Rising temperatures are causing soil to dump more carbon dioxide into the air
The Earth, essentially, is panting.
Nature is breathing. Trees inhale carbon dioxide and store that carbon in their leaves and branches. After they die, microbes in the soil gobble up their carbon-rich leftovers and exhale carbon dioxide back into the air, a process known as respiration. Rising temperatures are causing both processes to go faster. But — in an unexpected new finding — the two aren’t speeding up at the same rate. The microbes are working harder than the plants. The Earth, essentially, is panting. “Soils around the globe are responding to a warming climate, which in turn converts more carbon into carbon dioxide, which enters the atmosphere,” said Ben Bond-Lamberty, a researcher with the Joint Global Change Research Institute, a partnership between the Department of Energy’s Pacific Northwest National Laboratory and the University of Maryland. “Climate change is nudging up the temperature under which soils and ecosystems operate, with effects that are both predictable — such as faster activity — and uncertain — that is, microbial and plant communities might change.”
The effect of this growing imbalance not only puts more heat-trapping carbon dioxide into the air, but it also dwindles the strength of the soil as a natural place to store carbon. “It’s highly uncertain how long, and to what degree, the land will continue to function as a robust carbon sink,” said Bond-Lamberty, lead author of a new study in the journal Nature that examines this phenomenon. “But it’s also pretty clear that the terrestrial carbon sink can’t continue indefinitely if the climate continues to change.”
The scientists found that the rate at which microbes are pumping carbon dioxide into the atmosphere has jumped 1.2 percent between 1990 and 2014, a number that at first glance might seem small but — on a global scale in a relatively short period of time in history — is “massive,” they said. They based their conclusions on thousands of observations at hundreds of sites around the world.
“This is a finding based on observations in the real world,” Bond-Lamberty said. “This is not a tightly controlled lab experiment… We are only observing the Earth. We can’t manipulate it in the laboratory.” They pulled together all reported measurements of soil respiration and examined satellite data from the same time and place to determine “how plant productivity — which can be measured from satellite, or inferred based on land measurements — is changing, relative to soil respiration changes,” he said.
“Most studies that address this question look at one individual site,” said Vanessa L. Bailey, a senior research scientist at the Pacific Northwest National Laboratory and a co-author of the paper. “This study asks the question on a global scale. We’re talking about a huge quantity of carbon. Microbes exert an outsize influence on the world that is very hard to measure on such a large scale.”
As to why microbes are exhaling so much harder, “[It] could be more microbes, or it could be different ones,” Bailey said. One explanation for all that heavy breathing is that rising temperatures have made it easier for microbes to break down complex carbon. “So it could be that microbes eat different types of carbon as temperature increases, and this carbon is either now more available, or more extensively decomposed,” Bailey said. “What is happening is not a steady acceleration of ecosystem processes. It is a change in ecosystems’ carbon balance.”
Another recent study — this one by scientists at the University of Manchester in the UK — found that climate change is changing soil in other important ways. Heat and drought are spurring the spread of fast-growing drought-tolerant grass, which rob the soil of moisture. The researchers — led by Franciska de Vries from the university’s school of earth and environmental sciences — predicted that persistent drought could alter soil biology.
Bailey said that drought can pressure microbes to shift functions, prompting them to “decompose forms of carbon that were previously considered relatively stable and persistent in soil,” she said. “How microbes will respond to drought is a huge unknown, with large potential consequences for the stability, fertility and robustness of soils around the world.”
Humans can help keep soil healthy by preserving or regrowing forests, but it probably won’t be enough, Bond-Lamberty said, explaining that “this is a global response to a global process, and reversing it will depend on slowing, and eventually reversing, the rate at which we put greenhouse gases into the atmosphere,” he said.
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.