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New carbon data should produce better climate forecasts

Measurements for carbon dioxide input by plants and carbon dioxide released during respiration will help models

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TURIN, Italy Climate change forecasts have always been afflicted by multiple sources of uncertainty. Computer models are at the mercy of the data fed to them, and the sources of the data aren’t always so reliable, leading to large variations in forecasts of future temperatures. But now scientists have a little bit better input on two important issues: the amount of carbon dioxide inhaled by land plants and how respiration pumping CO2 back into the air depends on temperature.

At a news conference July 5 in Turin, Italy — coinciding with the Euroscience Open Forum conference scientists from the Max Planck Institute for Biogeochemistry in Jena, Germany, reported new findings from a global network of sensors called Fluxnet. Data from the sensors indicate that land-based plants gulp down between 115 billion and 131 billion metric tons of carbon each year in the carbon dioxide they consume during photosynthesis. Oddly enough, that’s pretty close to the informed guesses that experts have been using in their climate modeling. On the other hand, a separate but related study analyzing Fluxnet data found that CO2 output back into the air by the respiratory processes of living things is not as sensitive to temperature as scientists had been assuming.

Respiration output isn’t easy to estimate, and field studies have been in conflict with theoretical models on the effect of temperature on CO2 output, so that part of the global warming equation has contributed substantially to the uncertainty in climate forecasts. Of particular concern was the possibility that warming temperatures would elevate CO2 output by respiration in a feedback loop leading to runaway temperature rise. But the new analysis finds respiration to be less sensitive to temperature than some previous analyses had suggested. Consequently the most dire scenarios of rapid warming feedback now seem less likely.

With climate change science, though, no one new piece of data produces a simple conclusion. For one thing, respiration rates are complexly related to photosynthesis rates, and photosynthesis rates are sensitive to rainfall, which is itself affected by changes in various factors influencing climate.

“Climate change is not just global warming,” Markus Reichstein of the Planck institute and a coauthor on both papers said at the ESOF news conference. He said the new studies show that availability of water from precipitation is much more important to the carbon budget in ecosystems than has been generally recognized and warrants much more attention. And a lot of carbon is stored in soils, with a potential for substantial feedback to the atmosphere, he said.

So basically, the new studies (both published online July 5 in Science Express) do not change the general picture on climate change, but do provide better data for refined forecasts in the future.

Citations

C. Beer, M. Reichstein, E. Tomelleri, et al. Terrestrial Gross Carbon Dioxide Uptake: Global Distribution and Covariation with Climate. Science Express, published online July 5, 2010. Doi: 10.1126/science.1184984

M.D. Mahecha, M. Reichstein, N. Carvalhais, et al. Global Convergence in the Temperature Sensitivity of Respiration at Ecosystem Level. Science Express, published online July 5, 2010. Doi: 10.1126/science.1189587

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