Climate of Our Future

SciDev.net
The magnitude of the global carbon sink can be estimated by comparing the rate at which carbon is being emitted into the atmosphere — currently about 7 Gigatonnes a year, which we know from inventories of fossil fuel and biomass burning — with measurements of the rate of increase of carbon dioxide in the atmosphere.

The difference between the two represents the rate at which carbon is being absorbed by the Earth. Over the 40 years since atmospheric carbon dioxide measurements began, it is estimated that the earth has been able to absorb about half of the carbon dioxide that has been emitted by human activities.

Year-to-year fluctuations in the amount of carbon dioxide that is observed to accumulate in the atmosphere, however, indicate that the rate at which the global sink operates varies over relatively short periods of time.

During the 1990s, for example, the annual rate at which carbon dioxide increased in the atmosphere ranged from 0.9 ppm a year to 2.8 ppm a year. To understand the causes of this variation, the sink must be broken down into its component parts: the uptake and release of carbon from both the land and oceans.

Scientists can measure the land and ocean components of the global carbon sink by making use of the fact that the uptake and release of oceanic carbon dioxide does not alter certain features of the atmosphere, such as the concentration of oxygen; this is because the ocean uptake of carbon dioxide depends upon chemical reactions between carbon dioxide and water, which do not involve oxygen.

In contrast, the processes by which carbon dioxide is exchanged on land, namely photosynthesis and respiration, do influence oxygen exchange. In these processes, carbon dioxide and oxygen are directly related to each other as products and reactants in the same chemical and biological processes.

Using this and similar approaches, scientists have calculated that over the last two decades the oceans appear to be absorbing carbon at a relatively constant rate of around 2 Gt/yr. In contrast, the rate of absorption by the land appears to fluctuate; in the 1980s it was negligible, but in the 1990s, according to the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC), it was between one and two Gt a year.

These changes in the rate of absorption of carbon may be related to year-to-year fluctuations in the climate, perhaps associated with El Niño events, the general warming of the tropics that occurs about every four to seven years. For example, in warm and dry years typical of El Niño conditions forests act as net sources of carbon sources, whereas in the intervening cool and wet years they act as net sinks.

Furthermore even though the ocean sink is larger than usual during El Niño - mostly because outgassing of carbon dioxide from deep waters is prevented by changes in ocean circulation - the reduction in the land sink is much greater. As a result, El Niño years are in general those that see the greatest increases in the rate of carbon dioxide release into the atmosphere.