Module 09: What Happens to the CO2?


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).

Learning Objectives

Upon completion of this module you should be able to: 

  1. Outline the contemporary carbon cycle including sources and sinks for CO2.
  2. Explain the basic mechanisms for uptake of fossil fuel CO2 on land and in the oceans.
  3. Explain the concept of transient climate response to fossil fuel emissions.
  4. Describe carbon sink saturation mechanisms in the land and oceans.
  5. Explain the long-term fate of fossil fuel CO2 and its impact on climate.

Readings on the Global Carbon Cycle

Review Questions

  1. About what percentage of atmospheric CO2 is taken up each year by global photosynthesis? About what percentage of the global uptake of CO2 by photosynthesis is balanced by CO2 emitted due to respiration and decomposition?
  2. About what percentage of global fossil fuel combustion results in an increase of atmospheric CO2? How do we know?
  3. Describe two mechanisms by which the oceans can remove CO2 from the atmosphere.
  4. Describe four mechanisms by which land ecosystems can remove CO2 from the atmosphere.
  5. Explain why increases in photosynthesis do not necessarily lead to long-term removal of CO2 from the atmosphere.
  6. What factors limit the rate of long-term CO2 uptake by land ecosystems?
  7. What is meant by the phrase “carbon sink saturation?” Why do scientists expect this to occur and what are the implications for climate change over the coming decades?
  8. Describe the vertical distribution of fossil fuel CO2 dissolved in the oceans. How do we know? Explain the physical mechanisms that control it. 
  9. After fossil fuel combustion ends, what will happen to CO2 in the atmosphere?
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