Borneo Rainforest. Click image for more pictures of Borneo. (Photo by R. Butler)
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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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More rainforest news
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(03/13/2012) The combined impacts of deforestation and climate change will bring a host of new troubles for the world's tropical rainforests argues a new study in Trends in Ecology and Evolution. Drying rainforests due to climate change could lead to previously inaccessible forests falling to loggers, burning in unprecedented fires, or being overexploited by hunters.
(02/01/2012) New, radical theories in science often take time to be accepted, especially those that directly challenge longstanding ideas, contemporary policy or cultural norms. The fact that the Earth revolves around the sun, and not vice-versa, took centuries to gain widespread scientific and public acceptance. While Darwin's theory of evolution was quickly grasped by biologists, portions of the public today, especially in places like the U.S., still disbelieve. Currently, the near total consensus by climatologists that human activities are warming the Earth continues to be challenged by outsiders. Whether or not the biotic pump theory will one day fall into this grouping remains to be seen. First published in 2007 by two Russian physicists, Victor Gorshkov and Anastassia Makarieva, the still little-known biotic pump theory postulates that forests are the driving force behind precipitation over land masses.
(02/09/2012) Three thousand years ago (around 1000 BCE) several large sections of the Congo rainforest in central Africa suddenly vanished and became savannah. Scientists have long believed the loss of the forest was due to changes in the climate, however a new study in Science implicates an additional culprit: humans. The study argues that a migration of farmers into the region led to rapid land-use changes from agriculture and iron smelting, eventually causing the collapse of rainforest in places and a rise of grasslands. The study has implications for today as scientists warn that the potent combination of deforestation and climate change could flip parts of the Amazon rainforest as well into savannah.
(04/18/2007) The rapid expansion of soybean cultivation in the Amazon may be having a larger impact on climate than previously believed, according to research published last week in Geophysical Research Letters. Using experimental plots in the Amazon, a team of scientists led by Marcos Costa from the Federal University of Vicosa in Brazil found that clearing for soybeans increases the reflectivity or albedo of land, reducing rainfall by as much as four times relative to clearing for pasture land.
