Atmospheric composition, mass and structure - Atmosphere, Weather and Climate - page 17

Table 2.3 Anthropogenically induced changes in concentration of atmospheric trace gases.

Gas Concentration Annual Increase Sources

(%)

1850* 2000 1990s

Carbon dioxide 280 ppm 370 ppm 0.4 Fossil fuels

Methane 800 ppbv 1750ppbv 0.3 Rice paddies, cows,

wetlands

Nitrous oxide 280 ppbv 316 ppbv 0.25 Microbiological activity,

fertilizer, fossil fuel

CFC-11 0 0.27 ppbv ≈ 0 Freon †

HCFC-22 0 0.11 ppbv 5 CFC substitute

Ozone ? 10-50 ppbv ≈ 0 Photochemical reactions

(troposphere)

Notes : * Pre-industrial levels are derived primarily from measurements in ice cores where air bubbles are trapped as snow accumulates on
polar ice sheets.
† Production began in the 1930s.
Source : Updated from Schimel et al . (1996), in Houghton et al . (1996).

Figure 2.4 Global carbon reservoirs (gigatonnes of
carbon (GtC): where 1 Gt = 10 9 metric tons = 10 12
kg) and gross annual fluxes (GtC yr -1 ). Numbers
emboldened in the reservoirs suggest the net annual
accumulation due to anthropogenic causes.
Source : Based on Sundquist, Trabalka, Bolin and
Siegenthaler; after Houghton et al . (1990 and 2001).

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

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 atmospheric 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 productivity; inputs of ferti-
lizers 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

10 12 kg of carbon annually, representing 7 per cent
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 biogenic 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