Forests are an important buffer against climate change, absorbing some of the carbon-dioxide pollution released from burning fossil fuels. Fallen leaves and dead trees return carbon to the soil, which takes its brown color from the element.
But scientists have disagreed about how much of this huge store of locked-in carbon is at risk for release into the atmosphere.
“Young carbon” — such as that stored in leaves —rapidly returns to the air as microbes decompose plant matter. As the air warms, the decomposition speeds up, releasing more carbon. That process is well-known.
But deeper in the soil, older carbon is locked away as “humus” — the soft, brown material that makes forest floors spongy. Some scientists have asserted that this carbon will stay locked away even as the planet warms.
To test this idea, scientists took advantage of experimental forests maintained by the U.S. Energy Department and U.S. Forest Service in Wisconsin and North Carolina.
Since the late 1990s, scientists have blown carbon dioxide from large tanks into these forests; the gas carries a specific radioactive carbon signature, which can be easily traced.
Francesca Hopkins of the University of California-Irvine collected soil from the two forests in jars and then measured how much carbon dioxide the soil emitted as she warmed the containers. She also measured how much of the carbon dioxide was more than a decade old— meaning it had been locked away in humus for years.
She found that about one-third of the released carbon dioxide came from soils at least a decade old. As the soil heated up, that ratio stayed about the same, meaning that the older carbon was just as vulnerable to rising temperatures as the younger carbon.
“We now know for the top 15 centimeters [about six inches] of topsoil, most of that is going to be vulnerable to warming,” Hopkins said. “It’s going to increase its respiration rate as global temperatures warm.”
Hopkins called this accelerated release of carbon dioxide a contributor to a “vicious cycle” in which soil carbon dioxide triggers additional atmospheric warming, which in turn pushes the soil to release even more carbon dioxide.
“While that older material is not going to decompose really fast, there’s an awful lot of it,” said Susan Trumbore, the scientist who led the study.
The study increases concerns that temperate forests will flip from net absorbers of carbon dioxide to net emitters of the air-warming gas sooner rather than later, said Trumbore of the University of California-Irvine and the Max Planck Institute for Biogeochemistry in Germany.
The average surface temperature of the Earth has increased by 1.3 degrees Fahrenheit since 1900. In 2007, the Intergovernmental Panel on Climate Change called for limiting global warming to 3.6 degrees, a goal that looks increasingly difficult to reach as forests, melting permafrost in Arctic regions and the warming oceans absorb less and less of the greenhouse gas.
Hopkins said her results should focus attention on reducing carbon-dioxide pollution from burning fossil fuels. “We can control how much gasoline we burn and how much coal we burn, but we don’t have control over how much carbon the soil will release once this gets going,” she said.
The study was published online Monday by the Proceedings of the National Academy of Sciences.