5.1: Land Use Change and Climate Regulation
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The energy source that ultimately drives the earth's climate is the sun. The amount of solar radiation absorbed by the earth depends primarily on the characteristics of the surface. Although the link between solar absorption, thermodynamics, and ultimately climate is very complex, newer studies indicate that vegetation cover and seasonal variation in vegetation cover affects climate on both global and local scales. New generations of atmospheric circulation models are increasingly able to incorporate more complex data related to these parameters (Sellers et al. 1997). Besides regulating the atmosphere's composition, the extent and distribution 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).
The amount of solar radiation reflected by a surface is known as its albedo; surfaces with low albedo reflect a small amount of sunlight, those with high albedo reflect a large amount. Different types of vegetation have different albedos; forests typically have low albedo, whereas deserts have high albedo. Deciduous forests are a good example of the seasonal relationship between vegetation and radiation balance. In the summer, the leaves in deciduous forests absorb solar radiation through photosynthesis; in winter, after their leaves have fallen, deciduous forests tend to reflect more radiation. These seasonal changes in vegetation modify climate in complex ways, by changing evapotranspiration rates and albedo (IPCC 2001).
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 the atmosphere. This release of water from vegetation cools the air temperature. In the Amazon region, vegetation and climate is tightly coupled; evapotranspiration of plants is believed to contribute an estimated fifty percent of the annual rainfall (Salati 1987). Deforestation in this region leads to a complex feedback mechanism, reducing evapotranspiration rates, which leads to decreased rainfall and increased vulnerability to fire (Laurance and Williamson 2001).
Deforestation also influences the climate of cloud forests in the mountains of Costa Rica. The Monteverde 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 the mountaintops, reducing the ability for cloud forests to form. Removing the clouds from a cloud forest dries the forest, so it can no longer support the same vegetation or provide appropriate habitat for many of the species originally found there. Similar patterns may be occurring in other, less studied montane cloud forests around the world.
Different vegetation types and topographies have varying surface roughness, which change the flow of winds in the lower atmosphere and in turn influences climate. Lower surface roughness also tends to reduce surface moisture and increase evaporation. Farmers apply this knowledge when they plant trees to create windbreaks (Johnson et al. 2003). Windbreaks reduce wind speed and change the microclimate, increase surface roughness, reduce soil erosion, and modify temperature and humidity. For many field crops, windbreaks increase yields and production efficiency. They also minimize stress on livestock from cold winds.
- the amount of solar radiation reflected by a surface
- is the process whereby water is absorbed from soil by vegetation and then released back into the atmosphere
- surface roughness
- the average vertical relief and small-scale irregularities of a surface