Hurricane Mitch

Situated on steep slopes, montane and watershed forests are especially important in ensuring water flow and inhibiting erosion, yet during the 1980s, montane formations suffered the highest deforestation rate of tropical forests.

Aerial view of erosion in western Madagascar Does deforestation cause flooding? Not directly, according to a 2005 study by the UN's Food and Agriculture Organization (FAO) and the Center for International Forestry Research (CIFOR). The groups found that the frequency and extent of major floods has not changed over the last century despite significant reductions in forest cover. Instead, FAO and CIFOR say that deforestation does have a role in small floods and topsoil erosion by eliminating the buffering and soil-anchoring effects of forests. Further, the report accuses Asian governments of using deforestation as an excuse to deflect criticism over their poor handling of human settlement in areas unsuitable for habitation.

Additionally, the forest adds to local humidity through transpiration (the process by which plants release water through their leaves), and thus adds to local rainfall. For example, 50-80 percent of the moisture in the central and western Amazon remains in the ecosystem water cycle. In the water cycle, moisture is transpired and evaporated into the atmosphere, forming rain clouds before being precipitated as rain back onto the forest. When the forests are cut down, less moisture is evapotranspired into the atmosphere resulting in the formation of fewer rain clouds. Subsequently there is a decline in rainfall, subjecting the area to drought. If rains stop falling, within a few years the area can become arid with the strong tropical sun baking down on the scrub-land. Today Madagascar is largely a red, treeless desert from generations of severe deforestation. River flows decline and smaller amounts of quality water reach cities and agricultural lands. The declining rainfall in interior West African countries has in part been attributed to excessive clearing of the coastal rainforests. Similarly, new research in Australia suggests that if it were not for human influences—specifically widespread agricultural fires—the dry outback might be a wetter, more hospitable place than it is today. The effect of vegetation change from forests that favor rainfall to grassland and bush can impact precipitation patterns. Colombia, once second in the world with freshwater reserves, has fallen to 24th due to its extensive deforestation over the past 30 years. Excessive deforestation around the Malaysian capital of Kuala Lumpur, combined with the dry conditions created by el Niño, triggered strict water rationing in 1998, and for the first time the city had to import water.

There is serious concern that widespread deforestation could lead to a significant decline in rainfall and trigger a positive-feedback process of increasing dessication for neighboring forest cover; reducing its moisture stocks and its vegetation would then further the dessication effect for the region. Eventually the effect could extend outside the region, affecting important agricultural zones and other watersheds. At the 1998 global climate treaty conference in Buenos Aires, Britain, citing a disturbing study at the Institute of Ecology in Edinburgh, suggested the Amazon rainforest could be lost in 50 years due to shifts in rainfall patterns induced by global warming and land conversion.

The newly dessicated forest becomes prone to devastating fires. Such fires materialized in 1997 and 1998 in conjunction with the dry conditions created by el Niño. Millions of acres burned as fires swept through Indonesia, Brazil, Colombia, Central America, Florida, and other places. The Woods Hole Research Center warned that more than 400,000 square kilometers of Brazilian Amazonia were highly vulnerable to fire in 1998.

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Water wars?

Such losses of freshwater resources are considered one of the most immediate threats to national security in many countries. Freshwater—required for human consumption, agriculture, and industrial operations—or the lack thereof can have a tremendous effect on the social, economic, and political climate of a country. Realizing the importance of water, politicians of the future may try to secure their existing freshwater supplies or wage war to acquire other sources of water. Demand for water increases as the standard of living improves, so politicians of the future will look to guarantee freshwater supplies. Developing countries, where political and social conditions are often tense, will likely experience the most pressure from shrinking water supplies. In the future, wars may be fought over water, not oil. Already Egypt has made it known to its upstream neighbors—Sudan and Ethiopia—that it is willing to go to war over the Nile's water

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• How do rainforests help moderate flood and drought cycles?