Meeting ambitious climate mitigation targets

Research shows that reducing the emissions of short-lived climate pollutants (SLCPs) will play an important role in meeting the 2°C target of the Paris Agreement. Although these emissions will be partially reduced as a consequence of carbon dioxide (CO2) mitigation, it will not be enough. According to the IIASA Air Quality and Greenhouse Gases (AIR) Program, additional, dedicated SLCP policies that will also deliver other benefits such as improved health will be needed.

Anthropogenic climate change is largely driven by human-induced changes in the composition of the atmosphere, including long-lived greenhouse gases that have lifetimes of approximately eight years or more, and SLCPs that have lifetimes of approximately 20 years or less. The challenge with SLCPs is that reducing their emissions may lead to undesired warming in the atmosphere. However, many SLCPs are also considered air pollutants that have negative effects on human health, crop productivity, and ecosystems.

The IIASA AIR program has been a forerunner in analyzing the role that SLCPs could play in meeting ambitious climate mitigation targets. In 2017, researchers concluded a number of important research projects in this field. These include an improved estimate of current and past emissions of black carbon (one of the notorious SLCPs) [1]; an analysis of how changing emissions of aerosol precursors have influenced the radiative balance of the planet in past decades [2]; an innovative analysis of the contribution of solid fuel cook stoves to the aerosol load in the atmosphere [3]; and an analysis of the interactions between SLCP mitigation and the sustainable development goals [4].

Led by Zbigniew Klimont and coauthored by Lena Hoglund-Isaaskson, both from IIASA, Chapter 6 of the 2017 UN Environment Program (UNEP) Gap report is dedicated to the role that SLCPs could play in achieving ambitious long-term global temperature targets. The team, which consisted largely of AIR program researchers and their current collaborators, highlighted that reductions in SLCPs are most effective when accompanied by reductions in CO2. Since CO2 and SLCPs often have the same sources, synergies can be reaped in reduction efforts. The researchers however caution that some pollutants are not co-emitted and need to be targeted individually. Moreover, because they have a shorter lifespan and thus influence the atmosphere in characteristically different ways, it is neither useful nor appropriate to translate SLCF mitigation to carbon-equivalent units. The authors recommend that long-lived greenhouse gases and SLCPs should be considered and targeted separately with policies tied to specific, non-convertible measures.

The UNEP report concludes that SLCF potential emission reductions are significant. It states that SLCP reductions will be an integral part of a strategy that aims to keep global temperature increases to less than 2°C, and that early reductions would not only help to slow the rate of climate change in the short term, but also provide substantial health benefits. In the future, coreductions of SLCF because of CO2 reductions will play a role, but additional specific policies for SLCPs will be needed.

Regional baseline methane and black carbon [shaded background] emissions and mitigation potential by 2030 [Mt per year].

Over the past few years, the AIR program has also significantly extended and improved estimates of mitigation potentials and the costs of reducing SLCPs. This helps to better understand and design cost-effective strategies to reduce future climate change impacts in a complex world of interacting atmospheric processes and economic and environmental constraints.

References

[1] Klimont Z, Kupiainen K, Heyes C, Purohit P, Cofala J, Rafaj P, Borken-Kleefeld J, & Schöpp W (2017). Global anthropogenic emissions of particulate matter including black carbon. Atmospheric Chemistry and Physics 17 (14): 8681-8723.

[2] Myhre G, Aas W, Cherian R, Collins W, Faluvegi G, Flanner M, Forster P, Hodnebrog Ø, et al. (2017). Multi-model simulations of aerosol and ozone radiative forcing due to anthropogenic emission changes during the period 1990-2015. Atmospheric Chemistry and Physics 17 (4): 2709-2720.

[3] Huang Y, Unger N, Storelvmo T, Harper K, Zheng Y, & Heyes C (2017). Global radiative effects of solid fuel cook stove aerosol emissions. Atmospheric Chemistry and Physics Discussions 1-40.

[4] Haines A, Amann M, Borgford-Parnell N, Leonard S, Kuylenstierna J, & Shindell D (2017). Short-lived climate pollutant mitigation and the Sustainable Development Goals. Nature Climate Change 7 (12): 863-869.

[5] Klimont Z, Shindell D, Borgford-Parnell N, Höglund Isaksson L, Kallbekken S, Kuylenstierna J, Molina L, Srivastava L, et al. (2017). Bridging the gap – The role of short-lived climate pollutants. In: The Emissions Gap Report 2017-A UN Environment Synthesis Report. pp. 48-57 Nairobi: United Nations Environment Program (UNEP).