Focus on Arts and Ecology

It's a well-established fact that forests are some of the world's most important assets in the fight against climate change. They store vast amounts of carbon, while degrading or destroying them can release that carbon back into the atmosphere.

But research suggests global forests protect the climate in another way, as well — they can actually help cool down their environment.

"This is also important and comes into play when it comes to considering replanting forests and so on," said Quentin Lejeune of the Berlin-based climate science nonprofit Climate Analytics. "It's not only the carbon, but it can also influence the local climate."

A new study, led by Lejeune and just published in the journal Nature Climate Change, suggests that deforestation in the Northern Hemisphere has upset that cooling effect and helped make hot days even more intense. That's on top of the influence of ongoing human-caused climate change.

A variety of physical processes may help trees cool down the environment around them. The most important of these is likely a process called evapotranspiration — that's when water evaporates out of a plant's leaves and into the atmosphere. This process can help cool the local climate. But other processes may also come into play, Lejeune noted. For instance, trees help break up an otherwise smooth landscape. As a result, "it creates more turbulent edges when the wind is blowing over it" and helps to disperse heat, he said.

The new research, conducted with a suite of climate models, notes that the hottest days of the year have been gradually growing hotter since the Industrial Revolution. It concludes that deforestation actually accounted for more than half the warming that occurred over North America between 1920 and 1980, and about a third in Eurasia. Since the early 1980s, that contribution has fallen to about 30 percent, as the influence of climate change has continued to grow.

Overall, the researchers suggest that deforestation has contributed about a third of the local warming observed in the Northern Hemisphere since the 1860s. The study suggests that maximum annual temperatures may increase by about a tenth of a degree Celsius, on average, for any 10 percent decrease in forest cover across a landscape.

These findings, produced by the models, are still anywhere from three to six times smaller than the temperature effects actually observed in some locations, notes climate scientist Paul Stoy of Montana State University in a commentary on the new research, also published today in Nature Climate Change. Still, he adds, "it is a step in the right direction" for the models, and the study as a whole suggests that "forests can help mitigate extreme heat," he writes.

The study conflicts with some previous modeling research, which actually suggested that deforestation in much of the Northern Hemisphere has, itself, helped cool the climate. The models used in these studies primarily emphasized the way trees affect the absorption of sunlight, the authors say. Since most trees tend to be dark in color, removing forests can help literally brighten up the landscape — an effect that could potentially reflect more sunlight away from Earth, in the same way that snow cover reflects solar radiation.

But some of the models may not have adequately accounted for the effects of evapotranspiration and other physical processes by which trees can actually cool their local climate, as well, the authors write. The two processes — the cooling effect of the trees weighed against the warm sunlight they absorb — work against one another, and it hasn't always been clear which effect is the dominant one throughout the Northern Hemisphere, Lejeune said.

To investigate, he and his co-authors compared simulations from an ensemble of climate models to actual on-site temperature data taken from locations across the Northern Hemisphere. Then they conducted the rest of their study using only models that were able to accurately simulate the observed temperature changes.

The findings, relying on both the models and direct data comparisons, present a "significant advancement" in scientists' understanding of how forests fit in with global climate mitigation efforts, said Thomas O'Halloran, a forest and climate expert at Clemson University, in an email to E&E News. O'Halloran was not involved with the new study but has co-authored similar research, including a 2017 study that demonstrated the cooling influence of forest cover in the United States and Europe.

The results indicate that trees may play an even bigger role in global climate mitigation efforts than previously thought. Their importance as carbon sinks is well-documented already — now, the new research suggests that they may be important regulators of their local climates through other physical processes, as well.

"We need to understand how terrestrial ecosystems affect climate to ensure that climate mitigation activities that manipulate the land surface ... will achieve their intended goals," O'Halloran said. "The ultimate finding of this complex model-data analysis is almost intuitive — that forests help cool the surface on the hottest days — thus providing a valuable service to society."