Nature Journal Club

Markus Reichstein

Max Planck Institute for Biogeochemistry, Jena, Germany

A biogeochemist looks at where all the emitted carbon dioxide is going.

Humanity is currently performing a huge global experiment, emitting increasing amounts of CO2 into the atmosphere by burning fossil fuels. I find it astonishing that although we scientifically explore other planets, we still don’t understand Earth’s important carbon cycle.

Corinne Le Quéré at the University of East Anglia in Norwich, UK, and her team have put together the pieces of the contemporary global carbon cycle. They analysed observations and modelling results on fossil-fuel emissions and the terrestrial and ocean carbon cycle, which are the major contributors to the atmospheric carbon budget (C. Le Quéré et al. Nature Geosci. 2, 831–836; 2009).

The bottom line is that humans are emitting more CO2 than projected in the pessimistic scenarios outlined by the United Nations Intergovernmental Panel on Climate Change. The researchers find that only 40–45% of this CO2 remains in the atmosphere; the rest is ‘cleaned up’ by the ocean and land — the ‘carbon sink’. It would be interesting to know whether the fraction taken up by oceans and land remains constant, because any alterations will change the global climate–carbon-cycle feedback.

The study also indicates that we are moving towards saturation of the carbon sink, but the uncertainties are large. Many carbon pools and processes, particularly those below ground in the soil, are not well understood and are hardly accounted for in carbon-cycle models (P. Ciais Nature 462, 393; 2009).

The message from Le Quéré et al. is that more observations are needed, that data should be fully integrated with models, and that these efforts must be more targeted and coordinated if we are to understand what is going on with the Earth system in our huge experiment.

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