Last Wednesday PNAS published a study by Michiel Schaeffer (Environmental Systems Analysis group) and colleagues on the effects of CO2 and short-lived climate forcer mitigation. Their results reinforce that short-lived climate forcer measures are to be considered complementary rather than a substitute for early and stringent CO2 mitigation. These measures do not allow for more time for CO2 mitigation.
Climate change is one of the greatest challenges of our times. Human activities, like fossil-fuel burning, result in emissions of radiation-modifying substances that have a detectable, either warming or cooling, influence on our climate. These substances are often emitted by common sources. As climate policy is looking at options to limit emissions of all these substances, understanding their linkages becomes extremely important. For about two decades, policy-makers have considered options to avoid dangerous anthropogenic interference with the climate system. So far, many countries support limiting warming to below a 2 °C temperature limit, but the required global mitigation action to achieve this has been limited.
Anthropogenic global warming is driven by emissions of a wide variety of radiative forcers ranging from very short-lived climate forcers (SLCFs), like black carbon, to very long-lived, like CO2. These species are intricately linked. However, for reasons of simplification, this CO2-SLCF linkage was often disregarded in long-term projections of earlier studies. In this study the authors explicitly account for CO2-SLCF linkages and show that the short- and long-term climate effects of many SLCF measures consistently become smaller in scenarios that keep warming to below 2 °C relative to preindustrial levels.
Although long-term mitigation of methane and hydrofluorocarbons are integral parts of 2 °C scenarios, early action on these species mainly influences near-term temperatures and brings small benefits for limiting maximum warming relative to comparable reductions taking place later. Furthermore, the study shows that maximum 21st century warming in 2 °C-consistent scenarios is largely unaffected by additional black-carbon-related measures because key emission sources are already phased-out through CO2 mitigation. The study demonstrates the importance of coherently considering CO2-SLCF coevolutions. Failing to do so leads to strongly and consistently overestimating the effect of SLCF measures in climate stabilization scenarios. The results reinforce that SLCF measures are to be considered complementary rather than a substitute for early and stringent CO2 mitigation. Near-term SLCF measures do not allow for more time for CO2 mitigation. The authors disentangle and resolve the distinct benefits across different species and therewith facilitate an integrated strategy for mitigating both short and long-term climate change.
'Disentangling the effects of CO2 and short-lived climate forcer mitigation’, by Joeri Rogelj, Michiel Schaeffer et al. PNAS, 5 October 2014.