Since the 1950s, the concentration of atmospheric CO2 has increased at only about half the rate of fossil fuel combustion. Half of the fossil fuel CO2 resides in the air but the other half has been taken up by carbon “sink” processes in the land and oceans. About one-fourth of fossil fuel CO2 dissolves into the surface oceans, but the rate is limited by very slow mixing with cold dense water at depth which is not in physical contact with the air. The rest is stored in plants and soils on land. The land sink was a complete surprise and requires that worldwide plant growth exceeds death and decomposition by a few percent. Plants are growing faster than they are dying, over and above the rate of forest clearing and degradation, fires, and other forms of biomass destruction.
The land sink is a result of at least four major factors, some of them transient:
- CO2 fertilization that stimulates photosynthesis with death and decomposition delayed by the residence time of carbon in plants & soils;
- Nitrogen fertilization both intentional (agriculture) and unintentional (via air and water pollution);
- Regrowth of forests after agricultural abandonment, primarily in the developed world; and
- invasion of woody plants into boreal regions where warming has been strongest.
The behavior of carbon sinks in the future is less predictable than the climate. Carbon sinks are expected to weaken as climate warms. As fossil fuel emissions fall and people attempt to draw down CO2, carbon uptake in both the land and oceans are expected to decrease and possibly even reverse. This uncertain dependence of future carbon sinks on climate gives rise to an approximation that future temperature depends linearly on the total historical amount of fossil fuel emissions. In this approximation, every ton of carbon burned warms the climate by a constant amount. When emissions stop, so does warming.
In the long run (thousands of years), CO2 will be removed form the atmosphere very slowly by three processes. For the first few millennia, fossil fuel CO2 will invade the deep oceans which have yet to be exposed to air pollution. If the final fossil fuel pulse is big enough, this reservoir will be chemically saturated in about 3000 years. After that, chemical reactions with limestone and other calcareous sediment will increase the amount of dissolved CO2 the oceans can hold over a period of 10’s of thousands of years. If the historical pulse of fossil CO2 is even larger, weathering of igneous rocks on land will remove the rest over geologic time (100s of thousands of years).