Planting trees in tropics may provide strongest returns in cooling planet: Study

Planting trees in the tropics, rather than in higher latitudes, could be more rewarding in terms of cooling the planet, according to a new study on regional effects of afforestation.

Researchers, including those from the University of California Riverside, US, said that planting trees provides an overall positive effect for the world's climate by pulling out warming carbon from the atmosphere.

However, local effects on temperature not related to carbon varied significantly by region. For instance, trees at higher latitudes could have a slight heating effect, while the opposite is true in the tropics, they said.

"Our study found there is more cooling from planting trees in warm, wet regions, where they grow year-round. Tropical trees not only pull carbon dioxide from the air, they also cool while releasing water vapour," first author James Gomez, a graduate student at the University of California Riverside, said.

Using global climate models, the team found that adding trees yield weak global mean cooling of -17.77°C, which translate to a significant cooling of about -17.72°C in the tropics.

Regions, such as those in central Africa, can experience a cooling of up to -17.33°C, the team said.

"It's not that planting elsewhere doesn't help. It does, but the tropics offer the strongest returns per tree," Gomez said.

In a previous research, the team had looked at chemical effects resulting from trees interacting with the atmosphere and found that planting trees could cool the surface of the planet more than expected.

This study, published in the journal 'npj Climate and Atmospheric Science', helps understand the physical effects of tree planting which include evapotranspiration — or tree sweating —similar to how humans sweat to cool their body, the authors said.

Roots of a tree pull out water from soil, which then travels up through the trunk and into the leaves. Pores of the leaves open up so the tree can take in carbon dioxide for photosynthesis, and that is when part of the water pulled up from the soil evaporates into the air, they explained.

"It's just like the way sweating cools your body. In the tropics, there is constantly water available for trees, and that increases transpiration," Gomez said.

The researchers said trees can also lower the amount of sunlight reaching the planet's surface.

Trees give off water vapour and make the air more humid. More humidity can mean more clouds, and water vapour itself can soak up some sun, thereby reducing sunlight reaching the ground and providing a cooling effect, they said.

The researchers also found that, in some cases, trees can have a fire suppression effect.

"In tropical savannahs, and in other places around the world, trees are much more fire resistant than grasses," Gomez said.

"We find cooling is primarily from increased evapotranspiration and decreased downwelling solar radiation related to clouds and aerosols," the authors wrote.