Understanding The Global Carbon Budget

The carbon cycle is related to climate and climatic change because it controls carbon dioxide, the most important of the greenhouse gases. One of the world's preeminent experts on the topic, Dr. R. A. Houghton, has authored a synthesis paper on the topic, summarizing what is known about the global carbon budget and why it is important. The work is featured in the current issue of the Annual Review of Earth and Planetary Science.

In the paper, Dr. Houghton emphasizes that the key issue is to understand the processes responsible for adding carbon (sources) to the atmosphere and for removing it (sinks). Such understanding should lead to more accurate predictions of future concentrations of CO₂ and more accurate predictions of the rate and extent of climatic change. The recent past may be insufficient for prediction, however. Oceanic and terrestrial sinks that have lessened the rate of growth in atmospheric CO₂ until now may diminish as feedbacks between the carbon cycle and climate become more prominent.

Dr. Houghton comments, "Figuring out where all the carbon emitted from burning fossil fuels ends up is surprisingly difficult, especially when one recognizes that there are only three places it can go: the atmosphere, the oceans and land (plants and soil). The long-time effort to understand this distribution of carbon is giving way to a related question of whether and how the distribution of carbon will change as more carbon dioxide is added to that atmosphere and as the earth warms.

The natural processes on land and in the ocean that have removed carbon dioxide from the atmosphere for the last century may be starting to weaken. The oceans are becoming more acidic, and we see more fires in both tropical and northern forests. If these natural sinks for carbon diminish, global warming will occur more rapidly than predicted, and efforts to manage it will become that much more difficult."