One of the key points of contention over the climate benefit of biofuels is the impact of land use change (LUC) associated with biofuel feedstock production. LUC results in biogeochemical (e.g., soil organic carbon) and biogeophysical (e.g., surface albedo, evapotranspiration, and surface roughness) changes. Of the biogeophysical factors, surface albedo has been considered a dominant effect at the global scale.
A team at Argonne National Laboratory has now quantified land use change (LUC)-induced albedo effects for three major biofuels in the US, using satellite data products for albedo and vegetation observations. Published in the RSC journal Energy & Environmental Science, the analysis indicates that the land use change (LUC)-induced albedo effect is small for corn and miscanthus ethanol, but is significant for switchgrass ethanol, which is driven by the types, locations, and intensities of various land conversions to these biofuel feedstocks.
Climate impacts of changes in surface albedo from several LUC scenarios for cultivation of biomass for bio-based diesel fuel production varied significantly and could be substantial relative to those of petroleum-derived diesel. These studies suggested that LUC-induced albedo effects were potentially substantial and could even outweigh the biogeochemical effects evaluated in previous biofuel LCAs.
However, LUC-induced albedo effects of major biofuel production systems in the US have yet to be examined in the context of life-cycle GHG emissions of such biofuels encouraged by RFS and LCFS. We aim to address this issue by quantifying the albedo effects of major biofuels including corn ethanol and cellulosic ethanol derived from switchgrass and miscanthus, in the US, and to integrate the albedo effects into life-cycle GHG intensities of these biofuels on a common basis.—Cai et al.
Changes in surface albedo modify the absorption of incoming solar radiation, resulting in radiative forcings.
For the analysis, the Argonne team first defined LUC scenarios associated with biofuel production. They then retrieved albedo and land cover satellite observations and paired them on a geospatially consistent basis. They next analysed the albedo dynamics associated with specific land covers, and used them to drive a cloud- and aerosol-coupled radiation model to simulate the radiative forcings due to LUC-induced changes in albedo. Finally, they adopted a radiative forcing-based metric to evaluate LUC-induced albedo effects.
|Total LUC-induced albedo effects, in g CO2e/MJ, of corn, switchgrass, and miscanthus ethanol in the US. Cai et al. Click to enlarge.|
Among their findings:
With conversion from cropland/pasture land, grassland, forest, and shrubland to cornfields, corn ethanol has a net cooling albedo effect of -1.8 g CO2e/MJ. When the spatial variation of albedo effects was considered, the LUC-induced albedo effect could range from a warming effect of 2.0 g CO2e/MJ to a cooling effect of -5.7 g CO2e/MJ for corn ethanol.
A significant warming albedo effect of 12.1 g CO2e/MJ, on average, is found for switchgrass ethanol, as compared to life-cycle GHG emissions of about 17.3 g CO2e/MJ for switchgrass ethanol without albedo effects. The warming effect can vary from 3.2 to 21.0 g CO2e/MJ, mostly because of the spatial variation in the albedo effect of conversion from cropland/pasture land to switchgrass fields in various locations. The albedo warming effect more than offsets a GHG emission reduction of -3.5 g CO2e/MJ from increased SOC associated with the same LUCs for switchgrass ethanol production.
Miscanthus ethanol has a relatively small warming albedo effect of about 2.7 g CO2e/MJ, on average, as compared to life-cycle GHG emissions of about -6.8 g CO2e/MJ. The warming effect can vary from 0.1 to 5.4 g CO2e/MJ, mostly because of the spatial variation in the albedo effect of conversion from cropland/pasture land to miscanthus fields. The albedo warming effect is relatively small, compared to a GHG emission reduction of -20.1 g CO2e/MJ from the increased SOC associated with the same LUCs for miscanthus ethanol production.
With the LUC-induced albedo effect included, ethanol from corn, switchgrass, and miscanthus has life-cycle GHG emissions of 56, 29, and -4 g/MJ, respectively. These results translate to a GHG emission reduction of about 39%, 68%, and 104%, respectively, relative to petroleum-derived gasoline, which has a GHG emission intensity of 92 g CO2e/MJ.
|Integration of LUC-induced albedo effects with life-cycle GHG emissions, including LUC-induced SOC changes, for corn, switchgrass, and miscanthus ethanol. Cai et al. Click to enlarge.|
The albedo effects quantified in this study, which has been generally overlooked in traditional LCA, fill in an important biofuel GHG analysis gap and shed light on a fuller picture of the climate impacts of major U.S. biofuels. We aspire that this work raises awareness among policy makers and other stakeholders regarding an additional effect that can be considered when estimating biofuel life-cycle greenhouse gas emissions and offers them insight into routes to estimating equivalent GHG emissions from albedo shifts stemming from LUC associated with large-scale expansion in production of biofuels in the U.S and challenges associated with these estimations.
The LUC-induced albedo effect is small for corn and miscanthus ethanol, but is significant for switchgrass ethanol, reducing its GHG emission reduction relative to petroleum gasoline from about 81% without the albedo effects to 68% with the effects. These findings show that the albedo effect is biofuel feedstock- specific and can be significant for some biofuels. These results may be helpful for regulatory agencies like the US EPA and California Air Resources Board to better assess the magnitude and the major drivers of this biogeophysical effect for production of major biofuels in the US.—Cai et al.
H. Cai, J. Wang, Y. Feng, M. Wang, Z. Qina and J. B. Dunn (2016) “Consideration of land use change-induced surface albedo effects in life-cycle analysis of biofuels” Energy Environ. Sci. doi: 10.1039/C6EE01728B