Between 40 and 60 percent of all anthropogenic emissions of carbon dioxide appear to remain in the atmosphere. The remainder of the excess (about 4 gigatonnes of carbon per year (GtC/year)) is sequestered within the global carbon cycle – where and by what processes is still not clear. A variety of studies into ocean carbon fluxes cautiously support earlier estimates for a net ocean sink in the order of 2 GtC/year, although estimates vary between 1.4 and 3 GtC/year. "Biological pumping" to deep oceans, and incorporation in forests and soils (carbon dioxide fertilization effect) provide for other carbon dioxide sinks. However, the geographical and seasonal distributions of ocean carbon sources and sinks are complex and can vary significantly with year-to-year climate variability.
Estimates for terrestrial carbon storage based on CLIMAP data suggest a net terrestrial sink, mainly in boreal and temperate forests, or more than 700 GtC since the last glacial maximum.
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.