Climate policy and negotiations have historically focused on the role of forests in sequestering carbon dioxide and mitigating global warming. A new study expanded this focus to review three biophysical mechanisms by which forests influence climate at different latitudes. Through evapotranspiration, canopy roughness, and albedo, forests influence climate and promote stability by reducing extreme temperatures and flooding in all seasons and at all latitudes, researchers found.
For Emilia Pramova, a researcher at the forest management company OpenForests, studies like this one can help policymakers abandon “carbon tunnel vision” and recognize the role of forests in promoting biodiversity, resilience, and social well-being, as well as other adaptations to climate change.
Cooling Tropical Forests
Although all latitudes benefit from forest cover, “the message comes out very clearly: The biggest benefits are in the tropics,” said Louis Verchot, one of the authors of the paper and principal scientist at the International Center for Tropical Agriculture in Colombia.
Tropical forests have an enormous amount of biomass and “are photosynthesizing every day of the year…pulling in sunlight and exchanging with water back to the atmosphere,” said Michael Coe, coauthor of the study and director of the tropics program at Woodwell Climate Research Center.
That mechanism, he explained, “is essentially an air-conditioning system.” For example, where Coe works in Brazil, deforested areas are “5°C warmer on average during the year” than forested areas.
In addition to photosynthesis, the team found that evapotranspiration in tropical forests increases cloud coverage and promotes rainfall by releasing biogenic volatile organic compounds, which are “extremely reactive in the atmosphere. They create cloud condensation nuclei [and] change the droplet distribution within clouds, which makes them brighter and maintain the hydrological cycle,” said Verchot.
Most of the air masses passing through the Amazon rain forest, for example, come from the Atlantic Ocean. By the time they get to the western Amazon and the Andes, “60% to 70% [of their mass] has fallen someplace [as rain], been pumped back to the atmosphere by trees, and has fallen out again,” explained Verchot.
A forest canopy’s irregular surface (canopy roughness) also mitigates the effects of warmer temperatures. Canopy roughness leads to increased air turbulence, which redistributes heat from the forest floor to the atmosphere. According to Coe, when areas lose canopy roughness (as they do when they are deforested), the result has “a frying pan effect. It puts a lid on the atmosphere that keeps the heating local.”
Forests typically have low albedo and higher temperatures, as thick vegetation absorbs more energy than grasslands or bare soil. The warming effect of low albedo is overcome in tropical forests, however, by evapotranspiration and canopy roughness, resulting in year-round cooling.
Economic and Social Resilience
The combined effects of carbon sequestration and biophysical controls are “necessary to guide policy decisions that support global climate mitigation, local adaptation and biodiversity conservation,” the authors write. Verchot said policymakers in tropical countries in particular “need to look at these opportunities [for forest conservation] and incorporate them into their development model.”
For Mateo Estrada, an Indigenous Siriano leader and environmental coordinator for the Organization of Indigenous Peoples of the Colombian Amazon, the new research helps create “spaces for dialogue where companies, governments, and urban dwellers can recognize the importance of the forests for their economic success and survival.” For example, Bogotá, a city with more than 7 million people, might lose around 60% of its annual rainfall if deforestation continues in the Amazon.
Indigenous communities “are protecting very large chunks of forests and have a very important climate role,” said Coe. Estrada believes that the best way to protect tropical forests is “to solve the necessities of people that live there…we need new economies based on our Traditional Knowledge,” citing the need to protect Indigenous traditions, protect the genetic biodiversity of the region through patents, and protect support for local tourism and conservation efforts.
Around the world, forests provide local climate stabilization while also sequestering carbon dioxide. Protecting, expanding, and improving the management of these ecosystems—especially tropical forests—is one of the best strategies for mitigating and adapting to global warming. “These forests are one of our greatest assets; [they are] one of the best ways we can stabilize climate and help to save lives by just doing nothing. Just keep them in place,” concluded Coe.
—Santiago Flórez (@rflorezsantiago), Science Writer