Biomass energy sources are among the most promising, most hyped and most heavily subsidized renewable energy sources. They have real potential to heighten energy security in regions without abundant fossil fuel reserves, to increase supplies of liquid transportation fuels and to decrease net emissions of carbon into the atmosphere per unit of energy delivered. However, increased exploitation of biomass energy also risks sacrificing natural areas to managed monocultures, contaminating waterways with agricultural pollutants, threatening food supplies, and increasing net emissions of carbon to the atmosphere. The opportunities are real, but the concerns are also justified. As investments in biomass energy increase, there needs to be an active, continuing research on strategies for balancing the pros and cons of biomass energy. In particular, the contributions of biomass energy to energy security and net forcing of climate are not always coherent. A critical priority for research on biomass energy systems should be identifying the sites and technologies where biomass energy can and cannot lead to a net negative forcing of climate change. Our research team at the Carnegie Institution works to test the hypothesis that a dominant determinant of net climate forcing from biomass energy comes from characteristics of the land converted to biomass energy production. As a consequence, defining the climate-protective domain for biomass energy depends as strongly on the details of the deployment map as on production technologies and efficiencies.