"A century of gradual reforestation across the American East and Southeast has kept the region cooler than it otherwise would have become, a new study shows.
The pioneering study of progress shows how the last 25 years of accelerated reforestation around the world might significantly pay off in the second half of the 21st century.
Using a variety of calculative methods and estimations based on satellite and temperature data from weather stations, the authors determined that forests in the eastern United States cool the land surface by 1.8 – 3.6°F annually compared to nearby grasslands and croplands, with the strongest effect seen in summer, when cooling amounts to 3.6 – 9°F.
The younger the forest, the more this cooling effect was detected, with forest trees between 20 and 40 years old offering the coolest temperatures underneath.
“The reforestation has been remarkable and we have shown this has translated into the surrounding air temperature,” Mallory Barnes, an environmental scientist at Indiana University who led the research, told The Guardian.
“Moving forward, we need to think about tree planting not just as a way to absorb carbon dioxide but also the cooling effects in adapting for climate change, to help cities be resilient against these very hot temperatures.”
The cooling of the land surface affected the air near ground level as well, with a stepwise reduction in heat linked to reductions in near-surface air temps.
“Analyses of historical land cover and air temperature trends showed that the cooling benefits of reforestation extend across the landscape,” the authors write. “Locations surrounded by reforestation were up to 1.8°F cooler than neighboring locations that did not undergo land cover change, and areas dominated by regrowing forests were associated with cooling temperature trends in much of the Eastern United States.”
By the 1930s, forest cover loss in the eastern states like the Carolinas and Mississippi had stopped, as the descendants of European settlers moved in greater and greater numbers into cities and marginal agricultural land was abandoned.
The Civilian Conservation Corps undertook large replanting efforts of forests that had been cleared, and this is believed to be what is causing the lower average temperatures observed in the study data.
However, the authors note that other causes, like more sophisticated crop irrigation and increases in airborne pollutants that block incoming sunlight, may have also contributed to the lowering of temperatures over time. They also note that tree planting might not always produce this effect, such as in the boreal zone where increases in trees are linked with increases in humidity that way raise average temperatures."
-via Good News Network, February 20, 2024
Reblogging to show the temperature maps that are featured in the original study (and the Guardian article about it), because the visual difference really is so striking and so encouraging.
As you look at these maps of forests vs. temperature trends, remember that the temperature map is showing large-scale, very long-term averages, especially on the temperature map. Because of that, the map data doesn't reflect how very, very big a difference it can make on a local scale, e.g. those 9°F summer temperature conditions. And those local scale changes are the changes that people actually live in.
This is hugely
Forest Age vs. Warming Maps
Pictured: Guardian graphic. Source: Barnes, et al, 2024, ‘A Century of Reforestation Reduced Anthropogenic Warming in the Eastern United States.’ Note: Forest age data from North American Carbon Program. Age estimates as of 2019 at 1km resolution. Temperature data from Delaware Air Temperature & Precipitation Dataset.
Source: The Guardian, February 17, 2024. And the original study is here, from the journal Earth's Future, first published February 13, 2024.
(Also, btw, for any non-US and/or non-geography people, don't worry about the fact that there aren't any forests in the middle of the country. That's the Great Plains. Like we definitely did turn most of it into cropland, but it's not supposed to have forests.)
This is huge.
Even the small pockets of new reforestation elsewhere in the country are usually correlated with small pockets of cooling. (And of course correlation by itself does not equal causation, but that's what the rest of the study is for.)
This is genuinely strong evidence that the massive tree planting campaigns of the last 25 years are going to pay off dramatically much sooner than we thought.
The study found that the coolest forests were ones planted planted 20 to 40 years ago.
That means that trees planted in the 90s through 2004 are in that stage and causing the most cooling right now.
It also means that the ongoing, absolutely massive recent reforestation efforts are going to pay off a lot between now and 2050.
That means campaigns like China's 2022 pledge to plant or conserve 70 billion trees by 2030. Or India's annual tree-planting drive, which in 2021 saw 250 million trees planted in just one day. Or Kenya's new tree-planting holiday, started in 2023, to plant 100 million trees each year.
This study also gives strong evidence that newer forests don't have vanishingly few benefits compared to old growth forests - they do have benefits (if not as many), just different ones. It also, I would argue, suggests that tree planting efforts don't have to be ecologically perfect to make a real difference. They certainly were not nailing native plant biodiversity and ecological best practices in the US in the 1930s!
And as we learn (and actually implement) more and more about how to do reforestation right - more biodiversity, native plants only, actual forests and not just tree plantations - the benefits of reforestation will only increase.
Just want to add that many of these forests in the southeast are working forests! They are planted, thinned, and cut for timber production, often loblolly pine. Working forests can be great for wildlife, and many timber companies work closely with researchers to improve management for wildlife without sacrificing yield! At the same time, more specialist species often cannot thrive in these ecosystems.
The southeastern US used to be predominantly longleaf savanna, and longleaf specialists like red-cockaded woodpeckers and gopher frogs cannot persist in working forests. That said, we need timber production, and doing so in the most ecologically sound way is clearly beneficial to both wildlife and the climate!
It's really cool to see good stuff coming out of the southeast. It is one of the most biodiverse regions in the US, and there are so many people here working hard to keep it that way.
