Wedge decomposition analysis: Application to SRES and Post-SRES Scenarios

We introduce a general methodology for displaying the gross assumptions behind any carbon emissions trajectory, relative to a reference trajectory, and we apply this methodology to a limited number of IPCC SRES scenarios [1]. These scenarios have been used widely for climate change analysis. We examine four scenarios (A1B, A2, B1, B2) together with three paired “post-SRES” scenarios [2, 3] that achieve CO2 stabilization at 550 ppm by 2100. Our analysis is guided by the concept of the “stabilization wedge” introduced in a recent paper by Pacala and Socolow [4], which measures the quantitative contributions of specific technologies and strategies over the next 50 years in units of 1 GtC/yr reductions in 2050. We find that autonomous carbon-emissions reduction activity in the SRES scenarios account for a large number of “virtual” wedges, ranging from 8 to 35 in 2050. Roughly half of the virtual wedges in each scenario is due to energy efficiency improvements and structural change in the economy; most of the remaining half is due to high penetrations of non-fossil energy technologies. Post-SRES scenarios require only 2 to 4 “real” wedges in 2050, accounted for largely by greater non-fossil energy, greater energy efficiency, and CO2 sequestration. Our results reveal that the SRES and post-SRES scenarios share a number of common assumptions. In particular we find that the baseline development path, i.e., the number of virtual wedges and “autonomous” trends in absence of any climate policies, play a central role in determining the mitigation effort needed for achieving climate stabilization.