Paleoclimate studies indicate that the behaviour of the global carbon cycle may be significantly different under altered climate regimes. During the last glaciation, for example, the deep oceans appear to have been a major source of atmospheric carbon dioxide while large areas (such as Africa) and coastal areas were large sinks. Since the Holocene Optimum, these roles have been reversed. These changing source/sink patterns are consistent with ice core records of atmospheric carbon dioxide concentrations; ocean carbonate studies suggest that ocean carbon fluxes may still not be in equilibrium today. Hence there is considerable concern that carbon cycle models development to understand the current carbon cycle may not accurately represent behaviour of terrestrial and ocean carbon fluxes during the next century as the boundary conditions change.
(2) If emissions of carbon dioxide were halted today, it would take more than a century for the atmospheric level of carbon dioxide to approach its pre-industrial level. About 15% of all the carbon dioxide which has been emitted already will remain airborne for thousands of years, causing warming indefinitely. The slow warming of the ocean creates a lag between emissions and their full effect on temperature so the consequences of past emissions are not yet entirely apparent.