Global tree cover could expand in a warming climate. But the calculus of forests’ climate effects is far from straightforward, and emerging research suggests a more forested world won’t necessarily be a cooler world. LongReads
These are strange times for the Indigenous Nenets reindeer herders of northern Siberia. In their lands on the shores of the Arctic Ocean, bare tundra is thawing, bushes are sprouting, and willows that a generation ago struggled to reach knee height now grow 3 meters tall, hiding the reindeer. Surveys show the Nenets autonomous district, an area the size of Florida, now has four times as many trees as official inventories recorded in the 1980s.
That might seem like surprisingly good news for curbing global warming. Forests often have a cooling effect, releasing organic compounds and water vapor that promote the formation of clouds. And more, faster growing trees would absorb more atmospheric carbon and lock it away in wood. That prediction now appears to have been realized, says INPE climate researcher Luciana Gatti. Drawing on measurements of atmospheric carbon collected during 590 research flights over the Amazon between 2010 and 2018, she reported in a July 2021 study that the southeastern Amazon—a region often called the “arc of deforestation,” where agriculture has gobbled vast swaths of trees—had flipped from sink to source. So, in consequence, had the Amazon as a whole.
Much of the simulated increase occurred in boreal forests of the north, where longer growing seasons and thawing permafrost help trees gain ground. But forests also expanded in arid continental interiors in the subtropics. This phenomenon appears to be widespread across western North America and in the Russian Far East, she says. Schepaschenko concurs. In Siberia, he says, fires have helped fuel the northward spread of forests into the tundra. “The flames remove moss and lichen cover, allowing [tree] seeds to reach mineral soil.”suggesting a more forested future are right, however, it’s not yet clear just how beneficial those trees might be for curbing global warming.
Together, these two processes currently help cool Earth’s surface by 0.4°C to 0.6°C, Lawrence says, with each contributing about half of the reduction. But the impact depends, in part, on when during the forest’s life cycle it is measured. A young forest, for example, might warm the atmosphere because of its albedo effect. But it could become net cooler as the trees age and store more carbon.