4 March 2024

Reforestation curbed climate change in the eastern U.S.

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Reforestation that began in the 1930s was able to mitigate the effect of climate, explains an Indiana University study. Trees cooled the eastern U.S. while the rest of the country became warmer

by Matteo Cavallito

 

The spread of trees driven by the significant reforestation experienced by the United States in the past century allegedly helped counteract rising temperatures. A phenomenon that highlights the potential expressed by forested areas in climate mitigation. This is supported in a study by Indiana University which has been published in the journal Earth’s Future.

The eastern U.S. has cooled by 0.3 degrees in 100 years

Between the late 18th and early 20th centuries, a statement released by the researchers points out, the United States experienced extensive deforestation related to logging and the expansion of agriculture and cattle grazing with losses of more than 90 percent of the canopy in some areas. Beginning in the 1930s, the trend was reversed with a gradual return of trees. Since then, regrowth has affected about 15 million hectares of forest.

During the same period, the scientists note, global warming caused temperatures to rise by an average of 0.7°C across North America. Between 1900 and 2000, however, the East Coast and Southeast cooled by about 0.3 degrees. Why? Previous studies have suggested that the cooling could be caused by various factors, including agricultural activity and increased precipitation. However, researchers say, these studies had not considered forests as a possible explanation for the anomalous and widespread cooling.

Trees were the crucial factor

Using a combination of data from satellites and 58 meteorological towers to compare forests with nearby grasslands and croplands, the authors examined the effects of changes in forest cover on land surface temperatures. “Eastern US forests cool the land surface by 1–2°C annually compared to nearby grasslands and croplands, with the strongest cooling effect during midday in the growing season, when cooling is 2–5°C,” the study states.

Younger forests, between 20 and 40 years old, “have the strongest cooling effect on surface temperature.”

Moreover, historical analysis of land cover and daily meteorological data from 398 weather stations from 1900 to 2010 “showed that the cooling benefits of reforestation extend across the landscape.” Specifically, “Locations surrounded by reforestation were up to 1°C 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 US.”

Forests are a resource for temperature mitigation

Other factors such as, for example, changes in agricultural irrigation also contributed to the lowering of temperatures, the researchers note. But reforestation ultimately played the most important role in driving the cooling anomaly. In addition to turning the spotlight back on the well-known mitigation potential of trees, the study thus highlights “reforestation’s potential as a local climate adaptation strategy in temperate regions.”

In these areas, they explain, the spread of trees “could provide a complementary set of benefits: mitigating climate change by removing carbon dioxide from the atmosphere, while also helping with adaptation to rising temperatures by cooling surface and air temperatures over large areas.”

However, they say, in different environments, such as snow-covered boreal regions, adding trees could have a warming effect while in some places reforestation could also affect precipitation and other processes leading to disadvantages. Therefore, land management policies must consider various environmental factors when assessing the usefulness of forests as a climate adaptation tool.