World would be 1°C warmer without cooling effect of tropical forests

Tropical forests prevent the climate warming by storing carbon in their trees and cooling air when water evaporates from their leaves

Environment 24 March 2022

By Chen Ly

Tropical forests cool the planet by over 1°C – a finding that highlights the importance of protecting and reforesting equatorial regions.

Forests can affect local and global climates in a variety of ways, notably by taking in carbon from the atmosphere and locking it in trees and soil.

“But forests are not just carbon sponges,” says Deborah Lawrence at the University of Virginia. “Their biophysical structure interacts with the atmosphere and cools the surface of the Earth in other ways as well.”

These mechanisms include reflecting some sunlight back into space and evapotranspiration – when moisture within the tree is released as water vapour through leaves. The latter involves taking in heat from the nearby environment, so it cools the air surrounding a forest.

Irregularities in the surface formed by forest canopies also play a role. And finally, trees can produce chemicals known as biogenic volatile compounds, which also affect the climate.

Lawrence and her colleagues decided to investigate the local and global-scale climate effects of forests taking into account all of the ways in which they contribute to cooling, including their importance for locking away carbon in biomass. To do this, the team collected and analysed published data on the biophysical and carbon dioxide-absorbing effects of forests around the world.

They found that between 50 degrees north and south of the equator, forests have a global cooling effect of at least 1°C when both biophysical effects and carbon locking is considered. A third of this cooling can be attributed to biophysical mechanisms alone.

Further north and south, however, the team found that significant deforestation would lead to net global cooling because the snow and ice that would be uncovered reflects more sunlight than forests.

“[Though] we don’t really think we’d ever get such a large-scale deforestation occurring in the boreal,” says Lawrence. “Those forests are also important for many other reasons, including limiting extreme weather.”

At a local level, the team found that forests can be crucial for minimising the effects of extreme heat, drought, storms and flooding, which are all projected to become more prevalent, says Lawrence.

“We certainly should be protecting forests, but we need to think of them as adaptation tools as well,” she says. “In a world without them, we would already be much hotter and much more miserable.”

Pep Canadell at the Global Carbon Project, an organisation that studies greenhouse gas emissions, says: “Overall, I found nothing surprising from the paper, which is not to say it is not very relevant and indeed has an important point, which is our current accounting methods focus only on greenhouse gases and leave aside biophysical changes which we know are important in changing the climate too.”

Journal reference: Frontiers in Forests and Global Change, DOI: 10.3389/ffgc.2022.756115

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