Tropical Deforestation Affects Rainfall In The U.S. And Around The Globe

Now, a new study is offering insight into thelong-term impacts of these changes, particularly the effects oflarge-scale deforestation in tropical regions on the global climate.Researchers from Duke University, Durham, N.C., analyzed multiple yearsof data using the NASA Goddard Institute for Space Studies GeneralCirculation Computer Model (GCM) and Global Precipitation ClimatologyProject (GPCP) to produce several climate simulations. Their researchfound that deforestation in different areas of the globe affectsrainfall patterns over a considerable region.

Deforestation inthe Amazon region of South America (Amazonia) influences rainfall fromMexico to Texas and in the Gulf of Mexico. Similarly, deforesting landsin Central Africa affects precipitation in the upper and lower U.SMidwest, while deforestation in Southeast Asia was found to alterrainfall in China and the Balkan Peninsula. It is important to notethat such changes primarily occur in certain seasons and that thecombination of deforestation in these areas enhances rain in one regionwhile reducing it in another.

This finding contradicts earlierresearch suggesting deforestation would result in a reduction inprecipitation and increase in temperature in the Amazon basin, butcarry no detectable impact on the global water cycle.

"Our studycarried somewhat surprising results, showing that although the majorimpact of deforestation on precipitation is found in and near thedeforested regions, it also has a strong influence on rainfall in themid and even high latitudes," said Roni Avissar, lead author of thestudy, published in the April 2005 issue of the Journal ofHydrometeorology.

Specifically, deforestation of Amazonia wasfound to severely reduce rainfall in the Gulf of Mexico, Texas, andnorthern Mexico during the spring and summer seasons when water iscrucial for agricultural productivity. Deforestation of Central Africahas a similar effect, causing a significant precipitation decrease inthe lower U.S Midwest during the spring and summer and in the upperU.S. Midwest in winter and spring. Deforestation in Southeast Asiaalters rainfall in China and the Balkan Peninsula most significantly.

Eliminationof any of these tropical forests, Amazonia, Central Africa or SoutheastAsia, considerably enhances rainfall in the southern tip of the ArabianPeninsula. However, the combined effect of deforestation in all threeregions shifts the greatest precipitation decline in the U.S. toCalifornia during the winter season and further increases rainfall inthe southern tip of the Arabian Peninsula.

Improved understandingof tropical forested regions is valuable to scientists because of theirstrong influence on the global climate. The Amazon Basin literallydrives weather systems around the world. The tropics receive two-thirdsof the world's rainfall, and when it rains, water changes from liquidto vapor and back again, storing and releasing heat energy in theprocess. With so much rainfall, an incredible amount of heat isreleased into the atmosphere - making the tropics the Earth's primarysource of heat redistribution.

"Deforestation does not appear tomodify the global average of precipitation, but it changesprecipitation patterns and distributions by affecting the amount ofboth sensible heat and that released into the atmosphere when watervapor condenses, called latent heat," said Avissar. "Associated changesin air pressure distribution shift the typical global circulationpatterns, sending storm systems off their typical paths." And, becauseof the Amazon's location, any sort of weather hiccup from the areacould signal serious changes for the rest of the world like droughtsand severe storms.

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

Theresearchers caution that their results are based on numericalsimulations performed with a single general circulation model and thatreproducing the experiment with other computer models using differentatmospheric variables would be beneficial. Current research isattempting to answer why deforestation has such a major influence onprecipitation patterns during respective seasons.