Human-caused climate change from greenhouse gases is warming soils so much they are releasing their own carbon, creating a cascading effect that will speed up global warming.
That's the finding by a major new study published this week in the journal Nature.
The study found that rising temperatures will drive the loss of at least 55 trillion kilograms of carbon from the soil by mid-century, adding an additional 17 percent on top of the already projected greenhouse gas emissions due to human-related activities during that period.
That's like adding another United States in terms of carbon release, matching all emissions by industry and transportation.
The study was based on 49 climate change experiments worldwide, including six in Minnesota, and was led by University of Minnesota adjunct professor T.W. Crowther of the university's College of Food, Agricultural and Natural Resources Sciences' Department of Forest Resources.
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The researchers found that carbon losses will be greatest in the world's colder places, at high latitudes, which had largely been missing from most previous research. In those regions, massive stocks of carbon have built up over thousands of years and slow microbial activity has kept them relatively secure.
The findings are different from past studies on soils and carbon which assumed the ground would act as a carbon sponge. But most of the previous research had been conducted in the world's temperate regions, where there were smaller carbon stocks to begin with. Studies that focused only on these regions would have missed the vast proportion of potential soil carbon losses, said Crowther, who conducted his research while a postdoctoral fellow at the Yale School of Forestry & Environmental Studies and at the Netherlands Institute of Ecology.
"Carbon stores are greatest in places like the Arctic and the sub-Arctic, where the soil is cold and often frozen. In those conditions microbes are less active and so carbon has been allowed to build up over many centuries," Crowther said. "The scary thing is, these cold regions are the places that are expected to warm the most under climate change."
The results are based on an analysis of data on stored soil carbon from dozens of climate warming experiments conducted over the past 20 years by more than 30 co-authors in different regions of the world. The study predicts that for one degree of warming, about 30 petagrams of soil carbon will be released into the atmosphere. A petagram is equal to 1,000,000,000,000 kilograms.
"This is a big deal because the Earth is likely to have warmed by 2 degrees Celsius by mid-century, releasing as much carbon over that time period as will be emitted from fossil fuel burning in the United States," said Peter Reich, a University of Minnesota professor and one of the study's researchers.
The study considered only soil carbon losses in response to warming. There are several other biological processes - such as faster plant growth as a result of carbon dioxide increases, or slower plant growth due to climate warming and drought - that could dampen or enhance the effect of the soil carbon feedback. Reich is leading several long-term experiments in Minnesota forests and grasslands to address those questions.
