Trees invading warming Arctic will cause warming over entire region, study shows

BERKELEY — Contrary to scientists' predictions that, as the Earth warms, the movement of trees into the Arctic will have only a local warming effect, University of California, Berkeley, scientists modeling this scenario have found that replacing tundra with trees will melt sea ice and greatly enhance warming over the entire Arctic region. Denali National Park is one area that will become more heavily forested as the result of global warming. A new study indicates that as trees move northward with increasing temperatures, they will enhance warming over the entire Arctic north above 60 degrees north latitude, accelerating the melting of sea ice. (Abigail Swann/UC Berkeley) But UC Berkeley graduate student Abigail L. Swann, along with Inez Fung, professor of earth and planetary science and of environmental science, policy and management, doubted this local scenario because, while broad-leaved trees are dark, they also transpire a lot of water, and water vapor is a greenhouse gas that is well-mixed throughout the Arctic. Taking account of this in a standard model of global warming, the researchers discovered that, while broad-leaved trees do absorb some additional sunlight, the water vapor they pump into the atmosphere causes a more widespread warming. "Broad-leaved deciduous trees are not as dark as evergreen trees and so are generally assumed to be less important. But broad-leaved trees transpire a lot more water through their leaves and are actually able to change the water vapor content and increase the greenhouse effect.