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Soil study hints at climate model revision

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  • Study into soil carbon casts doubt over the accuracy of current climate models
  • Levels of black carbon in two savannahs in Australia surprised researchers
  • Study author: "We must have underestimated other causes of warming dramatically"
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By Matthew Knight
For CNN
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LONDON, England (CNN) -- A new study investigating the amount of carbon in Australian soil has cast doubt over the accuracy of current climate models in predicting future levels of global warming.

Bush fires in Australia leave high levels of black carbon in the soil for hundreds of years.

Bush fires in Australia leave high levels of black carbon in the soil for hundreds of years.

The study, conducted by the Department of Crop and Soil Sciences at New York's Cornell University, examined 452 separate sites across two Australian savannahs in Queensland and Northern Territory.

Researchers measured the amount of black carbon and organic carbon present in the soil, and found that there were much higher levels of black carbon than expected.

Soil organic carbon -- which is created by the decomposition of vegetation -- takes only a few years to convert into carbon dioxide (CO2), whereas black carbon -- which is produced by burning organic matter -- takes on average 1000 to 2000 years to convert to CO2.

So the more black carbon there is in the soil, the less CO2 is released.

The study's findings are significant because annual emissions of CO2 from soils -- through photosynthesis and respiration -- are ten times greater than the ones created by burning fossil fuels. The type and quantity of carbon present in soils is important in accurately predicting future levels of CO2 in the atmosphere.

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Johannes Lehmann, associate professor of soil fertility management and soil biogeochemistry at Cornell University told CNN: "The broader implication for these findings is the recognition that soils are not the homogeneous mass of carbon that can be modeled as a black box. We really need to know what forms of carbon are in soils to be able to model them correctly because that has ramifications for predicting their response to warming."

Lehmann, the lead author of the paper which is published in the journal Nature Geosciences, said that the black carbon content of soils in the study ranged from zero to 82 percent of soil organic carbon.

The report states: "By including realistic stocks of black carbon in prediction models, carbon dioxide emissions are reduced by 18.3 and 24.4 percent in the two Australian savannah regions in response to a warming of three degrees Celsius over 100 years."

But Lehmann is not in the business of denying that climate change is happening.

"We already know that the earth is warming and CO2 is accumulating in the atmosphere to a much greater degree than even six years ago," he said.

"But that doesn't mean we should ignore that there are other effects that would actually lead to over-predictions. It means that if we overestimated the soil response to warming then we must have underestimated something else dramatically because we have a net effect of faster global warming. That is really something to think about."

Even though this is the largest survey of back carbon ever published Lehmann concedes that it is difficult to assess how much these new findings might change climate predictions.

"Extrapolating this data for the rest of the world is impossible. But if we recognize that we have such a big variation in Australia, and on average quite a high number, then it is reason enough to have a closer look at what the same assessment would tell us for the other continents.

"Vegetation burning is part of the ecosystem dynamics in many continents, and burning is expanding. For example, we have a lot more fires in the American West. There is reason to believe that black carbon makes up a substantial proportion of soil organic matter in other continents," he said.

Lehmann points out that over the next few months the Intergovernmental Panel on Climate Change will begin to bring together the latest data that will form the basis of their next assessment.

"It's important to lead these discussions on how to make these climate models more accurate and this paper shows that we should be studying black carbon more."

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