The impact of deforestation on global warming varies with latitude, according to new research from a team of scientists representing 20 institutions from around the world. The surprising finding, which researchers say calls for new climate-monitoring strategies, will be published in the Nov. 17 issue of the journal Nature.
"It depends where the deforestation is," said UC Davis atmospheric science Professor Kyaw Tha Paw U, a study co-author. "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," the study says.
Paw U and his colleagues found that 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 albedo, or reflection of sunlight. Surface temperatures in open, non-forested, 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," said Xuhui Lee, the study's principal investigator and professor of meteorology at the Yale School of Forestry & Environmental Studies. "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."
Paw U emphasized that the findings should not be viewed as a "green light" to cut down forests in high latitudes. "The intent is to clarify where we can see these regional effects using actual temperature measurements," he said. "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."
The 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.
The researchers collected temperature data 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," said Lee.
David Hollinger, a scientist with the USDA Forest Service and study co-author, said, "Another way to look at the results is that the climate cooling benefits of planting forests is compounded as you move toward the tropics."
The study, "Observed Increase in Local Cooling Effect of Deforestation at Higher Latitudes," was supported, in part, by grants from the U.S. Department of Energy and the Yale Climate and Energy Institute.