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Sky Valley, Georgia

Photo: Jeff Greenberg / Jeffrey Greenberg / Universal Images Group / Getty Images

Before colonization, the eastern United States was almost entirely covered in forests. Between the late 18th and early 20th centuries, over 90 percent of it was lost in some areas. Since the 1930s, there has been a lot of reforestation, and around 15 million hectares of forest have been created since then. A research team now reports that these trees played a crucial role in keeping the eastern United States cool.

In the phase since reforestation, average temperatures in North America rose by a total of 0.7 degrees Celsius - while the east coast and the southeast cooled by around 0.3 degrees Celsius between 1900 and 2000.

Although it is known what cooling effect forests have, the connection between this “warming hole” and reforestation in the region has not yet been investigated, the researchers explain. Their study has now been published in the American Geophysical Union's journal “Earth's Future”.

Knowledge also helps with tree planting in the city

The team led by environmental scientist Mallory Barnes from Indiana University used historical land cover data and daily weather data from 398 weather stations. In doing so, they established a connection between forest cover and near-surface temperature for the period between 1900 and 2010. Result: At the end of the 20th century, weather stations in forests showed temperatures on average up to 1 degree Celsius cooler than in areas that had not been reforested.

Other factors may also have played a role, such as different irrigation in agriculture. "We can't explain all of the cooling, but we believe that reforestation is an important part of the story," Barnes said in a statement.

The researchers also compared soil surface and air temperatures between forests and nearby croplands and grasslands. To do this, they used data from satellites and weather towers. According to this, forests in the eastern United States could cool the land surface by 1 to 2 degrees Celsius on average annually. The effect is strongest at midday in summer: a cooling effect of 2 to 5 degrees Celsius is then possible.

“This knowledge is important not only for large-scale reforestation projects to protect the climate, but also for urban tree planting,” says Barnes.

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