The former forests of the eastern United States have rebounded over the last century. In the process, they’ve kept temperatures stable, or even marginally declining, for tens of millions of people while the world as a whole heats up. Climate discussion of reforestation usually relates to how much carbon it can draw from the atmosphere. This finding suggests the regional effects should not be neglected while considering the global consequences.
Americans are much more likely to deny evidence for climate change, particularly those in the south-east, than counterparts elsewhere. Although this undoubtedly reflects social and historical factors, direct experience may also play a part.
While almost the entire world has been getting hotter, exposing most of the global population to increasing heat waves, things have been different in the eastern US. The reason, new research suggests, is the recovery of forests devastated in the 18th and 19th centuries.
This information could help calculate how much reforestation other regions would need to protect themselves from global trends.
“It’s all about figuring out how much forests can cool down our environment and the extent of the effect,” said Dr Mallory Barnes of Indiana University in a statement. “This knowledge is key not only for large-scale reforestation projections aimed at climate mitigation, but also plans for initiatives like urban tree planting.”
Around 300 years ago, what is now the eastern US was almost entirely forested. Timber cutting and clearing for agriculture removed most of it, but since the 1930s, 15 million hectares (37 million acres) have been actively restored or recovered through neglect.
Forests cool the air around them by transpiring water, just as we cool ourselves when we sweat, and create clouds at the same time. Other factors, such as the darkness of their leaves and surface roughness, can also have an influence, but away from the poles, transpiration tends to dominate.
During the period of maximum felling, the eastern US probably warmed up, but we don’t have good records for most of that time. As the forests returned, they brought with them a regional cooling effect. While the whole of North America warmed by 0.7°C (1.2°F) between 1900 and 2010, the designated East Coast and Southeast regions cooled by 0.3°C (0.5°F).
Barnes and co-authors are far from the first to notice this contradiction to the global trend; professional deniers love to point it out at every opportunity. However, its cause has been debated, with aerosols released as pollution, increased rainfall, and changes to agricultural activity proposed for blame.
“This widespread history of reforestation, a huge shift in land cover, hasn’t been widely studied for how it could’ve contributed to the anomalous lack of warming in the eastern U.S., which climate scientists call a ‘warming hole,'” Barnes said. “That’s why we initially set out to do this work.”
It’s not news that trees have a cooling effect – you can feel it on entering a leafy neighborhood – but the size of the effect needed measuring. Barnes and colleagues used data taken from both satellites and thermometers in towers to compare forests with nearby areas further off the ground than in previous studies. They discovered that even areas a substantial distance from the forest benefited from the cooling effects.
The team concluded that today, the eastern US’s forests provide 1°-2°C degrees cooling (1.8°-3.6°F) over the year, and much more in summer. Since only a small fraction of that would have been available from the scrawny pre-1930s forests, this means that without the regrowth, warming would have been close to the rest of the planet. They acknowledged, however, that other factors also contributed, with Barnes noting, “We can’t explain all of the cooling, but we propose that reforestation is an important part of the equation.”
Planting forests is widely hailed as the quickest and easiest way to slow global heating by drawing carbon dioxide from the atmosphere. Nevertheless, it faces criticism since fires can reverse that effect, and in some locations, forests store less carbon than the grasslands they replace.
The authors note similar caveats apply to the use of forests in different environments as well, pointing out that at high latitudes, trees could be warmer than snow-covered tundra. Young forests (20-40 years old) also have a greater cooling effect than old ones, so not all the benefits are permanent. “Nature-based climate solutions…will only be effective if they are accompanied by economy-wide decarbonization,” they write.
Nevertheless, if the study can be replicated, it would suggest that in the right places, reforestation – or leaving forests standing in the first place – could make a big difference.
The study is published in Earth’s Future.
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