Abstract
Climate benefit assessments of bioenergy crops often focus on biogeochemical impacts, paying little if any attention to biogeophysical impacts. However, land conversions required for large-scale bioenergy crop production are substantial and may directly affect the climate by altering surface energy balance. In the US, such land conversions are likely to be met in part by converting Conservation Reserve Program (CRP) grassland to bioenergy crops. Here, we converted three 22 year old CRP smooth brome grass fields into no-till corn, switchgrass, or restored prairie bioenergy crops. We assessed the biogeophysical climate impact of the conversions using albedo changes relative to unconverted reference CRP grassland. The corn and perennial fields had higher annual albedo than the grassland they replaced—causing cooling of the local climate. The cooling of the corn field occurred solely during the non-growing season—especially when surfaces were snow-covered, whereas the cooling of the perennial fields was more prominent during the growing season. Compared to biogeochemical impacts with fossil fuel offsets for the same land conversions over eight years, the annual albedo-induced climate benefits add ∼35% and ∼78% to the annual biogeochemical benefits provided from the switchgrass and restored prairie fields, respectively, and offset ∼3.3% of the annual greenhouse gas (GHG) emissions from the corn field. We conclude that albedo-induced climate mitigation from conversion of CRP lands to perennial but not annual bioenergy crops can be substantial, and future climate impact assessments of bioenergy crops should include albedo changes in addition to GHG balances in order to better inform climate policies.
Highlights
Land use and land management changes directly affect local to regional climate by altering surface energy balance through changes in albedo (∆α), sensible and latent heat energies, surface roughness and soil heat flux e.g. [1, 2], as well as greenhouse gas (GHG) balances through exchanges of CO2, CH4 and N2O between the land surface and the atmosphere e.g. [3]
Georgescu et al [10] estimated that the global warming impact (GWI)∆α benefit from converting tilled corn (Zea mays L.)-soybean (Glycine max L.) fields to switchgrass (Panicum virgatum L.) bioenergy crops across the Central US was six-fold higher than the annual biogeochemical GWI benefit that arises from offsetting fossil fuel use
Our findings suggest that converting Conservation Reserve Program (CRP)-Ref grasslands rather than existing corn bioenergy croplands to either switchgrass or restored prairie bioenergy crop results in GWI∆α-annual benefits of ∼−0.3 Mg CO2-eq ha−1 yr−1— almost all the benefits occur during the non-growing season
Summary
Land use and land management changes directly affect local to regional climate by altering surface energy balance through changes in albedo (∆α), sensible and latent heat energies, surface roughness and soil heat flux (biogeophysical processes) e.g. [1, 2], as well as greenhouse gas (GHG) balances through exchanges of CO2, CH4 and N2O between the land surface and the atmosphere (biogeochemical processes) e.g. [3]. Most climate impact assessments focus on the biogeochemical impacts, paying little if any attention to the biogeophysical impacts [4] due primarily to the difficulty of adequately reconciling biogeophysical impacts with global warming impact (GWI) metrics and challenges associated with the complex and non-linear effects of biogeophysical change on climate [5, 6] Because of their radiative nature, albedo-induced climate impacts can be expressed in GWI metrics for direct comparison with biogeochemical GWIs e.g. Albedo-induced GWIs (GWI∆α) from land use and land management changes have been increasingly studied [9,10,11,12,13,14], in high-latitude forests with seasonal snow cover where the influence of albedo on regional and global climate is potentially higher than the influences of biogeochemical impacts [7, 15]. Georgescu et al [10] and Caiazzo et al [12] did not include albedo changes and consequent GWI∆α in the presence of snow, which could be substantial and potentially counteract the growing season climate benefits e.g. [13, 21]
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