Forest plantations may not provide as many climate benefits as previously thought

Planting forests helps reduce further global warming by absorbing some of the carbon dioxide we emit into the atmosphere. But the global climate benefit could be about 15 to 30 percent smaller than previously estimated, due to other influences such as sunlight absorption by trees.

“We're not saying don't plant trees,” he says. james webber at the University of Sheffield, UK. It's just that the climate benefits aren't as big as we thought, he says.

The impact of trees will depend in part on what other actions are taken to address climate change. Weber and his colleagues showed that the more forests are planted, the greater the benefits. “It’s more positive and efficient to do other things at the same time,” he says.

It has long been known that plants have both warming and cooling effects. In particular, dark foliage can have a warming effect by absorbing light that would otherwise be reflected into the space. This effect is strongest when trees are replaced by snow or ice, but can occur in other situations as well.

Plants also release volatile organic compounds into the air. “Those are the chemicals that create the smell of the forest,” team members say james king also at the University of Sheffield.

These biogenic compounds can affect the climate in a variety of ways. One important example is that it can react with chemicals in the atmosphere that react with methane. “So the methane stays around longer, and methane is a powerful greenhouse gas,” Weber said.

Compounds emitted by plants can also react with nitrogen oxides to form ozone, another greenhouse gas.

These effects will lead to further warming. However, compounds of biological origin can also form aerosol particles that reflect sunlight and have a cooling effect.

To understand the overall climate impact of afforestation, the researchers incorporated these and other processes into a climate model in which all available land is forested. This means, for example, trees that are located in areas that are currently grasslands, but not in farmland or urban areas.

“To our knowledge, this is the first time this has been done on a global scale and based on a plausible reforestation scenario,” King said.

The researchers modeled two scenarios. For one, little is being done to tackle climate change other than planting trees. In this case, the warming avoided by CO2-absorbing forests is reduced by 23 to 31 percent, once other forest impacts are taken into account.

In the second, more optimistic scenario, strong action is taken to limit further warming. In this case, avoided warming was reduced by 14 to 18 percent.

One reason for the difference is that reducing fossil fuel emissions reduces aerosols from air pollution. In a polluted world, adding more aerosols from forests won't make much of a difference, but in a cleaner world, the cooling effect will be greater.

The team acknowledges that the model is still incomplete and does not include all feedback effects. For example, it includes the greenhouse effect of ozone, but not its effects on vegetation. High levels of ozone can kill trees, meaning less CO2 is removed from the atmosphere. The model also does not include the effects of wildfires.

“It's very complicated,” King says. “It's not really possible to consider all feedback in one study.”

“Importantly, this study shows that preventing deforestation is a much more efficient way to mitigate climate change compared to reforestation, and therefore should be prioritized. It’s a necessity,” he says. Stephanie Law Climate Scientist at WWF in Washington, DC.

Another feedback missing from the model is the cooling effect of water evaporating from leaves, which can be greater in tropical regions, Roe says. So the climate benefits of tree planting may be overestimated, she says, but the study doesn't yet tell the full story.

Additionally, tree planting has many other benefits for humans and wildlife, including reducing erosion, maintaining water supply and quality, providing food and jobs, and reducing extreme heat in the region. “Afforestation, especially reforestation in forest biomes with native species, is absolutely worth pursuing,” says Lo.

“We've always known that forests have a warming effect under certain conditions and a cooling effect under others. What this study shows is that forests have an overwhelming net The effect is a cooling effect.” thomas crouser in ETH Zurich Swiss.

“But most importantly, even if the effects of cooling were not as great, we still need to conserve our natural forests to support the planet's biodiversity and the billions of people who depend on it.” “There is,” he says.