There are strange times for the Indigenous Nenets reindeer herders. In their lands on the shores of the ocean, bare tundra is thawing, bushes are growing, and willows are 3 meters tall, hiding reindeer. The district has four times as many trees as official inventories, which was recorded in the 1980s.

In some places the trees are moving along a wide front, but in other places the gains are patchier, according to forest ecologists.

As the climate warms, trees are invading. In Norway, pine and Birch are moving in opposite directions. In Alaska, moss and lichen are being replaced by spruce. Research shows that forests are expanding along two-thirds of Earth's 12,000-kilometer-long northern tree line, the point where forests give way to tundra.

Forest gains are not limited to the far north. The main planet-warming gas CO 2 is enabling plants to use water more efficiently and thrive in drier soils, which is why some warmer, arid regions are seeing an increase in trees. Existing forests are able to add more leaves and wood because of the effects of CO 2.

In the tropics, 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 tropical losses could be more than offset by gains elsewhere, leading to a world with more and faster growing trees.

That may be good news for global warming. The formation of clouds can be promoted by the release of organic compounds and water vapor in forests. Faster growing trees would absorb more carbon and keep it out of wood.

Climate effects of forests are far from straightforward, and emerging research suggests a more forested world won't be a cooler world. Reducing the amount of sunlight that is reflected into space could enhance warming in some areas. It could offset any gains in carbon absorption over time.

Deborah Lawrence is an environmental scientist at the University of Virginia. She says that complexity is not captured by current metrics.

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

The steady erosion of tropical forests, one of the planet's major carbon sinks, has been the focus of much of the research on the balance sheet. The news has been bad in the Amazon, the world's most expansive tropical forest. Since the 1970s, it has shrunk by about 18%.

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

Carlos Nobre of Brazil's National Institute for Space Research warned in 2007, that the ongoing losses could flip the Amazon from being a global carbon sink to a significant new source. Simulations of the Amazon's hydrological cycle showed that it would make rainforests drier, reduce tree growth, and cause a net release of carbon to the atmosphere.

Luciana Gatti says that the prediction appears to have been realized. The southeastern Amazon is often referred to as the "arc of deforestation" because of the measurement of atmospheric carbon collected during 590 research flights. She says that the Amazon has hit a tipping point.

Warming temperatures have compounded the effects of the Amazon's carbon storage capacity. The conversion of forest to more open savannas is being accelerated by the longer dry seasons. Climate scientist Chris Jones and colleagues at the United Kingdom's Met Office have concluded that the Amazon's total carbon storage could drop by one-third in coming decades if regional temperatures rise by 4 degrees C.

Some tropical forests are sequestering large amounts of carbon. One longterm field study in the lowland forests on the island of Borneo recently found that plots with tree deaths are 20 tons more carbon efficient than plots without tree deaths.

Tropical forests are being affected by warming even though they are still intact. An international study that has tracked 300,000 trees in more than 500 plots of intact tropical forests over 30 years shows that their ability to capture CO 2 has declined since the 1990s. The decline began in the Amazon and has since spread to tropical Africa, according to co-author Simon Lewis. Many tropical forests are slowing their carbon intake because of remote-sensing techniques that assess changes in the total leaf area produced by trees and other plants.

Outside the tropics, this picture is bright. Lawrence says that the world can more than compensate for losses in the tropics because of climate trends.

World map centered on the Arctic.

Warming temperatures and longer growing seasons are causing trees and shrubs to grow on the tundra. Satellite data shows that from 2000 to 2020, tundra greening increased and vegetation decreased.

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

Studies show higher CO 2 levels are helping forests. A 2016 study headed by a researcher from Peking University found that between one-quarter and one-half of the planet's vegetated places showed an increase in leaf area since 1982. Simulations suggest CO 2 fertilization accounts for 70% of the increase in global forest biomass.

More CO 2 will prompt forests to expand into new areas in the future, according to other planet-scale simulations. Digital models allow researchers to explore how forests might respond to a variety of factors. A study by a Brown University Climatologist found that warming alone caused vegetation to decrease globally, with tropical losses from overheating exceeding nontropical gains from longer growing seasons. She added the fertilization effects of increased atmospheric CO 2. Global forest cover increased by 15% above preindustrial extent as a result of boosting CO 2 levels.

In the north, longer growing seasons and thawing permafrost help trees gain ground. In the subtropics, forests expanded in arid continental interiors.

She says that the greening in arid zones occurs even without significant increases in precipitation. That makes it possible for seedlings to take root where no one grows today.

Future forest expansion predictions are questioned by some researchers. They say that other factors could intervene. Growing global demand for food and resources could cause deforestation to accelerate, wiping out any gains. Chris Huntingford of the UK Centre for Ecology and Hydrology says that a shortage of key soil nutrients such as phosphorus could be a problem. In the Amazon, a study published in Nature Geoscience found that a lack of adequate phosphorus could cut forest gains from CO 2.

There is a big question about how a warmer climate will affect fires. Climate change will increase fire risks in tropical and temperate forests. Losses could also be seen in boreal forests. Global Forest Watch reported earlier this year that the forest lost more than 8 million hectares in the next two years due to fires.

Some forests could be able to store more carbon if there is fire. regenerating forests can produce stands that are denser or 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

A forest ecologist at Northern Arizona University has seen a tree. The charred remains of evergreen spruce forests in Alaska were replaced by faster growing and less flammable aspen and Birch trees that could store up to five times more carbon than their evergreen predecessors.

She says that this phenomenon is widespread in western North America and in the Russian Far East. The two of them concur. In Siberia, fires help fuel the northward spread of forests into the tundra.

It is not yet clear how beneficial a more forested future will be for global warming.

There is little doubt that forests can help cool the atmosphere. They can move large amounts of soil into the air. A typical tree releases up to 100 liters of water every day, the equivalent of flooding the entire U.S. land area.

The transpiration cools the air because it requires energy to convert liquid water to vapor. Clouds that can bring down temperatures are created by the released vapor and other organic compounds. fertilization could reduce transpiration by allowing trees to use water more efficiently, but researchers say it will remain a potent cooling force.

The forest canopy's relative thickness helps reduce temperatures. The leaves and branches cause air currents to swirl and mix.

Lawrence says that the two processes help cool the Earth's surface by about half.

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

forests can warm the planet by changing the albedo of land surfaces. Fresh snow has an albedo of between 0.8 and 0.9, which means they bounce a lot of solar energy back into space. A continuous canopy of broadleaf trees can have an albedo of just 0.15, meaning they absorb solar energy and use it to heat up. A canopy of conifers can have a lower albedo.

Dark canopies replacing snow-covered surfaces are expected to have a major impact on albedo in regions with a lot of snow. In arid regions, the shift can be dramatic, as trees shade sandy or rocky soils. The cooling contribution of a forest is likely to depend on a number of factors, including latitude, altitude, how fast the trees grow, and the age of the forest.

New forests are likely to have the greatest warming impact at high latitudes and high altitudes, where the snow cover is long lasting and the temperatures are cold. In the contiguous United States, Chris Williams of Clark University found that forests in the Rockies will cause net warming over the course of a century.

The impact depends on when the life cycle of the forest is measured. A young forest might warm the atmosphere because of its albedo effect. As the trees age and store more carbon, it could become net cooler.

In Israel, 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. The forest was created in the 1960s by planting 4 million pines in the Negev Desert. The forest is often promoted as a valuable carbon sink.

Climate gains are not clear-cut so far. The warming caused by the dark canopy of the pines has been shown to surpass the cooling from their carbon capture. He expects the cooling influence to catch up as the trees grow. He says that it might not happen until the 2040s, assuming the trees live that long.

The findings were unexpected. He cautions that the Yatir forest is unusual. The forest is almost black and the desert is white.

Policy implications can be found in the uncertainty surrounding how new forests will affect climate. Few large-scale tree planting projects look at the potential climate downsides from a changing albedo. If greening initiatives end up placing trees in locations that are counter productive for cooling the climate, it could really backfire.

There is a question of how governments should account for new forests when they tally up their contributions to complying with global climate agreements. The nations are given credit for protecting their forests. Russia calculates that 25% of its fossil fuel emissions are offset by its forests. The discovery that Russia's boreal forests are expanding and storing more carbon suggests the nation could go much further.

If new forests accelerate warming over the long term, what should we do? Is it still a good idea for nations to get credit?

Scientists say that such questions highlight the importance of gaining a more sophisticated understanding of how forests and Earth interact. Without a clear picture, I worry that we could be placing too much emphasis on forests as a climate solution, when we really need deep decarbonization of society.