Carbon dioxide is another variable component of Earth’s
atmosphere. A complex pattern of carbon dioxide sources
(places where it originates) and sinks (places where it goes)
determines how much carbon dioxide will be present at any
one time. Plants consume carbon dioxide in great quantities
as part of their metabolic process. Coral reefs are colonies
of tiny ocean organisms that build their protective shells
with carbonates produced from dissolved carbon dioxide.
Fires, decaying vegetation, and human burning of fossil
fuels all release carbon dioxide back into the atmosphere.
This balance between carbon dioxide sources and sinks
changes with time. As we’ll describe later, the amount of
carbon dioxide has varied historically but has been increasing
more rapidly for almost two centuries—since the industrial
revolution. This recent increase in carbon dioxide in
turn has had a direct impact on global temperature because
carbon dioxide is a powerful greenhouse gas.