Assessing the merits of bioenergy by estimating marginal climate-change impacts.

Purpose: Climate-change impacts can be mitigated through greater use of bioenergy, but the extent to which specific options actually reduce overall impacts needs to be assessed. Most bioenergy assessments have used proxy measures for assessing its merits. Here, a new approach is presented, whereby the contribution of bioenergy use is assessed through quantifying marginal changes in climate-change impacts that result from the implementation of a bioenergy option. Methods: Marginal climate-change impacts were calculated for one specific example of a bioenergy option, conversion of an unutilised mature forest into a production forest harvested repeatedly for bioenergy over successive 25-year rotations. The overall benefit of the option was assessed by including stand-level carbon dynamics, global carbon-cycle feedback, progressively changing radiative efficiency and marginal impact sensitivity of warming. It also includes a differentiated assessment of three kinds of climatic impacts: direct-warming, rate-of-warming and cumulative-warming impacts. Marginal impacts were calculated and summed over 100 years to assess the overall marginal impact of this bioenergy option. Results and discussion: Bioenergy use in this specific example led to a large initial loss of biomass carbon followed by an ongoing and accumulating benefit through fossil-fuel substitution. This caused adverse climatic impacts over the first two rotations as the effects of the on-site carbon loss dominated the overall impact, but the option became increasingly beneficial over longer time frames as the benefit of fossil-fuel substitution accrued and eventually dominated. Summed over 100 years, the bioenergy option reduced direct-temperature and rate-of-warming impacts whilst increasing cumulative-warming impacts. The average of the three kinds of impacts showed a slight mitigation benefit by reducing overall impacts. In the particular example, bioenergy use was assessed to have a more beneficial effect if the analysis was carried out under the assumption of higher-emission concentrations pathways, or if it assumed a steeper relationship between climate perturbations and impacts. Conclusions: The usefulness of any climate-change mitigation option ultimately relates to the marginal climate-change impacts it can avert. It is shown here that marginal impacts can be calculated in routine operation and that they can provide an objective and methodologically consistent assessment of the mitigation potential of bioenergy use. [ABSTRACT FROM AUTHOR]