The Earth relies on its vegetative cover to extract and hold onto carbon dioxide when a great deal of it finds its way into the atmosphere, as has happened with the burning of fossil fuels. The forests, which form the largest part of this land-based cover, are referred to as carbon sinks.

Now, new research shows that one of the planet’s largest and most important carbon sinks, the forests of northern Eurasia, may be pulling in carbon at a slower rate than in the past. What is even more worrying is the possibility that regions that were absorbing carbon may emerge as sources of carbon emissions as the permafrost melts.

In northern Eurasia, the annual net sink rate increased from the 1960s to the 2000s, but since then, the rate at which carbon is sequestered by the region has leveled and even showed signs of weakening, said Michael Rawlins, an assistant professor in the University of Massachusetts’ Department of Geosciences.

In a study published in Biogeosciences, Rawlins and his team compared estimates of the rate of carbon sequestration occurring in this region from nine models, and they all suggested a weakening carbon sink. In time, the emissions are projected to overtake the ability of forests to contain them and, as he put it, “it is thought that the region will switch from a net carbon sink to a net carbon source sometime this century.”

The carbon cycle represents a movement of carbon between different systems—forests, soil, ocean systems and the atmosphere. This entails both an uptake and a release of carbon by these systems. Sinks are those zones that take in more carbon than they emit, effectively storing the carbon and preventing it from contributing to global warming.

The burning of fossil fuels essentially reintroduces ancient carbon that is stored in the depths of the earth back into the carbon cycle. Although the oceans are currently the greatest carbon sink, terrestrial carbon sinks also play a significant role in keeping the carbon out of the atmosphere. “Land areas that sequester carbon help to offset the anthropogenic emissions of carbon dioxide,” Rawlins explained.

Race between sinks and sources
Northern Eurasia plays an important part in the global carbon cycle because of its large areas of forest and huge soil carbon reservoirs, he added. The net land carbon sink for northern Eurasia amounts to approximately 0.48 petagram of carbon per year, about 6.5 percent of global carbon emissions, according to Rawlins. “The region’s land carbon sink contributes to the sequestration of a significant percentage of the global emissions,” he said.

“I am surprised that the sinks keep taking in more carbon as the emissions keep increasing,” said Richard Houghton, a senior scientist at the Woods Hole Research Center. “You would expect that eventually they would stop taking it up.” That process may already be underway, and northern Eurasia may not be the only region where this slowing down of carbon uptake has been observed.

Other areas are also witnessing alterations to the carbon cycle that could aggravate global warming. A large part of the land carbon sink is in the tropics, and a recent paper in Naturefound that the Amazon carbon sink has accumulated carbon at a slower rate in recent years.

“If that was a universal phenomenon on land, then you would see more of the carbon emitted from fossil fuel and land-use change staying in the atmosphere,” Houghton said, “but we are not really seeing that yet.” Most of the climate projections are based on the carbon cycle, which accumulates carbon in proportion to the emissions, but if that doesn’t continue, all the projections are conservative and underestimate how climate would change.

A puzzle for climate models
The computerized climate models that were reviewed in the study also yielded great variations in their results. “Models do a good job at simulating some elements of the climate system, but they disagree on key aspects of the land-atmosphere CO2 exchange, and in particular the amount of carbon being sequestered,” Rawlins said in a statement.

Houghton also expressed concern about the huge variations in estimations from different models, but he commended the paper for highlighting the problem and addressing the question of what needs to be done to improve these models.

There is a critical lack of information about carbon cycle processes across northern Eurasia, Rawlins said. “It is a large region, sparsely populated and with very few research sites given the large area, ” he said. “Our results suggest that model improvements need to be targeted at processes controlling vegetation productivity and respiration from the soil.”

While the models may throw up different estimates for changes in the rate of carbon uptake, a slowdown is not good news, especially if it happens across a large expanse of the globe. “We would be in trouble then,” Houghton said. “Managing the carbon cycle is hard now, but it would be a lot harder without the land plants acting as a big sink.”

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500