The local level is where deforestation has the most immediate effect. With forest loss, the local
community loses the system that performed valuable but often under-appreciated services like ensuring the regular flow of clean
water and protecting the community from flood and drought. The forest acts as a sort of sponge, soaking up rainfall brought by tropical storms while anchoring soils and releasing water at regular intervals. This regulating feature of tropical rainforests can help moderate destructive flood and drought cycles that can occur when forests are cleared.
When forest cover is lost, runoff rapidly flows into streams, elevating river levels and subjecting downstream villages, cities, and agricultural fields to flooding, especially during the rainy season. During the dry season, such areas downstream of deforestation can be prone to months-long droughts which interrupt river navigation, wreak havoc on crops, and disrupt industrial operations.
Situated on steep slopes, montane and watershed forests are especially
important in ensuring water flow and inhibiting erosion, yet during the 1980s, montane forests suffered the
highest deforestation rate of tropical forests. (That trend changed in the late 1990s and 2000s, when upland forests recovered, while lowland areas bore the brunt of deforestation, largely due to agricultural expansion).
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 forest clearing with fire. 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 desiccation for neighboring forest cover; reducing its moisture
stocks and its vegetation would then further the desiccation 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 desiccated 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 Amazon were highly vulnerable to fire in 1998. That extent grew in 2005 and 2010 when the Amazon was hit by even worse droughts.
Deforestation for rubber in Laos. Click image for more pictures. (Photo by R. Butler)
Review questions:
How do rainforests help moderate flood and drought cycles?
N. Myers in "Deforestation Rates in Tropical Forests and Their Climactic Implications," Friends of the Earth, London, 1989 estimates that the tropical deforestation rate increased by 90% during the 1980s.
The "Estimated Annual Rates of Deforestation" chart is derived from Orr, D.W., Earth in Mind: On Education, Environment, and the Human Prospect, Washington, D.C.: Island Press, 1994; State of the World's Forests 1997 (SOFO) published by the United Nations Food and Agriculture Organization (FAO); and N. Myers in "Deforestation Rates in Tropical Forests and Their Climactic Implications," Friends of the Earth, London, 1989.
The U.N. FAO (State of the World's Forest 1997 (SOFO)) provides statistics revealing the high deforestation rates of tropical montane forest during then 1980s.
According to Salati, E. and Nobre, C.A ("Possible climatic impacts of tropical deforestation," in Tropical Forests and Climate, ed N. Myers., Dordrecht: Kluwer Academic Publishers, 1992) 50-80% of the moisture in the central and western Amazon is recycled.
Myers, N. in "The world's forests and their ecosystem services," in Nature's Services: Societal Dependence on Natural Ecosystems, ed G.C. Daily, Washington D.C.: Island Press, 1997 explains how moisture is transpired by plants and evaporated back into the atmosphere to form rain clouds.
A. Gioda reviews the importance of water throughout history in "A Short History of Water" Nature & Resources, Vol. 35, No. 1, Jan-Mar 1999.
Szollosi-Nagy, A., Najlis, P., and Bjorklund, G breadown the availability of global freshwater resources in "Assessing the world's freshwater resources," Nature & Resources, Vol. 34, No. 1, Jan-Mar 1998.
Albor, T. reported on severe flooding in the Philippines resulting from deforestation ("Illegal Logging blamed for Philippine Flood Toll," Christian Science Monitor 11/12/91).
Pearce attributes declining rainfall in interior West African countries to coastal forest loss (Pearce, F., "Lost Forests Leave West Africa Dry," The New Scientist 1-18-97.).
Vegetation change caused by ancient human agricultural fires may have impacted precipitation patterns in the Austalian outback according to Cowen, R. ("If You Don't Spare the Tree, You May Spoil More Than the Jungle," Christian Science Monitor. 1/13/98) and Johnson, B.J. et aL. ("65,000 years of vegetational change in central Australia and the Australian summer monsoon," Science Vol. 284, No 5417 (1150-1152), 14-May-1999).
In his article "Escaping Nature's Wrath" in the Chronicle Foreign Service (1998), P. Grunson discusses the role of environmental degradation on damage inflicted by Hurricane Mitch.
In his book, Ultimate Security: The Environmental Basis of Political Stability (Washington, D.C.: Island Press. 1996), Norman Myers discusses the importance of environmental stability in maintaining social and policial stability. He indicates that shrinking water supplies will have important political raminfications in the near future.
The decline of Colombia's freshwater resources is mentioned in Inter Press Service (IPS), "Environment-Colombia: Garbage, Guerrillas and Animal Smugglers" 1/7/98.
N. Myers brings up the concern that widespread deforestation could trigger a positive-feedback process of increasing dessication for neighboring forest cover in "The world's forests and their ecosystem services" in Nature's Services: Societal Dependence on Natural Ecosystems, ed G.C. Daily, Washington D.C.: Island Press, 1997.
At the 1998 global climate treaty conference in Buenos Aires, the Nautral Environment Research Coucil of the UK released a study forcasting the conversion of 2.8 million square kilometers of the Amazon rainforest to desert resulting from global climate change. This dire projection has been considered too extreme by many climate researchers since its release. Nevertheless the story was picked up by McCarthy, M. in "Amazon forest 'will be dead in 50 years'" The Independent. 11/11/98.
In its RisQue98 (Risco de Queimada, or "Risk of Burning" in Amazonia - 1998), the Woods Hole Research Center assessed the risk of forest fires and agricultural burning in Brazilian Amazonia for the second half of 1998 and found that more than 400,000 square kilometers were vulnerable.