Planting Forests at High Latitudes Can Have a Cooling Effect
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4 February 2010 Planting forests at high latitudes can have a cooling effect A new study based on high-resolution satellite observations of forests suggests the ‘albedo’, or reflectivity, of a forest needs to be carefully considered in afforestation schemes designed to mitigate climate change, i.e. carbon balance alone is not a good indicator of mitigation value. The authors argue that the Kyoto Protocol carbon accounting rules grossly overestimate the cooling caused by afforestation, because they do not consider albedo effects. In an effort to limit rising temperatures caused by carbon dioxide and other greenhouse gas (GHG) emissions, the Kyoto Protocol requests participating nations to account for the carbon sequestered, or stored, in afforestation as one of the many options to meet targets for emissions reduction in the period 2008-2012. Reafforestation converts non-forested land, such as cropland, grassland or degraded land, to forests. Trees absorb and store carbon from the air as they grow and can therefore mitigate the global warming. However, other factors significantly influence the impact of reafforestation on climate change, such as carbon emissions due to direct and indirect land use change. This study focused on the effect of the albedo of land surfaces, which is not accounted for in the Kyoto Protocol. Highly reflective land cover, such as snow, can reflect back most of the sun’s radiation and therefore has a high albedo. In contrast, forests absorb most of the incoming radiation and have a low albedo. This could have a comparative warming effect on surface air temperatures, depending on the age of the forest. The researchers used high resolution satellite observations to investigate the effects on climate change of planting forests on croplands across the globe. The projected cooling achieved from increased carbon storage was calculated after taking into account the warming caused by a decrease in surface albedo as cropland was changed to forest cover at the time of forest maturation. This effect is especially important at higher latitudes where snow-covered crop or grass land has a significantly higher albedo than snow-covered trees. Only individual afforestation projects on a local scale (10-100 hectares) were considered: it is suggested that this represents a more realistic representation of how afforestation might actually occur. Previous modelling studies at much coarser scales than this study have included large bands of afforestation on land that was not cropland and have therefore overestimated the decrease in albedo at higher latitudes. Previous large-scale modelling studies have suggested that afforestation in tropical latitudes causes a cooling effect on the climate and that afforestation schemes at mid latitudes have little effect. These studies suggest that at high latitudes, afforestation would contribute a warming effect on the climate, due to the darkening of the earth’s surface. The results of this study differ: it is suggested that the net effect of albedo in small-scale afforestation, anywhere on crops between latitudes 40 degrees South and 60 degrees North, does not cause warming. In addition, the results suggest the impact of afforestation schemes on the climate does not fall into clear latitudinal bands. There are significant variations in the albedo effects across the zones, although the albedo effect tends to be greater at higher latitudes. Nevertheless, albedo-linked warming in these temperate latitudes is not high enough to overcome the cooling effect caused by carbon sequestration at the time of forest maturation. The study also determined that the scale of change in land use for the afforestation schemes considered in this study were too small to influence cloud cover changes, and therefore albedo, over the forested areas. In comparison, other studies suggest large scale reafforestation schemes do affect cloud cover over forests. In addition, the authors caution that the study was not designed to determine the impact on climate of regional or continental scale land use change through afforestation. Source: Montenegro, A., Eby, M., Mu, Q. et al. (2009). The net carbon drawdown of small scale afforestation from satellite observations. Global and Planetary Change. 69: 195–204. Contact: [email protected] Theme(s): Climate change and energy, Forests Opinions expressed in this News Alert do not necessarily reflect those of the European Commission To cite this article/service: "Science for Environment Policy": European Commission DG Environment News Alert Service, edited by SCU, The University of the West of England, Bristol. 1 European Commission DG ENV News Alert Issue 183 February 2010 .