 grams of methane per day. In the USA, the 2010 standing herd of bovines amounted to 106 million animals. These methane emissions are further magnified by the millions of sheep and goats kept worldwide for human use. Some researchers believe the addition of methane from livestock has more than quadrupled over the last century.
Figure 6. Intensive cattle production is a source of significant methane. Image Source: Agricultural Research Service, United States Department of Agriculture
Landfills
Landfills are the second biggest source of methane in the United States. Methane is created in landfills when organic waste decomposes anaerobically (without oxygen). The amount of methane produced depends on factors like the type of waste, moisture content of the waste, and the design and management practices at the landfill. Some landfills reduce methane emissions by burning this gas to produce electrical energy which is sold to utility companies.
Fossil Fuel Extraction
The process of extracting of natural gas, petroleum, and coal from the lithosphere is a significant source of methane emissions (called fugitive emissions) into the atmosphere (Figure 7). Methane emissions also occur during the processing, storage, and distribution of these fossil fuels. Fossil fuel use because of human activities is the fourth largest source of methane emissions in the United States.
Figure 8. Strip coal mining operation in the United States. The mining of coal releases significant quantities of methane to the atmosphere. (Image Source: Wikimedia Commons, Photographer Stephen Codrington. This image is licensed under the Creative Commons Attribution 2.5 Generic license).
Methane Clathrates
Frozen methane clathrate deposits found at the bottom of seabeds and in deep permafrost is a natural source of potential greenhouse gases that may have a significant effect on our planet's climate. Recent scientific studies have found that significant amounts of methane may have been recently released into the atmosphere from methane clathrate deposits found in the Arctic. Research has identified that the release of this methane into the atmosphere may be occurring because of global warming. Further, NASA scientist James Hansen has suggested that methane clathrate emissions may continue to grow because a warmer planet leads to a positive feedback situation (Figure 8).
Figure 8. The future warming of methane clathrate deposits may release methane to the atmosphere creating more global warming. This positive feedback process with feed additional methane release because of the increase in temperature, This cycle could repeat itself many times. Image Source: Michael Pidwirny.
Nitrous Oxide
The average concentration of nitrous oxide in the atmosphere is now increasing at a rate of 0.2 to 0.3% per year (Figure 9). The role of nitrous oxide in the enhancement of the greenhouse effect is smaller than the other greenhouse gases already discussed, but still represents about 6% of climate forcing though human-induced greenhouse gas emissions. Sources for the increase of nitrous oxide in the atmosphere include land-use conversion, fossil fuel combustion, biomass burning, and soil fertilization. Most of the nitrous oxide added to the atmosphere each year due to human activities comes from agricultural soils, where nitrogen-rich fertilizer and manure is converted to nitrous oxide by soil bacteria. Nitrous oxide is also released into the atmosphere when fossil fuels and biomass are burned. 
Figure 9. The following graph illustrates the rise in atmospheric nitrous oxide from 1978 to 2010. An extrapolation into the immediate future would suggest continued annual increases. (Image Copyright: Michael Pidwirny, Data Source: National Oceanic and Atmospheric Administration's (NOAA) Climate Monitoring and Diagnostics Laboratory, http://www.esrl.noaa.gov/gmd/hats/combined/N2O.html).
Chlorofluorocarbons and 'F-gases'
'F-gases' are man-made gases that included chlorofluoracarbons (CFCs), hydrofluorocarbons (HFCs) and sulphur hexafluoride (SF6). They are used as refrigerants, propellants and in electronics manufacture, but are highly persistent in the Earth's atmosphere. They are typically thousands of times more potent as greenhouse gases than carbon dioxide. While reductions in the use of CFCs have been underway in Western Nations for over twenty years, these chemicals are still used in some developing countries. The Montreal Protocol, the international agreement that phases out ozone-depleting substances, requires the end of chlorodifluoromethane production by 2020 in developed countries and 2030 in developing countries. CFCs have been gradually phased out in most nations and replaced by hydrofluorocarbons (HFCs) which avoid ozone depletion problems, but are still very potent greenhouse gases. The Kyoto Protocol aims to reduce emissions of these HFCs by tighter controls and the use of new alternatives such as using butane or propane as the coolant in refrigerators rather than HFCs.
Climate Forcing Effects
In climate science, the relative climate-forcing strength of different greenhouse gases is described relative to that of carbon dioxide. Methane is much more effective at absorbing infrared radiation (heat) and is thus a more powerful greenhouse gas. Yet its lifetime in the atmosphere is only about 10 years, compared to between 30 and 1000 years for a molecule of carbon dioxide. As such, the climate-forcing strength of a kilogram of methane on a 100 year time-horizon – its Global Warming Potential (GWP) – is 25. That is, every kilogram of methane in the atmosphere has the equivalent global warming potential of 25 kilograms of carbon dioxide. The GWP of nitrous oxide is 298.
Our impact on the global climate since the Industrial Revolution has been difficult to interpret. While emissions of greenhouse gases, like carbon dioxide and methane have had a net warming effect, emissions of sulphate aerosols have had a net cooling effect. The net effect is warmer global temperatures, but the complex interaction of these positive and negative influences on the Earth's climate system make predicting future effects difficult.
The problem is exacerbated by our poor level of understanding of exactly how some factors, like land surface albedo (the reflectivity of the land) and cloud cover, operate and interact with changes in global temperature. Another very important greenhouse gas is water vapor and, though human activities are not directly responsible for changes in its concentration in the atmosphere, an indirect increase through elevated surface and lower atmosphere (troposphere) temperatures may lead to an signifcant positive feedback process - making global temperatures even warmer.
The Future
While many developed nations have had carbon emission reduction programs underway for several years, often as part of the United Nations Framework Convention on Climate Change and its 'Kyoto Protocol', their combined emissions have continued to increase. In the coming decades, growth in carbon emissions is likely to be dominated by emissions from developing and transition economies like China and India.
From a technological standpoint, the most straightforward methods of greenhouse gas mitigation are through increased energy efficiency, in production and/or use. Carbon Capture and Storage technologies have been touted as a way to continue fossil fuel based energy generation while reducing emissions of carbon dioxide to the atmosphere. On methane emissions reduction, there is promising research in ruminant feeds, which is leading to per capita cutbacks in animal methane output. In the case of rice farming, changes in land management and crop varieties also hold significant potential.
References
- Hansen, James. 2009. Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity. Bloomsbury Press, New York.
- Holmes, A. and D. Duff. 1993. Holmes Principles of Physical Geology. Fourth Edition. Routledge, Taylor & Francis Group Ltd., Oxford.
- NOAA. 2010. Emissions of Potent Greenhouse Gas Increase Despite Reduction Efforts U.S. Dept. of Commerce, Washington DC.
- Peake, S. and J. Smith. Climate Change: From Science to Sustainability. Second Edition. Oxford University Press, Oxford.
- Pittock, A.B. 2009. Climate Change: The Science, Impacts and Solutions. Second Edition. CSIRO Publishing, Collingwood, Australia.
- Ruddiman, W.F. 2008. Earth's Climate: Past and Future. Second Edition. W.H. Freeman and Company, New York.
- Schmidt, G. 2004. Methane: A Scientific Journey from Obscurity to Climate Super-Stardom. NASA Goddard Space Center.
- Shakhova, N., I. Semiletov, A. Salyuk, D. Kosmach, and N. Bel’cheva (2007), Methane release on the Arctic East Siberian shelf, Geophysical Research Abstracts, 9, 01071.
- Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (Eds.). 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Cambridge University Press, Cambridge, United Kingdom.
- Takahashi, J. and B.A. Young (Editors). 2002. Greenhouse Gases and Animal Agriculture. Proceedings of the 1st International Conference on Greenhouse Gases and Animal Agriculture, Obihiro, Japan, 7-11 November, 2001. Elsevier Sciences, Amsterdam, The Netherlands.
Citation
Reay, D., & Hogan, C. (2012). Greenhouse gas. Retrieved from http://www.eoearth.org/view/article/153147
- See more at: http://www.eoearth.org/view/article/153147/#sthash.UkE2OoDf.dpuf
Q
Principle 2d
Carbon Cycling
click the image to enlarge the graphic
1. The first involves the sediments at the bottom of the oceans: Carbon dioxide dissolves into cold ocean water at high latitudes. That CO2 is then carried to the deep ocean by sinking currents where it stays in sediments for hundreds of years. Thus deep ocean currents pump carbon from the atmosphere into the sea for storage.
2. Plants also reduce carbon in the atmosphere through photosynthesis: They use energy from the sun to combine CO2 from the atmosphere and water from the soil to make carbon-rich carbohydrates like glucose.Thus plants extract CO2 from the atmosphere and accumulate it in their tissues. Conversely, cutting down trees, or deforestation, and the burning of fossil fuels increases CO2 in the atmosphere.
3. Some carbon is transformed into calcium carbonate (limestone), the largest carbon reservoir on Earth. Read more…
Carbon Cycling
This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. All numbers are in gigatons. (Diagram adapted from U.S. DOE, Biological and Environmental Research Information System.)
The level of greenhouse gases in the atmosphere is controlled by cycles. Those cycles move carbon between the ocean, land, different life forms, and the atmosphere. Here are three examples:
1. The first involves the sediments at the bottom of the oceans: Carbon dioxide dissolves into cold ocean water at high latitudes. That CO2 is then carried to the deep ocean by sinking currents where it stays in sediments for hundreds of years. Thus deep ocean currents pump carbon from the atmosphere into the sea for storage.
2. Plants also reduce carbon in the atmosphere through photosynthesis: They use energy from the sun to combine CO2 from the atmosphere and water from the soil to make carbon-rich carbohydrates like glucose.Thus plants extract CO2 from the atmosphere and accumulate it in their tissues. Conversely, cutting down trees, or deforestation, and the burning of fossil fuels increases CO2 in the atmosphere.
3. Some carbon is transformed into calcium carbonate (limestone), the largest carbon reservoir on Earth.
These are three examples of cycles that directly or indirectly relate affect our climate. Of these, the carbon cycle has been involved in long-term climatic change in geological history. Carbon cycling involves many carbon-containing compounds, including short-term organic, long-term organic, and inorganic compounds.
Biological processes involving the carbon cycle have relatively short time spans influenced by photosynthesis and diurnal (day-night) and annual cycles. On a longer time span, carbon dioxide is moved from the atmosphere and the ocean and into terrestrial deposits through biologic and geologic processes.
Carbon cycle
Published: May 31, 2010, 7:46 pm
Updated: May 7, 2012, 11:29 am
Author: Michael Pidwirny
Figure 1: Carbon cycle. (Source: PhysicalGeography.net)
All life is based on the element carbon. Carbon is the major chemical constituent of most organic matter, from fossil fuels to the complex molecules (DNA and RNA) that control genetic reproduction in organisms. Yet by weight, carbon is not one of the most abundant elements within the Earth's crust. In fact, the lithosphere is only 0.032% carbon by weight. In comparison, oxygen and silicon respectively make up 45.2% and 29.4% of the Earth's surface rocks.
Carbon is stored on our planet in the following major sinks (Figure 1 and Table 1): (a) as organic molecules in living and dead organisms found in the biosphere; (b) as the gas carbon dioxide in the atmosphere; (c) as organic matter in soils; (d) in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk; and (e) in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.
Photosynthesis and respiration
Ecosystems gain most of their carbon dioxide from the atmosphere. A number of autotrophic organisms have specialized mechanisms that allow for absorption of this gas into their cells. With the addition of water and energy from solar radiation, these organisms use photosynthesis to chemically convert the carbon dioxide to carbon-based sugar molecules. Each year, photosynthesis by terrestrial plants moves about 110 petagrams (1 petagram = 1015 grams = 1012 kilograms = 1 billion metric tons; so 110 petagrams = 110 billion metric tons) of carbon from the atmosphere to the biota. These molecules can then be chemically modified by these organisms through the metabolic addition of other elements to produce more complex compounds like proteins, cellulose, and amino acids. Some of the organic matter produced in plants is passed down to heterotrophic animals through consumption.
Figure 1: Carbon cycle. (Source: PhysicalGeography.net)
Carbon is released from ecosystems as carbon dioxide gas by the process of respiration. Respiration takes place in both plants and animals and involves the breakdown of carbon-based organic molecules into carbon dioxide gas and some other compound byproducts. The detritus food chain contains a number of organisms whose primary ecological role is the decomposition of organic matter into its abiotic components. Each year, respiration by organisms other than detrivores returns to the atmosphere almost half (50 petagrams or 50 billion metric tons) of the carbon dioxide that is absorbed by photosynthesis. Another portion of the carbon that flows from the atmosphere to the biota becomes part of the detritus food chain. Partially decomposed organic matter becomes part of the soil carbon storage pool. Eventually, the organic material in the soil is decomposed to its constituents, water and carbon dioxide, which return to the atmosphere. This flow of carbon is known as decay and accounts for about 60 petagrams (60 billion metric tons). Together with respiration, these flows account for most but not all of the carbon removed from the atmosphere by photosynthesis.
Carbon in the oceans
| Table 1: Estimated major stores of carbon on the Earth. | Sink | Amount in Billions of Metric Tons | |
| Atmosphere | 578 (as of 1700) - 766 (as of 1999) | ||
| Soil Organic Matter | 1500 to 1600 | ||
| Ocean | 38,000 to 40,000 | ||
| Marine Sediments and Sedimentary Rocks | 66,000,000 to 100,000,000 | ||
| Terrestrial Plants | 540 to 610 | ||
| Fossil Fuel Deposits | 4000 | ||
CO2 + H2O ? H2CO3
This reaction has a forward and reverse rate. These two reqctions achieve a chemical equilibrium in which they both occur at equal rates, thus mainting a relatively stable ratio of CO2 to H2CO3. Another reaction that is important in controlling the acidity (i.e. pH levels) of the oceans is the release of hydrogen ions and bicarbonate:
H2CO3 ? H+ + HCO3−
This reaction buffers seawater against large changes in pH.
Certain forms of sea life biologically fix bicarbonate with calcium (Ca+2) to produce calcium carbonate (CaCO3). This substance is used to produce shells and other hard body parts by organisms such as coral, clams, oysters, some protozoa, and some algae. When these organisms die, their shells and body parts sink to the ocean floor where they accumulate as carbonate-rich deposits. After long periods of time, these deposits are physically and chemically altered into sedimentary rocks. Ocean deposits are by far the biggest sink of carbon on the planet (Table 1).
Carbon in the lithosphere
Carbon is stored in the lithosphere in both inorganic and organic forms. Inorganic deposits of carbon in the lithosphere include fossil fuels like coal, oil, and natural gas, oil shale, and carbonate-based sedimentary deposits like limestone. Organic forms of carbon in the lithosphere include litter, organic matter, and humic substances found in soils. Some carbon dioxide is released from the interior of the lithosphere by volcanoes. Carbon dioxide released by volcanoes enters the lower lithosphere when carbon-rich sediments and sedimentary rocks are subducted and partially melted beneath tectonic boundary zones.
Human role in the carbon cycle
Until recently, the flow of carbon stored in fossil fuels to the atmosphere was minuscule—nearly zero. The fossil fuel storage represented a “dead-end” for the carbon cycle. The Industrial Revolution increased the use of coal, oil, and natural gas. Burning fossil fuels completes the process of break-down back to carbon dioxide and water. In 2000, humans burned about 5.1 billion short tons (4.6 billion metric tons) of coal, 28.1 billion barrels of oil, and 89 trillion cubic feet of natural gas, which caused about 6.5 petagrams (6.5 billion metric tons) of carbon to flow from the fossil fuel storage pool to the atmosphere.
The combustion of fossil fuels is not the only flow in the carbon cycle affected by economic activity. Prior to the expansion of human civilization, the amount of carbon stored in biota changed very slowly from year to year because the amount taken up through photosynthesis was nearly equal to the amount emitted through respiration and decomposition. But human activity has disturbed the biotic storage pool. Over the last several hundred years, humans have reduced the area covered by forests, a process known as deforestation. By reducing the number of trees through burning and/or chopping them down and allowing them to decay, deforestation reduces the amount of carbon stored in the biota. This carbon flows to the atmosphere. In the 1990’s, deforestation and other changes in land use caused 1-2 petagrams (1-2 billion metric tons) of carbon to flow from the biota to the atmosphere annually.
The other important set of flows moves carbon from the atmosphere to the ocean and from the ocean to the atmosphere. For a long time, these two flows were approximately equal. This balance was created and maintained by the spontaneous flow of carbon from the storage of high concentration to the storage with the lower concentration. These movements created an equilibrium between the amount of carbon in the atmosphere and ocean.
This equilibrium has been disrupted by the combustion of fossil fuels and deforestation. These two flows add carbon to the atmosphere, which causes the concentration of carbon to increase in the atmosphere relative to the ocean. The increased atmospheric concentration of carbon causes carbon to flow spontaneously from the atmosphere to the ocean. The size of this flow is limited by a negative feedback loop, termed the Revelle Factor, which slows the flow of carbon from the atmosphere to the ocean relative to the flow of carbon to the atmosphere. As carbon dioxide dissolves in the ocean, it reduces the ocean’s pH (makes it more acidic). The lower pH slows the rate at which carbon dioxide dissolves in the ocean. Currently, the flow of carbon from the atmosphere to the ocean is about 2 petagrams (2 billion metric tons) greater than the flow of carbon from the ocean to the atmosphere.
The “missing” carbon
Despite the scientific certainty that the global carbon cycle is governed by the law of conservation, scientists are not able to “balance” the storages and flows. That is, summing the best estimates for the flows of carbon to and from the atmosphere indicates that there is less carbon in the atmosphere than expected. During the 1990’s, the atmosphere was missing about 3 petagrams (3 billion metric tons) of carbon per year. This missing carbon is associated with an unknown carbon sink.
Figure 2. Spatial distribution of estimated increase in net primary production from 1982 to 1999. (Source: Climate-Driven Increases in Global Terrestrial Net Primary Production from 1982 to 1999. Ramakrishna R. Nemani, Charles D. Keeling, Hirofumi Hashimoto, William M. Jolly, Stephen C. Piper, Compton J. Tucker, Ranga B. Myneni, and Steven W. Running (6 June 2003). Science 300 (5625), 1560.)
The unknown carbon sink is either an unknown mechanism that removes carbon from the atmosphere and/or a known mechanism that removes carbon faster than estimated by scientists. There are several hypotheses concerning the unknown carbon sink. Many are based on negative feedback loops that include the atmospheric concentration of carbon dioxide. One hypothesis is that the increasing concentration of carbon dioxide in the atmosphere increases net primary production, and this speeds the rate at which carbon is pulled from the atmosphere. Experiments indicate that plants grow faster at higher concentration of carbon dioxide, but it is not clear whether this increase is significant in the real world. If the growth of plants is not limited by the availability of carbon in the atmosphere, increasing its concentration will not increase growth. On the other hand, the mechanism may be boosted by human activities that increase the availability of nitrogen to plants.
Another hypothesis for the unknown carbon sink focuses on climate. The increasing concentration of carbon dioxide in the atmosphere is partially responsible for the global increase in temperature. As the world gets warmer, this could enhance plant growth, which would speed the rate at which plants remove carbon from the atmosphere via net primary production (Figure 2). Recent research suggests that climatic changes have enhanced plant growth in northern mid-latitudes and high latitudes. Global changes in climate have eased several critical climatic constraints to plant growth, such that net primary production increased 6% (3.4 petagrams of carbon over 18 years) globally. The largest increase was in tropical ecosystems. Amazon rain forests accounted for 42% of the global increase in net primary production, owing mainly to decreased cloud cover and the resulting increase in solar radiation.
Alternatively, the increase in temperature could accelerate the rate of decay. Decay frees up nutrients that were previously “tied up” in the organic material. If these nutrients are limiting in a Leibigian sense, the increased supply could accelerate net primary production and therefore speed the rate at which carbon dioxide is removed from the atmosphere. But if the nutrients were not limiting, accelerated rates of decay would increase the flow of carbon dioxide to the atmosphere.
Recent trends in atmospheric carbon dioxide
Even though scientists cannot balance the global carbon cycle, it is clear that the amount of carbon entering the atmosphere is greater than the amount of carbon leaving the atmosphere. Over the last 30 years, the amount of carbon stored in the atmosphere has increased, which we see as a significant increase in the atmospheric concentration of carbon dioxide (CO2) (Figure 3). The so-called Mauna Loa curve shows that between 1959 and 2008, the concentration of carbon in the atmosphere increased from about 317 parts per million (ppm) to 380 ppm. This increase is worrisome because the amount of carbon dioxide in the atmosphere influences the amount of heat retained, which may alter global climate. Notice too that the increase in is not steady. Within each year, the concentration of carbon dioxide rises and falls. This intrannual cycle allows us to watch the planet “breathe.”
Figure 3. The Mauna Loa atmospheric carbon dioxide curve: 1958-2008. (Image Source: Wikimedia, Artist Robert A. Rohde. Data Source : Keeling, C.D. and T.P. Whorf. 2005. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.).
The Mauna Loa atmospheric CO2 measurements constitute the longest continuous record of atmospheric CO2 concentrations available in the world. The Mauna Loa site is considered one of the most favorable locations for measuring undisturbed air because possible local influences of vegetation or human activities on atmospheric CO2 concentrations are minimal and any influences from volcanic vents may be excluded from the records. The methods and equipment used to obtain these measurements have remained essentially unchanged during the monitoring program that has been running for more than 50 years.
Further Reading
- Houghton, Richard. Understanding the Global Carbon Cycle. The Woods Hole Research Center.
- Nemani, Ramakrishna R., Keeling, Charles D., Hashimoto, Hirofumi, Jolly, William M., Piper, Stephen C., Tucker, Compton J., Myneni, Ranga B.,and Running, Steven W. 2003. Climate-Driven Increases in Global Terrestrial Net Primary Production from 1982 to 1999. Science, 300(5625):1560.
- PhysicalGeography.net
Pidwirny, M. (2012). Carbon cycle. Retrieved from http://www.eoearth.org/view/article/150923
- See more at: http://www.eoearth.org/view/article/150923/#sthash.kf7GCTpl.dpuf
Q
In response to warming, Eastern forests inhaling more carbon dioxide than they're exhaling
Caitlyn Kennedy
May 30, 2014
Scientists have known for many years that rising temperatures are lengthening the growing season in many northern and mid-latitude forests. New research indicates that in the eastern United States, increased carbon uptake has outpaced a simultaneous increase in carbon dioxide “exhaled” into the atmosphere through respiration. Overall, it seems that eastern forests are acting as increasing “sinks” for carbon dioxide.
One reason for the increased carbon uptake is that trees up and down the eastern half of the country are leafing out earlier in the spring. The map shows a trend toward earlier spring green-up over twelve years (2001-2012), based on both satellite vegetation data and ground observations from long-term research sites in forests across the Northeast. Shades of green indicate locations where the onset of spring is occurring earlier. White indicates very little change, while pink shows locations where the onset of spring is occurring later in the year. On average, spring is arriving about 10 days earlier than it used to only two decades ago.
The shift toward earlier spring leaf out is due to warming in the U.S. East, and has been mirrored by a delay in when trees drop their leaves in autumn. In a recent study published in Nature Climate Change, researchers found that enhanced “greening activity” during this extended growing season increased the amount of carbon that forests removed from the atmosphere through photosynthesis—the process by which plants convert carbon dioxide in the air into sugar molecules to use for food and to grow.
But just like animals do, plants and soil organisms burn sugars for energy and “exhale” carbon dioxide as a byproduct, a process known as respiration; warming also increases ecosystem respiration. While respiration increased in eastern forests during the extended growing season, the forests absorbed more carbon dioxide than they released, leading to a total net increase in carbon storage.
It’s not a given that the increased carbon uptake would dominate increased respiration. In northern (boreal) forests, which are primarily dominated by evergreen species, previous research found an overall decrease in carbon uptake despite an extended growing season. That’s a sign that that these two ecosystems have responded very differently to a warming climate, although additional research is needed to know exactly why.
Increased carbon uptake in forests may help reduce the growth rate of atmospheric carbon dioxide and slow future warming. But at the same time, climate change is increasing the vulnerability of many U.S. forests to fire, insect infestations, drought, and disease outbreaks. These disturbances raise the potential for large releases of carbon dioxide back into the atmosphere. Despite recent increases in forest growth due to elevated carbon dioxide and temperature changes, it remains unclear how future net forest carbon storage in the United States will respond to accelerating mortality and trends in land use and forest management.
References:
Keenan, T. F., Gray, J., Friedl, M. A., Toomey, M., Bohrer, G., Hollinger, D. Y., Munger, J.M., O'Keefe, J., Schmid, H.P., Wing, I.S., Yang, B., and Richardson, A. D. (2014). Net carbon uptake has increased through warming-induced changes in temperate forest phenology. Nature Climate Change, advance online publication. doi:10.1038/nclimate2253
Joyce, L. A., S. W. Running, D. D. Breshears, V. H. Dale, R. W. Malmsheimer, R. N. Sampson, B. Sohngen, and C. W. Woodall, 2014: Ch. 7: Forests. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 175-194. doi:10.7930/J0Z60KZC.
Q
Principle 2e
Aerosols
Aerosols are simply particles that are airborne. They have a complex effect on the Earth's energy balance: (1) They can cause cooling, by reflecting incoming sunlight back out to space. (2) They can cause warming, by absorbing and releasing heat energy in the atmosphere.
Small solid and liquid particles can be carried into the atmosphere through a variety of natural and man-made processes, including volcanic eruptions, sea spray, forest fires, and emissions generated through human activities (things like exhaust from your car or a power plant). Read more…
Aerosols
Aerosols are simply particles that are airborne. They have a complex effect on the Earth's energy balance: (1) They can cause cooling, by reflecting incoming sunlight back out to space. (2) They can cause warming, by absorbing and releasing heat energy in the atmosphere. 
Small solid and liquid particles can be carried into the atmosphere through a variety of natural and man-made processes, including volcanic eruptions, sea spray, forest fires, and emissions generated through human activities (things like exhaust from your car or a power plant).
While the role of aerosols in the climate system has been known since the late 1800s, their complex interactions on climate are still being studied. Some may have a cooling effect in the short term but a warming effect in the long term. Black carbon, an important aerosol, is now considered to be the second largest contributor to global warming after carbon dioxide. However, since black carbon remains in the atmosphere for only days to weeks, reducing it before it enters the atmosphere may prove to be an efficient way to slow near-term climate change. (In contrast, CO2 stays in the atmosphere for hundreds of years.) However, when black carbon falls on snow or ice, it reduces the albedo of those surfaces. Albedo, meaning "whiteness", relates to the ability of a surface to reflect solar energy. Therefore, black carbon contributes to a positive feedback loop: as albedo is reduced, more radiation is absorbed by the surface, which leads to an increased rate of melting of snow and ice, which leads to even more reduced albedo. Burning of forests and savannahs and combustion of fossil fuels account for about 80% of black carbon, commonly known as soot, in the atmosphere.
watch the video
click the image to enlarge the graphic
When viewed from space, a number of patterns emerge from Earth’s aerosols—some driven by nature and others by man.
Nature generates broad swaths of particles—detectable by satellites—over both water and land. The strong winds of the “roaring forties” latitudes, for example, create a heavy band of airborne salt north of Antarctica. A thinner and more evenly dispersed veil of aerosols—primarily salt from whitecaps and sulfates from microalgae—usually covers most of the world’s oceans. Over land, massive plumes of dust blow above deserts.
Meanwhile, the eastern portion of the United States and urban areas in Europe are hotspots for the production of human-made aerosols. Plumes of industrial aerosols — typically sulfates from coal power plants and black and organic carbon from vehicle traffic — rise from cities such as New York, Pittsburgh, London, and Berlin.
The western portion of the United States is comparatively clear, though some areas experience aerosol loads that rival the worst conditions in the East. Industrial aerosols, dust, and wildfire smoke frequently pollute the air in the Los Angeles Basin. Agriculture can produce heavy loads of soil dust, especially in California’s San Joaquin and Imperial valleys, and the largest localized source of dust in the western U.S. is Owens Dry Lake, a river bed that was drained to provide water for LA. Likewise, the port of Houston has some of the most aerosol-laden air in the world.
Aerosols and Incoming Sunlight (Direct Effects)
Source: http://earthobservatory.nasa.gov/Features/Aerosols/page3.php
The Sun provides the energy that drives Earth’s climate, but not all of the energy that reaches the top of the atmosphere finds its way to the surface. That’s because aerosols—and clouds seeded by them—reflect about a quarter of the Sun’s energy back to space.
Aerosols play an important role in Earth’s climate. Most aerosols are brighter than land or ocean, and cool the Earth by reflecting sunlight back to space. (NASA astronaut photograph.)
Different aerosols scatter or absorb sunlight to varying degrees, depending on their physical properties. Climatologists describe these scattering and absorbing properties as the “direct effect” of aerosols on Earth’s radiation field. However, since aerosols comprise such a broad collection of particles with different properties, the overall effect is anything but simple.
Although most aerosols reflect sunlight, some also absorb it. An aerosol’s effect on light depends primarily on the composition and color of the particles. Broadly speaking, bright-colored or translucent particles tend to reflect radiation in all directions and back towards space. Darker aerosols can absorb significant amounts of light.
Pure sulfates and nitrates reflect nearly all radiation they encounter, cooling the atmosphere. Black carbon, in contrast, absorbs radiation readily, warming the atmosphere but also shading the surface. Organic carbon, sometimes called brown carbon or organic matter, has a warming influence on the atmosphere depending on the brightness of the underlying ground. Dust impacts radiation to varying degrees, depending on the composition of the minerals that comprise the dust grains, and whether they are coated with black or brown carbon. Salt particles tend to reflect all the sunlight they encounter.
Black carbon aerosols, similar to the soot in a chimney, absorb sunlight rather than reflecting it. This warms the layer of the atmosphere carrying the black carbon, but also shades and cools the surface below. (Maps adapted from Chung, 2005.)
Aerosols can have a major impact on climate when they scatter light. In 1991, the eruption of Mount Pinatubo in the Philippines ejected more than 20 million tons of sulfur dioxide—a gas that reacts with other substances to produce sulfate aerosol—as high as 60 kilometers (37 miles) above the surface, creating particles in the stratosphere. Those bright particles remained above the clouds and didn’t get washed from the sky by rain; they settled only after several years.
Climatologists predicted global temperatures would drop as a result of that global sulfate infusion. They were right: Following the eruption, global temperatures abruptly dipped by about a half-degree (0.6°C) for about two years. And Pinatubo isn’t a unique event. Large, temperature-altering eruptions occur about once per decade.
Large volcanic eruptions may lift sulfate aerosols into the stratosphere, which usually cools the global climate for the following year or two. (Graph by Robert Simmon, based on aerosol data from GISS and temperature data from the UAE CRU.)
In addition to scattering or absorbing radiation, aerosols can alter the reflectivity, or albedo, of the planet. Bright surfaces reflect radiation and cool the climate, whereas darker surfaces absorb radiation and produce a warming effect. White sheets of sea ice, for example, reflect a great deal of radiation, whereas darker surfaces, such as the ocean, tend to absorb solar radiation and have a net warming effect.
Aerosols, particularly black carbon, can alter reflectivity by depositing a layer of dark residue on ice and other bright surfaces. In the Arctic especially, aerosols from wildfires and industrial pollution are likely hastening the melting of ice.
Dark aerosols dramatically change the reflectivity of the Earth’s surface when they land on snow. Black ash covered the summit of New Zealand’s Mount Ruapehu after an eruption in 2007, but was soon covered by fresh snow. Long-term accumulation of black carbon aerosols in the Arctic and Himalaya is leading to increased melting of snow. (Photograph ©2007, New Zealand GeoNet.)
Scientists believe the cooling from sulfates and other reflective aerosols overwhelms the warming effect of black carbon and other absorbing aerosols over the planet. Models estimate that aerosols have had a cooling effect that has counteracted about half of the warming caused by the build-up of greenhouse gases since the 1880s. However, unlike many greenhouse gases, aerosols are not distributed evenly around the planet, so their impacts are most strongly felt on a regional scale.
Despite considerable advances in recent decades, estimating the direct climate impacts of aerosols remains an immature science. Of the 25 climate models considered by the Fourth Intergovernmental Panel on Climate Change (IPCC), only a handful considered the direct effects of aerosol types other than sulfates.
Q
Principle 2f
Feedback Loops
A big change in any one component of the climate system can influence the entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system—changes more rapid and on a larger scale than projected by current climate models.
Feedback loops are found in many natural processes. For example: The relationship between snow and ice cover and greenhouse gases trapped under snow and ice generates a positive feedback loop that releases additional greenhouse gases as the planet warms (see the diagram at right). Read more…
Feedback Loops
The woolly mammoth was one of the large mammals that became extinct in North America at the onset of the Younger Dryas approx. 13,000 years ago.
A big change in any one component of the climate system can influence the entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system—changes more rapid and on a larger scale than projected by current climate models.
Feedback loops are found in many natural processes. For example: The relationship between snow and ice cover and greenhouse gases trapped under snow and ice generates a positive feedback loop that releases additional greenhouse gases as the planet warms
Abrupt climate change triggered by feedback loops in the climate system have occurred many times in Earth’s history. One of the most well-known examples of abrupt change is referred to as the Younger Dryas event or "Big Freeze", and it occurred 14,500 years ago as the Earth's climate began to shift from a cold glacial world to a warmer interglacial state. Partway through this transition, temperatures in the Northern Hemisphere suddenly returned to near glacial conditions for about 1,300 years.
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What is causing Arctic sea ice decline?
Readers sometimes ask us, “What are the reasons behind Arctic sea ice decline?” In summer months, ice extent has declined by more than 30 percent since the start of satellite observations in 1979. But is climate change really the culprit, or could other factors be contributing?
Scientists have long suspected that the decline in summer sea ice was too strong to be caused by natural variations, such as weather patterns that cause fast changes in ice extent. Climate model simulations have shown that sea ice will decline as the Arctic gets warmer. But Arctic ice has declined faster than models predicted, raising the possibility that massive sea ice loss was caused at least in part by natural variations in weather.
This month, researchers at the Max Planck Institute for Meteorology in Germany released a new study examining all the possible drivers for the observed sea ice retreat. Study lead Dirk Notz said, “There has always been the chance that we simply observed a freak encounter of unusual natural variability with very low ice cover.”
Looking for a smoking gun
Notz said, “The whole study was based on ruling out one possible cause after the other.” The team looked at each possible cause of ice loss, using statistical analyses to determine whether a particular cause could explain the ice decline. The scientists first considered natural variability, or the effect of short-term and seasonal weather conditions and winds. While variable winds and weather clearly play a role in how much ice melts each summer, they found that the amount of variability was far too low to explain the intensity of the decline.
The researchers then looked at the possibility that ice loss was leading to more ice loss, in a vicious cycle known as a positive feedback loop. As ice declines, the increased areas of open water absorb more heat, which can then melt more ice. This is one reason that researchers are concerned about declining sea ice. However, the new study concluded that such a feedback cannot explain the amount of ice loss that has already occurred. Notz said, “After each year with an extreme ice loss, the ice cover always recovered somewhat. Self acceleration is not a major player in the Arctic at the moment.”
Notz and colleagues then looked at other possible drivers for ice decline, ranging from solar radiation and cosmic rays, to volcanic eruptions, wind patterns, and oceanic heat transport. For all of these potential causes, their analysis showed that none of these were correlated with ice extent.
“In the end, only the increase in CO2 remained on our list of possible drivers,” Notz said, “We find a clear, physically plausible correlation of increasing CO2 and decreasing sea-ice cover.” While other research has previously shown a connection between sea ice decline and global warming, most of those studies focused on climate models. The new study provides an independent confirmation that sea ice is in fact declining because of human-caused climate change.
The Arctic Ocean has lost more than 30 percent of its summer ice cover in the last thirty years. Scientists have long thought that climate change is to blame, but a new study provides more evidence for that idea. Credit: Patrick Kelley, U.S. Coast Guard. High Resolution Image
Q
Principle 2g
Local Relevance
Climate is Complex: Warm West, Cool East U.S.
Source: http://climatenexus.org/learn/science-primers/warm-west-cool-east-us-temperature-divide 
While California and the western U.S. have experienced record drought and high temperatures in recent years, many areas in the eastern U.S. have experienced extreme or even record-breaking cold temperatures.
So why do the West and East experience such different temperatures, even as the global average temperature continues to rise? The answer has to do with how the heat getting added to the climate system is distributed and affecting regional weather patterns. In the U.S., the prolonged cold spells in the East, drought in California, and record heat in the Northwest and Alaska are all connected to the atmospheric polar jet stream. There is a growing body of scientific evidence suggesting that rising temperatures in the Arctic may be linked to the strong and persistent jet-stream pattern that has led—three winters in a row—to this warm West and cool East divide.
Why are we seeing the “warm West and cool East” divide?
The polar jet stream in the Northern Hemisphere is a fast-flowing and narrow current of air that circles the mid to high-latitudes. In recent years, the polar jet stream has slowed, its winds have weakened, and its north-south waves, known as Rossby waves (which bring Arctic air to the U.S.) may be getting bigger and occurring more frequently. For three consecutive winters, starting in December 2012, the jet stream has had significant north-south Rossby waves. In the East, the jet stream has been dipping unusually far south, creating a low “trough” that brings down cool Arctic air. Meanwhile, in the West, high pressure conditions over the northeast Pacific have confronted cool Arctic air from the sunken jet stream, creating a “ridge” of high atmospheric pressure causing the jet stream to divert towards the North. As a result, rain and snow that would normally fall on the West coast is instead re-routed to Alaska and as far north as the Arctic Circle. The exceptional persistence and unusual recurrence of this pattern has led scientists to nickname the ridge and trough as the “Ridiculously Resilient Ridge” and the “Terribly Tenacious Trough.”
How might climate change be linked to the divide?
There is strong agreement in the scientific community that the most immediate cause of the drought in California—and western heat more broadly—is the "Ridiculously Resilient Ridge.” The RRR, as it’s called for short, is a product of unusual jetstream behavior that scientists are linking to many complex factors including Arctic warming and warmer atmospheric conditions over the northeast Pacific.
A recent study led by Noah Diffenbaugh investigates how climate change might be playing a role in the California drought and extreme west coast heat by analyzing atmospheric conditions over the Pacific. In the study, Diffenbaugh and his team find that the region of high atmospheric pressure over the Pacific Ocean responsible for diverting the jet stream is much more likely to form in the presence of modern greenhouse gas concentrations.
As for the remarkable cold in the East, Arctic researchers have recently begun to study the effects of rapid warming in the Arctic and their possible link to jet stream behavior and mid-latitude weather conditions. The Arctic Ocean has experienced rapid sea ice loss due to global warming at a rate of 11 percent per decade since 1979. This has created an expanse of newly available ocean water to absorb the Sun’s energy. The extra energy has created conditions that are conducive to even more warmth, creating a positive feedback effect known as Arctic amplification. With changes in cloud cover, more atmospheric water vapor, and more heat creeping in from warmer latitudes below, the Arctic has been warming at a rate twice as fast as lower latitudes.
In 2012, climate scientists Jennifer Francis and Stephen Vavrus proposed that Arctic amplification driven by sea ice loss could significantly affect mid-latitude weather by decreasing the temperature gradient between mid-latitudes and the Arctic, causing the jet stream’s winds to slowdown and waves to meander. A 2015 paper by Francis and Vavrus tests this theory further and finds that as the Arctic warms faster than the rest of the planet, the frequency of extreme weather events caused by persistent jet stream patterns will increase.
Scientists are identifying additional mechanisms that may be contributing to the unusual mid-latitude weather. All sea ice studies agree that the loss of sea ice warms the Arctic atmosphere and increases moisture content, but scientists have identified different ways that the atmosphere may be responding to the loss of ice and resulting temperature increase. For example, a team led by Judah Cohen has proposed a chain of events where less Arctic sea ice and more open water cause the atmosphere to heat, which leads to more snow cover in Siberia in October, cooling the atmosphere and forcing the meandering jet stream pattern we are currently seeing.
While the scientific community is still investigating whether sea ice loss and Arctic amplification are the ultimate causes of the jet-stream pattern responsible for the divide between the warm West and cool East, the link between a slow, wavy jet-stream and extreme U.S. weather is well-established, and there is evidence supporting Francis and Vavrus’s theory that the decreased temperature gradient between mid-latitudes and the Arctic is slowing the jet stream waves.
How unusual are the opposing temperature extremes in the U.S.?
Average global temperatures in 2014 were record hot, but in the U.S. there was a record-setting split between regions experiencing unusually hot and cold temperatures. Almost one-fourth (23.2 percent) of the U.S. experienced temperatures “much above normal,” while 18.6 percent saw temperatures “much below normal.” NOAA’s definition of “temperature extreme” is when regions experience temperatures at the top or bottom 10 percent of the historical range of recorded values. Since the U.S. temperature record began in 1895, only a handful of years have a pattern similar to 2014, where more than 10 percent of the country was experiencing extreme warmth while a similarly large or larger area experienced extreme coolness. Among these years, 2014 experienced an unprecedented range of simultaneous, opposed temperature extremes. As the jet stream continues to stick in a position where the eastern U.S. experiences Arctic conditions and the western U.S. gets little relief from extreme heat and drought, people are starting to discuss the extent to which climate change may be at work.
How does this fit into the bigger picture?
The unusual jet stream behavior has created a situation where the eastern U.S. is by far the coldest place in the world compared to average temperatures, leading some to argue that it might be more appropriate to say “global weirding” than warming. From the U.S. perspective, this may be true: the U.S. is experiencing a weird divide between extreme heat in the West and cold in the East. But it’s also important to remember that even as U.S. winter temperatures get unusually cold, overall temperatures in the U.S. are actually unusually warm. While select regions may experience cooler temperatures trends as the global temperature ratchets up, long-term trends and projections clearly indicate continued warming in all regions of the U.S. including the east, and globally.
Principle 2h
Misconceptions about this Principle
The Misconception
Isn’t it true that the planet cooled mid-century during a period of post World War II industrialization when CO2 emissions were rising?
The misconception goes something like this: If CO2 really is responsible for global warming, then why don’t we see a consistent rise in temperatures during the 20th century since CO2 levels were rising throughout the century and especially during the post WWII period? Instead we see a cooling period from the 1940s to the 1970s.
The Science
CO2 was increasing during the period following WWII, but so were sulphate aresols, which had a pronounced cooling effect because they reflect incoming solar energy back into space and lead to cooling.
The science says: Although temperatures increased overall during the 20th century, three distinct periods can be observed. Global warming occurred both at the beginning and at the end of the 20th century, but a cooling trend is seen from about 1940 to 1975. As a result, changes in 20th century trends offer a good framework through which to understand climate change and the role of numerous factors in determining the climate at any one time. Read more…
Source: http://www.skepticalscience.com/global-cooling-mid-20th-century.htm
The Science
CO2 was increasing during the period following WWII, but so were sulphate aresols, which had a pronounced cooling effect because they reflect incoming solar energy back into space and lead to cooling.
The science says: Although temperatures increased overall during the 20th century, three distinct periods can be observed. Global warming occurred both at the beginning and at the end of the 20th century, but a cooling trend is seen from about 1940 to 1975. As a result, changes in 20th century trends offer a good framework through which to understand climate change and the role of numerous factors in determining the climate at any one time.
Early and late 20th century warming has been explained primarily by increasing solar activity and increasing CO2 concentrations, respectively, with other factors contributing in both periods. So what caused the cooling period that interrupted the overall trend in the middle of the century? The answer seems to lie in solar dimming, a cooling phenomenon caused by airborne pollutants.
The main culprit is likely to have been an increase in sulphate aerosols, which reflect incoming solar energy back into space and lead to cooling. This increase was the result of two sets of events.
Industrial activities picked up following the Second World War. This, in the absence of pollution control measures, led to a rise in aerosols in the lower atmosphere (the troposphere).
A number of volcanic eruptions released large amounts of aerosols in the upper atmosphere (the stratosphere). Combined, these events led to aerosols overwhelming the warming trend at a time when solar activity showed little variation, leading to the observed cooling. Furthermore, it is possible to draw similar conclusions by looking at the daily temperature cycle. Because sunlight affects the maximum day-time temperature, aerosols should have a noticeable cooling impact on it. Minimum night-time temperatures, on the other hand, are more affected by greenhouse gases and therefore should not be affected by aerosols. Were these differences observed? The answer is yes: maximum day-time temperatures fell during this period but minimum night-time temperatures carried on rising.
The introduction of pollution control measures reduced the emission of sulphate aerosols. Gradually the cumulative effect of increasing greenhouse gases started to dominate in the 1970s and warming resumed.
As a final point, it should be noted that in 1945, the way in which sea temperatures were measured changed, leading to a substantial drop in apparent temperatures. Once the data are corrected, it is expected that the cooling trend in the middle of the century will be less pronounced.
Principle 2
Knowledge Check
To pass this knowledge check you will need to have read the main paragraphs for each topic of the principle.