Most climate-concerned people know that trees can help slow global warming by removing carbon dioxide from the atmosphere, but a recent study published in the journal Science shows the climate cooling benefits of planting trees may be overestimated.

“Our study showed that there is a strong cooling from the trees. But that cooling might not be as strong as we would have thought,” Maria Val Martin, a researcher at the University of Sheffield in the U.K., said.

Darker forests can warm the Earth because they reduce the albedo of the land they cover, meaning they absorb more sunlight and reflect less solar radiation back into space. So more heat is held by the Earth’s surface.

In addition, trees play a more complex role in the Earth and its atmosphere than just sequestering carbon dioxide. They also release organic compounds, such as isoprene and monoterpenes.

These compounds can react with various oxidants, including the hydroxyl radical that breaks down methane, a greenhouse gas that is roughly 80 times more potent at warming the climate than carbon dioxide over a 20-year period. The reaction with the organic compounds released by forests leads to a reduction in hydroxyl concentrations, which decreases the destruction of methane and increases the concentrations of the greenhouse gas in the atmosphere.

Given methane’s potency in warming the climate, even a modest change can have a significant impact on atmospheric warming, said James Weber, a lecturer at the University of Reading in the U.K. and the lead study author. Consequently, the climate benefits from planting trees will be greater if methane is also being reduced in the atmosphere through other means.

“Reforestation has a part to play, but it will be more efficient if we do it while also cutting greenhouse gas emissions and anthropogenic pollution,” Weber said. 

The compounds that trees emit also react to nitrogen oxides, creating the greenhouse gas ozone, which can warm the atmosphere, but can also lead to the production of aerosol particles that reflect solar radiation back into space, creating a cooling effect.

“Really, [we’re] saying let’s do it, but let’s do it as part of a broader package of sustainable measures, not ‘planting trees is our only option,’” Weber said.

To understand the impact of forestation on climate, the team of researchers compared two scenario models. In one, tree planting was one of the few climate change mitigation strategies and emissions of organic compounds from plants led to an increase in the greenhouse gases ozone and methane in the atmosphere. 

With both the alterations in a forest’s ability to reflect sunlight back into space and the scattering of some light away from the Earth by organic aerosols accounted for, forests created a warming effect that counteracted about 31 percent of the cooling the trees caused by removing carbon dioxide from the atmosphere.

In the other scenario, which involves a more inclusive effort to mitigate climate change, including environmental management and lower energy-intensive consumption that prevented higher concentrations of greenhouse gasses from being emitted into the atmosphere, only 14 to 18 percent of carbon removal was offset after forest warming was considered. 

The models don’t account for other events that can have effects on forests, such as with wildfires and drought, Weber said. 

The focus should be on not only restoring woods and planting trees, but also preserving current forests, said Sassan Saatchi, senior scientist of the Jet Propulsion Laboratory at the California Institute of Technology and an adjunct professor at the University of California.

Saatchi says that tree planting makes more sense for some areas than others, and in some places, like California, preserving forests also means removing trees to help prevent forest fires and to help forests better survive in the long run. 

“That’s the key thing that we are looking for,” Saatchi said. “How do we really make the mitigation plans that we have long term, because we don’t want to just do something that in 10 years, we destroy again.”