Forests may raise local temperature, professor says

Planting trees may not be a universal solution to global warming, Yale School of Forestry & Environmental Studies professor Xuhui Lee said before an audience of about 50 at a talk at Kroon Hall Wednesday.

While trees can lower temperature by removing carbon from the atmosphere, forests also absorb large amounts of heat radiation from the sun, Lee said. A forest, he added, may either increase or decrease local temperatures depending on the climate in which it grows.

“[Planting trees] may not always be a good way forward in fighting climate change,” he said.

Lee offered the Yatir Forest in Israel as an example of the several factors that determine whether a forest increases or decreases local temperature. While the forest removes a significant amount of carbon dioxide from the atmosphere, it absorbs significantly more of the sun’s radiation than the treeless areas surrounding it because of the dark color of the trees, he said. The heat radiation the forest absorbs — 50 watts per square meter — is far greater than the heat the forest removes by absorbing carbon dioxide.

Further complicating matters, heat removed by evaporation and other processes may offset the absorbed radiation. Ultimately, the Yatir Forest is an average of five degrees Celsius cooler than the adjacent shrub land, he said.

“This is the paradox,” he said.

Lee is trying to understand the impact of these factors through several research projects. He presented data from one of his studies comparing temperatures observed in neighboring grasslands and forests. Forests above the 45th parallel — the latitude of New York’s border with Canada — are notably warmer than their adjacent grasslands because grasslands covered in snow absorb far less radiation, he said. But below this latitude, he added, where less snow typically falls, there is no notable temperature difference.

Four audience members interviewed said they found the presentation interesting and engaging.

Jennifer Miller FES ’15, who is studying wildlife in Rajasthan, India, said she found the lecture relevant to her research.

“Climate scientists have predicted that grasslands [there] will be turning into forests,” she said. “I’m trying to predict how that will influence wildlife species in [this] area.”

Chen Quan FES ’12 said people need to discuss details like these before debating about climate change’s policy implications.

Comments

  • prion

    Where exactly is the paradox? This is exactly what you would expect from the basics of radiative emissivity and evaporative heat loss from basic physics and chemistry courses.