Forests affect climate in three different ways: they take up CO2 from the atmosphere and cool the planet; they evaporate water to the atmosphere and increase cloudiness, which also cools the planet; and they are dark and absorb a lot of sunlight, warming the Earth.
The carbon offsetting programs that promote planting trees are taking only the first effect into account.
When the changes to the surface properties are also taken into account, it is clear only tropical rainforests are strongly beneficial to slow down global warming. In the tropics, in addition to absorbing carbon dioxide, trees promote clouds which help to cool the planet.
In other locations, specifically in the seasonally snow-covered high latitude area, the warming from the darkening of the surface either cancels or exceeds the net cooling from the other two effects.
These new results have initiated a lively scientific discussion on the effectiveness of terrestrial carbon sequestration.
Clearly, more studies will be needed to confirm these results and narrow the uncertainties that are inherent in any single modelling study. Climate policy on terrestrial sequestration should follow later.
Based on the results from this new study, one may think that deforestation outside of the tropics could be an effective strategy to combat climate change.
In dealing with our environment, a broader view should be taken and narrower criteria should be avoided to prevent environmentally harmful consequences. Apart from their role in climate, forests provide natural habitat to plants and animals, preserve the biodiversity, produce economically valuable timber and firewood, and protect watersheds.
Dr Govindasamy Bala is an atmospheric scientist at the Lawrence Livermore National Laboratory in Livermore, California.