Climate change could lead to more forests. But will they cool the planet as well? Science | Science

These are very strange times for the Indigenous Nenets reindeer farmers of northern Siberia. The Arctic Ocean’s shores are awash with bare tundra. Bushes are sprouting and willows that once stood only a few meters high can now reach 3 meters. This hides the reindeer. Surveys show that the Nenets autonomous area, which is roughly the same size as Florida has four times more trees than the 1980s official inventories.

Dmitry Schepaschenko, a forest ecologist at the International Institute for Applied Systems Analysis, Austria, has mapped the greening in Siberian tundra. “A few trees appear here and there, and some shrublike trees become higher.”

As the Arctic Circle warms, trees are growing everywhere. Norway is seeing birch, pine and other trees move in a poleward direction, eclipsing all the tundra. In Alaska, spruce have taken over from moss & 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 the map, below).

Forest gains aren’t just limited to the far north. Lower latitudes are seeing an increase in trees in warmer, more arid areas. This is partly due to increasing carbon dioxide (CO) concentrations.₂)—the main planet-warming gas—are enabling plants to use water more efficiently and thrive in drier soils. CO has fertilizing properties.₂ 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. Some studies predict that these tropical losses will be more than offset with gains elsewhere, which could lead to more trees and faster growth in the future.

This might seem like good news to curb global warming. Forests can often produce a cooling effect by releasing organic chemicals and water vapor, which promotes the formation clouds. Faster growing trees would absorb more atmospheric CO2 and store it 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. For example, new forests could increase warming by reflecting less sunlight into space. That could lead to a reduction in carbon absorption over time.

“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 accountResearchers must forecast how future climate will impact forests. This includes not only analyzing current trends like development-driven deforestation but also how powerful forces like wildfires and warmer temperatures could affect forests. These factors can sometimes help or hinder their ability to absorb 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. It has declined by 18% overall since the 1970s due to deforestation.

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.

Luciana Gattini, INPE climate researcher and Luciana Gatti say that the prediction appears to be now true. She used measurements of atmospheric carbon taken during 590 research flights above the Amazon between 2010-2018 to report in a July 2021 article. 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. As a result, the Amazon as a system had become a mess. “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 periods are stressing trees and increasing fire risk, which accelerates the conversion of forest to 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.

Some tropical forests continue to store large amounts of carbon in the meantime. One example is a long-term field study that was done in Borneo’s lowland forests. It found that 1-ha plots with no tree deaths, held an average of 20 tonnes more carbon today than in 1958. This is primarily due to CO.₂ fertilization.

However, ongoing warming is harming tropical forests, even if they are still intact. A study that tracked 300,000 trees over more than 500 plots in intact tropical forests over 30 year found that they are capable of capturing CO even without deforestation.₂ peaked in the 1990s and has since declined by one-third. According to Simon Lewis, a University College London plant ecologist, the decline started in Amazon and has been extended to tropical Africa since 2010. Remote-sensing techniques, which measure changes in the total area of leaves produced by trees and other plants, suggest that many tropical forests are slowing down their carbon intake.

This is a grim pictureOutside the tropics, the sky is brighter. 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.)

Studies showing higher levels of CO are one source of optimism.₂ 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.

More CO will be produced in the future₂ will also prompt forests to expand into new areas, other planet-scale simulations indicate. These digital models allow researchers explore how forests might respond in response to a variety of factors, such as changes in global temperatures and atmospheric CO concentrations. One such study was published in December 2021. 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. However, this finding was reversed when she included fertilizing effects of higher atmospheric CO₂. Overall, increasing CO₂ levels to about 560 parts per million—or double preindustrial levels— increased global forest cover by 15% above preindustrial extent.

The boreal forests of northern Canada saw the largest simulated increase in tree growth, with longer growing seasons and thawing of permafrost. Forests also expanded in the subtropics’ arid continental interiors.

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. This allows seedlings take root in places where none are currently growing.

Some researchersSome question optimistic projections of future forest expansion. They also warn that other factors could be at play. For example, deforestation could increase to meet growing global demand for food resources and resources, wiping away any global gains. A shortage of key soil nutrients like phosphorus could also neutralize CO₂ fertilization, especially in tropical forests, says Chris Huntingford of the UK Centre for Ecology & Hydrology. A 2019 study on Amazonia, for instance, was published in Nature GeoscienceKatrin Fleischer, an ecosystem scientist at the Technical University of Munich, conducted the study and found that a deficiency in phosphorus could reduce forest gains from CO.₂ fertilization in half.

Another question is how a warmer, dryer climate will impact wildfires. Modeling studies have shown that climate change will increase fire risk in temperate and tropical forests. Boreal forests could also be at risk. Global Forest Watch earlier this year reported that boreal forests could lose more than 8,000,000 hectares in 2021. This is 30% less than 2020. This was mainly due to wildfires.

Plant ecologists believe that fire could allow some boreal forests not to store as much carbon as they do now. That’s because regenerating forests can produce stands that are denser or comprise more vigorous species better adapted to fire.

Michelle Mack, a Northern Arizona University Forest Ecologist, has seen the 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.”

She says that this phenomenon is widespread in western North America and the Russian Far East. Schepaschenko concurs. Schepaschenko agrees. He says that fires in Siberia have helped to fuel the northward spread 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 it is by moving large amounts moisture from soils to 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 down the air because energy is needed to convert liquid water to vapor. The vapor released by trees along with other organic compounds can create clouds that can lower temperatures. (CO₂ fertilization could reduce transpiration by allowing trees to use water more efficiently, but researchers say it will remain a potent cooling force.)

Temperatures are also reduced by the forest canopy’s relative roughness. Air currents are created by the leaves and branches, which cause heat to be dissipated 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.

It turns out that forests can also warm the planet by altering the reflectivity or albedo of land surfaces. Gleaming surfaces like fresh snow have an average albedo of 0.8-0.9 (on a scale of zero to one), which means that they reflect a lot more solar energy back into space. A continuous canopy of broadleaf plants can have an albedo as low as 0.15. This means that the trees absorb solar energy, and then radiate it as heat. A canopy of conifers might have a lower albedo at 0.08.

High-altitude, boreal, and high-altitude areas that receive a lot snow will see a dramatic shift in albedo. As dark canopies replace the snow-covered surfaces, this will have a significant effect on albedo. The shift can also be dramatic in arid regions due to the shade trees provide for highly reflective sandy or rock 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.

New forests tend to have the greatest warming impact at high altitudes and high latitudes. This is because snow cover is thick and long-lasting. Trees grow slower in these areas, which reduces their ability to absorb carbon. Chris Williams, a Clark University geographer, discovered that the Rocky Mountains forests will result in net warming in the contiguous United States. However, the study found that the Pacific Coast forests will experience net cooling.

But the impact depends, in part, on when during the forest’s life cycle it is measured. For example, a young forest might heat the atmosphere due to its albedo effect. However, it could become net cool as the trees age and store greater carbon.

Dan Yakir, an earth system scientist at the Weizmann Institute of Science, has been watching the Yatir Forest’s balancing act, which is located approximately two hours from Tel Aviv, for the past two hours. In the yellow sands and hills of Mount Hebron, workers created the forest by planting 4 million Aleppo trees. Today, the forest is often promoted to be a valuable carbon-sink.

Yakir states that there have not been any clear-cut climate gains. His measurements of biomass as well as albedo show that the warming resulting from the dark canopy of pines has far outpaced the cooling caused by carbon capture. He expects that the cooling effect will catch up as the trees grow. But the crossover might not occur until the 2040s, he says—and that assumes the trees live that long.

Yakir states that the findings were not expected. Yakir cautions that the Yatir forests are in some ways unusual. For example, “The forest is almost black and the desert almost white. … Everything makes our case extreme.”

The uncertaintyThe question of how new forests will impact climate is not only a scientific problem. It also has policy implications. For example, few large-scale tree planting projects assess the potential climate impacts of 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 issue of how governments should account new forests when tallying their contributions to global climate accords such as the Paris agreement. Usually, credit is given to nations for expanding or protecting their forests. Russia, for instance, estimates that nearly one quarter of its fossil fuel emissions can be offset by its large, 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.

What if the growth of new forests is actually a catalyst for warming, and not a slowing down? Should nations continue to 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.”