THE CARBON CYCLE: The carbon cycle describes the movement of carbon, in its many forms, between the earth, atmosphere, oceans, and the animals, plants and bacteria that live there. For example, much of the carbon stored in trees and soils is released into the atmosphere when forests are cleared and cultivated. Sometimes this release happens very quickly, like when a forest fire burns. Sometimes it happens slowly, as dead plants decompose. When forests regrow on cleared land, trees draw carbon from the atmosphere and store it again in the plants and soil. If the global totals for photosynthesis (plants taking CO2 from the air and using it for energy, giving off oxygen) and respiration (animals taking in oxygen and using it to make energy, giving off CO2) are not equal, carbon accumulates, either on land or in the atmosphere. The rates of photosynthesis and respiration are not known, and they're not measured well enough, but there does appear to be an imbalance, known as the "missing sink" of carbon. Yet the carbon cycle must be a closed system, which means there is a fixed amount of carbon; we just don't know where the missing carbon is yet. Understanding why there is an imbalance, and where it occurs, is critical to combating the threat of global warming.
ABOUT GLOBAL WARMING: Global warming refers to an increase in the earth's average temperature -- which has risen about 1 degree F over the past 100 years. A warmer earth may lead to changes in rainfall patterns, and a rise in sea level, for example, as polar glaciers melt. Some of this rise is due to the greenhouse effect: certain gases in the atmosphere trap energy from the sun so that heat can't escape back into space. Without the greenhouse effect, the earth would be too cold for humans to survive, but if it becomes too strong, the earth could become much warmer than usual, causing problems for humans, plants and animals.