Geography Reference
In-Depth Information
Over timescales of millions of years, rock weathering (as
described above and see also Chapter 3) adds carbon to river water
flowing into the oceans. This carbon can be extracted by marine
animals for their shells and bones and when they die the carbon in
the sediment can drift to the ocean floor. Some of this can be
returned to the ocean surface by deep upwelling currents, but some
can build up as a deep deposit. The slow movement of the Earth's
plates means that ocean floors eventually get swept under the con-
tinents (see Chapter 3 for an explanation) and when this happens
the sediment can be heated, melted and released back into the
atmosphere (e.g. during volcanic eruptions). Additionally, carbon
can get locked away within the Earth's surface for a long time
when animals and plants die and the carbon enters the ground (e.g.
in a peatland). Over long time periods this deposit can get buried
and compressed and form coal or oil. Human extraction of this
'fossil carbon' for fuel then releases the carbon back to the atmo-
sphere when it is burnt.
Currently our burning of fossil fuels releases approximately 6.5
gigatonnes (thousand million tonnes) of carbon per year. Defor-
estation in the tropics releases a further 1.5 gigatonnes per year.
However, the sums do not quite add up since atmospheric
carbon dioxide concentrations are only increasing by 3 giga-
tonnes per year. It seems that the additional carbon is being taken
back up by the oceans or by additional photosynthesis with the
latest data suggesting about half of this additional carbon for each.
However, these additional sinks for carbon might not continue
to absorb such a high proportion of the carbon we have been
releasing into the atmosphere for much longer. There is probably
a threshold maximum amount of additional carbon that the
oceans and vegetation can take up and once this buffer has been
used up the rate of change in temperature might climb even
further. As an alternative to fossil fuels the use of biofuels has
been proposed whereby plants are grown to be used as fuel. The
process is meant to be 'carbon neutral' in that the same amount
of carbon released into the atmosphere from burning was taken
up by the plant in its growth. However, as Box 2.1 illustrates,
the story is not simple.
