Borneo Rainforest. Click image for more pictures of Borneo. (Photo by R. Butler)
CLIMATIC ROLE OF FORESTS
By Rhett Butler
| Last updated July 22, 2012
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. In turn, this warm carries moisture from forest trees into to atmosphere, where it condenses as rain. In other words, tropical forests cool local climate and help generate rainfall. Conversely, 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.
In 2007 two Russian physicists proposed a new theory for explaining the role forests have in generating rainfall over land masses. The concept, known as the pump theory holds that it is condensation from forests, and not temperature differences, that drives the winds which bring precipitation over land. The theory is hotly contested.
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Norman Myers explains the albedo connection in "The world's forests and their ecosystem services," In Nature's Services: Societal Dependence on Natural Ecosystems ed G.C. Daily, Island Press, Washington D.C. 1997.