Does Planting Trees Really Help Cool the Planet?

Restoring forests—particularly in tropical regions—may have a stronger cooling effect than previously thought. However, even if all the trees lost since the mid-1800s were replanted, it wouldn’t be enough to fully offset the warming caused by human activities. Reducing greenhouse gas emissions is still crucial.

A new modeling study from the University of California, Riverside—published in Communications Earth & Environment—suggests that restoring forests to their preindustrial levels could reduce global average temperatures by 0.34°C, or about a quarter of the warming Earth has already undergone.

A Vision for Replanting 1 Trillion Trees

The scenario examined involves increasing tree coverage by around 12 million square kilometers—about 135% the size of the United States—and aligns with global reforestation estimates of planting 1 trillion trees. Since industrialization began, the planet is thought to have lost nearly half its trees, or about 3 trillion.

“Reforestation isn’t a cure-all,” said lead author and climate scientist Bob Allen. “It’s a powerful tool, but it must be combined with substantial cuts in emissions.”

Unlike earlier research that primarily focused on trees’ role in carbon absorption, this study also considered their broader atmospheric influence. Trees emit biogenic volatile organic compounds (BVOCs), which interact with other gases to form particles that reflect sunlight and enhance cloud formation—both of which contribute to additional atmospheric cooling. Most climate models overlook these chemical effects.

“When these chemical interactions are factored in, the overall cooling effect becomes more pronounced,” said lead researcher Bob Allen. “It’s an essential piece of the climate puzzle.”

Tropical Forests Lead the Way in Climate Cooling Benefits

However, the cooling benefits of reforestation aren’t uniform across the globe. The study found that tropical forests offer the most advantages, providing stronger cooling with fewer negative side effects. Trees in these areas are better at absorbing carbon, emit higher levels of BVOCs, and cause less surface darkening—a phenomenon that can lead to warming in forests at higher latitudes.

Reforestation also influences regional air quality. The researchers observed a 2.5% decrease in atmospheric dust across the Northern Hemisphere in their restoration model.

In tropical zones, the rise in BVOC emissions had mixed outcomes: while they contributed to more particulate matter and increased aerosol formation—potentially worsening air quality—they also led to reduced ozone levels, indicating cleaner air in that respect.

These regional differences suggest that reforestation doesn’t need to be widespread to be effective.

“Even smaller-scale efforts can significantly influence local climates,” noted Antony Thomas, a co-author and graduate student in UCR’s Department of Earth and Planetary Sciences. “We don’t need to restore every forest all at once to see meaningful change.”

Ambitious Reforestation Faces Major Land Use and Feasibility Challenges

The researchers admit that the reforestation scenario they modeled is highly ambitious and unlikely to happen in full. It assumes trees could be replanted across all former forested areas, which would require converting developed land—such as homes, farmland, and pastures—back into forest. This presents significant challenges related to food production and competing land uses.

“With 8 billion people to feed, we have to be strategic about where we replant,” Allen explained. “The best potential lies in tropical regions, but unfortunately, those are also the places where deforestation is still happening today.”

They point to Rwanda as a successful example of aligning conservation with economic goals. In that country, tourism revenues from protected forests are funneled back into local communities, creating economic incentives to preserve rather than clear the land.

This research started as a class project in Allen’s graduate-level climate modeling course at UC Riverside and later expanded into a full collaborative study. It used Earth system models and land-use data to assess the real-world potential of large-scale forest restoration.

The takeaway is one of cautious optimism: restoring forests can play a meaningful role in climate mitigation—but it must complement, not replace, deep reductions in fossil fuel emissions.

“Climate change is a real and pressing issue,” said co-author Antony Thomas. “And every effort toward restoration, no matter how small, contributes to the solution.”

Read the original article on: Phys Org

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