Trees are part of the climate solution - but not the whole answer

Replanting the world’s forests could significantly reduce global warming, particularly in tropical regions, according to a new modeling study.

But even if every tree lost since the 19th century were replaced, the cooling impact would still fall short of reversing the full effects of human-caused climate change. Emissions cuts, the authors stress, remain essential.

The study, led by researchers at the University of California, Riverside, shows that restoring global forests to their preindustrial extent could lower average global temperatures by 0.34 degrees Celsius. That’s about one-quarter of the warming the planet has already experienced.

A climate strategy with limits

The modeling is based on a hypothetical increase in forest cover of 12 million square kilometers – about 135% the size of the United States. That estimate is roughly equivalent to the widely discussed goal of planting one trillion trees.

Scientists estimate that the Earth has lost nearly half of its trees – about three trillion – since the dawn of industrial society.

“Reforestation is not a silver bullet,” said Robert Allen, a climate scientist at UC Riverside and the study’s lead author. “It’s a powerful strategy, but it has to be paired with serious emissions reductions.”

While earlier studies have focused mostly on how trees absorb carbon dioxide from the atmosphere, this research broadens the lens.

It accounts for chemical processes triggered by trees. These include the release of naturally occurring compounds called biogenic volatile organic compounds (BVOCs) that can enhance cooling in other ways.

Forests help cool the atmosphere

Trees, especially those in tropical regions, release BVOCs that interact with atmospheric gases to create particles that reflect sunlight and promote cloud formation. Both effects have a net cooling influence on the planet’s climate. However, most climate models do not factor in these chemical interactions.

“When you include these chemical effects, the net cooling impact becomes more significant,” Allen said. “It’s a crucial part of the picture.”

The researchers found that tropical forests in particular offer a stronger cooling effect than those in other regions.

In the tropics, trees absorb more carbon, emit more BVOCs, and contribute less to the surface-darkening effect. When dark tree canopies replace snow-covered ground in high-latitude forests, this darkening effect actually increases warming.

Climate change close to home

In addition to influencing global temperatures, planting trees has implications for local air quality and regional climates.

The team found that their restoration scenario led to a 2.5% reduction in atmospheric dust across the Northern Hemisphere. In tropical areas, BVOC emissions produced mixed air quality results – contributing to an increase in fine particles associated with poor health outcomes, while also reducing harmful ozone levels.

These findings suggest that smaller-scale, localized reforestation efforts can still yield meaningful environmental benefits.

Antony Thomas is a graduate student in UC Riverside’s Department of Earth and Planetary Sciences and co-author of the study.

“Smaller efforts can still have a real impact on regional climates,” he said. “Restoration doesn’t have to happen everywhere at once to make a difference.”

Balancing forests and farmland

Despite the model’s optimistic numbers, the researchers acknowledge that reforesting all previously wooded areas is highly unlikely. Much of that land is now used for farming, ranching, or urban development, raising difficult questions about land use and food security.

“There are eight billion people to feed,” Allen said. “We have to make careful decisions about where trees are planted. The best opportunities are in the tropics, but these are also the areas where deforestation continues today.”

The team cites Rwanda as a case study for how conservation can coexist with development. In Rwanda, tourism revenues tied to forest protection are reinvested in local communities, giving people a financial incentive to preserve forests instead of cutting them down.

From classroom to climate research

The idea for the study began in a graduate-level climate modeling course at UC Riverside and eventually grew into a full-scale research project.

Using Earth system models and global land-use data, the team set out to evaluate just how much planting trees could move the needle on climate change.

The findings are cautiously hopeful: forest restoration is a valuable part of the climate solution, but it can’t do the job alone.

“Climate change is real,” Thomas said. “And every step toward restoration, no matter the scale, helps.”

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