Did Reforestation Forestall Climate Change In Eastern U.S ?

I boat sets sail on Lake Champlain on September 4, 2023, when 
it was 90 degrees. The trees in the foreground probably
made it feel a bit cooler.

They say trees are great for the environment.

One large scale example of the benefit from trees might have come over the past five decades or so in the eastern United States.  

First, the mystery, as reported in The Guardian. 

"While the U.S., like the rest of the world, has heated up since industrial times due to the burning of fossil fuels, scientists have long been puzzled by a so-called 'warming hole; over parts of the U.S. Southeast, where temperatures have flatlined, or even cooled, despite the unmistakable broader warming trend."

Most of the East was deforested for farms and such with European settlement. The lack of forests continued well into the 19th century. But starting in the 1920s and accelerating after World War II,   forests re-grew.  

The Guardian continues:

"'The reforestation has been remarkable and we have show this has translated into the surrounding air temperature,' said Mallory Barnes, an environmental scientist at Indiana University who led the research. The 'warming hole' has been a real mystery and while this doesn't explain all of it, this research shows there is a really important link to the trees coming back."

Tree blunt heat because water that was drawn up by red roots are released into the air via leaves as vapor. That water vapor cools the surrounding air a little bit. 

The renewed forests cooled the eastern U.S. by around two or three degrees each year. The cooling effect is most noticeable in the summer, as the trees are estimated to have cooled the air by roughly four to eight degrees Fahrenheit. 

The Guardian notes that trees probably weren't the only things keeping the East a little on the cool side. Pollution before the Clean Air Act purified the atmosphere in the late 20th century probably blocked sun, keeping temperatures from rising too much. 

It's possible agricultural irrigation might have had a small cooling effect, too. 

 POSSIBLE VERMONT EFFECTS?

In the age of climate change, Vermont has warmed more than places further south along the Appalachian chain and southeastern United States.

Still,  we might have partly gone through that cooling phase due to reforestation, but just a little earlier than the Southeast. 

After clearing the state for lumber and farms, only around 20 percent of Vermont remained forested by the late 1880s. Most of that was at high, rather inaccessible elevations.

During the 1990s, the trend reversed, but Vermont didn't become fully reforested until the 1960s or so. Since then, around 75 to 80 percent of Vermont was forested.

Remember how I said especially in the Southeast, the cooling effect of forests was most noticeable in the summer?

In Burlington climate records, I noticed that there was a relative peak during the 1930s and 1940s in the number of days in the summer that reached 90 degrees or more.  As more of the state became forested, the number of 90 degree days, as measured in Burlington, declined through the 1950, 1960s and 1970s.

At Burlington, there were six consecutive years from 1944-49 that each had eight or more days of 90 degree temperatures. Such a long streak of hot summer weather didn't happen again until 2015 through 2023, with each of those summers having eight or more 90 degree days. Climate change was very likely one major contributor to these very recent hot spells. 

The hot weather of the 1940s probably wasn't entirely due to a lack of forest. Drought can allow the air to heat more efficiently than wet ground can.  As noted earlier in this post, air pollution in the 1950s into the 1970s might have dimmed the sun enough to keep 90 degree weather at bay. There might have been just a persistent weather pattern in the 1940s that created hot days.

But there's no denying trees are probably helping us cool off at least a little on hot summer days.  In Vermont, and pretty much everywhere else, trees can help mitigate the effects of a warming planet, especially in the summer. 

One of the reasons the city of Burlington, and so many other cities across the United States plant trees is for that cooling effect in the summer.  

It's probably weird that we're talking about heat waves on a February day that brought morning temperatures to near zero.  But it will be summer soon enough. If you want to stay cool, thank a tree near you.