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Surface albedo is a measure of the extent to which the Earth’s surface reflects or absorbs sunlight. Lighter surfaces, such as bare ground covered with snow, have a relatively high albedo and reflect almost all the energy (heat) of incoming solar radiation. Darker surfaces, such as deep oceans and dense forests, have a relatively low albedo and absorb almost all the heat of incoming solar radiation. Consequently, when changing conditions cause a decrease in surface albedo—for example, when trees are planted on lands that receive substantial winter snowfall—they can increase the capacity of the affected area to absorb heat from the sun, with an overall warming effect.

Afforestation and reforestation to increase carbon sequestration are permitted activities under the Clean Development Mechanism of the Kyoto Protocol and other climate change mitigation programs. Project developers can earn carbon credits or offsets for the amounts of carbon sequestered by the trees they plant. Recent research, however, has shown the importance of changes in surface albedo caused by tree planting, which should be considered in tandem with the increases in carbon sequestration achieved by such projects 20,21,22,23,24 Tree planting increases carbon sequestration; but, depending on local and regional conditions, it can also decrease surface albedo.25

The recent scientific studies cited above have shown that, in the Earth’s middle and high latitudes, the warming effect of lower surface albedo that results from afforestation or reforestation can have an unintended net warming effect. Before the advent of industrialization, humans were already affecting the climate by clearing forests to plant crops, an activity that increased surface albedo.26 Thus, the net result of preindustrial changes in land use was negative climate forcing (cooling). According to one recent study,27 the rate of cooling induced by preindustrial deforestation was similar in magnitude to the current rate of positive climate forcing (warming) caused by anthropogenic emissions of ozone (O3), nitrous oxide (N2O), and halocarbons.

Researchers have also compared the magnitude of the positive climate forcing caused by modern-day tree planting (as a result of lowered albedo) with the negative forcing that results from increased carbon sequestration by newly planted trees. Their results indicate that, in the middle and high latitudes, the warming effects of afforestation and reforestation activities could partially or completely offset the cooling effects—especially in snow-covered areas, where the difference in albedo between snow-covered bare ground and partially snow-covered forested areas is greatest. This finding has policy implications in general for climate change mitigation efforts and specifically for the valuation of carbon credits earned through reforestation and afforestation activities.

The albedo effect is less significant in tropical zones, where research models indicate that deforestation causes warming.28 In tropical forest areas, loss of tree cover reduces leafy surface area and, as a result, slows the release of water vapor into the atmosphere. Slower evaporation rates, in turn, have a negative effect on cloud formation above tropical rain forests. Because the clouds reflect incoming solar radiation, they lower surface temperatures. With less cloud cover, more incoming solar radiation reaches the Earth’s surface, and temperatures rise.

At the National Center for Atmospheric Research, researchers using the Parallel Climate Model (sponsored by the U.S. Department of Energy) have modeled climate change for three future periods— 2000-2033, 2033-2066, and 2066-2100—considering in one case only the effects of atmospheric forcing and in another case the combined effects of atmospheric forcing and changes in land cover.29 They found that forest-to-agriculture conversion in the Amazon rain forest causes significant warming, although the effects are not uniform across all tropical forests. (For example, in Indonesia, temperature changes from forest loss are minor in future scenarios, because the Asian monsoon compensates for the decrease in cloud formation relative to that over undisturbed forests.)

In the middle and, especially, high latitudes, there is increasing evidence that the radiative cooling effect derived from carbon sequestration by newly planted trees could be partially or completely offset by the radiative warming effect of reduced albedo. For example, the IPCC in its latest assessment notes that replacement of tundra vegetation by coniferous evergreen trees is likely to reduce regional albedo significantly and lead to a warming effect greater than the cooling effect of increased carbon sequestration.30 In contrast, tree planting in tropical latitudes, where vegetation and cloud formation are linked, can double the radiative cooling benefit by reducing incoming solar radiation while also increasing carbon sequestration.