In America’s southeast, except for most of Florida and Virginia, “temperatures have flatlined, or even cooled,” due to reforestation, even as most of the world has grown warmer, reports The Guardian.
A study finds that that the relatively cool temperatures are due to “the vast reforestation of much of the eastern US following the initial loss of large numbers of trees in the wake of European settlement in America. Such large expanses have been reforested in the past century – with enough trees sprouting back to cover an area larger than England – that it has helped stall the affect of global heating.”
“The reforestation has been remarkable and we have shown this has translated into the surrounding air temperature,” said Mallory Barnes, an Indiana University professor who ran the research project. “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.”
The Guardian notes that
There was a surge in deforestation from the start of the US’s early colonial history, as woodland was razed for agriculture and housing, but this began to reverse from around the 1920s as more people began to move into cities, leaving marginal land to become populated again with trees. The US government, meanwhile, embarked upon an aggressive tree-planting program, with these factors leading to about 15m hectares of reforested area in the past century in the eastern US.
The expanded forests cool the southeastern U.S. “mainly through the trees’ transpiration, in which water is drawn up through the roots to the leaves and then released into the air as vapor, slightly cooling the surrounding area…By poring over data from satellites and weather stations located across the eastern US from 1900 to 2000, Barnes and her colleagues found reforested areas have provided this cooling impact on a grand scale, with most of this effect occurring within 400 meters of the trees.”
The reforested areas cool the eastern U.S. by about 2 to 4 degrees Fahrenheit. The cooling effect is strongest in the peak of summer, when trees lower temperatures by 4 to 9 degrees.
The world’s forests are recovering elsewhere, too, helping slow the Earth’s warming — at least in temperate and polar areas. This isn’t the first time this happened. In the 16th and 17th Century, forests expanded in the Americas, as European diseases wiped out much of the Native American population, resulting in their land reverting to forest. 215,000 square miles of new woodlands grew, enough to consume 27 billion tons of carbon dioxide.
But in this century, the expansion of forest is due to happier causes, such as more efficient agriculture, and the replacement of horses with automobiles. The automobile restored New England’s forests, which had mostly disappeared by 1910, but now cover much of the region. Today, Vermont is 78% forested, but in 1910, it was mostly un-forested.
England has slightly more forest now than it did during the Black Death around 1350, even though England today has a dozen times as many people as it did back then. Scotland has many times more forest than it did in 1350. The United Kingdom as a whole has three times as much forest as it did at the start of the 20th century.
China’s forests have grown by about 234,000 square miles over the last 30 years, an area the size of Ukraine. The European Union has added an area the size of Cambodia to its woodlands. Many tropical forests are still shrinking, but temperate forests are expanding slightly faster than tropical forests are shrinking, meaning that trees are still removing at least as much carbon dioxide from the atmosphere as they used to.
Even in some tropical countries, forests are expanding. Costa Rica has 150% more forest than it did in 1987, and about as much forest as it did in 1961 — although still less than it had in the 1940s and 1950s.
