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:

• It redistributes heat energy on the Earth through latent heat energy exchange.
• The condensation of water vapor creates precipitation that falls to the Earth's surface providing needed fresh water for plants and animals.
• It helps warm the Earth's atmosphere through the greenhouse effect.

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.

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

• 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

Deforestation in Amazonia
Published: March 30, 2007, 3:24 pm
Updated: April 5, 2013, 5:29 pm
Author: Philip M. Fearnside


Massive defoestation in Mato Grosso, Brazil. Source: Pedro Biondi

Introduction

Transamazon Highway. (Source: Philip M. Fearnside)

Tropical forests in Amazonia are being cleared rapidly, representing an important worldwide element of land-use and land-cover change. While some processes are common to forests throughout the world, others are not. Amazonian clearing is dominated by large cattle ranchers, with an increasing role being played by soybeans. Small farmers and estate crops such as oil palm have less relative importance here than elsewhere. Deforestation in Brazilian Amazonia has a significant contribution from “ulterior” motives such as land speculation, money laundering and tax evasion. Infrastructure projects, especially highway construction and improvement, represent key governmental decisions unleashing chains of activity that escape from government control. Deforestation sacrifices environmental services such as maintenance of biodiversity, water cycling and carbon stocks. The substantial impact of this deforestation on loss of environmental services has so far not entered into decision-making on infrastructure projects, making strengthening of the environmental assessment and licensing system a high priority for containing future loss of forest.

History
Deforestation has been a feature of Amazonian landscape since long before the arrival of Europeans in the 1500s. Indeed, no forest in the region can be considered “virgin” in the sense of being unaffected by past human activities. Prior to decimation of their populations by disease and violence from the Europeans, indigenous peoples maintained extensive areas of agriculture and they enriched the surrounding forest with useful species such as Brazil nuts. These human influences would be merged with forest regrowth during the lapse of three centuries before non-tribal populations reached levels sufficient to begin exerting significant pressure on the forest. Contrary to the claims of some, this history of past human disturbance in no way diminishes the rationale for conserving Amazonian forests today. Likewise, the exuberant forests that now stand on formerly cleared areas do not justify the myth of a future recovery-that forests being cleared today may one day regrow to their former stature. In practice, secondary forests are recleared for cattle pasture or other uses long before they regain the biomass and diversity of “primary” forests.


Fig. 1. Map describing major ecoregions of Brazil, indicating ten sampling localities of An. darlingi. Ecoregions are: north Amazon River (NAR); south Amazon River (SAR); Cerrado (CER); coastal Atlantic Forest (CAF); interior Atlantic Forest (IAF). These ecoregions are from 10 states: AM: Amazonas, AP: Amapá, ES: Espírito Santo, GO: Goiás, MT: Mato Grosso, PA: Pará, PR:Paraná, RJ: Rio de Janeiro, SP: São Paulo, TO: Tocantins. http://dx.doi.org/10.1016/j.meegid.2012.04.002

In Brazil, deforestation over the course of several centuries destroyed the Atlantic forest of the south-central part of the country (note: the names of Brazil’s regions treat Rio de Janeiro as the “center” of the country). The pace of clearing was especially dramatic in the case of state of Paraná, where the forest was almost completely cleared in less than 30 years in the middle of the 20th century. At the beginning of this period prominent citizens frequently made statements to the effect that Paraná’s forests were so vast that human efforts would “never” put more than a dent in them. The similarity of these statements to those sometimes made today with reference to the Amazon forest is evident, as is the irony of their baselessness.

Deforestation in Amazonia has proceeded with a succession of different forces in different periods. The Amazon rubber boom lasted from the invention of the pneumatic tire in the 1880s to the beginning of commercial rubber production from plantations in Southeast Asia in 1914. During this period “agricultural colonies” such as those in the 35,000-square-kilometer (km2) Zona Bragantina near Belém, in the state of Pará, supplied the rapidly growing urban centers, and, to a certain extent, the population engaged full-time in exploitation of the natural rubber trees in the Amazonian interior. Much of the agricultural land was abandoned to secondary forest when the rubber boom collapsed. More recent clearing surges occurred with the opening of the Belém-Brasília Highway in the late 1950s, and especially the Transamazon Highway in 1970 (the event often taken as the beginning of the “modern” period of Amazonian clearing). The Transamazon Highway was settled by small farmers, many of whom were brought from other parts of Brazil by the federal government and settled in official colonization projects. This much-publicized initiative was soon overshadowed in terms of its impact on deforestation by the large cattle ranchers who received generous tax incentives and subsidized financing from the government though the Superintendency for the Development of Amazonia (SUDAM). Large and medium-sized ranchers continue to account for the bulk of clearing in Brazilian Amazonia. The relative role of small versus large actors is an important difference between different locations in Brazilian Amazonia, between different historical periods, and between Brazil and other countries.


Current rates and causes


Arc of Deforestation in the region of Terra do Meio. (Source: Philip M. Fearnside)

Global generalizations about the role of “poverty” in tropical deforestation generally do not apply to Brazil, where most clearing is done by the rich. Cattle ranching is the use put to the great majority of land cleared, either immediately upon clearing in the case of large ranchers, or after a harvest or two of an annual crop in the case of small farmers. Although the government incentives programs of the 1970s and 1980s have been either discontinued or have diminished in importance, government infrastructure investment and agricultural credit continue to encourage clearing. Logging has a key role in serving as a source of funds for landholders to pay for deforestation. Logging also provides initial access roads, which can then be used and improved by those who later deforest the areas. Unlike the clear cutting that is done for timber harvesting in temperate and boreal forests, logging in Amazonia is always selective because only a few species have commercial value. The disturbed forest that remains after logging is much more susceptible to fire than are unlogged forests.

Deforestation in Amazonia is often not “rational” from the normal financial perspective of paying an attractive return on money invested, at least when only legal money flows are considered. In practice, deforesters make their decisions based on the combined total of all benefit streams, including those that may be undeclared and/or illegal. Investment in Amazonian land can serve as a means of laundering money from illegal sources such as drug trafficking, corruption, sale of stolen goods and income from legitimate activities that is undeclared to tax authorities. For money from these sources the sale of any beef or other products produced in the Amazonian landholdings represents legal income, whereas the investment needed to produce it is highly variable and easily underdeclared to tax authorities.


Burning and deforestation of the Amazon forest to make grazing lands. Source: (NASA LBA-ECO Project)

“Ulterior” motives for clearing also include potential returns from land speculation. The value of land in Amazonia has generally climbed upward and invariably shoots to much higher levels where a road is built or improved. Buying land or claims to land at low prices and reselling it later at much higher prices can give greater returns to a landholder than do the ranching and agriculture that are undertaken during his tenure. Clearing a portion of the land is the only practical way to maintain claim to the area and avoid it being invaded by landless peasants, taken over by another large operator, or confiscated by the government for agrarian reform. Prior to the 1994 “Plano Real” economic reform, Brazil’s rate of inflation was much higher than it is today and land speculation represented a more powerful force, as was shown by the drop by over 50% in the deforestation rate in 1996 and 1997 in parallel with falling land values resulting from the removal of inflation as a driving force. Today, although there is no longer a need to invest funds in real estate as an escape from inflation, profits can still be made by individual landholders when infrastructure projects lead to increasing land values.

A state of lawlessness prevails in substantial areas in Amazonia, leading to distinctive “leaps” in the deforestation frontier. Most notorious is the “Terra do Meio”, or “Middle Lands” to the west of the Xingu River encompassing the Iriri River basin. This area, the size of Switzerland, has effectively been outside of the control of the Brazilian government and is the realm of drug traffickers, illegal loggers and grileiros, or large land thieves who appropriate land through fraudulent (and sometimes violent) means. Declaration of a series of reserves in the area in 2005, following the assassination of Sister Dorothy Stang, is a hopeful sign that the lawless condition is subject to change. The activities of grileiros continue in other locations, most recently increasing in the southern part of the state of Amazonas.

A new and increasing force driving deforestation is export commodities such as soybeans and beef. Soybeans have taken over much of the cerrado (central Brazilian savanna) and are advancing in some rainforest areas such as Santarém, Pará. Soybeans have a large indirect force on deforestation by providing economic justification for highway construction projects that spur forest loss through cattle ranching, logging and land speculation. Beef export has traditionally not been a force behind deforestation in Brazil (in sharp contrast to the “hamburger connection” of Central America) because the presence of foot-and-mouth disease made major markets in Europe, Japan and North America unwilling to import beef in frozen form. Since 1996 states in Brazil have successively been certified as free of foot-and-mouth disease, starting in the extreme south of the country and now including three of the nine Amazonian states. The impact of beef export on deforestation already affects all of Amazonia, even though most of the area is still uncertified and cannot export beef directly. Instead, beef produced outside of Amazonia, for example in the state of São Paulo, can be exported while the people in São Paulo eat beef raised in Pará.

Over 80% of both the cumulative clearing and the current clearing activity is concentrated in the “arc of deforestation,” which is a band along the eastern and southern edges of the forest. Deforestation advances from this band towards the center of the region. However, in addition to the expansion from existing clearings in the arc of deforestation, smaller clearings that appear far from the existing frontier have an importance that is much greater than their small area would suggest. These new clearings can serve as the seeds for much greater clearing activity in the future in the remaining blocks of undisturbed forest. Decisions on building highways that open these areas to migration and to investment are critical in speeding the deforestation process.


Future paths


Intact Amazon forest near Manaus. (Source: Philip M. Fearnside)

The future path of deforestation depends on human decisions. It is not foreordained that the Amazon forest will be destroyed, although this is obviously the endpoint if present trends continue unchanged. Various modeling efforts have projected clearing patterns in Amazonia and agree that vast areas would be cleared if trends continue and planned infrastruture projects are built. Attempts to model what might happen under hypothetical “governance” scenarios are less convincing, since they rely on simple assumptions of restraint and obedience to environmental regulations that are at variance with observed behavior to date. Nevertheless, the potential to change deforestation behavior is real, both through creation of protected areas and through command and control measures to repress illegal clearing. An important historical example is the deforestation licensing and control program that was carried out by the state government in Mato Grosso over the 1999-2001 period, where clearing patterns indicated that the program had a real effect on deforestation rates. This is important as a demonstration that deforestation can be controlled if government authorities are serious about doing so. The example is historical, as the election of Brazil’s largest soybean entrepreneur as state governor in 2002 put an end to the program in practice.

So far the principal measures that have been used to limit deforestation are creation of reserves and repression by fining those who clear without required authorizations. These are the measures that agencies such as the Brazilian Institute for the Environment and Renewable Natural Resources (IBAMA) are empowered to implement. However, the better-funded government agencies such as the Ministry of Transportation continue to plan and build roads and other infrastructure projects without regard for impact on deforestation. While the environmental licensing process for infrastructure projects may lead to compensatory actions, such as creation of reserves and funding of programs for monitoring and enforcement of regulations, the licensing has on many occasions shown itself to be incapable of stopping damaging projects and either diverting development efforts to less-damaging alternatives to achieve the same objectives or forcing a rethinking of the wisdom of the objectives themselves. Environmental impact studies (EIS) have only been required in Brazil since 1986 and the system is still subject to continual challenges and (often successful) attempts to build infrastructure with either no EIS (e.g., the proposed BR-319 Highway) or with an inadequate EIS that is effectively replaced with a package of parallel activities that escape from the legal requirement of serving as prerequisites for the infrastructure (e.g., the proposed BR-163 Highway). Building and improving highways is one of the principal ways that government decisions affect the deforestation process; the consequences of these decisions are much more far reaching than are the effects of any compensatory programs such as environmental education, promotion of agroforestry and the like. One of the most basic measures needed is strengthening the environmental impact assessment and licensing system such that it serves its intended role of informing major decisions before they are made, rather than as a formality to legalize decisions that have already been made.
Indigenous lands and various categories of parks and reserves represent a primary defense against deforestation.

Indigenous peoples, whose officially recognized lands represent approximately 20% of Brazilian Amazonia, actively defend their territories against invasion. An essential complement to this is the general understanding that invasion of indigenous land will not be rewarded by a land title or by any sort of compensation. In many places in the arc of deforestation the only forest that remains standing is what is in indigenous areas.

Since the mid 1990s tremendous progress has been made in “demarcating” indigenous areas, that is, marking the limits of these areas on the ground (as opposed to merely drawing them on a map). At the same time, large areas have been protected in newly created conservation units, both by federal and state governments. These include an increasing number of “sustainable use” reserves, although a number of new reserves have also been created in the “integral protection” category. Conservation units now total roughly 10% of Brazilian Amazonia. Decreeing a reserve, even if only on paper, can have a significant effect in discouraging both small squatters and large grileiros from clearing forest with the expectation of eventually receiving legal title to their claims. In the longer term “paper parks” are not enough—the reserves must be adequately guarded. Expansion of the reserve network must be done quickly because the opportunity to create reserves is often lost once deforestation advances into an area. Reserve creation is a critical step in guiding the development of Amazonia towards a future that maintains substantial areas of tropical forest.

Impacts
Amazonian deforestation causes serious impacts that, if counted in decision making, would often be seen to outweigh the benefits derived from clearing. Loss of biodiversity is one such impact, which implies both loss of the value of the direct uses to which the species lost might be put by humans and the loss of existence values that are independent of such direct uses. Deforestation impacts are magnified by the pattern of clearing that leaves the remaining forest divided into fragments, which then degrade both biodiversity and carbon stocks.


BR-319 Highway. (Source: Philip M. Fearnside)

Deforestation also affects water cycling, because conversion of forest to cattle pasture results in the water running off into the rivers and flowing directly to the ocean without being recycled through the trees. A significant part of the rainfall in Amazonia, especially in the dry season, depends on water that has been recycled through the trees of the forest. This water is critical to preventing droughts from exceeding the forest’s tolerance for water stress, leading to death of trees and degradation of the forest through fire and further drought. Water from Amazonia is also critical to maintaining rainfall in the heavily populated regions in south-central Brazil, such as São Paulo, and in neighboring countries such as Argentina.

Global warming receives a substantial input of greenhouse gases from Amazonian deforestation, although the majority of the gases released by humanity as a whole come from burning fossil fuels such as petroleum and coal rather than from deforestation. However, over three-fourths of Brazil’s contribution to this global problem is the result of Amazonian deforestation. Half of the dry weight of the trees is carbon, and when forest is cut this carbon is released to the atmosphere either as carbon dioxide or as methane, both from burning and from decomposition of wood that fails to burn. The fact that most deforestation is for cattle pastures that do little either for the national economy or for providing employment to the population offers an opportunity to slow deforestation as part of a program for mitigating global warming. The value of the damage done by greenhouse-gas emissions from deforestation far exceeds the value of the timber, beef and other products that are sold as a result of the clearing.

Alternatives
Discussion of “alternatives to deforestation” often focuses on the extraction of non-timber forest products such as rubber and Brazil nuts, sustainable forest management for timber, and increasing the production and sustainability of land uses in already deforested areas, for example by encouraging agroforestry. These “alternatives” all have their places, but they also have limitations and are often misrepresented with respect to their role in slowing deforestation. The economic logic that activities expand when they prove to be profitable applies to those undertaken in deforested areas, and what may be intended as an “alternative” can, if it proves highly successful, advance into the forest and have the opposite effect on deforestation. Proposals for intensified pasture production and for extensive production of biofuels are subject to these reservations. The same applies to annual crops: the notion that farmers and ranchers who reap greater per-hectare yields will be satisfied with their harvests and refrain from further deforestation is unrealistic as a basis for programs aimed at reducing deforestation. Unfortunately, people do not stop clearing when their stomachs are full, but instead use their increased wealth to expand their clearings.

Uses of standing forest such as sustainable (i.e., low intensity) harvesting of both non-timber and timber products often produce low financial returns. This is to be expected, but the rationale for promoting and subsidizing these uses lies in their role in maintaining environmental services, not in their efficiency as commodity sources. These activities are valuable as transitional mechanisms for maintaining traditional populations who can serve as forest guardians. The same activities can also serve as parts of future mechanisms whereby environmental services of standing forest become the primary means of generating income to support the rural population, and the basis of the economy is shifted from its present reliance on forest destruction. Among the environmental services of the forest, carbon storage is the closest to becoming the basis of substantial financial flows through international negotiations, but the equally valuable services of maintaining biodiversity and water cycling can eventually be expected to gain similar status.

Further Reading

• Publications on Human Carrying Capacity, Agroecosystems, Deforestation, and Development Planning in Brazilian Amazonia, National Institute for Research in the Amazon (INPA).
• Expanding Deforestation in Mato Grosso, Brazil, NASA Earth Observatory
• Deforestation in the Amazon, Rhett A Butler, Mongabay.com.

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.

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