The albedo climate impacts of biomass and carbon plantations compared with the CO2 impact

Changes in land use and the consequent changes in land-cover properties modify the interactions between the land surface and the atmosphere locally and regionally (Kabat et al., 2004). Important factors in these interactions are the biochemical fluxes of CO2 and other trace gases, and the biophysical fluxes of energy and water vapor. Modeling studies, well validated with detailed observations, show that changing land-use in the past centuries influenced local, regional, and probably also global climate patterns (e.g. IPCC, 2001). Historically, land-use mediated climate change appears to be an important factor (Brovkin et al., 1999). In mid to high latitudes, for example, land-use changes influence surface-air temperature because of the large difference in surface albedo between different land covers, such as cropland and forest in snow-covered conditions (Robinson and Kukla, 1985; Bonan et al., 1995; Harding and Pomeroy, 1996; Sharrat, 1998). Emission scenarios, required to estimate future climate change, nowadays often include detailed changes in land-use patterns and the consequent changes in sources and sinks of trace gases (e.g. Strengers et al., 2004). The biophysical consequences on the climate systems are, however, often neglected. It is therefore important to examine the role of land-use changes in determining future climates (Pielke Sr et al., 2002). Future land-use change does not only include deforestation and afforestation as a consequence of expanding or contracting agriculture. Other land uses, such as plantations for carbon sequestration or energy production (to substitute fossil fuels), are likely to become more important. We define a “carbon plantation”, or C-plantation, as the land cover resulting from the planting, managing, and harvesting of trees on formerly non-forested lands with the specific aim of achieving a maximum net uptake of CO2 from the atmosphere into vegetation, litter, or soil. A “biomass plantation” refers to the land cover resulting from the activities aimed at providing biomass as a primary energy carrier. Currently, integrated assessment modelers are beginning to include such land uses in studies of mitigation strategies, but only the impact on CO2 fluxes is included (Leemans et al., 1996). However, if applied at a large scale, biomass and C-plantations might not only influence climate change by reducing global greenhouse gas (GHG) concentrations, but also have an impact on the energy fluxes between the land surface and the atmosphere. This was initially brought forward for C-plantations by Betts (2000), who estimated the effect of coniferous C-plantations in the northern hemisphere on planetary albedo. His calculations suggest that the global warming by albedo changes associated with high-latitude C-plantations would outweigh global cooling by carbon sequestration. Here we will compare the importance of the biophysical climate impact of future land-use changes for two major mitigation options: production of biofuels and permanent C-plantations. We will largely focus on the impact of surface albedo changes as the obvious non-CO2 impact of land-use change on climate in the extra-tropics. Our analysis is crucial to decide if such biophysical effects also need to be included in integrated assessment models for an adequate evaluation of the effectiveness of mitigation options that modify the land surface.