Regional temperature change potentials for short-lived climate forcers based on radiative forcing from multiple models

Atmos. Chem. Phys., 17, 10795–10809, 2017 https://doi.org/10.5194/acp-17-10795-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Regional temperature change potentials for short-lived climate forcers based on radiative forcing from multiple models Borgar Aamaas1 , Terje K. Berntsen1,2 , Jan S. Fuglestvedt1 , Keith P. Shine3 , and William J. Collins3 1 CICERO Center for International Climate Research, PB 1129 Blindern, 0318 Oslo, Norway 2 Department of Geosciences, University of Oslo, Oslo, Norway 3 Department of Meteorology, University of Reading, Reading RG6 6BB, UK Correspondence to: Borgar Aamaas () Received: 14 February 2017 – Discussion started: 14 March 2017 Revised: 5 July 2017 – Accepted: 14 August 2017 – Published: 14 September 2017 Abstract. We calculate the absolute regional temperature change potential (ARTP) of various short-lived climate forcers (SLCFs) based on detailed radiative forcing (RF) calculations from four different models. The temperature response has been estimated for four latitude bands (90– 28◦ S, 28◦ S–28◦ N, 28–60◦ N, and 60–90◦ N). The regional pattern in climate response not only depends on the relationship between RF and surface temperature, but also on where and when emissions occurred and atmospheric transport, chemistry, interaction with clouds, and deposition. We present four emissions cases covering Europe, East Asia, the global shipping sector, and the entire globe. Our study is the first to estimate ARTP values for emissions during Northern Hemisphere summer (May–October) and winter season (November–April). The species studied are aerosols and aerosol precursors (black carbon, organic carbon, SO2 , NH3 ), ozone precursors (NOx , CO, volatile organic compound), and methane (CH4 ). For the response to BC in the Arctic, we take into account the vertical structure of the RF in the atmosphere, and an enhanced climate efficacy for BC deposition on snow. Of all SLCFs, BC is the most sensitive to where and when the emissions occur, as well as giving the largest difference in response between the latitude bands. The temperature response in the Arctic per unit BC emission is almost four times larger and more than two times larger than the global average for Northern Hemisphere winter emissions for Europe and East Asia, respectively. The latitudinal breakdown likely gives a better estimate of the global temperature response as it accounts for varying efficacies with latitude. An annual pulse of non-methane SLCF emissions globally (representative of 2008) lead to a global cooling. In contrast, winter emissions in Europe and East Asia give a net warming in the Arctic due to significant warming from BC deposition on snow. 1 Introduction Climate is influenced by a multitude of emissions with varying impacts (e.g. Myhre et al., 2013). Emissions of shortlived climate forcers (SLCFs), such as black carbon (BC), organic carbon (OC), SO2 , NH3 , NOx , CO, and volatile organic compounds (VOCs), affect the composition of the atmosphere primarily on timescales of days to a few months. CH4 is included in the definition because its lifetime of around 10 years is shorter than timescales for stabilizing the climate (Aamaas et al., 2016). The variation in the geographical pattern of SLCF emissions has changed over time, with emissions typically being high in the early phases of industrialization, and then gradually being reduced due to air quality concerns and technological improvements. Nevertheless, emissions are still growing in many parts of the world, and there is a growing focus politically to develop a mitigation strategy for the SLCFs to achieve both improved air quality and slowing global warming (Schmale et al., 2014; Shindell et al., 2012; Stohl et al., 2015). Due to the short atmospheric lifetimes, emissions of SLCFs lead to a spatial pattern in radiative forcing (RF) that is more inhomogeneous than for emissions of longlived greenhouse gases such as CO2 . While we focus on RF from large emission regions, Bowman and Henze (2012) and Henze et al. (2012) showed that radiative forcing efficiencies Published by Copernicus Publications on behalf of the European Geosciences Union. 10796 B. Aamaas et al.: Regional temperature change potentials for short can vary by 1000 % for much smaller emission regions. It is well established that there is not a close relationship between the RF pattern and the surface temperature response pattern, due to modifications by heat transport in the atmosphere and ocean and the spatial variability in climate feedbacks (e.g. Boer and Yu, 2003). However, as shown by Shindell and Faluvegi (2009) and Shindell (2012), it is possible to establish relationships between the RF pattern caused by a certain constituent and the response in broad latitude bands. Recently, Najafi et al. (2015), have shown from observational and model data that there is a distinct difference in the Arctic response to the overall forcing by ozone, aerosols, and land use compared to other latitude bands. Emission metrics are simple tools based on comprehensive model simulations that relate emissions to a certain response (physical climate change or economic damage), e.g. Fuglestvedt et al. (2003) and Tol et al. (2012). The most widely used emission metric, the global warming potential (GWP), is given by the integrated RF (over a time horizon of H years) in response to a pulse emission. Shine et al. (2005) introduced the Global Temperature Change Potential (GTP) using the surface temperature change (after a time horizon of H years) for the response. Emissions metrics have typically estimated a global effect due to global emissions (e.g. Aamaas et al., 2013). A first step going beyond global means was to quantify the global response based on regional emissions for SLCFs (Fuglestvedt et al., 2010; Collins et al., 2013; Aamaas et al., 2016). By introducing the concept of regional temperature potentials (RTPs), Shindell and Faluvegi (2010) extended the metric concept to include regional responses (in terms of surface temperature change in broad latitude bands) from regional RFs. In addition to the regionality, the timing of the SLCFs emissions matter. This is potentially important, since the photochemistry in the atmosphere, lifetime, atmospheric transport, and forcing efficiency is likely to vary between the seasons. As some sources (e.g. domestic heating and agricultural waste burning) have a large seasonal cycle, using seasonal RTP metrics might have a significant impact on the evaluation of cost effectiveness of mitigation measures. Here we use detailed multimodel calculations of the relationship between emission location and the resulting specific RF (RF per Tg yr−1 emissions) for SLCFs (Bellouin et al., 2016; Sect. 2.1) and the regional climate sensitivities (e.g. Shindell and Faluvegi, 2009) to estimate ARTPs for a range of aerosols, aerosol precursors, and ozone precursors (BC, OC, SO2 , N (...truncated)