Journal Article

Disentangling the effects of CO₂ and short-lived climate forcer mitigation

Shindell, Drew T., Rogelj, Joeri, Hare, William, Klimont, Zbigniew, Velders, Guus J. M., Schellnhuber, Hans Joachim, Schaeffer, Michiel, Meinshausen, Malte, Amann, Markus

2014

Overview

Significance 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. Some, like soot (black carbon), are very short lived, whereas others, like carbon dioxide (CO ₂), are very persistent and remain in the atmosphere for centuries to millennia. Importantly, 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. Our study disentangles these linkages and therewith helps to avoid crucial misconceptions: Measures reducing short-lived climate forcers are complementary to CO ₂ mitigation, but neglecting linkages leads to overestimating their climate benefits. 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 CO ₂. These species are often released from common sources and are therefore intricately linked. However, for reasons of simplification, this CO ₂–SLCF linkage was often disregarded in long-term projections of earlier studies. Here we explicitly account for CO ₂–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, we find 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 CO ₂ mitigation. Our study demonstrates the importance of coherently considering CO ₂–SLCF coevolutions. Failing to do so leads to strongly and consistently overestimating the effect of SLCF measures in climate stabilization scenarios. Our results reinforce that SLCF measures are to be considered complementary rather than a substitute for early and stringent CO ₂ mitigation. Near-term SLCF measures do not allow for more time for CO ₂ mitigation. We disentangle and resolve the distinct benefits across different species and therewith facilitate an integrated strategy for mitigating both short and long-term climate change.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: Air pollution / benefits / black carbon / burning / carbon / carbon dioxide / Carbon dioxide emissions / challenges / climate / Climate change / Climate change policy / Climate models / consistent / cooling / copenhagen accord / cumulative carbon emissions / emissions / Emissions policy / Emissions reduction / environmental policy / fossil fuels / Global climate models / Global warming / greenhouse-gas emissions / human health / humans / pathways / Physical Sciences / Pollutant emissions / soot