Carbon cycle.

Once the plausible scenarios for CO{sub 2} buildup have been devised, it is necessary to determine exactly which interacting biogeochemical processes control the global distribution of carbon and its stocks. This involves uptake by green plants (since CO{sub 2} is the basis of photosynthesis, more CO{sub 2} in the air means faster rates of photosynthesis), changes in the amount of forested area, the types of vegetation planted, and the way in which climatic change affects natural ecosystems. The growth rate of photosynthesizers, such as grain plants and trees, may well increase. On the other hand, weeds and vegetation that harbour disease-bearing insects would also become more vigorous. Moreover, since there is a slow removal of CO{sub 2} from the atmosphere, largely accomplished through chemical processes in the ocean that take from decades to centuries, the rates at which climatic change modifies mixing processes in the ocean also need to be taken into account. There is considerable uncertainty over just how much CO{sub 2} will remain in the air, but most present estimates put the so-called airborne fraction at about 50 percent, which suggests that, over the time frame of a century or two at least, something like half the CO{sub 2} injected into the atmosphere will remain and exacerbate the greenhouse effect.