History of contributions of planetary studies to the science of climate change
Published: May 13, 2008, 7:48 pm
Updated: July 27, 2012, 8:55 am
Author: Spencer Weart
American Institute of Physics


Venus as imaged by the Magellan mission to Venus. This global view of the surface of Venus is centered at 180 degrees east longitude. (Source: NASA)

A planet is not an object in the laboratory that scientists can subject to different pressures and radiations, comparing how it reacts to this or that. We have only one Earth, and that makes climate science difficult. We can learn much by studying how past climates were different from that of today, and observing how the climate changes in reaction to humanity's "large scale geophysical experiment" of emitting greenhouse gases may teach us a great deal. But these are limited comparisons—different breeds of cat, but still cats. Fortunately, our solar system contains planets with radically different atmospheres.

By the mid 1950s, scientists knew that the atmosphere of Mars was unbreathable—composed mainly of carbon dioxide (CO2), very tenuous and cold, and occasionally stirred up with yellowish dust storms. If Mars had features resembling "canals", as some suspected, they were not full of water for water could not exist as a liquid on the planet's surface. The Mariner 4 spacecraft of 1965, sending back blurry pictures that showed a surface scarred with craters like the Moon, confirmed that the planet was an unlikely abode of life. As for Venus, radio observations published in 1958 showed an amazingly hot climate, with temperatures upwards of 600° Kelvin (K), around the melting point of lead. "It was very disappointing to many people," one of the discoverers recalled, "who were reluctant to give up the idea of a sister planet and perhaps even the possibility of life." Some astronomers worked up arguments that the radio measurements were misleading, representing something in the upper atmosphere, so that life might still exist on Venus. The matter was settled in 1962 when the spacecraft Mariner 2 flew past the planet and detected an unquestionably hot surface.

Already back in 1940, Rupert Wildt had made a rough calculation of the greenhouse effect, caused by the great amount of CO2 others had found in telescope studies of Venus, predicting the effect could raise the surface temperature above the boiling point of water. But raising it as high as 600°K seemed impossible, and nobody mounted a serious attack on the problem; after all, there were very few people in the field of planetary astronomy in those decades. Finally in 1960 a young doctoral student, Carl Sagan, took up the problem and found a solution that made his name known among astronomers. Using what he later recalled as "embarrassingly crude" methods, taking data from tables designed for steam boiler engineering, he confirmed that Venus could indeed be a greenhouse effect furnace. The atmosphere would have to be almost totally opaque, and this "very efficient greenhouse effect" couldn't all be due to CO2—he pointed to absorption by water vapor as the likely culprit.


Dr. Carl Sagan, best known for his acclaimed public television series

Sagan concluded that "Venus is a hot, dry, sandy... and probably lifeless planet." He proposed, most significantly, that the situation was self-perpetuating—the surface of the planet was so hot that whatever water the planet possessed remained in the atmosphere as vapor, helping maintain the extreme greenhouse effect condition. However, it was later found that Sagan was mistaken, for Venus's atmosphere has little water. Today's explanation of the planet's strong greenhouse effect is that Venus has a much denser CO2 atmosphere than assumed by astronomers in the past. But mistakes in science can be as useful as valid results when they stimulate further work and ultimately point in the right direction.

A few researchers tried putting a feedback between temperature and water vapor into a simple system of equations; the results were strange. In 1969, Andrew Ingersoll reported "singularities"—mathematical points where the numbers went out of bounds. This signaled "a profound change in the physical system which the model represents," and Ingersoll pointed to CO2 as the key ingredient in the effect. (The Soviet Venera 4 had penetrated Venus’s atmosphere in 1967 and showed it was mostly CO2, and in 1978 the American Pioneer spacecraft found it was almost entirely CO2.) Sagan had estimated that Venus had started out with roughly the same amount of CO2 as the Earth. On our planet, most of the carbon is locked up in minerals and buried in sediments. The surface of Venus, by contrast, was so hot and dry that carbon-bearing compounds evaporated rather than remaining in the rocks, releasing great amounts of the greenhouse gas into the atmosphere. Perhaps Venus had once enjoyed a climate of the sort hospitable to life, but as water had gradually evaporated into the warming atmosphere, followed by CO2, the planet had fallen into its present hellish state? In a 1971 paper, James Pollack argued that Venus might once have had oceans like Earth's. It seemed that such a "runaway greenhouse" could have turned the Earth too into a furnace, if the starting conditions had been only a little different.

In the late 1970s Michael Hart pursued the idea with a more complex computer model, and concluded that the balance was exceedingly delicate. Hardly any planets in the universe, he said, orbited in the narrow "habitable zone" around a star where life could flourish. For our solar system, the orbits in which a planet would be too close to the Sun—so that at some point the planet would suffer a runaway greenhouse effect from which it could never recover—were separated by only a 5% gap from orbits in which the planet would be so far away that runaway glaciation would freeze any ocean solid. The Earth, then, was a lucky place. However, Hart's calculations were riddled with untested assumptions, and many scientists denied that our situation was so extremely precarious. Nonetheless, Hart defended his ideas energetically among his colleagues, and to the public, including an appearance on television in "Walter Cronkite's Universe." Later calculations disproved Hart's conclusions—a Venus-type runaway on our planet is scarcely possible, even if we burn all available fossil fuels.)

The atmosphere of Venus was filled not only with CO2 but also with an opaque haze. Its nature was unknown, and in the 1960s scientists could only say that the haze was probably caused by some kind of tiny particles. "The clouds on Venus had long been a mystery," as one expert recalled, "in which stratospheric aerosols now appeared to play a key role. The unraveling of the precise role of aerosols in the Venus atmosphere would certainly benefit studies of chemical contamination of Earth's atmosphere." In the early 1970s, ground-based telescope observations produced extraordinarily precise data on the optical properties of these aerosols, and at last they were identified—the haze was made up of sulfur compounds.

The greenhouse effect of the sulfates could be calculated, and by the late 1970s, NASA climate modeler James Hansen stated confidently that the sulfates, together with CO2, "are responsible for the basic climatic state on Venus." (CO2 was by far the largest factor, and the exact effects of sulfates would be debated in many subsequent studies.) Hansen had originally become interested in the greenhouse effect when, in response to Sagan's primitive calculations, he tried to derive a better explanation of why the planet's atmosphere was so hot. Hansen's findings about sulfate aerosols strengthened his belief that these particles could have a significant effect on Earth's climate as well. Sulfates were emitted by volcanoes and, increasingly, by human industry, so perhaps Venus had things to tell us about climate change at home.


The planet Mars as imaged by The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) in April, 2003. (Source: NASA)

And then there was Mars—the Red Planet—which also inspired important new thinking about the Earth's atmosphere, even before spacecraft observations. In the 1960s, NASA asked a group of scientists to derive a means of detecting life on Mars. A few noticed that life on Earth makes its presence blatantly evident by driving the atmosphere far from chemical equilibrium. In particular, the abundant oxygen in the air would swiftly drain away, by combining with surface minerals, but the oxygen is renewed by daily emissions from plantlife. Telescopic studies found practically no oxygen in the Red Planet's atmosphere, and, overall, the Martian atmosphere showed no signs of any chemical disequilibrium. Biochemist James Lovelock dismayed his peers by arguing that this showed any search for life on the planet would be fruitless. The sterile atmosphere of Mars, so strikingly different from the Earth's, helped Lovelock, and eventually others, to recognize that life plays a central role in determining the nature of our own planet's atmosphere.

In 1971 the spacecraft Mariner 9, a marvelous jewel of engineering, settled into orbit around Mars and saw... nothing. A great dust storm was shrouding the entire planet. While such storms are rare for Mars, this one was no misfortune, but great luck. Observers immediately saw that the dust had profoundly altered the Martian climate, warming the planet by tens of degrees. The dust settled after a few months, but its lesson was clear—haze could warm an atmosphere. More generally, one studying the climate of any planet would have to take dust very seriously. In particular, it seemed that on Mars the temporary warming had reinforced a pattern of winds, which kept the dust stirred up. It was a striking demonstration that feedbacks in a planet's atmospheric system could flip weather patterns into a drastically different state. This was no longer speculation but an actual event in full view of scientists—"the only global climatic change whose cause is known that man has ever scientifically observed."

Crude speculations about such radical instabilities in the Earth’s own climate had been published independently by two scientists a few years before, about the same time as Ingersoll’s calculation of the Venus runaway greenhouse effect. Pursuing such thoughts before Mariner arrived at Mars, Carl Sagan had made a bold prediction. He suggested that the Red Planet's atmosphere could settle in either of two stable climate states. Besides the current "ice age" there was another possible state, more clement, which might even support life. The prediction seemed to be validated by crisp images of the surface that Mariner beamed home after the dust cleared. The canals some astronomers had imagined were nowhere to be seen, but geologists did see strong signs that vast water floods had ripped the planet in the far past. It was a deadly blow to the old, comforting belief that planets had naturally stable climates, and reinforced the “runaway greenhouse” speculations about Venus that were emerging around the same time.

Calculations by Sagan and his collaborators now suggested that the planet's climate system was balanced so that it could have been flipped from a state with oceans to the present frigid desert, or even back and forth between the two states, by relatively minor changes—changes in its orbit, in the strength of the Sun's radiation, or in the reflection of sunlight off the polar ice cap. These were also, as the authors remarked, "fashionable variables in theories of climatic change on Earth." (Not until 2004 did direct observations on the surface of Mars prove that the planet had indeed once carried standing water, although much farther in the past than Sagan had guessed.)

Speculations published recently about the Earth’s climate suggested possibilities as spectacular as anything proposed for Mars. A small change could start a warming in which the Earth's polar ice caps would shrink, lowering the planet's reflectivity and pushing the warming further into a self-sustaining climate shift. Much the same thing could perhaps happen on Mars, releasing the CO2 frozen at its poles, starting a greenhouse effect process that would melt the ice buried in the soil. In fact, some kind of drastic climate shift had happened on Mars, if evidence of ancient floods is correct. In these arguments, dust stood at the fore, since storms that deposited dust on the polar caps and darkened them seemed the most likely mechanism for pushing the planet into its warm phase.

In later years, spacecraft observed the exotic weather of Venus and Mars in great detail. Meanwhile, as computer models of atmospheres improved, the Earth's two neighbors, plus more exotic planets such as Jupiter, occasionally served as testbeds to probe the limits of the modelers' methods. If a set of equations gave plausible results for such utterly different atmospheres, it created more confidence for their applications on Earth. But the main lesson was a larger one. The idea that feedbacks involving the greenhouse effect could have huge consequences for a planet's climate was no longer mere speculative theory—it was an observation of real events.

Further Reading

• Benestad, Rasmus and Pierrehumbert, Ray. 2006. Lessons from Venus. RealClimate.
• Math Program Cracks Cause of Venus Climate Change, Jet Propulsion Lab, National Aeronautics and Space Administration (NASA).
• Mark A. Bullock and David H. Grinspoon, Global Climate Change on Venus, March 1999, Scientific American Magazine.
• Bruce M. Jakosky1,2 and Roger J. Phillips, Mars' volatile and climate history, Nature 412, 237-244 (12 July 2001).
• Climate History of Mars, California Space Institute, University of California Office of the President.

NASA - What's the Difference Between Weather and Climate?
Soource: https://www.nasa.gov/mission_pages/noaa-n/climate/climate_weather.htm



Latest three month average temperature and precipitation anomalies for the United States.
Credits: NOAA

The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere "behaves" over relatively long periods of time.

When we talk about climate change, we talk about changes in long-term averages of daily weather. Today, children always hear stories from their parents and grandparents about how snow was always piled up to their waists as they trudged off to school. Children today in most areas of the country haven't experienced those kinds of dreadful snow-packed winters, except for the Northeastern U.S. in January 2005. The change in recent winter snows indicate that the climate has changed since their parents were young.

If summers seem hotter lately, then the recent climate may have changed. In various parts of the world, some people have even noticed that springtime comes earlier now than it did 30 years ago. An earlier springtime is indicative of a possible change in the climate.

In addition to long-term climate change, there are shorter term climate variations. This so-called climate variability can be represented by periodic or intermittent changes related to El Niño, La Niña, volcanic eruptions, or other changes in the Earth system.

What Weather Means
Weather is basically the way the atmosphere is behaving, mainly with respect to its effects upon life and human activities. The difference between weather and climate is that weather consists of the short-term (minutes to months) changes in the atmosphere. Most people think of weather in terms of temperature, humidity, precipitation, cloudiness, brightness, visibility, wind, and atmospheric pressure, as in high and low pressure.

In most places, weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season. Climate, however, is the average of weather over time and space. An easy way to remember the difference is that climate is what you expect, like a very hot summer, and weather is what you get, like a hot day with pop-up thunderstorms.

Things That Make Up Our Weather
There are really a lot of components to weather. Weather includes sunshine, rain, cloud cover, winds, hail, snow, sleet, freezing rain, flooding, blizzards, ice storms, thunderstorms, steady rains from a cold front or warm front, excessive heat, heat waves and more.

In order to help people be prepared to face all of these, the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS), the lead forecasting outlet for the nation's weather, has over 25 different types of warnings, statements or watches that they issue. Some of the reports NWS issues are: Flash Flood Watches and Warnings, Severe Thunderstorm Watches and Warnings, Blizzard Warnings, Snow Advisories, Winter Storm Watches and Warnings, Dense Fog Advisory, Fire Weather Watch, Tornado Watches and Warnings, Hurricane Watches and Warnings. They also provide Special Weather Statements and Short and Long Term Forecasts.

NWS also issues a lot of notices concerning marine weather for boaters and others who dwell or are staying near shorelines. They include: Coastal Flood Watches and Warnings, Flood Watches and Warnings, High Wind Warnings, Wind Advisories, Gale Warnings, High Surf Advisories, Heavy Freezing Spray Warnings, Small Craft Advisories, Marine Weather Statements, Freezing Fog Advisories, Coastal Flood Watches, Flood Statements, Coastal Flood Statement.

Who is the National Weather Service?
According to their mission statement, "The National Weather Service provides weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy. NWS data and products form a national information database and infrastructure which can be used by other governmental agencies, the private sector, the public, and the global community."

To do their job, the NWS uses radar on the ground and images from orbiting satellites with a continual eye on Earth. They use reports from a large national network of weather reporting stations, and they launch balloons in the air to measure air temperature, air pressure, wind, and humidity. They put all this data into various computer models to give them weather forecasts. NWS also broadcasts all of their weather reports on special NOAA weather radio, and posts them immediately on their Interactive Weather Information Network website at: http://iwin.nws.noaa.gov/iwin/graphicsversion/bigmain.html.
 
What Climate Means
In short, climate is the description of the long-term pattern of weather in a particular area.

Some scientists define climate as the average weather for a particular region and time period, usually taken over 30-years. It's really an average pattern of weather for a particular region.

When scientists talk about climate, they're looking at averages of precipitation, temperature, humidity, sunshine, wind velocity, phenomena such as fog, frost, and hail storms, and other measures of the weather that occur over a long period in a particular place.

For example, after looking at rain gauge data, lake and reservoir levels, and satellite data, scientists can tell if during a summer, an area was drier than average. If it continues to be drier than normal over the course of many summers, than it would likely indicate a change in the climate.

Why Study Climate?
The reason studying climate and a changing climate is important, is that will affect people around the world. Rising global temperatures are expected to raise sea levels, and change precipitation and other local climate conditions. Changing regional climate could alter forests, crop yields, and water supplies. It could also affect human health, animals, and many types of ecosystems. Deserts may expand into existing rangelands, and features of some of our National Parks and National Forests may be permanently altered.


An example of a Monthly Mean Outgoing Longwave Radiation (OLR) product produced from NOAA polar-orbiter satellite data, which is frequently used to study global climate change.
Credits: NOAA


The National Academy of Sciences, a lead scientific body in the U.S., determined that the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. Yet, there is still some debate about the role of natural cycles and processes.

Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases – primarily carbon dioxide, methane, and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertanties exist about exactly how Earth's climate responds to them. According to the U.S. Climate Change Science Program (http://www.climatescience.gov), factors such as aerosols, land use change and others may play important roles in climate change, but their influence is highly uncertain at the present time.

Who Studies Climate Change?
Modern climate prediction started back in the late 1700s with Thomas Jefferson and continues to be studied around the world today.

At the national level, the U.S. Global Change Research Program coordinates the world's most extensive research effort on climate change. In addition, NASA, NOAA, the U.S. Environmental Protection Agency (EPA) and other federal agencies are actively engaging the private sector, states, and localities in partnerships based on a win-win philosophy and aimed at addressing the challenge of global warming while, at the same time, strengthening the economy. Many university and private scientists also study climate change.

What is the U.S. Global Change Research Program?
The United States Global Change Research Program (USGCRP) was created in 1989 as a high-priority national research program to address key uncertainties about changes in the Earth's global environmental system, both natural and human-induced; to monitor, understand, and predict global change; and to provide a sound scientific basis for national and international decision-making.

Since its inception, the USGCRP has strengthened research on global environmental change and fostered insight into the processes and interactions of the Earth system, including the atmosphere, oceans, land, frozen regions, plants and animals, and human societies. The USGCRP was codified by Congress in the Global Change Research Act of 1990. The basic rationale for establishing the program was that the issues of global change are so complex and wide-ranging that they extend beyond the mission, resources, and expertise of any single agency, requiring instead the integrated efforts of several agencies.

Some Federal Agencies Studying Climate
In the 1980s the National Weather Service established the Climate Prediction Center (CPC), known at the time as the Climate Analysis Center (CAC). The CPC is best known for its United States climate forecasts based on El Niño and La Niña conditions in the tropical Pacific.
 

Image Above: The operational SST anomaly charts are useful in assessing ENSO (El Niño - Southern Oscillation) development, monitoring hurricane "wake" cooling, and even major shifts in coastal upwelling.
Credits: NOAA

CPC was established to give short-term climate prediction a home in NOAA. CPC's products are operational predictions or forecasts of how climate may change and includes real-time monitoring of climate. They cover the land, the ocean, and the atmosphere, extending into the upper atmosphere (stratosphere). Climate prediction is very useful in various industries, including agriculture, energy, transportation, water resources, and health.

NASA has been using satellites to study Earth's changing climate. Thanks to satellite and computer model technology, NASA has been able to calculate actual surface temperatures around the world and measure how they've been warming. To accomplish the calculations, the satellites actually measure the Sun's radiation reflected and absorbed by the land and oceans.NASA satellites keep eyes on the ozone hole, El Nino's warm waters in the eastern Pacific, volcanoes, melting ice sheets and glaciers, changes in global wind and pressure systems and much more.

At the global level, countries around the world have expressed a firm commitment to strengthening international responses to the risks of climate change. The U.S. is working to strengthen international action and broaden participation under the support of the United Nations Framework Convention on Climate Change.

Today, scientists around the world continue to try and solve the puzzle of climate change by working with satellites, other tools and computer models that simulate and predict the Earth's conditions.

For information about the U.S. Global Change Research Program, please visit:
http://www.usgcrp.gov/

For information about NASA's study of Earth's climate, please visit on the Internet:
http://www.nasa.gov/vision/earth/features/index.html

For a review of 2004's Global Temperature, please visit:
http://www.nasa.gov/vision/earth/lookingatearth/earth_warm.html

For information about NASA, please visit on the Internet:
http://www.nasa.gov

For information about the National Weather Service, please visit on the Internet:
http://www.nws.noaa.gov/

For immediate watches and warnings, visit the NWS Interactive Weather Information Network website at:
http://iwin.nws.noaa.gov/iwin/graphicsversion/bigmain.html

To find a NOAA weather radio station near you:
http://www.nws.noaa.gov/nwr/

For a glossary of weather terms, please visit the National Weather Service Weather Glossary on the Internet at:
http://www.nws.noaa.gov/glossary/


Rob Gutro
NASA's Earth-Sun Science News Team/SSAI

NASA Goddard Space Flight Center, Greenbelt, Md., and excerpts from NOAA's CPC web page, and the U.S. EPA web page. 2/2005 

Edits: Dr. J. Marshall Shepherd, NASA/GSFC, Drew Shindell, NASA/GISS, Cynthia M. O'Carroll, NASA/GSFC
Last Updated: May 13, 2015
Editor: NASA Administrator