Geoscience Reference
In-Depth Information
global system on Earth will adapt as human activity enhances the greenhouse effect, even
if the adaptations are not favourable to human life.
The carbon cycle
Carbon is forever moving around. Each year, roughly 128 billion tonnes is released as car-
bon dioxide into the atmosphere by processes on land, and nearly as much is immediately
absorbed again by plants and by the weathering of silicate rocks. At sea, the figures are
comparable, though slightly more goes in than comes out. The system would be more or
less in balance were it not for volcanic emissions and the 5 billion tonnes released each year
by burning fossil fuels. The total amount of carbon held in the atmosphere is quite small
- just 740 million tonnes, only slightly more than that held in plants and animals on land
and slightly less than that held by living things in the ocean. By comparison, the amount of
carbon stored in solution in the oceans is vast at 34 billion tonnes, and the amount stored
in sediments is 2,000 times greater still. So the physical processes of solution and precip-
itation may be even more important in the carbon cycle than the biological ones. But life
seems to hold some key cards. Carbon incorporated by phytoplankton would be released
back to sea water and hence the atmosphere very quickly were it not for the physical prop-
erties of copepod faecal pellets. These tiny planktonic animals excrete their waste in small,
dense pellets which can slowly sink into the deep ocean, removing them, at least temporar-
ily, from the cycle.
