Long-term Climate Impact of Albedo in Willow Biofuels

Short-rotation willow (Salix sp.) is a candidate second-generation biofuel which has generated particular interest in the northeastern US. This crop can be grown on marginal, abandoned agricultural land as both a novel income source and as a low carbon biofuel. If willow is to be considered a tool for climate mitigation, studies of willow must also take into account not only uptake and output of greenhouse gases, but also the biogeophysical impacts of land use change. Small differences in surface reflectance, or albedo, across different crops and covers can scale up to significant impacts on the global energy budget. Similarly, small changes to annual albedo caused by loss of winter snow cover can scale up to major changes in the long-term climate impact. Both current and future albedo differences will influence the actual climate impact of any use of willow as a biofuel.

Here, we show that the albedo impact of transition from old crop field to willow is slightly cooling (-109 GWP-albedo). In contrast, reforestation of old fields would have a warming effect (758 GWP-albedo), but planting with alternative biofuels such as Panicum sp. or Miscanthus sp. would not (-405 GWP-albedo; -705 GWP-albedo). We then used model predictions of future snow cover over the next 100 years to see how these effects might change under four different RCP scenarios. We found that as climate warmed in the future, the warming impact of forests lessened (509 to 738 GWP-albedo by 2070) and the cooling impact of willow decreased (-76 to -106 GWP-albedo by 2070). Final project results will demonstrate the relative importance of albedo versus carbon sequestration in willow climate mitigation projects in upstate New York.