“You go on Mount Lemmon right now and you’ll see increased numbers of drying and dying trees,” UA professor Brian Enquist says.

Ron Medvescek / Arizona Daily Star 2015

Don’t count on North American forests to bail the planet out of climate change, says a new study whose authors include four University of Arizona researchers.

For years, scientists and government agencies have said forests are carbon sinks, absorbing carbon dioxide emitted into the atmosphere by fossil fuel burning.

The Environmental Protection Agency says U.S. forests removed 11.5 percent of all C02 emissions in 2014. The Forest Service pegs the removal rate at 10 percent to 20 percent in a given year. Globally, a 2011 study published in Science magazine estimated that forests and other land-based ecosystems remove 25 percent to 30 percent of all greenhouse gases.

But the new study concludes that warmer weather nationwide and drier weather in regions such as the Southwest will reduce North American forests’ growth rate through the 21st century. Climate change probably will slowly reduce the rate at which forests absorb carbon, so forests may become sources of carbon, not sinks, the study finds.

“For most of recent history, forests have played a significant role mitigating the effects of greenhouse gas emissions,” the study says. But the possibility that rising temperatures can reverse this trend has the potential “to accelerate climate change beyond critical tipping points,” it adds.

The study combined computer model forecasts of rising global temperatures with an analysis of 2 million tree ring records, covering 1900 to 1950 at 1,457 sampling sites across North America.

“It’s like a thermostat gone bad,” said one of the study’s UA researchers, Margaret Evans, an assistant professor in dendrochronology at the school’s Laboratory of Tree-Ring Research, in a news release. “Forests act as a carbon sink by taking carbon dioxide out of atmosphere, but the more the climate is warming, the slower the trees are growing, the less carbon they suck up, the faster the climate is changing.”

The study’s results don’t mean we should give up on forests as a carbon buffer, she added in an email. Instead, the study suggests that society needs to reduce fossil fuel emissions to keep forests operating as they have.

The ecosystem is telling us global warming is a much more immediate problem than people realize, and “we are closer to a tipping point than we thought,” in which northern forests are starting to respond negatively instead of positively to higher temperatures, Evans said.

In the U.S. southwest, “We are in one of the bull’s-eye areas of climate change, where we’ve already been seeing definite temperature increases as well as more pronounced droughts,” said Brian Enquist, another of the study’s UA researchers. This area is already seeing a disproportionate reduction in forest growth, he said.

“You go on Mount Lemmon right now and you’ll see increased numbers of drying and dying trees, from increased temperatures and decreased precipitation,” said Enquist, a professor of ecology and evolutionary biology.

The study was published last week in the journal Ecology Letters. It also relied on researchers in Montana, Pennsylvania and Switzerland and was partially financed by the UA.

The study is “very significant” in estimating future carbon budgets — the amount of CO2 that can be emitted into the atmosphere and still keep temperatures from rising no more than the goal of 2.7 to 3.6 degrees Fahrenheit set by the 2015 Paris climate accords, said Diana Liverman, co-director of the UA’s Institute for the Environment.

“If forests die back or reforestation is more difficult, then it will make it more difficult to use forests to mitigate emissions … or may make forests a larger source of emissions,” Liverman said.

The 1998 Kyoto Protocol, the world’s first big emissions-reduction effort, allowed for carbon credits for reforestation and creation of new forests, but not for protecting existing forests, Liverman said. But the Paris climate accord gives countries credit for protecting existing and new forests.

The Paris commitments probably don’t assume that climate change will reduce the forests’ ability to absorb carbon, she said. So a country that hopes to meet its commitments through forest restoration or other land use management may have to try harder at that, or have to do more to mitigate fossil fuel combustion, she said.

The study predicted the slowest future forest growth rates — up to 75 percent slower in some areas — would occur in the interior West, including the Southwest, the Rockies, interior Canada and Alaska. Growth rate increases would be limited to coastal areas in the Pacific Northwest, the Florida Panhandle, northeastern Quebec and the Maritime Provinces of Nova Scotia, New Brunswick and Prince Edward Island, the study says.

This study makes a very good case that we can’t assume that forests will remain carbon sinks into the future, said Mark Harmon, an Oregon State University forest science professor who wasn’t involved with the study. But predicting forests’ future response is challenging, partly because so many “moving parts” are involved and because the response is unlikely to be uniform across the forests, he said.

“What is not understood is whether the processes releasing carbon will also slow in the future,” he said.

A Forest Service ecologist, Sean Healey, said the threat that this study identifies to forests’ ability to store carbon is certainly credible. But Healey noted that the study didn’t consider other disturbances and changes that can affect forests.

Also, forests’ growth rate isn’t the biggest factor affecting their ability to store carbon, said Healey, of the service’s Rocky Mountain Research Station in Ogden, Utah. More important are the trees’ age and the impacts of various disturbances, such as fires and insects — “all the cataclysms that might happen to a forest, some related to climate, some not,” Healey said.

Evans said the relationship between temperature and the carbon-absorbing behavior of forests has long been a matter of scientific uncertainty.

A commonly held theory is that there is what’s known as a “boreal greening effect” in the far north’s boreal forests from higher temperatures and atmospheric CO2 concentrations. The theory is that those forests would then remove more CO2 from the atmosphere, reducing climate change impacts.

The researchers were startled to find no evidence through their computer models that boreal greening will occur, they said. They found boreal forest growth is limited by high temperatures because “higher temperatures mean drought stress, trees pushed to their physiological limits and not able to grow as well,” the UA’s Evans said.

Also, some of the study’s predictions about the northern forests are already coming true, researchers said. In Alaska, trees once projected to respond positively to warmer weather are already responding negatively, literature from other research and NASA satellite data show, Evans said.

“That’s shockingly sort of scary to us,” Evans said.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter: tonydavis987.