Reforestation in Eastern United States has Cooled Our Climate

Indiana University researchers find measurable differences in our climate due to reforestation

Article on phys.org: “A century of reforestation helped keep the eastern US cool, study finds”

https://phys.org/news/2024-02-century-reforestation-eastern-cool.html

Great article. I’ve excerpted key portions below. 

But …

They don’t distinguish between the effects of shade and evapotranspiration.

Evapotranspiration refers to the way in which plants soak up water from the ground and push it out through their leaves, causing it to evaporate and causing a cooling effect. When you experience the cool temperatures under a shade tree or in a forest, it’s not just the shade, but the evaporating water.

You would expect the authors to acknowledge the effects of water cycles, even if it’s hard to distinguish or tease out or quantify the effects of 1) shade, 2) evaporative cooling and 3) heat removal in the cloud layer as water condenses and radiates heat outward into space. 

Instead, they undercut the value of forests in northern climes by referencing the fact that evergreen boreal forests may have a warming effect, due to the albedo effect. The albedo effect refers to the tendency of dark objects (such as evergreen trees) to absorb heat, whereas light objects (such as snow) reflect heat back out into space. If you cut down evergreen trees, the sunlight hits the snow, reflecting more sunlight.

Such analyses disregard the effect of water cycles (e.g., evaporative cooling) on climate. This favors the timber industry by suggesting that evergreen trees contribute to global warming due to their dark color, as opposed to the white snow which would reflect sunlight if you cut down the trees. 

But the Indiana University study is on the right track by measuring the historical tendency of forests to cool the eastern US. 

****

Key portions of article:

The researchers found that forests in the eastern U.S. today cool the land's surface by 1 to 2 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 2 to 5 degrees Celsius (3.6 to 9 degrees Fahrenheit)—providing relief when it's needed most.

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 1 degree Celsius (1.8 degrees Fahrenheit) cooler than locations that did not undergo reforestation. Spots up to 300 meters (984 feet) away were also cooled, suggesting the cooling effect of reforestation could have extended even to unforested parts of the landscape.

 'Why hasn't the eastern United States warmed at a rate commensurate with the rest of the world?'" Barnes said. "We can't explain all of the cooling, but we propose that reforestation is an important part of the equation."