Soil-based emissions pose a US-size risk to climate change
Around 55 trillion kilograms of carbon stored in soil could re-enter the atmosphere by 2050 if climate change continues, according to the first worldwide study on carbon feedback, published in Nature. The global emissions equivalent of another US, the study explains that as plants and animals die, their carbon-based bodies become part of the soil, storing millions of tonnes of greenhouse gasses in the earth. However, a type of feedback loop triggered by climate change could result in that carbon re-entering the atmosphere. The phenomenon and scale of carbon feedback was not considered in COP21 agreements. 

A lot of carbon stored inside the Earth may soon re-enter the atmosphere, according to a new, global study on soil-based carbon. The massive release of carbon could be the equivalent of adding another fully industrialized country the size of the United States to the map over the coming decades.

Plants, animals, and microbes absorb much of the carbon dioxide released into the Earth's atmosphere by burning fossil fuels. As these plants and animals die, their carbon-based bodies become part of the soil, storing the equivalent of millions of tons of greenhouse gases in the dirt. As atmospheric carbon dioxide is a primary driver of climate change, some have pointed to this soil absorption as a helpful carbon sink, somewhat alleviating the human-caused warming of the planet. But as surface temperatures continue to rise, a lot of the carbon stored in the soil may be set to re-enter the atmosphere in a big way.

The new study warns that a sort of feedback loop caused by climate change will cause a massive amount of carbon formerly trapped underground to return into the atmosphere in the form of even more greenhouse gases. These soil-based emissions could be a major obstacle to keeping climate change in check in coming decades, say the researchers.

"The exchange of carbon (C) between the soil and atmosphere represent a prominent control on atmospheric C concentrations and the climate," they wrote in a paper published in the journal Nature. "These processes are driven by the organisms (plants, microbes, and animals) that live in the soil, the activity of which could be accelerated by anthropogenic warming."

As the Earth gets warmer, say the researchers, microbes and other organisms living in the soil will increase their respiration rate, producing greenhouse gases such as carbon dioxide and methane, which are two leading causes of warming. These emissions could create a feedback loop as the greenhouse gases warm the planet further, leading to further respiration in the soil, which creates more warming, and so on.

"If climate change isn't stopped, an additional 55 trillion kilograms of carbon will be released into the atmosphere by the year 2050," said Tom Crowther, the paper's lead author and a researcher with the Netherlands Institute of Ecology, said in a statement.

That represents "about 17 percent more than the projected emissions due to human-related activities during that period," he said.

The massive emissions jump could present a significant stumbling block to international efforts to keep the global temperature from rising more than 2 degrees above pre-industrial levels — the commonly held goal of the nearly 200 countries who signed the Paris Agreement on climate change, which went into force in early November.

Climate scientists had not considered the scale of carbon feedback during those negotiations because so little was known about the phenomenon.

Previous studies on the subject had shown conflicting data from highly localized regions, leading to mixed and unclear results. But this study was different, incorporating data from around the globe.

"With data from more than 40 institutes around the world, covering 20 years, our scope is now finally worldwide," said Dr. Crowther.

The results of the worldwide study? For every degree of warming, about 30 petagrams (30 trillion kilograms) of carbon could be released into the atmosphere through this soil feedback mechanism, the paper warns.

While the scale of the problem is clear, the precise numbers are still uncertain, say researchers, especially since increased CO2 production will also lead to more plants to absorb carbon, which would mitigate the feedback to a certain extent. But the research team says that despite the large margin of error, the overall trend provides "strong empirical evidence" for the existence of a carbon-climate feedback phenomenon that will only get worse if nothing is done to slow or stop climate change.

"This is really critical, because if the additional release of carbon is not counterbalanced by new uptake of carbon by plants then it's going to exacerbate climate change and increases the urgency to immediately reduce greenhouse gas emissions," said Jonathan Sanderman, a scientist with the Woods Hole Research Center who studies soil changes under climate change, reported the Washington Post.

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While climate accords and a general trend towards cleaner energy are expected to slow global warming, and, by extension, the feedback caused by climate change, many have expressed concerns that President-elect Donald Trump will contribute to climate change by defunding current climate policies and pulling out of the Paris Agreement. The United States is currently the second-largest carbon emitter, after China.

"Now that this longstanding scientific query has been answered at last, we should adjust international climate models accordingly, and do this as quickly as possible," said Crowther. "The same goes for policy."