Effects of forests

The Kyoto Protocol allows emissions of greenhouse gases to be offset by the planting of new forests ('Kyoto forests'). The reasoning is that forests can offset emissions by capturing ('sequestering') atmospheric carbon dioxide (see top panel of graphic). However, trees not only absorb carbon dioxide, they have other effects on climate. In particular, because they reflect different amounts of sunshine compared to the underlying surface, they can alter the amount of sunlight that is absorbed. Dark green forests absorb more of the incoming solar radiation than arable cropland and will tend to warm the planet. The effect is greatest during the winter months when large unforested areas are covered in highly reflective snow, but when much of a forest canopy would remain uncovered. To compare the effect on climate of surface reflectivity changes with that due to the capacity of the trees to sequester carbon, the reflectivity effect can be expressed as equivalent amounts of carbon emission. A map of the equivalent emissions is shown in the middle panel. As expected, regions where the surface reflectivity effect is most important are at high northern latitudes in areas that have a winter covering of snow. In some northern forest locations (e.g. North America and eastern Asia), the changes in reflectivity reverse the beneficial effects on climate from the uptake of carbon dioxide from the atmosphere (bottom panel). Despite large uncertainties in the calculations, the results do clearly show that the beneficial effect on climate of afforestation may be offset by changes in the surface reflectivity as dark trees replace lighter land cover. Consequently, in many areas, the climate benefits of planting extra trees will not be as great as their carbon 'sink' potential suggests. [Text and graphics courtesy of Hadley Centre, UK]