Forests, carbon and global climate

In recent centuries, human activities have fundamentally altered many of the Earth's biogeochemical cycles. Among the first recognized and most prominent of these changes has been the modification of the global carbon cycle. The dramatic release of carbon trapped by both prehistoric ecosystems (i.e. fossil fuels) and modern-day vegetation has led to a 31% increase in concentrations of atmospheric carbon dioxide (CO₂) from a preindustrial concentration of ca. 280 ppm to 368 ppm in 2000. This concentration has certainly not been exceeded during the past 420 000 years, and probably not during the past 20 million years. Moreover, the rate of increase in CO₂ concentration during the past century is at least an order of magnitude greater than the world has seen for the last 20 kyr (Prentice et al. 2001). CO₂ is the most important of the greenhouse gases that are increasingly trapping solar heat and warming the global climate. In addition to climatic warming, this extra CO₂ may have a number of effects on terrestrial ecosystems, from increasing plant growth rates and biomass to modifying ecosystem composition by altering the competitive balance between species.