What Controls the CO2 Concentration?

The atmosphere's carbon dioxide (CO2) concentration has risen about 30% since preindustrial times, enhancing the natural greenhouse effect. The prediction of future climate change requires quantitative understanding of the sources and sinks of CO2 (and other greenhouse gases), and their contributing natural and human influences. There is a natural cycling of carbon in the environment, causing a large exchange of carbon among the atmosphere, the land vegetation and soils, and the oceans and marine biosphere. Evidence from ice core data suggests that this natural cycle has been roughly in balance, with only minor variation, since about 10,000 years ago.

Over the past two centuries, the CO2 concentration has risen from preindustrial levels at an increasing rate. Although the anthropogenic emissions are only about 5% as large as the natural fluxes, the rising concentration indicates that emissions from land use change (e.g., deforestation, biomass burning, agricultural expansion) and from fossil fuel combustion (i.e., use of coal, oil, and natural gas) have become increasingly larger than can be accommodated by natural removal processes. Projections of these trends unabated suggest that the atmospheric CO2 concentration may reach double its preindustrial value by the middle of the next century, leading ultimately to an average global temperature increase of 1.5°C to 4.5°C.

Over the past few years, quite by surprise, the rise in CO2 concentration has slowed significantly, indicating either that the sources have decreased or the sinks have increased, or both. While there has been a world wide recession, inventories indicate that emissions of CO2 from fossil fuel use have not decreased nearly enough to account for this. Biomass burning may also be somewhat reduced, but does persist. The reduction in the increase of the CO2 concentration could be due to changes in the natural cycle; however, measurements of fluxes into and out of the ocean suggest that the ocean is not taking up the additional CO2, and recent ENSO (El Niñ o-Southern Oscillation) events appear to be correlated with small increases in atmospheric CO2. Other research has shown that interannual fluctuations in climate can modify carbon uptake and release by respiration. Recent studies also suggest that mid-latitude forests may be taking up more carbon than previously estimated. Future research within the USGCRP to better understand the global carbon cycle is vital to understanding and predicting climate change, and assessing potential response options.

Forest Photo © 1992, Paul Grabhorn