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  • Deforestation in the Amazon:Effects on

    Climate Change

    GG 612 Global Climate ChangeOctober 31, 1999 Prof. Myneni

    Ashley L. Vandegrift

  • 2

    Forests are most often defined as ecosystems with a minimum of 10% crown cover of

    trees and/or bamboo, generally associated with wild flora, fauna and natural soil conditions and

    not subject to agricultural practices.1 The changing area of the worlds forests, including natural

    forests and forest plantations, was estimated to be 3,454 million hectares in 1995, or about one-

    fourth of the land area of the Earth. (see Chart 1) About 55% of these are located in developing

    countries.2 (see Chart 2) While tropical forests occupy less than 7% of the earths terrestrial

    surface, they are home to as many as 30 million species of plants and animals, or more than half

    of all life forms on our planet. 3 Their importance is thus undeniable. For example, of the

    roughly 3,000 plants identified as having cancer-fighting properties, 70% grow in the rainforest.

    In addition, a single acre of tropical rainforest supports between 60 and 80 tree species. 4 Of these

    forests, the Brazilian Amazon is the largest contiguous region of tropical forest in the world,

    making it an extremely important global resource, which is sensitive to land-use changes and

    destruction. The rate and future course of these land-use changes, or forest clearing, in the

    Brazilian Amazon is closely linked to human-use systems that replace the vegetation.

    Deforestation often refers to a change in land-use with the depletion of tree crown cover to less

    than 10%.5 More specifically, deforestation is the complete destruction of forest cover through

    clearing for agriculture, cattle ranching, small holder agriculture, large-scale commodity crop

    production and logging. Further, forest degradation through less-than-destructive logging, forest

    land farming and fuelwood gathering is estimated to amount to more than the area of

    deforestation, working to lower forests overall productive capacity. As global deforestation rates

    have continued to soar, causing rapid and potentially severe impacts, so have concerns of how

    this process effects other natural cycles. Forests are the single most important repository of

    terrestrial biological diversity, as well as home to many animals and indigenous peoples.

    Deforestation causes a loss of this biodiversity and habitat, as well as disrupts the natural carbon

    cycle. Most importantly, it is the release of CO2 into the atmosphere due to deforestation, which

    has in turn, led to an increase in climate change in regions distant from the perturbed area.

    Amazonian deforestation rates are now used to determine human effects on the global carbon

    cycle. Thus, climate change is becoming one of the most serious effects of large-scale

    deforestation in the Amazon.

    The global carbon cycle is made up of many carbon flows and stocks, proving that the

    interactions between land surface and the atmosphere are significant and complex. Natural stocks

    include oceans, fossil fuel deposits, the atmosphere, and the Earth itself when the element is

    present in rocks and sediments, swamps, wetlands, forests and soils. Hundreds of billions of tons

    of carbon as CO2 is absorbed from, or emitted to, the atmosphere through natural processes.

  • 3

    These flows include plant photosynthesis, respiration and decay, as well as oceanic absorption

    and release of CO2. Currently, the atmosphere contains about 750 billion tons of carbon. (see

    Chart 3) This amount is the result of an increase by about 25% in the past 100 years, or an

    increase of about 1.5 ppm per year. 6 Further, the whole Amazonian hylaea retains about

    115 * 109 tons of carbon. This is of the order of nearly 20% of the carbon of the entire

    atmospheres CO2. If the entire Amazon forest was replaced by a much less voluminous, man-

    made vegetation, there would be a net increase of about 8% in global atmospheric CO2 levels.7

    CO2, an important greenhouse gas, forms when the carbon in biomass oxidizes as it burns or

    decays. This is mostly due to human activities that alter land-uses and fossil fuel combustion.

    Land-use changes include slash-and-burn agriculture; clearing land for permanent pasture,

    cropland or human settlements; the development of infrastructure, such as roads and dams;

    intentional and accidental forest burning; and unsustainable logging and fuelwood collection.

    Land-use changes thus cause both carbon sinks and sources. A stock that is taking up carbon is a

    sink. For example, forests accumulate carbon over decades and centuries. About two-thirds of

    the globes terrestrial carbon, exclusive of that sequestered in rocks and sediments, is sequestered

    in the standing forests, forest plants, leaf and forest debris, and in forest soils.8 A stock that is

    releasing carbon is a source of emissions, and if enough carbon is released, the forest will

    eventually become a net source. The release of CO2 will occur if the original ecosystem stored

    more carbon than the modified ecosystem, thus, causing deforestation to act as a source of CO2.

    Further, the amount of carbon released is a result of deforestation at different rates according to

    the method of clearance and subsequent land-use. (see Chart 4) Throughout the 1980s, it is

    believed that deforestation accounted for 1.6 billion tons of carbon emissions. 9 If forests are

    simply cut and allowed to decay, then most of the carbon is released into the atmosphere within

    ten to twenty years. This release is the forests most important contribution to climate change,

    even though human activities in tropical forests also produce other greenhouse gases, such as

    methane, carbon monoxide, nitrous oxide and ozone, which can act as sources as well. One of

    the main differences between CO2 and these other gases is that newly added CO2 will remain in

    the atmosphere from decades to thousands of years, while the others will breakdown much faster.

    It is now widely believed that the accumulation of CO2 in the atmosphere, partly due to

    deforestation, is responsible for the observed global warming of around 0.5C in the past 100

    years.10

    Global warming is now attributed to the steady increase of atmospheric trace gases

    produced largely by human activities. These gases are referred to as greenhouse gases because

    they cause radiative forcing, by letting in most of the incoming solar radiation that heats the

  • 4

    Earths surface, yet also prevent the outgoing thermal radiation from escaping to space, thus

    trapping some of the surface heat energy. Aerosols also contribute to climate change. Most

    aerosols reflect solar radiation back to space, working to cool the earths surface. Although

    aerosols are injected into the atmosphere from natural sources, their concentration can also be

    increased by human activities such as deforestation and over-grazing, which emit dust from the

    top soil into the air. The concentration of greenhouse gases, especially CO2, has fluctuated

    naturally over geological time scales. The temperature of the Earth has responded to these

    fluctuations by switching between ice age and interglacial conditions. In addition to these slow

    natural variations, atmospheric concentrations of greenhouse gases can be altered by the burning

    of fossil fuels, clearing of forests, and many other industrial activities. Human activities can

    produce a change in atmospheric composition, thus leading to radiative forcing of the climate

    system and an eventual change in climate. While much of the existing climate change may be

    due to these natural variations, it is now widely believed that the current trends are actually due to

    these anthropogenic causes. For example, for the first time ever, the Intergovernmental Panel on

    Climate Change has concluded that the observed increase in global average temperature over the

    last century is unlikely to be entirely natural in origin and that the balance of evidence

    suggests that there is a discernible human influence on global climate.11 It has been measured

    that the globally averaged temperature of the air at the Earths surface has warmed between 0.3

    and 0.6C since the late 19th century.12 (see Chart 5) Further, all 10 of the warmest years on

    record have occurred in the last 15 years, while the 1990s have already been warmer than the

    1980s by almost 0.1C. 13 The warming has been greatest at night, over land, in the mid-to-high

    latitudes of the northern hemisphere, while the warming during the northern winter and spring has

    been stronger than during other seasons. Other evidence of global warming since the 19th century

    include the observed rise in sea level, the shrinkage of mountain glaciers, a reduction of northern

    hemisphere snow cover, and increasing sub-surface ground temperatures. Much support for this

    evidence rests on measurements from tree rings, ice cores, and corals. It is expected that mean

    surface temperature of the earth will most likely rise by 1 to 2C over the next 50 to 100 years, if

    current CO2 trends continue. 14 Because CO2 changes can drive climate change, it is important to

    examine those activities which effect CO2 levels. Due to the fluctuations in carbon resulting from

    land-use changes, deforestation has become a major cause of present and future climate change.

    In turn, these climate changes could also

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