Global processes  (Page 2/9)

Land use change and climate regulation

The energy source that ultimately drives the earth's climate is the sun. The amount ofsolar radiation absorbed by the earth depends primarily on the characteristics of the surface. Although the linkbetween solar absorption, thermodynamics, and ultimately climate is very complex, newer studies indicate thatvegetation cover and seasonal variation in vegetation cover affects climate on both global and local scales. Newgenerations of atmospheric circulation models are increasingly able to incorporate more complex data relatedto these parameters ( Sellers et al . 1997 ). Besides regulating the atmosphere's composition, the extent anddistribution of different types of vegetation over the globe modifies climate in three main ways:

• affecting the reflectance of sunlight ( radiation balance );
• regulating the release of water vapor ( evapotranspiration    ); and
• changing wind patterns and moisture loss ( surface roughness    ).

Vegetation absorbs water from the soil and releases it back into the atmosphere through evapotranspiration    , which is the major pathway by which water moves from the soil to theatmosphere. This release of water from vegetation cools the air temperature. In the Amazon region, vegetation and climateis tightly coupled; evapotranspiration of plants is believed to contribute an estimated fifty percent of the annualrainfall ( Salati 1987 ). Deforestation in this region leads to a complex feedback mechanism, reducing evapotranspiration rates, whichleads to decreased rainfall and increased vulnerability tofire ( Laurance and Williamson 2001 ).

Deforestation also influences the climate of cloud forests in the mountains of Costa Rica. TheMonteverde Cloud Forest harbors a rich diversity of organisms, many of which are found nowhere else in the world. However,deforestation in lower-lying lands, even regions over 50 kilometers way, is changing the local climate, leaving the"cloud" forest cloudless ( Lawton et al . 2001 ). As winds pass over deforested lowlands, clouds are lifted higher, often above themountaintops, reducing the ability for cloud forests to form. Removing the clouds from a cloud forest dries the forest, soit can no longer support the same vegetation or provide appropriate habitat for many of the species originally foundthere. Similar patterns may be occurring in other, less studied montane cloud forests around the world.