Characterizing the Climate Effects of Biofuel Cultivation

Large scale deployment of new biofuel crops has the potential to influence climate through biogeophysical and biogeochemical mechanisms operating at the land surface. In turn, climatic variability influences the productivity of biofuel crops and thus their potential contribution as a source of energy. In order to characterize this two-way interaction between biofuels and climate, we are conducting a series of modeling experiments within the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM).

Key questions that this study attempts to address include 1) In what ways (e.g. at what spatial and temporal scales, under what land cover and management scenarios) does a transition to biofuel crops represent a climate stabilizing versus a climate destabilizing endeavor? 2) Which vegetation properties and management choices are most influential in determining key climatic outcomes associated with biofuels? 3) Are biofuel crop yields robust to changing climatic conditions?

Our approach is to develop new biofuel plant functional types (PFTs) for the land surface component of CESM - the Community Land Model (CLM) - and to examine climatic implications of future biofuel deployment scenarios within the coupled land-atmosphere framework of CESM. CLM represents plant functional types with more than 50 parameters that describe aerodynamic, physiological, optical, and biogeochemical properties etc. We are constraining these values with the best available observational data. We are also making structural modification to the model in order to represent unique features of biofuel crops and their management, such as changes to the phenology and carbon allocation schema.

In addition to seeking observational data to constrain parameters, we are conducting systematic sensitivity analysis on the default c4 grass parameters in CLM. By limiting the analysis to a handful of representative single-point sites, we are able to run several hundred 30-year simulations in less than one wall clock hour, permitting a thorough exploration of the parameter space and its influence on climate forcing terms such as energy and carbon fluxes. This analysis reveals those parameters that are most and least influential in terms of climate forcing, helping to prioritize the search for observational data and leading the way to characterizing the uncertainty in biofuel crops' influence on climate.

Future biofuel land-use and greenhouse gas concentration scenarios will be selected to examine biofuel cultivation under a range of climatic conditions. This will permit us to test biofuels' resilience to climatic change in terms of productivity, water-use efficiency, and nitrogen-use efficiency as well as examine climate feedbacks resulting from biofuel cultivation under various conditions.