Climate change could lead to a net expansion of global forests. But will a more forested world actually be cooler?
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.
In some places the trees are advancing along a wide front, but in other places the gains are patchier, says forest ecologist Dmitry Schepaschenko of the International Institute for Applied Systems Analysis in Austria, who has mapped the greening of the Siberian tundra. “A few trees appear here and there, and some shrublike trees become higher.”
All around the Arctic Circle, trees are invading as the climate warms. In Norway, birch and pine are marching poleward, eclipsing the tundra. In Alaska, spruce are taking over from moss and lichen. Globally, recent research indicates forests are expanding along two-thirds of Earth’s 12,000-kilometer-long northern tree line—the point where forests give way to tundra—while receding along just 1% (see map, below).
Forest gains are not confined to the far north. At lower latitudes, some warmer, arid regions are also seeing an uptick in trees, in part because increasing concentrations of carbon dioxide (CO2)—the main planet-warming gas—are enabling plants to use water more efficiently and thrive in drier soils. And the fertilizing effects of CO2 are enabling existing forests to add more leaves and wood, increasing their biomass.
It’s a startlingly different picture from what is unfolding in the tropics, where hundreds of thousands of hectares of forest are lost each year to chainsaws and fire, and climate change is stressing the remaining trees. But those tropical losses could be more than offset by gains elsewhere, some studies predict, leading in the coming decades to a world with more and faster growing trees.
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.
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