A new study published Thursday in the journal Science has determined that if organic carbon in deep layers of soil warms at a rate similar to surface layers it could result in a dramatic increase in carbon dioxide emissions by the end of the century, if not sooner.
According to research by scientists at the Department of Energy's Lawrence Berkeley National Laboratory, deeper stores of carbon are more sensitive to warming than previously thought.
"Our calculations suggest that by 2100 the warming of deeper soil layers could cause a release of carbon to the atmosphere at a rate that is significantly higher than today, perhaps even as high as 30% of today's human-caused annual carbon emissions depending on the assumptions on which the estimate is based," said Caitlin Hicks Pries, a postdoctoral researcher in Berkeley Lab's Climate and Ecosystem Sciences Division.
To conduct the study, the scientists set up a soil research station in the foothills of California's Sierra Nevada mountains, which represent temperate forest soils; a category that makes up nearly 14% of the total soil area worldwide. Here they built six small soil plots and rung them each with heating cables sunk more than six feet underground. Then they warmed three of the plots 4° Celsius (7.2° Farenheit) for more than two years and monitored soil respiration for the duration of the experiment using several different approaches. Using a "business-as-usual" scenario, Intergovernmental Panel on Climate Change simulations of global average soil temperature predict that soil will warm 4° Celsius by 2100.
The researchers found that of the 34 to 37% increase in CO2 released at the three warmed plots, 40% of the increase was due to CO2 that came from below 15 centimeters.
Hicks told Fusion the researchers were surprised by the magnitude of the soil's response to warming.
"We did not expect the warmed soils to lose about 35% more carbon than the control soils, especially since this experiment occurred during California's drought, which we expected would reduce the response," she said.
The implications of this study, the first to extensively look at carbon stocks in soil below 20 centimeters in depth—which contain more than 50% of the planet's soil-based organic carbon—are extremely worrisome, according to the study:
The results shed light on what is potentially a big source of uncertainty in climate projections. Soil organic carbon harbors three times as much carbon as Earth's atmosphere. In addition, warming is expected to increase the rate at which microbes break down soil organic carbon, releasing more CO2 into the atmosphere and contributing to climate change.
Co-author Margaret Torn said the research uproots the assumption that carbon in "subsoil" is more stable to warming than that in topsoil.
"Deeper soil layers contain a lot of carbon, and our work indicates it's a key missing component in our understanding of the potential feedback of soils to the planet's climate," she said in a statement.
Hicks explained the difference between positive feedback to climate change and negative feedback, and how this makes the cumulative impact of warmer soil harder to determine.
"This study shows that soils are potentially a larger positive feedback to climate change than expected because soil carbon stored in deeper soils is similarly responsive to warming as surface soils," she said. "Thus there is the potential that warmer soils will put more CO2 into the atmosphere, which would increase climate change—but there is also the possibility that warming will enhance plant growth, which would take CO2 out of the atmosphere. We did not measure that plant response, so without knowing how the plants respond, we cannot definitively say what this will mean for climate change."
Hicks said they are in the process of starting a global network of soil warming experiments in order to test the response of deeper soils in different soils and ecosystems.
Studies predicting global warming over the rest of the century show that a 4° Celsius average increase could occur as soon as 2060, and is likely by 2100 if drastic actions to reduce greenhouse gas emissions aren't taken. The planet has already warmed more than 1° Celsius above pre-industrial levels and carbon dioxide concentrations continue to rise above the 400 ppm threshold that scientists initially warned as being catastrophic.