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Tropical deforestation affects rainfall in North America

Tropical Deforestation Affects US Climate

Tropical Deforestation Affects US Climate
By Mike Bettwy
Goddard Space Flight Center release
September 20, 2005

Today, scientists estimate that between one-third and one-half of our planet’s land surfaces have been transformed by human development.

Now, a new study is offering insight into the long-term impacts of these changes, particularly the effects of large-scale deforestation in tropical regions on the global climate. Researchers from Duke University, Durham, N.C., analyzed multiple years of data using the NASA Goddard Institute for Space Studies General Circulation Computer Model (GCM) and Global Precipitation Climatology Project (GPCP) to produce several climate simulations. Their research found that deforestation in different areas of the globe affects rainfall patterns over a considerable region.

Deforestation in the Amazon region of South America (Amazonia) influences rainfall from Mexico to Texas and in the Gulf of Mexico. Similarly, deforesting lands in Central Africa affects precipitation in the upper and lower U.S Midwest, while deforestation in Southeast Asia was found to alter rainfall in China and the Balkan Peninsula. It is important to note that such changes primarily occur in certain seasons and that the combination of deforestation in these areas enhances rain in one region while reducing it in another.

This finding contradicts earlier research suggesting deforestation would result in a reduction in precipitation and increase in temperature in the Amazon basin, but carry no detectable impact on the global water cycle.

“Our study carried somewhat surprising results, showing that although the major impact of deforestation on precipitation is found in and near the deforested regions, it also has a strong influence on rainfall in the mid and even high latitudes,” said Roni Avissar, lead author of the study, published in the April 2005 issue of the Journal of Hydrometeorology.

Specifically, deforestation of Amazonia was found to severely reduce rainfall in the Gulf of Mexico, Texas, and northern Mexico during the spring and summer seasons when water is crucial for agricultural productivity. Deforestation of Central Africa has a similar effect, causing a significant precipitation decrease in the lower U.S Midwest during the spring and summer and in the upper U.S. Midwest in winter and spring. Deforestation in Southeast Asia alters rainfall in China and the Balkan Peninsula most significantly.

Elimination of any of these tropical forests, Amazonia, Central Africa or Southeast Asia, considerably enhances rainfall in the southern tip of the Arabian Peninsula. However, the combined effect of deforestation in all three regions shifts the greatest precipitation decline in the U.S. to California during the winter season and further increases rainfall in the southern tip of the Arabian Peninsula.

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NASA Satellite Data Used to Assess Amazon Deforestation – 15-September-2005
The Amazon, a vast tropical forest stretching across South America, is so large that is virtually impossible to study the evolving landscapes within the basin without the use of satellites. Scientists have used satellite imagery of the Amazon for more than 30 years to seek answers about this diverse ecosystem and the patterns and processes of land cover change. This technology continues to advance and a new study shows that NASA satellite images can allow scientists to more quickly and accurately assess deforestation in the Amazon.

Amazon deforestation lower than last year says Brazil – 27-August-2005
Yesterday Brazil announced that 3,515 square miles (9,103 square kilometers) of Amazon rainforest were destroyed between August 2004 and July 2005, a marked decline from the 7,229 sq. mi. (18,723 sq. km.) in the same period a year earlier. While the government has tried to take credit for the drop, analysts say the slowing is more likely the result of lower commodity prices, giving farmers less incentive to clear forest land.

Improved understanding of tropical forested regions is valuable to scientists because of their strong influence on the global climate. The Amazon Basin literally drives weather systems around the world. The tropics receive two-thirds of the world’s rainfall, and when it rains, water changes from liquid to vapor and back again, storing and releasing heat energy in the process. With so much rainfall, an incredible amount of heat is released into the atmosphere – making the tropics the Earth’s primary source of heat redistribution.

“Deforestation does not appear to modify the global average of precipitation, but it changes precipitation patterns and distributions by affecting the amount of both sensible heat and that released into the atmosphere when water vapor condenses, called latent heat,” said Avissar. “Associated changes in air pressure distribution shift the typical global circulation patterns, sending storm systems off their typical paths.” And, because of the Amazon’s location, any sort of weather hiccup from the area could signal serious changes for the rest of the world like droughts and severe storms.

Clearly, land-cover changes in tropical regions carry potentially significant consequences on water resources, wildfire frequency, agriculture and related activities at various remote locations. And while greenhouse gas emissions and pollutants receive considerable attention, this study shows that land-cover change is another important parameter that needs to be considered in climate policies, especially since deforestation rates in tropical Africa, Southeast Asia, and South America have remained constant or have increased over the past two decades. Land-cover change, depending on its nature, can either mitigate or exacerbate greenhouse warming.

The researchers caution that their results are based on numerical simulations performed with a single general circulation model and that reproducing the experiment with other computer models using different atmospheric variables would be beneficial. Current research is attempting to answer why deforestation has such a major influence on precipitation patterns during respective seasons.

Additional images from the NASA press release

Deforestation detections in Mato Grosso for 2002 (August 2001-2002), 2003 (August 2002-August 2003), and 2004 (August 2003-August 2004) are shown in yellow, blue, and red, respectively. Remaining forest cover in 2004 is shown in green. Background values for non-forested regions are Nomalized Difference Vegetation Index (NDVI) values from the MOD13 16-day composite from May 8-23, 2004 (julian days 129-144). Single-pixel deforestation clusters were removed based on lower reliability of single pixel clusters from field validation results. Click image to enlarge. Credit: Doug Morton, University of Maryland-College Park

From the rainforests around the Amazon River (top) in Brazil, through the central highlands and into Bolivia to the southwest, numerous fires were burning throughout the region on September 8, 2002. The fires were detected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, and their locations are marked with red dots in this true-color image. Thick smoke and clouds are shrouding the highlands in the southern portion of the image. At bottom left, a portion of Lake Titicaca can be seen. Click image to enlarge. Credit: Jesse Allen, based on data from the MODIS Rapid Response Team, NASA-GSFC

On August 11, 2002, the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite detected scores of fires (marked with red dots) burning in northern Mato Grosso in west-central Brazil. Smoke blankets the tropical Amazon forests at the left of the image as fires and deforestation continue to encroach along the margins of the disappearing forests. Click image to enlarge. Credit: Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC

A Landsat Image of the Amazon River, Brazil, on November 30, 2000. Click image to enlarge. Credit: NASA,, Center for Global Change and Earth Observations, Michigan State University

This article is adapted from a NASA press release.