Reforestation in US East helped keep it cool

Reforestation: A young tree sprouting upward from the mossy floor of a forest.
Much of the U.S. warmed during the 20th century. But the eastern part of the country remained mysteriously cool. Now a new study suggests that a century of forest growth, due to widespread reforestation, likely helped keep the eastern U.S. cool as the rest of the country warmed. Image via Pexels/ Lauri Poldre.

AGU posted this story originally, earlier this year. Edits by EarthSky.

Widespread 20th-century reforestation in the eastern United States helped counter rising temperatures due to climate change, according to new research. The authors highlight the potential of forests as regional climate adaptation tools, which are needed along with a decrease in carbon emissions.

Mallory Barnes is the lead author of the study and an environmental scientist at Indiana University. She said:

It’s all about figuring out how much forests can cool down our environment and the extent of the effect. This knowledge is key not only for large-scale reforestation projections aimed at climate mitigation, but also for initiatives like urban tree planting.

The peer-reviewed AGU journal Earth’s Future published on February 13, 2024.

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Deforestation to reforestation

Before European colonization, the eastern United States was almost entirely covered in temperate forests. From the late 18th to early 20th centuries, timber harvests and clearing for agriculture led to forest losses. Those losses exceeded 90% in some areas. In the 1930s, there were efforts to revive the forests, coupled with the abandonment and subsequent reforestation of subpar agricultural fields. Those kicked off an almost century-long comeback for eastern forests. About 15 million hectares of forest have since grown in these areas.

Kim Novick is an environmental scientist at Indiana University and co-author of the new study. Novick said:

The extent of the deforestation that happened in the eastern United States is remarkable, and the consequences were grave. It was a dramatic land cover change, and not that long ago.

A warming hole

During the period of regrowth, global warming was well underway. Temperatures across North America rose 0.7 degrees Celsius (1.23 degrees Fahrenheit) on average. In contrast, from 1900 to 2000, the East Coast and Southeast cooled by about 0.3 degrees Celsius (0.5 degrees Fahrenheit), with the strongest cooling in the southeast.

Previous studies suggested the cooling could be caused by aerosols, agricultural activity or increased precipitation. But many of these factors only explained highly localized cooling. Despite known relationships between forests and cooling, studies had not considered forests as a possible explanation for the anomalous, widespread cooling. Barnes said:

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.’ That’s why we initially set out to do this work.

Trees are cool

Barnes, Novick and their team used a combination of data from satellites and 58 meteorological towers to compare forests to nearby grasslands and croplands. This allowed an examination of how changes in forest cover can influence ground surface temperatures and in the few meters of air right above the surface.

The researchers found that forests in the eastern U.S. today cool the land’s surface by one to two degrees Celsius (1.8 to 3.6 degrees Fahrenheit) annually. The strongest cooling effect occurs at midday in the summer, when trees lower temperatures by two to five degrees Celsius (3.6 to 9 degrees Fahrenheit). Trees provide relief when it’s needed most.

Using data from a network of gas-measuring towers, the team showed this cooling effect also extends to the air. They found forests lower the near-surface air temperature by up to one degree Celsius (1.8 degrees Fahrenheit) during midday. (Previous work on trees’ cooling effect has focused on land, not air, temperatures.)

Reforestation cooling extends into unforested areas

The team then used historic land cover and daily weather data from 398 weather stations to track the relationship between forest cover and land and near-surface air temperatures from 1900 to 2010. They found that by the end of the 20th century, weather stations surrounded by forests were up to one degree Celsius (1.8 degrees Fahrenheit) cooler than locations that did not undergo reforestation. Spots up to 300 meters (984 feet) away also cooled. That suggests the cooling effect of reforestation could have extended even to unforested parts of the landscape.

Other factors, such as changes in agricultural irrigation, may have also had a cooling effect on the study region. The reforestation of the eastern United States in the 20th century likely contributed to, but cannot fully explain, the cooling anomaly, the authors said.

Barnes said:

It’s exciting to be able to contribute additional information to the long-standing and perplexing question of, ‘Why hasn’t the eastern United States warmed at a rate commensurate with the rest of the world?’ We can’t explain all of the cooling, but we propose that reforestation is an important part of the story.

A strategy for climate change?

Reforestation in the eastern United States is generally regarded as a viable strategy for climate mitigation due to the capacity of these forests to sequester and store carbon. The authors note that their work suggests that eastern United States reforestation also represents an important tool for climate adaptation.

However, in different environments, such as snow-covered boreal regions, adding trees could have a warming effect. In some locations, reforestation can also affect precipitation, cloud cover and other regional scale processes in ways that may or may not be beneficial. Land managers must therefore consider other environmental factors when evaluating the utility of forests as a climate adaptation tool.

Bottom line: According to new research, reforestation in the eastern United States helped counter rising temperatures in the 20th century.

Source: A Century of Reforestation Reduced Anthropogenic Warming in the Eastern United States


Read more: Sea level rise creating ghost forests in U.S. East

Read more: Wildfires turn world’s largest forests into carbon emitters

April 17, 2024

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