Deforestation for rubber in Laos. Click image for more pictures. (Photo by R. Butler)
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IMPACT OF DEFORESTATION: LOCAL AND NATIONAL CONSEQUENCES
By Rhett Butler | Last updated July 22, 2012
LOSS OF LOCAL CLIMATE REGULATION
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.
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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. However a 2007 study reached a different conclusion that forests do impact the occurrence and severity of destructive floods.
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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.
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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. In the 1990s Egypt 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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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.
Image from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, the image shows vegetation 'greenness' during the 2010 drought, between July and September, compared to average conditions for the same period between 2000 and 2009. The redder the image the less 'green' the forest. Image courtesy of NASA. Click to enlarge. |
Review questions:
- How do rainforests help moderate flood and drought cycles?
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