Science - July 7, 2010

Accurate figure for CO2 uptake by vegetation

Laborious calculations have led to an improvement in climate models. Tropical forests account for one third of all CO2 uptake.

Fluxnet measurement pole.
The vegetation covering our planet absorbs 123 billion tons of carbon every year. This, the most accurate measurement up to now, got researcher Elmar Veenendaal (Nature Conservation and Plant Ecology) into this week's Science. Which surprised him a little as this figure is not that new. It is hardly any different from the common rule of thumb that the gross uptake of carbon is twice the biomass production on Earth. According to Veenendaal, the article's importance lies in the improved accuracy of the figure and its consequences for the carbon-climate models.
Plants use light to help convert CO 2 into biomass. We are now better able to estimate how much is stored - the so-called gross primary production - thanks to an international network of measurement sites. The group Veenendaal is part of, an international research team led by the Max Planck Institute of Biogeochemistry in Jena, uses five different models to calculate carbon uptake. Each of these models uses different methods for scaling up local data to obtain global figures for carbon uptake. The answer to the question of what the world's gross carbon uptake is turns out to be 123 billion tons a year. Tropical forests account for a third of that. Savannahs, Veenendaal's speciality, account for a quarter of that uptake.

The researchers went on to make further investigations. 'We compared those empirical models with the process-driven models. These are models that try to describe the physiological processes, for instance, that determine the uptake of carbon.' Process models inevitably contain a lot of assumptions. The most recent calculations allow those assumptions to be checked. This allows a kind of calibration of the process models, which in turn means better forecasts. Veenendaal: 'If you have a better understanding of the process, you are in a better position to analyse what might happen if rain patterns were to change, for example, as the Earth heats up. Our research is therefore helping to make the predictions for the climate scenarios more reliable. The IPCC is due to produce a new report in a couple of years' time. At the moment everyone is busy improving those models and reducing uncertainty.'  And that, says Veenendaal, is definitely worth a publication in Science.