Forests could actually turn into a source of CO2, according to a study that paired climate forecasts with an analysis of records on more than two million trees across North America.
Scientists have considered forests a potential barrier to climate change, since plants on land take up about 25 percent of our carbon dioxide emissions. As trees in colder areas are exposed to warmer temperatures and more CO2 emissions, they will grow faster and absorb more emissions, helping to mitigate the effects of a primary greenhouse gas, the theory goes.
But, in an alarming twist, global warming is likelier to limit forests' capacity for absorbing emissions in many parts of the continent, a study released today in the journal Ecology Letters finds. After combining climate projections with the tree records, researchers found no evidence for the boreal greening hypothesis. In fact, they found a risk of a negative feedback loop, as trees in their model reacted poorly to warmer temperatures due to drought and other disturbances.
That means as trees die faster than they can take up CO2 emissions, releasing trapped carbon, forests could become a net source of carbon, accelerating climate change. The study found that we could reach such a tipping point as early as 2050.
“It’s one more piece of evidence that this is a serious problem and it’s going to take a real serious global coordination to stop it,” said Noah Charney, the study's first author and a postdoctoral research associate in the University of Arizona’s Department of Ecology and Evolutionary Biology. “There isn’t necessarily going to be some balance that’s going to keep us in check.”
The study employed widely used climate projections and tapped 1,457 sample sites across the continent. From those, tree records were collected from 1900 to 1950, a period Charney said was relatively climatically stable. The model included projections for 13 climate zones across North America and took into account that forests in different parts of the continent might respond differently to warming temperatures, he said.
The model projected forest growth rates up to 75 percent slower for trees along the Rocky Mountains in southwestern parts of the U.S. up through Canada and Alaska. Although increases in growth were seen in certain coastal areas, such as the Pacific Northwest and the Florida panhandle, some of the negative simulated effects are already becoming reality in Alaska, for example. Given the prevalence of boreal regions in Europe and Asia, the researchers are confident that the results hold in other parts of the world.
Carbon storage on land has long posed a central question for climate change scientists, said David Archer, a professor of geophysical sciences at the University of Chicago who studies global warming. Archer, who wasn't involved in the study, said he agreed with the researchers’ conclusion: Even though some forests currently store carbon, and will continue to do so as temperatures rise in the future, it's unclear how long that effect will last before warming leads to negative responses.
There are significant challenges in modeling change in forests and forecasting responses to climate conditions that haven't been seen before. Nonetheless, Archer said, the study adds to evidence that emissions need to be monitored, especially when considering the impact climate change could have on carbon stored in soil.
“If you count both sides of the land equation, I would not be at all surprised for carbon to start emerging rather than disappearing from land in response to climate in the future," Archer said. "That just seems to be inevitable to me."