Climate impact of Finnish air pollutants and greenhouse gases using multiple emission metrics

Atmos. Chem. Phys., 19, 7743–7757, 2019 https://doi.org/10.5194/acp-19-7743-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Climate impact of Finnish air pollutants and greenhouse gases using multiple emission metrics Kaarle Juhana Kupiainen1 , Borgar Aamaas2 , Mikko Savolahti1 , Niko Karvosenoja1 , and Ville-Veikko Paunu1 1 Finnish Environment Institute, Mechelininkatu 34a, P.O. Box 140, 00251 Helsinki, Finland 2 CICERO Center for International Climate Research, PB 1129 Blindern, 0318 Oslo, Norway Correspondence: Kaarle Juhana Kupiainen (, ) Received: 11 October 2018 – Discussion started: 13 December 2018 Revised: 26 March 2019 – Accepted: 6 May 2019 – Published: 11 June 2019 Abstract. We present a case study where emission metric values from different studies are applied to estimate global and Arctic temperature impacts of emissions from a northern European country. This study assesses the climate impact of Finnish air pollutants and greenhouse gas emissions from 2000 to 2010, as well as future emissions until 2030. We consider both emission pulses and emission scenarios. The pollutants included are SO2 , NOx , NH3 , non-methane volatile organic compound (NMVOC), black carbon (BC), organic carbon (OC), CO, CO2 , CH4 and N2 O, and our study is the first one for Finland to include all of them in one coherent dataset. These pollutants have different atmospheric lifetimes and influence the climate differently; hence, we look at different climate metrics and time horizons. The study uses the global warming potential (GWP and GWP∗ ), the global temperature change potential (GTP) and the regional temperature change potential (RTP) with different timescales for estimating the climate impacts by species and sectors globally and in the Arctic. We compare the climate impacts of emissions occurring in winter and summer. This assessment is an example of how the climate impact of emissions from small countries and sources can be estimated, as it is challenging to use climate models to study the climate effect of national policies in a multi-pollutant situation. Our methods are applicable to other countries and regions and present a practical tool to analyze the climate impacts in multiple dimensions, such as assessing different sectors and mitigation measures. While our study focuses on short-lived climate forcers, we found that the CO2 emissions have the most significant climate impact, and the significance increases over longer time horizons. In the short term, emissions of especially CH4 and BC played an important role as well. The warming impact of BC emissions is enhanced during winter. Many metric choices are available, but our findings hold for most choices. 1 Introduction The Paris Agreement and its target of “holding the increase in the global average temperature to well below 2 ◦ C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 ◦ C above pre-industrial levels” (UNFCCC, 2015) provides an important framework for individual countries to consider the climate impacts and mitigation possibilities of its emissions. Globally, CO2 and greenhouse gas emissions are key components in achieving the targets of the agreement, but the role of short-lived climate forcers (SLCFs) should also be studied as additional drivers of the surface temperatures. The climate effect of emission reductions of air pollutants, particularly black carbon and tropospheric ozone, have been a focus of research in last few years (Shindell et al., 2012; Bond et al., 2013; Smith and Mizrahi, 2013; Stohl et al., 2015). Since air pollutants can either cool or warm the climate on different timescales depending on the species, emission reduction policies from a climate perspective have to be designed to take into account the net effect of multiple pollutants (UNEP/WMO, 2011; Stohl et al., 2015). The pollutants considered to have most climate relevance are termed short-lived climate pollutants (SLCP) or short-lived climate forcers (SLCF), depending on the context. However, there is no common agreement on the definition of SLCPs or SLCFs. In this study we use the terms as in the Intergovernmental Panel on Climate Change’s (IPCC) special report Global Warming of 1.5 ◦ C (IPCC, 2019) where Published by Copernicus Publications on behalf of the European Geosciences Union. 7744 K. J. Kupiainen et al.: Climate impact of Finnish air pollutants and greenhouse gases (1) SLCFs refer to both cooling and warming species and include methane (CH4 ), ozone (O3 ) and aerosols (i.e., black carbon, BC, organic carbon, OC, and sulfate) or their precursors, as well as some halogenated species, and (2) SLCPs refer only to the warming SLCFs. Policies focusing on SLCPs have been suggested as supplements to greenhouse gas reductions (UNEP/WMO, 2011; Shindell et al., 2012, 2017; Rogelj et al., 2014; Stohl et al., 2015). Modeling studies by UNEP/WMO (2011) and Stohl et al. (2015) suggested that the climate response of SLCF mitigation is strongest in the Arctic region. The Arctic region is of particular interest, since in the past 50 years the Arctic has been warming twice as rapidly as the world as a whole and has experienced significant changes in ice and snow covers as well as permafrost (AMAP, 2017). AMAP (2011, 2015) as well as Sand et al. (2016) demonstrated that emission reductions of SLCFs in the northern areas have the largest temperature response to the Arctic climate per unit of emissions reduced, with the Nordic countries (Denmark, Finland, Iceland, Norway and Sweden) and Russia having the largest impact when compared to the other Arctic countries, the United States of America and Canada. Shindell et al. (2017) and Ocko et al. (2017) have argued for assessing both near- and long-term effects of climate policy. However, comparing the climate impacts of SLCFs, CO2 and other pollutants is not straightforward. Emission metrics are one way of enabling a comparison as they provide a conversion rate between emissions of different species into a common unit, for example CO2 -equivalent emissions. Common emission metrics are the global warming potential (GWP) (IPCC, 1990) and the global temperature change potential (GTP) (Shine et al., 2005). The GWP compares the integrated radiative forcing (RF) of a pulse emission of a given species relative to the integrated RF of a pulse emission of CO2 . Since the United Nations Framework Convention on Climate Change (UNFCCC) reporting procedure uses the GWP with a 100-year time horizon (GWP100) as a reporting guideline, it has become the most common metric to report greenhouse gas emissions. The GTP is an alternative to GWP and it compares the temperature change at a point in time due to a pulse emission of a species relative to the temperature change of a pulse emission of CO2 . The GTP combines the changes in the radiative forcing induced by the different species with the temperature response (...truncated)