69The Role of Greenhouse Gases
As carbon moves through the system, different components can move at
different speeds. Scientists break these reaction times down into two cat-
egories: short-term cycles and long-term cycles. In short-term cycles, carbon
is exchanged quickly. Examples of this include the gas exchange between
the oceans and atmosphere (evaporation). Long-term cycles can take years to
millions of years to occur. Examples of this would be carbon stored for years
in trees or carbon weathered from a rock being carried to an ocean, being
buried, incorporated into plate tectonic systems, and then later released into
the atmosphere through a volcanic eruption.
Throughout geologic time, the earth has been able to maintain a balanced