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Rainforest canopy in the late afternoon. (Photo by R. Butler)
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GLOBAL CONSEQUENCES
Tropical rainforests play a vital role in the functioning of the planet's natural systems. The forests regulate
local and global weather through their absorption and creation of rainfall and their exchange of atmospheric gases.
For example, the Amazon alone creates 50-80 percent of its own rainfall through transpiration. Cutting the rainforests
changes the reflectivity of the earth's surface, which affects global weather by altering wind and ocean current
patterns, and changes rainfall distribution. If the forests continue to be destroyed, global weather patterns may
become more unstable and extreme.
CLIMATIC ROLE OF FORESTS
As previously discussed, tropical rainforests play a vital role in local climate regulation by their interaction
with water cycles. However, rainforests also have a significant effect on global weather. Rainforests, like all
forms of vegetation, affect the "surface albedo" or reflectivity of a surface by absorbing more heat than bare soil. Norman Myers (1997) explains,
Much of the energy that converts surface moisture into water vapor comes
from the sun's radiational heating of the land surface. The energy thus depends on surface albedo, or relevant
degree of reflectant "shininess" of the land surface (Gash and Shuttleworth 1992). In turn, the albedo
depends on the vegetation, which absorbs more heat than does bare soil. Over thick vegetation, vigorous thermal
currents take moisture (provided by the same plant cover) up into the atmosphere, where it condenses as rain. Because
of its influence on convection patterns and wind currents, and hence on rainfall regimes, the albedo effect constitutes
a basic factor in controlling climate.
The loss of forest vegetative cover means less heat absorption translating to less moisture being taken up into the atmosphere.
Rainfall is also affected when forest-clearing fires create air pollution and release tiny particles, known as aerosols, into the atmosphere. While aerosols can both heat and cool the air, depending on their size, shape, and color, high concentrations of biomass-burning aerosols directly impact local climate by increasing cloud formation but decreasing rainfall, according to research by NASA. In areas with lots of smoke, "cloud droplets form around the aerosol particles, but may never grow large enough to fall as rain," say researchers with NASA's Goddard Space Flight Center who studied the effect. Thus large forest fires have the effect of further reducing rainfall, leaving burned areas more prone to dryness and future fires.
In the long run, these changes explain why deforested regions may experience a decline in rainfall.
Tropical deforestation can also affect weather in other parts of the world. A 2005 study by NASA found that deforestation in the Amazon region of South America influences rainfall from Mexico to Texas and in the Gulf of Mexico, while forest loss in Central Africa affects precipitation patterns in the upper and lower U.S Midwest. Similarly, deforestation in Southeast Asia was found to impact rainfall in China and the Balkan Peninsula.
Soybeans may worsen drought in the Amazon rainforest
Expansion of agriculture in the Amazon may impact climate
Amazon drought extends into second year
Will Amazon drought worsen in 2007?
Changes in forest cover could significantly affect climate
Temperate forests may worsen global warming
Review questions:
- Why does local rainfall decline with deforestation?
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Continued: Atmospheric Role of Forests
Unless otherwise specified, this article was written by Rhett A. Butler [Bibliographic citation for this page]
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