Reply to 'Emissions accounting for biomass energy with CCS'

Sanchez et al. reply

Our Letter¹ assesses the impact on regional carbon emissions if biomass energy is used to replace fossil fuels in the electricity system, and carbon capture and storage (CCS) is used to sequester most of the emissions associated with electricity production. Our bioenergy assessment prioritizes, but does not rely solely on, bioenergy production from wastes and residues, which do not compete with food crops and would otherwise be burned or left to decompose, releasing their carbon into the atmosphere as CO₂. Most of these feed stocks minimize the impact of biomass regrowth and uptake rates. For feed stocks such as forest residues that may take many years to grow back, there will be some amount of short-term radiative forcing that was not accounted for in our analysis.

As Gilbert and Sovacool suggest², it is important not to count the same CO₂ emissions reductions in two separate sectors when quantifying economy-wide emissions. Our analysis avoids this accounting error by using a simplified methodology, ascribing all changes in atmospheric CO₂ — from plant growth to combustion in a bioenergy and CCS (BECCS) plant — to the electricity sector. Should BECCS be adopted widely, it will be important to allocate emissions credits among all relevant actors across sectors.

Several forms of complementary analyses inform roadmaps for sustainable bioenergy production. In addition to the bottom-up engineering-economic analysis performed in our Letter¹, our team at the Renewable and Appropriate Energy Laboratory at UC Berkeley, USA, and others, have engaged in commodity-chain theoretical bioenergy analysis, producing quantitative indirect land-use change estimates³, and evaluations of previous efforts through meta-analysis⁴. Based on this work, we agree with Gilbert and Sovacool² that monitoring and verification should be a critical part of any long-term strategy for mitigating climate change. Of particular concern is whether the cultivation or extraction of biomass for energy will degrade or enhance the ecological productivity and related carbon flows of the land⁵.

Each step of energy extraction, preparation, combustion, and disposal demands a rigorous assessment of carbon impacts. This statement applies not only to bioenergy, but also carbon capture technologies. In addition to oft-cited concerns about sustainable bioenergy production, risks of CO₂ leakage from long-term geologic sequestration raises additional uncertainties about BECCS and other CCS strategies⁶. However, the choice of counterfactual is critical to any bioenergy analysis, including assumptions of population, future diet, and crop productivity⁷. Recent research shows that biofuel production can provide emissions benefits over non-bioenergy land-use decisions, including forest recovery on marginal land⁸. Geologic storage of carbon through CCS can proceed for decades and potentially millennia if properly managed, which may be more stable than other carbon sequestration options from biomass. The emissions benefits of BECCS — encompassing displaced fossil-fuel CO₂ emissions from energy production and geologic CO₂ sequestration — may improve the desirability of biomass production for bioenergy over other land-use decisions, but more research is needed to directly compare it with other sequestration strategies. Moving forward, supportive policy should incentivize land-use decisions that are beneficial for the climate⁹.