Across the world's oceans there is a continual cycle of equilibration of dissolved CO2 in water with CO2 in the atmosphere. About 88 Pg C/year is released from the surface of the world's oceans, with an annual uptake by the oceans of 90 Pg C. Consequently, the net uptake by oceans is estimated to be ~2 Pg C/year.
The carbon that dissolves in our oceans occurs in three main forms. In addition to CO2, it is also found as bicarbonate and carbonate ions. About 90% exists as bicarbonate, and about 8% as carbonate.
Like terrestrial vegetation, the world's oceans have provided a substantial buffer to increases in atmospheric CO2 emissions arising from human activities. Sabine and Feely (Chapter 3, this volume) examine the various oceanic sources and sinks for CO2, the net air-sea flux of CO2 over time and the potential changes in the strength of the oceans as a sink for CO2 in response to increasing atmospheric CO2 concentrations and changes in climate in the 21st century.
The oceans contain the bulk of the world's natural carbon, far more than land or the atmosphere. They also have a huge capacity to store CO2 released by human activity. It is estimated that ~118 Pg anthropogenic CO2-C is accumulated in the world's oceans between 1800 and 1994 - equivalent to almost half of all emissions related to fossil fuel and cement manufacturing over that time. However, this sink capacity is controlled by a complex mix of factors and, over the next few decades to centuries, the maintenance of this capacity is far from guaranteed. For instance, the efficiency with which the oceans can take up CO2 at the surface - known as the Revelle factor - has decreased since the pre-industrial era in response to reductions in pH, themselves caused by elevated atmospheric CO2 concentrations. As mentioned earlier, rising sea surface temperatures may also have significant effects on the oceanic CO2 sink. As water temperatures increase, the solubility of CO2 is reduced and the likelihood of stratification (and so nutrient limitation of phytoplankton) is increased, both leading to an overall reduction in oceanic CO2 uptake.
Such impacts of global warming and elevated atmospheric CO2 concentrations represent a positive feedback effect that, other things being equal, will result in a reduction in the carbon sink strength of the
(<0.1 Pg C/year) but long-term sinks for atmospheric CO2.
Smith and Ineson (Chapter 4, this volume) review our current understanding of CO2 fluxes in soils, the sensitivity of these fluxes to land use and the potential impacts of increasing global temperatures on these fluxes as the 21st century progresses.
They go on to discuss the potential of soils to help mitigate anthropogenic climate change through land management aimed at protecting the existing sink and, where possible, increasing the soil carbon sink strength. With anthropogenic CO2-C emissions already standing at ~6.3 Pg/year, and set to go on increasing, achieving a stable CO2 concentration in the atmosphere that avoids potentially dangerous climate change is a huge challenge. However, the use of carbon sequestration in soils in the short term (next 20 years) as part of a wider suite of measures to offset increasing emissions may allow stabilization at reasonable levels (450-600 ppm) by 2010.
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