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Soil study muddies climate change debate

Climate change may not be as severe as predicted, suggests an international study that shows current modelling of carbon dioxide emissions from soils are overestimated by as much as 20%.

The view, reported in the latest Nature Geoscience journal, is based on a study of Australian soils that finds the amount of carbon dioxide (CO2) released by Australian soils is much lower than previously believed.

The finding has major implications for climate change predictions as annual carbon emissions from soils are estimated to be more than all human-made CO2 emissions combined.

The Australian and US researchers say emissions from soils are lower because they contain a much higher proportion of charcoal, or black carbon, than estimated by previous models.

"Current models of global climate change .. are inaccurate if a larger fraction of soil organic carbon than postulated has a very slow decomposition rate," they write.

Co-author Dr Evelyn Krull, of CSIRO Land and Water, says charcoal, which is formed in the aftermath of bushfires, is a very stable form of carbon that can last for millennia.

"In effect it's a carbon sink," Krull says.

Inflated predictions

Under the commonly used RothC model, the proportion of black carbon is calculated to be about 6.6%, she says.

Krull says in their study of 452 soil samples from the Australian National Soil Archive and two landscape transects of about 3000km in Queensland and the Northern Territory, charcoal content ranged from zero to 82%.

She says the average proportion of charcoal present for all 452 soil samples was 20.4%.

The team found by including realistic estimates of charcoal in their climate prediction models, the amount of CO2 predicted to be released from two Australian savannah regions under a 3ºC warming scenario was 18.3% and 24.4% lower than previously calculated.

For Australia, a proportion of 20% charcoal in soils would lead to a 135 teragram (135 billion kilograms) overestimation on a continental scale.

"On an annual basis, an inflated prediction from topsoils alone equates to ... 84% of CO2 emissions associated with aviation for Australia using values obtained for 2006," the paper says.

Krull, who has analysed soil samples from across the globe since the paper was prepared, says she has found soils from countries around the same latitude as Australia have similar charcoal content.

She says this means that current scenarios predicted by climate change modeling "are making it look worse than it actually would be".

This highlights the need for a global initiative to analyse soils worldwide for charcoal content so that modeling can be more accurate, she says.