Global Effects

Carbon dioxide (CO2) is considered to be the principal gas responsible for the "greenhouse effect." One of the most important roles of forests in ameliorating the "greenhouse effect" is in absorbing carbon from the atmosphere, thereby reducing the buildup of carbon dioxide. The effect is as follows: energy in the form of visible light and UV (ultraviolet) rays from the sun passes freely through the atmosphere. It heats up the Earth, but is restricted in its ability to escape when reradiated in the form of infrared radiation because it is absorbed by CO2 and other gases contained in the Earth's atmosphere. Atmospheric CO2 increased linearly from 1850-1960, but has increased exponentially since then. Widely dismissed as far-fetched only a few years ago, global warming is currently recognized as real and dangerous (Bishop and Landell-Mills 2002).

Green plants take up CO2 from the atmosphere and use it in photosynthesis to produce sugar and other plant compounds for growth and metabolism. Long-lived woody plants store carbon in wood or other tissues. Through the process of decomposition the carbon in the wood may be released to the atmosphere after the plants die in the form of CO2 or methane. As plant material decomposes in the soil, part of the carbon in plant tissue can form part of the soil organic matter, serving as another more or less permanent carbon sink. However, when a forest is cleared for agriculture, pasture, or other purposes, all the carbon stored in the trees and soil is released into the atmosphere.

Due to sampling and measurement problems, measurements of carbon stocks are not very accurate. For the past 20 years, scientists have been attempting to calculate the global carbon stocks of tropical forests, as well as the changes in these stocks as changes in land use occur. Recently, satellite data and remote sensing have been used to characterize ground cover, providing more accurate estimates of changes in vegetation each year (Loveland and Belward 1997).

Globally, forests contain more than half of all terrestrial carbon, and account for about 80% of carbon exchange between terrestrial ecosystems and the atmosphere. Forest ecosystems are estimated to absorb up to 3 billion -tons of carbon annually. In recent years, however, a significant portion of this has been returned through deforestation and forest fires. For example, tropical deforestation in the 1980s is estimated to have accounted for up to a quarter of all carbon emissions from human activities (FAO 2001a).

The carbon stock estimates by the International Panel on Climate Change (IPCC 2000) are listed in Table 1.2. Tropical forests are by far the largest carbon (C) stock in vegetation, while boreal forests represent the largest C stock

Table 1.2. Global carbon stocks in vegetation and soil carbon pools to a depth of 1 m (IPCC 2000)



Global carbon stocks (Gt C)

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