The first solidly evidence-based estimate for how much carbon the world’s plants suck up each year has been produced by an international team of scientists. Their new figure of 123 billion tons per year is remarkably close to the tentative previous numbers, but important differences in the details that combine to give this overall figure will require even state of the art climate models to be revised.

Christian Beer and his colleagues have come up with a new number for terrestrial ‘gross primary production’ (GPP) – the total amount of carbon dioxide that plants ‘breathe’ in every year. Their new number – published in Science and presented at the ESOF meeting in Turin – comes from a combination of observational data and modelling.

“The old estimate was based on a number of very hypothetical assumptions,” says Markus Reichstein, one of the authors of the new paper and a researcher at the Max Planck Institute for Biogeochemistry in Jena, Germany. “By chance it isn’t far away from what we got. The point is we now have a much more solid empirical estimate.”

In addition, the work by Beer, Reichstein, et al shows that many assumptions about exactly where and how the 123 billion tons of carbon is taken up by plants are wrong. “The distribution across biomes is quite different,” says Reichstein.

The new study shows that tropical forests take up 34% of the terrestrial GPP with savannahs close behind on 26%. Although savannahs might seem a surprising second place in the GPP league, their sheer size makes them an important player.

In addition the work reveals quite how constrained by water availability carbon dioxide uptake by plants is. Rainfall appears to influence carbon uptake on around 40% of vegetated land, rising to 70% in savannahs, shrublands, grasslands and farms. This means such areas will be extremely susceptible to the changes in precipitation that are expected to result from climate change. Conversely, tropical forests may be more robust than previously thought.

Reichstein says that the next IPCC and climate models will all have to be adjusted as researchers continue to gain a firmer grasp on both the carbon and the water cycles and what their overlap means for GPP.

Image: flux towers, like this one in Germany, were important data collectors for this research / image courtesy of Max Planck Institute for Biogeochemistry.