Climate change could expand forests. But will they cool the planet? | Science

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A version of this story appeared in Science, Vol 376, Issue 6595.Download PDF

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.

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. New forests could enhance warming in some areas, for example, by reducing the amount of sunlight that is reflected into space. Over time, that could offset any gains in carbon absorption.

“Forests are not just carbon sponges,” says environmental scientist Deborah Lawrence of the University of Virginia. But that complexity, she adds, “is not adequately captured by current carbon-centric metrics.”

To account for how forests will affect future climate, researchers must not only tally current trends, such as development-driven deforestation, but also forecast how powerful forces such as surges in wildfire and warmer temperatures might affect forests, sometimes helping and sometimes harming their ability to soak up atmospheric carbon.

Historically, researchers have focused much of their attention on the losses side of the balance sheet, for example by quantifying the steady erosion of tropical forests, one of the planet’s major carbon sinks. In the Amazon, the world’s most expansive tropical forest, the news has been almost unremittingly bad. Overall, it has shrunk by about 18% since the 1970s because of deforestation.

Forests can have global effects of both cooling (carbon uptake, cooling from clouds & turbulence) & warming (lower albedo).
V. Altounian/Science

In 2007, meteorologist Carlos Nobre of Brazil’s National Institute for Space Research (INPE) warned the ongoing losses could flip the Amazon from acting as a global carbon sink to a significant new source. Simulations of the Amazon’s hydrological cycle, he found, showed deforestation would make rainforests drier, reduce tree growth, and promote tree losses, resulting in a net release of carbon to the atmosphere.

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 Nature 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. “We have hit a tipping point,” she says.

The years since 2018 have been “even worse” for the Amazon’s carbon storage capacity, Gatti says, as warming temperatures have compounded the effects of deforestation. Longer dry seasons are stressing trees and increasing fire risks, accelerating the conversion of forest to more open savannas. Overall, the Amazon’s total carbon storage could drop by one-third in coming decades if regional temperatures rise by 4°C, modeling studies conducted by climate scientist Chris Jones and colleagues at the United Kingdom’s Met Office conclude.

In the meantime, however, some tropical forests are continuing to sequester large amounts of carbon. For example, one longterm field study in the lowland forests on the island of Borneo recently reported that intact 1-hectare plots, where tree deaths remain infrequent, hold an average of 20 tons more carbon today than they did in 1958, primarily because of CO2 fertilization.

But ongoing warming is working against tropical forests, even those that are still intact. An international study that has tracked 300,000 trees in more than 500 plots of intact tropical forests over 30 years finds that even without deforestation, their ability to capture CO2 peaked in the 1990s and has since declined by one-third. The decline began in the Amazon, and since 2010 has extended to tropical Africa, says co-author Simon Lewis, a plant ecologist at University College London. Remote-sensing techniques that assess changes in the total leaf area produced by trees and other plants also suggest many tropical forests are slowing their carbon intake.

This bleak picture brightens outside the tropics. In cooler regions, research suggests climate trends are driving gains in both forest extent and productivity that could more than compensate for losses in the tropics— “presuming,” Lawrence says, “that the world can meet its goals for limiting deforestation.” (So far, it’s not clear it will.)

World map centered on the Arctic.

Greening trend

Around the Arctic, trees and shrubs are encroaching on tundra thanks to warmer temperatures and longer growing seasons. From 2000 to 2020, increases in tundra greening (left) outstripped vegetation decreases (right), according to a recent analysis of satellite data.

Map of the Arctic showing tundra greening (most land in Arctic circle besides Greenland) and tundra browning (less extensive).
(Graphic) K. Franklin/Science; (Data) Logan Berner and Scott Goetz/Northern Arizona University; NASA’s Arctic Boreal Vulnerability Experiment

One source of optimism are studies showing higher CO2 levels are already helping forests add biomass. For example, a widely cited 2016 study headed by remote sensing researcher Zhu Zaichun of Peking University found that between one-quarter and one-half of the planet’s vegetated places showed an increase in leaf area since 1982, whereas just 4% showed a decline. Simulations by Zhu’s team suggest CO2 fertilization accounts for 70% of the increase in global forest biomass.

In the future, more CO2 will also prompt forests to expand into new areas, other planet-scale simulations indicate. These digital models allow researchers to explore how forests might respond to a wide array of factors, including changes in global temperatures and atmospheric carbon concentrations. One such study, published in December 2021 in JGR Atmospheres by climatologist Jennifer Kowalczyk of Brown University, found warming alone caused vegetation to decrease globally, with tropical losses from overheating exceeding nontropical gains from longer growing seasons. But that finding flipped when she added the fertilizing effects of increased atmospheric CO2. Overall, boosting CO2 levels to about 560 parts per million—or double preindustrial levels— increased global forest cover by 15% above preindustrial extent.

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.

That was somewhat surprising, Kowalczyk says, because the greening in arid zones “occurs even without significant increases in precipitation.” Instead, she says, the extra atmospheric CO2 allows trees to reduce water loss, because they don’t need to open their stomata so wide to take in CO2. That enables seedlings to take root where none grow today.

Some researchers question optimistic predictions of future forest expansion. One issue, they say, is that other factors could intervene. Deforestation, for example, could accelerate to satisfy growing global demand for food and resources, wiping out any global gains. And a shortage of key soil nutrients such as phosphorus could neutralize CO2 fertilization, especially in tropical forests, says Chris Huntingford of the UK Centre for Ecology & Hydrology. In the Amazon, for example, a 2019 study published in Nature Geoscience, led by ecosystem modeler Katrin Fleischer of the Technical University of Munich, found that a lack of adequate phosphorus could cut forest gains from CO2 fertilization in half.

Another big question is how a warmer, drier climate will influence wildfires. Modeling studies have long warned that climate change will increase fire risks in tropical and temperate forests. Boreal forests could also see losses. In an early portent, Global Forest Watch reported earlier this year that boreal forests lost more than 8 million hectares in 2021, 30% more than in 2020, with wildfires mostly to blame.

But fire could also enable some boreal forests to store more carbon, not less, plant ecologists say. That’s because regenerating forests can produce stands that are denser or comprise more vigorous species better adapted to fire.

Burned evergreen trees, surrounded by new growth of grasses and young saplings.
Wildfire can destroy forests, but in some it can clear the way for faster growing trees that help curb climate change by absorbing carbon.Juan Carlos Munoz/NPL/Minden Pictures

Michelle Mack, a forest ecologist at Northern Arizona University, has seen this arboreal phoenix in action. After fires devastated evergreen spruce forests in Alaska in 2004, their charred remains were replaced by faster growing and less flammable aspen and birch—deciduous trees that could ultimately store up to five times more carbon than their evergreen predecessors. “I thought there was no way these forests could recover the carbon they lost in the fire,” Mack says. “But these deciduous trees did so rapidly.”

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.”

Even if models suggesting a more forested future are right, however, it’s not yet clear just how beneficial those trees might be for curbing global warming.

On the plus side, there’s little doubt that forests can help cool the lower atmosphere. One way they do this is by moving large amounts of moisture from soils into the air. A typical tree may “sweat” up to 100 liters of water every day, and the planet’s estimated 3 trillion trees release some 60,000 cubic kilometers each year, the equivalent of flooding the entire U.S. land area with about 6 meters of water.

This transpiration cools the air, as energy is required to convert the liquid water to vapor. And the released vapor, together with other organic compounds produced by trees, helps create clouds that can bring down temperatures. (CO2 fertilization could reduce transpiration by allowing trees to use water more efficiently, but researchers say it will remain a potent cooling force.)

The relative roughness of a forest canopy also helps reduce temperatures. The leaves and branches cause air currents to swirl and mix, helping dissipate heat higher into the atmosphere.

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.

A coniferous forest swathed in mist.
Forests produce water vapor and other compounds that promote cloud formation, helping cool the planet.Philippe Henry/Biosphoto/Minden Pictures

But it turns out forests can warm the planet, too, primarily by changing the reflectivity, or albedo, of land surfaces. Gleaming surfaces such as fresh snow have an albedo of 0.8 to 0.9 (on a scale from zero to one), meaning they bounce a lot of solar energy back into space. In contrast, a continuous canopy of broadleaf trees can have an albedo of just 0.15, meaning the trees absorb solar energy and radiate it in the form of heat. A canopy of conifers can have an even lower albedo: 0.08.

In boreal and high-altitude regions that get a lot of snow, the expansion of forests is expected to have a major impact on albedo, as dark canopies replace snow-covered surfaces. In arid regions, the shift can also be dramatic, as trees shade highly reflective sandy or rocky soils. But whether the warming caused by albedo changes will ultimately outstrip a forest’s cooling contribution is likely to depend on several factors, including latitude, altitude, how fast the trees grow, and the age of the forest.

Overall, new forests are likely to have their greatest warming impact at high latitudes and high altitudes, where snow cover is extensive and long-lasting and cold temperatures mean trees grow more slowly, reducing their ability to capture carbon. In the contiguous United States, for example, a study by geographer Chris Williams of Clark University found that over a 100-year span, forests growing in the Rocky Mountains will cause net warming, whereas those growing in lower, less snowy areas east of the Mississippi River and on the Pacific Coast will cause net cooling.

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.

In Israel, earth system scientist Dan Yakir of the Weizmann Institute of Science has been watching this balancing act play out in the Yatir Forest, a 2-hour drive from Tel Aviv. Workers created the forest in the 1960s, planting some 4 million Aleppo pines in the yellow sands of the Negev Desert on the slopes of Mount Hebron. Today, the forest is often promoted as a valuable carbon sink.

So far, however, any climate gains are not clear-cut, Yakir says. His measurements of biomass and albedo have shown that, so far, the warming caused by the dark canopy of the pines now exceeds the cooling from their carbon capture. As the trees grow, however, he expects the cooling influence to catch up. But the crossover might not occur until the 2040s, he says—and that assumes the trees live that long.

The findings were unexpected, Yakir says. And he cautions that the Yatir forest is, in some ways, unusual. For example, “The forest is almost black and the desert almost white. … Everything makes our case extreme.”

The uncertainty surrounding how new forests will affect climate poses not just a scientific challenge: It has policy implications, too. Few large-scale tree planting projects, for example, assess the potential climate downsides from a changing albedo. As a result, such greening initiatives “could really backfire” if they “end up placing trees in locations that are counterproductive for cooling the climate,” Williams says.

Then there is the question of how governments should account for new forests when they tally up their contributions to complying with global climate agreements such as the Paris accord. Typically, nations are given credit for protecting or expanding their forests. Russia, for example, calculates that roughly one-quarter of its fossil fuel emissions are offset by its huge, carbon-absorbing forests. And Schepaschenko’s discovery that Russia’s boreal forests are expanding and storing even more carbon suggests the nation could go much further. “We have the potential to turn [new forests] into a massive carbon capture hub,” the nation’s minister of development of the Far East and Arctic, Aleksey Chekunkov, told Bloomberg last year.

But what if new forests turn out to accelerate warming over the long term, not slow it? Should nations still get credit?

Such questions, scientists say, highlight the importance of gaining an even more sophisticated understanding of how forests and Earth’s climate interact. Without a clearer picture, Williams says, “I really worry that we could be placing too much emphasis on forests as a climate solution, when what we really need is deep decarbonization of society.”

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