- Trees are largely heralded as a natural climate solution, but in some places, planting trees darkens the Earth’s surface and, in turn, brings unintended warming.
- Using data from satellites and models of the atmosphere, researchers examined the effects of tree planting in wet, mid-latitude regions such as the eastern United States and southeast China.
- In these regions, forests create favorable conditions for the formation of clouds, which then reflect the sun’s radiation during the day, cooling the atmosphere; when clouds were included in their models, the researchers found that forests in wet, mid-latitude areas produced a cooling effect.
- However, the lead researcher says, this cloud cooling effect is just one of many factors to consider when deciding if tree planting will be an effective and enduring climate solution in a given location.
Planting trees seems to be a simple solution to the climate crisis. As trees photosynthesize, they capture carbon dioxide and lower the concentration of greenhouse gases in the atmosphere, cooling the planet. But in some places, planting trees darkens the Earth’s surface and could bring unintended warming.
“When you wear a black T-shirt rather than a white T-shirt … that actually has a warming effect, right?” Sara Cerasoli, a Ph.D. candidate at Princeton University, told Mongabay.
When viewed from above, forests are darker than many other landscapes such as grasslands, savannas, and areas covered in snow. In these places, planting trees can decrease what’s known as albedo — the amount of sunlight or radiation reflected from the Earth’s surface. With less sunlight reflected away and more absorbed, the surface and the atmosphere above it warm up.
In wet tropical regions, reflecting the sun is not an issue, Cerasoli said, because the forests sequester a lot of carbon they offset (or make up for) the albedo effect. But the opposite is true at high latitudes such as in the boreal regions. There, the reflection of the sun’s radiation from brighter surfaces has a significant cooling effect, and adding trees could increase climate change rather than mitigate it.
But what effect do forests have on the places in between, in the mid-latitude regions such as the temperate forests of the eastern United States or southeast China?
To answer this question, Cerasoli and colleagues used data from satellites and models of the atmosphere to examine the effects of tree planting in wet, mid-latitude regions. Their findings are published in the Proceedings of the National Academy of Sciences.
“What makes our study new,” Cerasoli said, “is the focus on clouds.”
Trees release water vapor into the lower part of the atmosphere as they photosynthesize, creating favorable conditions for clouds to form. Clouds, like snow or other light surfaces, reflect the sun’s radiation during the day, which results in cooling.
“We had a combination of satellite data and modeling that enabled us to prove that forests are associated with a higher cloud coverage,” Cerasoli said.
When the reflection from clouds was included in their models, the researchers found that forests in wet, mid-latitude areas produced a cooling effect. So planting trees in these regions is beneficial thanks to the combined action of carbon sequestration and extra cooling from cloud reflectance — a one-two punch in the fight against climate change.
However, taking the step from the study to incorporating it into policy requires further investigation, Cerasoli says. There is a lot to consider before planting trees.
Planting native trees, supporting local biodiversity, involving local communities, and considering the economy are all factors that experts say are important to ensure tree-planting projects succeed and endure.
“We can’t just plant trees to draw down carbon;” Jean-François Bastin, associate professor at the University of Liège in Belgium, said in an October 2020 TED-Ed talk, “we need to restore depleted ecosystems.”
Globally, more carbon is stored in soil than in all the Earth’s plants and the atmosphere combined. Afforestation (planting in places where forests aren’t historically found, such as in savannas) can release these carbon stores, resulting in a net loss of carbon from the ecosystem. So although afforestation is mentioned in the study as a promising climate solution for wet, mid-latitude climates, many experts advise caution when introducing trees to novel locations.
“Tree planting now dominates political and popular agendas and is often presented as an easy answer to the climate crisis, as well as a way for corporate companies to mitigate their carbon emissions,” Kate Hardwick, conservation partnership coordinator at Royal Botanical Gardens Kew, who was not involved in the new study, said in a statement. “But sadly, it isn’t as simple as that … When people plant the wrong trees in the wrong place, it can cause considerably more damage than benefits, failing to help people or nature.”
Cerasoli, S., Yin, J., & Porporato, A. (2021). Cloud cooling effects of afforestation and reforestation at midlatitudes. Proceedings of the National Academy of Sciences, 118(33). doi:10.1073/pnas.2026241118
Banner image of clouds above Shenandoah National Park in the Eastern United States. Photo by Mark Collins via Flickr (CC BY 2.0)
Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough
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