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.
On land, the most important regions for sucking up carbon from the atmosphere are the tropical rainforests of Amazonia, the Congo basin and Southeastern Asia, and the boreal forests and Arctic tundra.2 Collectively known as the ‘green lungs’ of the planet, these regions have vast quantities of carbon locked up in vegetation and soil.3 Sizeable fractions of boreal forest and tundra regions have an added store of carbon in their underlying permafrost layer. In a warming climate, thawing of permafrost could thus release large amounts of carbon as CO2 or, in swamps and bogs, as CH4, which would further amplify climate change4.
In the oceans, two important carbon ‘hot-spots’ exist in the North Atlantic Ocean and the Southern Ocean around Antarctica. Here, excess carbon moves from the surface into deep waters where it is stored over timescales of centuries to millennia. Changes in the oceanic circulation in these areas, which might happen as temperatures rise, could decrease the oceans’ capacity to store carbon. Initial studies suggest that this is already happening in the Southern Ocean, which calls into question whether, in the future, carbon sinks will continue to operate or will saturate and perhaps even become carbon sources5.
The challenges for research are clear, starting with understanding the carbon cycle as an integral component of the global climate system. For example, further studies are needed to elucidate the key processes that transform carbon in terrestrial and marine ecosystems, and to understand how the carbon cycle is coupled both to the cycles of nutrients, such as nitrogen and phosphorus, and to the hydrological cycle.
Also warranting further attention is the multitude of climate system–carbon cycle feedbacks that operate on timescales ranging from days to to geological epochs. This cannot be achieved without improving our modelling tools. Here, the international research community has a long-term commitment to the improvement of Earth-system models — in other words, global climate models with a closed carbon cycle3,6.
Complex models must be constrained with real-world observations. Long-term observations of key carbon hot-spots are thus imperative. One operational example is the Zotino Tall Tower Observatory facility in the Siberian taiga forest, which includes a 300-m-tall mast for measuring regional atmospheric greenhouse gases, reactive chemistry, aerosols and meteorology.7 A similar observatory will be established in the Amazon forest in the short term. These ground-based measurements should be complemented with repeated air- and space-borne remote-sensing systems.
Perhaps most challenging of all will be managing the carbon cycle such that it continues to keep the planet in a stable climatic state. From a natural-science perspective, this will involve developing non-fossil-fuel-based energy sources such as biofuels, as well as finding ways of sequestering carbon such as by afforestation or air capture and storage.
Technologies that might have multiple benefits, such as using biomass taken from the farming cycle, are especially worthy of investigation. Biomass can potentially be used either as a low-carbon fuel or as a means of storing carbon. The latter case has the added benefit of generating products that could be traded for credits on the carbon market. Biomass that is transformed into long-lasting carbon materials can effectively remove atmospheric CO2, at least for the lifetime of the products, and as such is termed ‘carbon negative’8.
The global carbon cycle and its management cannot be studied from only a natural-science perspective. 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. Scientific assessment of any management options thus clearly needs to accommodate the multitude of socioeconomic drivers in the modern world. Addressing this in a rational, scientific way poses a huge challenge that must be met in order to steer the Earth systems within acceptable bounds over the next 100 years and beyond.
To assess which anthropogenic emissions of carbon dioxide are compatible with the goal of limiting global warming to 2 °C, our climate model must include changes in the carbon cycle. Earth-system simulations by the Max Planck Institute for Meteorology demonstrate substantially reduced ‘permissible’ carbon dioxide emissions during the twenty-first century when a coupled carbon cycle is included (Roeckner, E. et al. Clim. Change doi:10.1007/s10584-010-9886-6, 2010).