The regional temperature implications of strong air quality measures

Abstract

Abstract. Anthropogenic emissions of short-lived climate forcers (SLCFs) affect both air quality and climate. How much regional temperatures are affected by ambitious SLCF emission mitigation policies is, however, still uncertain. We investigate the potential temperature implications of stringent air quality policies by applying matrices of regional temperature responses to new pathways for future anthropogenic emissions of aerosols, methane (CH4), and other short-lived gases. These measures have only a minor impact on CO2 emissions. Two main options are explored, one with climate optimal reductions (i.e., constructed to yield a maximum global cooling) and one with the maximum technically feasible reductions. The temperature response is calculated for four latitude response bands (90–28∘ S, 28∘ S–28∘ N, 28–60∘ N, and 60–90∘ N) by using existing absolute regional temperature change potential (ARTP) values for four emission regions: Europe, East Asia, shipping, and the rest of the world. By 2050, we find that global surface temperature can be reduced by -0.3±0.08 ∘C with climate-optimal mitigation of SLCFs relative to a baseline scenario and as much as −0.7 ∘C in the Arctic. Cutting CH4 and black carbon (BC) emissions contributes the most. The net global cooling could offset warming equal to approximately 15 years of current global CO2 emissions. On the other hand, mitigation of other SLCFs (e.g., SO2) leads to warming. If SLCFs are mitigated heavily, we find a net warming of about 0.1 ∘C, but when uncertainties are included a slight cooling is also possible. In the climate optimal scenario, the largest contributions to cooling come from the energy, domestic, waste, and transportation sectors. In the maximum technically feasible mitigation scenario, emission changes from the industry, energy, and shipping sectors will cause warming. Some measures, such as those in the agriculture waste burning, domestic, transport, and industry sectors, have large impacts on the Arctic, especially by cutting BC emissions in winter in areas near the Arctic.

A continued role of short-lived climate forcers under the Shared Socioeconomic Pathways

“…The AGTP is an emission-metric-based emulator of the climate response and a well-established method that enables us to quantify and compare global temperature impacts of a large number of sources and scenarios in a transparent and, in terms of computer resources, cost-effective manner. The approach is described in detail in the literature (Fuglestvedt et al, 2010;Shine et al, 2005;Aamaas et al, 2013); here we give a brief outline. The AGTP gives the global-mean surface temperature response per kilogram of species emitted as a function of time after an emission pulse, i.e., an instantaneous one-off emission.…”

“…The AGTP for methane includes the direct forcing, as well as the effect of OH-induced changes in its lifetime and effects on tropospheric ozone and stratospheric water vapor. See Aamaas et al (2013) for details and analytical expressions for the AGTP of individual species. For CO 2 and methane, we calculate the global-mean F for year 2014 global concentrations (i.e., the year that is considered present-day in our emissions data; see below) using the equations from Etminan et al (2016).…”

“…Next, we quantify the temperature response to temporally evolving emissions from 1900 to 2100. The AGTP framework can readily be extended from pulse-based calculations since any scenario can be viewed as a series of pulse emissions and analyzed through convolution (Aamaas et al, 2013). The temperature response T at time t for species i is (for each region and sector) given by…”

“…For the aerosols and their precursors, we use the multi-model results from the Ae-roCom Phase II experiment (Myhre et al, 2013a), while for CO 2 , NH 3 , and ozone precursors, we use the uncertainties from the IPCC AR5 (Myhre et al, 2013b). For further details, see Aamaas et al (2019) and Lund et al (2017). Our temperature responses are also influenced by uncertainties in emissions and climate sensitivity.…”

“…While some SLCFs with a warming contribution to temperature change can, in part, be mitigated individually (in particular methane), improving air quality requires consideration of all relevant species. Removal of all present-day anthropogenic aerosols may add as much as 0.5 • C of additional global near-term warming according to recent work (Hienola et al, 2018;Samset et al, 2018;Aamaas et al, 2019). Due to co-emission, species such as sulfur dioxide (SO 2 ) are also commonly affected by measures to reduce climate warming even if these have LLGHGs as the primary target.…”