The global carbon cycle is the pathway by which carbon moves through the Earth system, including the land, oceans, atmosphere and biosphere. Some components of the Earth system, such as the oceans and land, at times act as reservoirs of carbon by storing it for long periods, and at other times act as carbon sources by releasing it back into the atmosphere (Fig. 1). Human emissions of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) are interfering with — and altering — this pathway. Now, more than ever, understanding the global carbon cycle in all its complexity is a pressing research issue.
Since the onset of the industrial revolution, more than two hundred years ago, greenhouse gases have been released into the atmosphere through human activities such as burning fossil fuels and deforestation. Atmospheric concentrations of carbon-based compounds, such as CO2 and CH4, are now far higher than they have been at any stage during the past several thousand years. However, of the carbon released in greenhouse-gas emissions, only about 40% remains in the atmosphere. The rest is absorbed by the oceans and the land biosphere.
On time scales of up to a few thousand years, the atmosphere, oceans, vegetation and soils rapidly exchange carbon in vast amounts through a multitude of physical, chemical and biological processes. Many of these processes will either slow or accelerate the growth of greenhouse-gas levels in response to warming, and thus represent a positive or negative feedback, respectively, between the global carbon cycle and the climate. On land, for instance, warmer temperatures can lead to enhanced soil respiration, thereby increasing the release of CO2 back into the atmosphere. Conversely, in northern latitudes, warmer temperatures can increase the length of the growing season and foster enhanced CO2 uptake by the vegetation1.
How can scientists keep an eye on the cycling of carbon through such a complex system? Some regions are especially important in maintaining the global carbon cycle and can provide vital clues to the overall health of the Earth system.
>> The disruption of the global carbon cycle is tightly linked to human development, and to the need for energy and food resources on land and in the seas.