(Courthouse News Service) Once believed to play a crucial part in combating climate change because of their ability to store and absorb carbon dioxide, new research shows North American forests have reached 78 percent of their capacity to capture carbon.

The study, published Friday in the journal Nature Communications, looks at 140,000 plots across the United States and Canada. The researchers studied the growth of forests in recent decades and made projections about the future.

“There’s a lot of hope that our forests will soak up the carbon dioxide we’re producing, but the capacity of our forests is limited,” lead researcher Kai Zhu from the University of California, Santa Cruz said.

Forests are what’s known as a “carbon sink,” a place that can offset harmful carbon emissions causing climate change. But the new study reveals they have limits that are quickly being reached.

In a best-case scenario, forests across the continent will only gain 22 percent more space to sequester carbon over the next 60 years, the study found.

“Reducing deforestation in the tropics is much easier than expanding forests in North America,” Zhu said. “That option is very limited.”

The study was based on the historical growth of North American forests in past two decades, and their projected growth in the future. Zhu and the others used data from 2000-2016, as well as observations from 1990-1999.

Using that model, they then made predictions about forest conditions under different climate change scenarios in future decades.

While past studies about forests’ ability to capture carbon have been based on satellite data and simulation models, this one is comes from ground-based measurements of the forests.

The predictions used best-case scenarios based on projections from the Intergovernmental Panel on Climate Change. Zhu acknowledged, however, that disturbances like wildfires and development will cause a loss in forests’ ability to capture carbon.

“The future potential is pretty limited,” Zhu said, noting pine forests in the eastern and southeastern U.S. are the main potential for growth.

“If mitigation depends on forests, this has implications for conservation we have to think about.”