UC DAVIS (US) — The impact of deforestation on global warming varies with latitude, a finding that calls for revised climate-monitoring strategies, a new study shows.
“It depends where the deforestation is,” says KyawTha Paw U, professor of atmospheric sciences at University of California, Davis. “It could have some cooling effects at the regional scale, at higher latitudes, but there’s no indication deforestation is cooling lower latitudes, and in fact may actually cause warming.”
“Because surface station observations are made in grassy fields with biophysical properties of cleared land, they do not accurately represent the state of climate for 30 percent of the terrestrial surface covered by forests,” according to the study, published in the journal Nature.
Deforestation in the boreal region, north of 45 degrees latitude, results in a net cooling effect. While cutting down trees releases carbon into the atmosphere, it also increases an area’s reflection of sunlight (its albedo).
Surface temperatures in open, nonforested, high-latitude areas were cooler because these surfaces reflected the sun’s rays, while nearby forested areas absorbed the sun’s heat. At night, without the albedo effect, open land continued to cool faster than forests, which force warm turbulent air from aloft to the ground.
“People are debating whether afforestation is a good idea in high latitudes,” says Xuhui Lee, the study’s principal investigator and professor of meteorology at the School of Forestry & Environmental Studies at Yale University.
“If you plant trees you sequester carbon, which is a benefit to the climate system. At the same time, if you plant trees you warm the landscape because trees are darker compared to other vegetation types. So they absorb solar radiation.”
The findings should not be viewed as a “green light” to cut down forests in high latitudes, Paw U says. “The intent is to clarify where we can see these regional effects using actual temperature measurements. Besides absorbing carbon dioxide, forest ecosystems have a number of other valuable qualities, even if at certain latitudes they may be warmer than open areas.”
Researchers calculated that north of Minnesota, or above 45 degrees latitude, deforestation was associated with an average temperature decrease of 1.5 degrees Fahrenheit.
On the other hand, deforestation south of North Carolina, or below 35 degrees latitude, appeared to cause warming. Statistically insignificant cooling occurred between these two latitudes.
Temperatures data were collected from a network of specialized weather stations in forests ranging from Florida to Manitoba and compared results with nearby stations situated in open grassy areas that were used as a proxy for deforested land.
“The cooling effect is linear with latitude, so the farther north you go, the cooler you get with deforestation,” Lee says.
“Another way to look at the results,” says study co-author David Hollinger, a scientist with the USDA Forest Service, “is that the climate cooling benefits of planting forests is compounded as you move toward the tropics.”
The study was supported, in part, by grants from the U.S. Department of Energy and the Yale Climate and Energy Institute.
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