Forest cover plays big role in global warming measurements

Impacts of  deforestation on global warming vary with latitude

A high country forest near Vail. Colorado.

By Summit Voice

SUMMIT COUNTY — New research on the relationship between forest cover and surface temperatures suggest that deforestation in northern latitudes may have a regional cooling effect, but closer to the equator, cleared areas likely result in a net warming.

In a nutshell, the impacts of  deforestation on global warming vary with latitude, which may require new climate-monitoring strategies, The study by a team of 20 researchers from around the world is being published in the Nov. 17 issue of the journal Nature.

That’s because temperature readings are generally taken in grassy fields with biophysical properties of cleared land, so those readings “do not accurately represent the state of climate for 30 percent of the terrestrial surface covered by forests,” according to the study.

“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.”

“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.”

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.

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.”

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One Response

  1. It’s not really latitude that matters, but rather how long the snow season lasts in any given location. In the lower elevation of Oregon and Washington snow does not occur often or last long, so there would be no albedo-boost from deforestation. In areas with less snow, we should be protecting our old growth forests and letting the young forests grow to store more carbon.

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