US forests store record carbon as natural and human factors combine

U.S. forests have stored more carbon in the past two decades than at any time in the last century, an increase attributable to a mix of natural factors and human activity, finds a new study.

To unravel the cause behind this spike, researchers used nationwide forest data to examine how six environmental factors may have contributed to the increase in carbon sequestered by forests. They found that natural forces such as increasing temperatures, shifting precipitation, and carbon fertilization are among the largest contributors to carbon gains, but human drivers, like letting forests get older and planting trees, are also becoming bigger factors.

Since most decarbonization efforts focus on curbing active emissions, this new analysis aims to help researchers better separate what portion of carbon held by forests is related to human action and which portion isn’t, said Brent Sohngen, co-author of the study and a professor of environmental and resource economics at The Ohio State University.

“Identifying and separating these influences hasn’t really been done before,” said Sohngen. “But with this data, the U.S. can be much more explicit about its carbon accounting, and that ability will provide a lot more information and long-term benefits for people who manage their forests and try to create carbon sinks.”

The study is published in the Proceedings of the National Academy of Sciences.

How forests act as carbon sinks

Forests act as vital tools to slow warming of the planet, as the more carbon that trees sequester, the less there is in the atmosphere. Large areas such as the Amazon Rainforest and the Congo Basin are known as “passive” carbon sinks because they absorb more carbon than they release without the need for human intervention.

In temperate regions, forests like Ohio’s Wayne National Forest are more likely to become carbon sources because they require extensive active inputs, such as tree-planting or other forest management activities to remain a strong sink, said Sohngen.

“The forests that we aren’t managing are doing exactly what we want them to do, which is to be ecosystem buffers,” he said. “That’s a good thing, but as we hit global carbon thresholds, the strength and size of that sink is slowing down in all these forests.”

Key findings from the new study

The study looked at six drivers—temperature, precipitation, carbon dioxide, management, age composition and area—and the team was surprised by exactly how much natural factors influenced the total amount of carbon stored by U.S. forests. For instance, changes in temperature and precipitation from 2005 to 2022 led to an increase of 66 million metric tons of carbon sequestration per year.

During the same period, human intervention had both negative and positive effects, as human-caused deforestation reduced stored forest carbon by about 31 million tons per year, while activities like tree-planting and reforestation added about 23 million tons per year. Yet it was forest age—mostly structural changes in the peak growth stages of local trees—that helped lock in the most carbon, by 89 million metric tons per year.

Overall, these results suggest that while climate policies are doing their part to mitigate current climate challenges, scientists should also recognize the extent to which natural processes continue to shape our world.