Geoscience Reference
In-Depth Information
average time for a CO
2
molecule to be dissolved
in the ocean or taken up by plants is about
four years. Photosynthetic activity leading
to primary production on land involves 50
(
Figure 2.5
). Half of this increase has taken
place since the mid-1960s; currently,
atmospheric CO
2
levels are increasing by
1.5-2ppmv per year. The primary net source is
fossil fuel combustion, now accounting for
6.55
×
10
12
kg of carbon annually, representing 7
percent of atmospheric carbon; this accounts
for the annual oscillation in CO
2
observed in
the Northern Hemisphere due to its extensive
land biosphere.
The oceans play a key role in the global
carbon cycle. Photosynthesis by phytoplankton
generates organic compounds of aqueous
carbon dioxide. Eventually, some of the bio-
genic matter sinks into deeper water, where it
undergoes decomposition and oxidation back
into carbon dioxide. This process transfers
carbon dioxide from the surface water and
sequesters it in the ocean deep water. As a
consequence, atmospheric concentrations of
CO
2
can be maintained at a lower level than
otherwise. This mechanism is known as a
'biologic pump'; long-term changes in its
operation may have caused the rise in atmos-
pheric CO
2
at the end of the last glaciation.
Ocean biomass productivity is limited by the
availability of nutrients and by light. Hence,
unlike the land biosphere, increasing CO
2
levels will not necessarily affect ocean produc-
tivity; inputs of fertilizers in river runoff may
be a more significant factor. In the oceans, the
carbon dioxide ultimately goes to produce
carbonate of lime, partly in the form of shells
and the skeletons of marine creatures. On land,
the dead matter becomes humus, which may
subsequently form a fossil fuel. These transfers
within the oceans and lithosphere involve very
long time scales compared with exchanges
involving the atmosphere.
As
Figure 2.4
shows, the exchanges between
the atmosphere and the other reservoirs are
more or less balanced. Yet this balance is not
an absolute one; between
AD
1750 and 2008 the
concentration of atmospheric CO
2
is estimated
to have increased by 38 percent, from 280 to
387ppm, the highest value for 650,000 years!
10
12
kg C/year. Tropical deforestation
and fires may contribute a further 2
×
10
12
kg
C/year; the figure is still uncertain. Fires
destroy only above-ground biomass, and a
large fraction of the carbon is stored as charcoal
in the soil. The consumption of fossil fuels
should actually have produced an increase
almost twice as great as is observed. Uptake and
dissolution in the oceans and the terrestrial
biosphere primarily account for the difference.
Carbon dioxide has a significant impact on
global temperature through its absorption and
re-emission of radiation from the earth and
atmosphere (see Chapter 3C). Calculations
suggest that the increase from 320ppm in the
1960s to 387ppm (
AD
2008) raised the mean
surface air temperature by 0.6
×
C (in the
absence of other factors). The rate of increase
of CO
2
since
AD
2000 has been about 2ppm/yr
compared with less than 1ppm in the 1960s
and 1.5ppm in the 1980s.
Research on deep ice cores taken from
Antarctica has allowed changes in past
atmospheric composition to be calculated by
extracting air bubbles trapped in the old ice.
This shows large natural variations in CO
2
concentration over the ice age cycles (
Figure
2.7
). These variations of up to 100ppm were
contemporaneous with temperature changes
that are estimated to be about 10
°
C. These
long-term variations in carbon dioxide and
climate are discussed further in Chapter 13.
°
•
Methane
(CH
4
) concentration (1,775ppbv) is
more than double the pre-industrial level
(750ppbv). It was increasing by about 4-5ppbv
annually in the 1990s but this has dropped
to near zero since 1999-2000 (
Figure 2.7
). For
unknown reasons, concentrations increased
again in 2008. Methane has an atmospheric
lifetime of about nine years and is responsible
