Local Effects

The role of forests and their vegetation in maintaining lower ambient temperatures or higher relative humidity are thought to affect both local and regional climatic conditions (Nobre et al. 1991). This may be important for maintaining or enhancing the productivity of agriculture in adjacent areas (Lopez 1997).

Even more dramatic can be the influence of large extensions of tropical forest on the hydrologic regime. For example, a classic work by Salati and Vose (1984) showed that rainfall in the Amazon is internally recycled, with about 50% of rain coming from condensation of water vapor from evapotranspiration from the forest canopy. When a sizeable amount of forest is cut down, the remaining forest is less able to evaporate and transpire, causing a decrease in rainfall that may eventually result in changes in the vegetation from forest to savanna or woodland (Salati and Nobre 1992). However, the effects of deforestation on rainfall are not that clear. Deforestation also changes the surface albedo and aerodynamic drags, which in turn affects temperatures, cloudiness, air circulation, etc., resulting in a highly scale-dependent and non-linear system (Chomitz and Kumari 1995). Comprehensive reviews of results obtained at different scales using micro-scale empirical studies, meso-scale climate models, and general circulation models show that it is no longer clear whether deforestation reduces rainfall. Some reviews conclude that while the assumption that deforestation affects local climate is plausible and cannot be totally dismissed, the magnitude and outcome of the effect remain to be clearly demonstrated, and are likely to be relatively minor (Nasi et al. 2002).

However, deforestation has been considered responsible for declines in rainfall in several areas of the humid tropics, such as near the Panama Canal, northwestern Costa Rica, southwestern Ivory Coast, western Ghats of India, northwestern Peninsular Malaysia, and parts of the Philippines (Myers 1988; Meher-Homji 1992; Salati and Nobre 1992). In northwestern Peninsular Malaysia, two states have experienced disruption of rainfall regimes, causing 20,000 ha of rice paddy fields to be abandoned and another 72,000 ha to significantly decline in productivity (Myers 1997).

The impact of deforestation on hydrologic flows is a major concern throughout Central America. Sedimentation of reservoirs, dry season water shortages, flooding, and the severity of damage caused by Hurricane Mitch in 1998 have all been widely attributed, at least in part, to deforestation (Pagiola 2002). When vegetation is removed, soil aggregates break down and the soil becomes less permeable to water. As a result, there is less water stored in the soil and more erosion and surface runoff during rainstorms. Consequently, floods are more common during rainstorms and water flow in streams decreases during dry spells.

Both rural and urban populations perceive water services as ecosystem functions that should be maintained. A study carried out by the Tropical Agriculture Research and Higher Education Center, CATIE, in Costa Rica found that most Costa Ricans agree to pay for the environmental services (ES) provided by forests. The same study shows that the ES that Costa Ricans value most is water protection; followed by biodiversity protection, carbon sequestration, and scenic beauty (35, 25, 20, and 20%, respectively) (Nasi et al. 2002).

The poor sectors of human populations worldwide are particularly vulnerable to climate change. Not only are they more dependent on the weather for their livelihoods (for example, through rain-fed agriculture) but also they tend to reside in tropical areas that are likely to suffer the most from rising temperatures and sea levels (Pagiola 2002). Moreover, the poor generally lack the financial and technical resources that could allow them to adjust to global warming.

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