Geoscience Reference
In-Depth Information
Carbon cycle
At the heart of the AOGCMs is the carbon cycle and estimating what happens to anthropo-
genic carbon dioxide and methane emissions. As about half of all our carbon emissions are
absorbed by the natural carbon cycle and do not end up in the atmosphere, but rather in the
oceans and the terrestrial biosphere, the Earth's carbon cycle is complicated, with both
sources and sinks of carbon dioxide.
Figure 13
shows the global carbon reservoirs in giga-
tonnes or 1,000 million tonnes (GtC) and fluxes (the ins and outs of carbon in GtC per
year). These indicated figures show the changes since the industrial revolution. Evidence is
accumulating that many of the fluxes can vary significantly from year to year.This is be-
cause in contrast to the static view conveyed in figures like this one, the carbon system is
dynamic, and coupled to the climate system on seasonal, inter-annual, and decadal times-
cales. What has become clear is that the surface ocean and the land biosphere both take up
about 25 per cent each of our carbon emission every year. However, as the oceans continue
to warm they can hold less dissolved carbon dioxide, which means that their uptake will re-
duce. Also as we continue to deforest and substantially alter land use then the land bio-
sphere ability to absorb carbon diminishes.
