The oceans are a very important element in both the carbon budget and the heat balance of the earth.
The ocean's capacity to absorb heat is about 1000 times more than that of the atmosphere, and it has absorbed about 20 times more heat than the atmosphere since 1960. Between a quarter and a third of the carbon dioxide produced by human activities in the last two decades has been absorbed by the oceans. Together with similar rates of uptake by land vegetation, this has reduced the rate of accumulation of carbon dioxide in the atmosphere to about 55 per cent of the amount released by human activities since 1959, thereby slowing the rate of climate change. Of the quantity absorbed in the oceans, about 40 per cent is being sequestered in the Southern Ocean by water masses sinking from the sea surface as part of the overturning circulation.
The uptake of carbon dioxide in the ocean is enhanced by the marine carbonate buffer system, which transforms CO2 into carbonate ions through a series of reactions. This means that the ocean is able to absorb far more CO2 than would be possible based on solubility alone. However, the buffering capacity of the ocean will saturate as the CO2 concentration in the surface ocean increases, and the strength of the ocean CO2 sink will therefore decrease in the future, causing an increasingly large fraction of ANTHROPOGENIC CO2 emissions to stay in the atmosphere.
As a result of global warming, the heat content of the upper layers of the ocean is increasing. A growing number of reconstructions of surface ocean temperature over the past 1000 to 2000 years shows that the sharp temperature rise over the past century is now beyond the bounds of natural variability.
As shown in the figure below, ocean temperature changes are not uniform. Because of possible changes to ocean currents (which may be a natural occurrence) some areas may show surface cooling. One such region is the subarctic region of the North Atlantic Ocean. The surface ocean cooling trend in that region may counteract warming that is predicted to occur there over the next decade, thus delaying the onset of substantial global warming in that region for a few years.
Change in ocean heat content from 1955 to 2003
Linear trends (1955–2003) of change in ocean heat content per unit surface area (watts per square metre) for the 0 to 700 metre layer, based on the work of Levitus et al. (2005). The linear trend is computed at each grid point using a least squares fit to the time series at each grid point. The contour interval is 0.25 watts per square metre. Red shading indicates values equal to or greater than 0.25 watts per square metre and blue shading indicates values equal to or less than -0.25 watts per square metre.
Source: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 5 Observations—oceanic climate change and sea level, Figure 5.2, p. 391.
Temperature is also important because it determines how much CO2 dissolves into sea water. Seawater that is colder can hold more CO2. Ocean warming may therefore cause a slight decrease in the uptake of CO2 by the oceans.
When CO2 dissolves into water it forms a weak acid, lowering the pH of the water. Since the industrial revolution, it is estimated that surface ocean pH has dropped by about 0.1, and it is projected to drop by a further 0.14 to 0.35 units by 2100 as the atmospheric CO2 levels increase. Note that pH is a logarithmic scale so a change of 1 pH unit means a 10-fold change in acidity.
The ability of many marine creatures (such as corals) to build their shells and coverings (composed mainly of calcium carbonate) is affected by the pH of the water and concentration of carbonate ions. If the ocean carbonate ion concentration becomes undersaturated, marine organisms will no longer be able to form their calcium carbonate shells. As shelled organisms die and accumulate at the bottom of the sea, much of the carbon remains there, and this slowly removes carbon from the atmosphere–biosphere cycle. Such sediments lock up carbon for millions of years. The exposure of carbonaceous sediments millions of years later after earth movements eventually allows some of the carbon to return to the atmosphere. It is possible that ocean acidification will lead to undersaturation and dissolution of calcium carbonate in parts of the surface ocean by 2100. Model simulations suggest that the greatest decrease in pH and carbonate ion concentrations will be at low- and mid-latitudes, but that undersaturation may occur in the Southern Ocean, which already has low carbonate ion concentrations, within a few decades.
The oceans will not continue to absorb CO2 at the same rate that they have been. As the concentration of CO2 in surface waters rises, the uptake of CO2 from the atmosphere will slow. Also important, however, is the amount of photosynthesis by phytoplankton taking place in the upper waters of the sea (photosynthesis absorbs CO2). If the rate of marine photosynthesis increases, this may offset some of the chemical changes that decrease CO2 absorption by the water.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report—the physical science basis, Chapter 5, Observations—oceanic climate change and sea level; Chapter 7, Couplings between changes in the climate system and biogeochemistry, Section 7.3.4, 'Ocean carbon cycle processes and feedbacks to climate'.