At this week's climate conference in Montreal there have been a number of proposals to plant trees for the purpose of absorbing carbon emissions and helping mitigate climate change. However, a new study from the Carnegie Institution's Department of Global Ecology and Lawrence Livermore National Laboratory says that careful consideration should be given as to where these forests are planted. Planting trees in temperate regions could actually contribute to global warming.
The study, using complex climate modeling software to simulate changes in forest cover and then measuring the impact on global climate, found that northern forests tend to warm the Earth because they absorb a lot of sunlight without losing much moisture. The situation is different in the tropics where higher temperatures result in higher rates of evapotranspiration, the process by which forests release water into the atmosphere. Tropical forests may have a net cooling impact relative to northern forests.
The research has important implications for the greenhouse gas debate. The United States wants any future agreement on climate to include provisions for tradable carbon credits whereby industrial countries could exceed emissions limits by planting forests and exchanging carbon allotments with forested countries. These new findings suggest that reforestation programs should focus on planting trees in the tropics and not in temperate or boreal regions.
Papua New Guinea has made just such a proposal at the UN conference. The plan calls for wealthy countries to pay developing countries to preserve their rainforests arguing that these ecosystems store massive amounts of carbon dioxide as well as providing important ecological services and serving as a repository for most of the planet's biodiversity.