About half of the extra carbon dioxide that modern human activity has released into the air has been absorbed by the land and oceans. The processes, regions or systems that absorb greenhouse gases are called SINKS
. Obviously sinks are important in influencing the total quantity of greenhouse gases in the atmosphere, and so any reduction in their capacity will result in increased global warming even if emissions into the air do not increase.
The oceans are an important sink, and so too is the photosynthesis performed by vegetation on the land. Land vegetation releases almost as much carbon as it absorbs, through respiration required for plant metabolism and maintenance, and through natural decay. However, the uptake of CO2 by vegetation through photosynthesis is currently significantly greater than the release through respiration, which means that the world is becoming 'greener' (apart from the effects of land clearing). This is largely a response to the increased CO2 concentration in the atmosphere, which, all other factors being constant, facilitates photosynthesis by increasing the amount of CO2 that diffuses into the leaf for a given amount of water loss from the leaf. In some circumstances, soil can also be a carbon sink—this depends largely on management practices, natural disturbances, and adjustments to changes in inputs and outputs arising from land use change. The figure below illustrates the main sources and sinks in the global carbon cycle.
The global carbon cycle
The global carbon cycle for the 1990s, showing the main annual fluxes in GtC per year: pre-industrial 'natural' fluxes in black and 'anthropogenic' fluxes in red. The net terrestrial loss of -39 GtC is inferred from cumulative fossil fuel emissions minus atmospheric increase minus ocean storage. The loss of -140 GtC from the 'vegetation, soil and detritus' compartment represents the cumulative emissions from land use change, and requires a terrestrial biosphere sink of 101 GtC. Net anthropogenic exchanges with the atmosphere are from Column 5 'AR4' in Table 7.1. Gross fluxes generally have uncertainties of more than +/-20% but fractional amounts have been retained to achieve overall balance when including estimates in fractions of GtC per year for riverine transport, weathering, deep ocean burial, etc. 'GPP' is annual gross (terrestrial) primary production. Atmospheric carbon content and all cumulative fluxes since 1750 are as of end 1994.
Source: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 7 Couplings between changes in the climate system and biogeochemistry, Figure 7.3, p. 515.
Increased atmospheric carbon dioxide concentration is expected to cause ocean acidification, which reduces the oceans' capacity to absorb CO2 as they become saturated. As a result, the oceans may become less effective in absorbing CO2. There are also signs of an additional relative weakening of oceanic sinks as a result of changes in wind, surface air temperatures and water fluxes.
The terrestrial uptake of CO2 is expected to be reduced in the future as the biosphere sink capacity starts to saturate and other factors become limiting (e.g. water, nutrients or temperature). The saturation of ocean and land sinks means that a greater fraction of ANTHROPOGENIC CO2 emissions will remain in the atmosphere, resulting in an additional increase in the atmospheric concentration of CO2 and therefore further global warming. As terrestrial ecosystems respond to anthropogenic climate change, including different levels of warming and drying in different regions, it is likely that some areas that have been sinks will become SOURCES through decreases in net primary production, increased occurrence of wild fires, and changes in ecosystem composition.
Methane is a more powerful greenhouse gas than CO2 on a weight basis, although it has a much shorter residence time in the atmosphere. Additional sources of methane may eventuate with increased temperatures, because warmer conditions may promote the release of methane from oceanic and terrestrial stores. With global warming greater than 1°C there is an increased possibility of positive feedback from methane hydrates. This could theoretically release thousands of gigatonnes of carbon currently stored as methane hydrates in the ocean, due to subsurface warming of the sediments in areas of rapid penetration of warmer water from the ocean surface.
Another potential source of methane is from melting permafrost. Such melting is occurring in boreal forests of the northern circumpolar region, but the likelihood and extent of methane releases from this are largely unknown.
The Climate Institute, Evidence of accelerated climate change, November 2007.
Intergovernmental Panel on Climate Change, Contribution of Working Group I to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 7 Couplings between changes in the climate system and biogeochemistry.