Transport impacts on atmosphere and climate: Metrics J.S. Fuglestvedt a , K.P. Shine b, * , T. Berntsen a , J. Cook b , D.S. Lee c , A. Stenke d , R.B. Skeie a , G.J.M. Velders e , I.A. Waitz f a CICEROdCenter for International Climate and Environmental ResearchdOslo, P.O. Box 1129, Blindern, 0318 Oslo, Norway b Department of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, UK c Dalton Research Institute, Department of Environmental and Geographical Sciences, Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester M1 5GD, UK d DLR, Institut fu ¨r Physik der Atmospha ¨re, Oberpfaffenhofen, D-82234 Wessling, Germany e Netherlands Environmental Assessment Agency, P.O. Box 303, 3720 AH Bilthoven, The Netherlands f Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA article info Article history: Received 18 July 2008 Received in revised form 15 April 2009 Accepted 16 April 2009 Keywords: Transport Climate change Global warming potential GWP Global Temperature Change Potential (GTP) Radiative forcing abstract The transport sector emits a wide variety of gases and aerosols, with distinctly different characteristics which influence climate directly and indirectly via chemical and physical processes. Tools that allow these emissions to be placed on some kind of common scale in terms of their impact on climate have a number of possible uses such as: in agreements and emission trading schemes; when considering potential trade-offs between changes in emissions resulting from technological or operational devel- opments; and/or for comparing the impact of different environmental impacts of transport activities. Many of the non-CO 2 emissions from the transport sector are short-lived substances, not currently covered by the Kyoto Protocol. There are formidable difficulties in developing metrics and these are particularly acute for such short-lived species. One difficulty concerns the choice of an appropriate structure for the metric (which may depend on, for example, the design of any climate policy it is intended to serve) and the associated value judgements on the appropriate time periods to consider; these choices affect the perception of the relative importance of short- and long-lived species. A second difficulty is the quantification of input parameters (due to underlying uncertainty in atmospheric processes). In addition, for some transport-related emissions, the values of metrics (unlike the gases included in the Kyoto Protocol) depend on where and when the emissions are introduced into the atmosphere – both the regional distribution and, for aircraft, the distribution as a function of altitude, are important. In this assessment of such metrics, we present Global Warming Potentials (GWPs) as these have tradi- tionally been used in the implementation of climate policy. We also present Global Temperature Change Potentials (GTPs) as an alternative metric, as this, or a similar metric may be more appropriate for use in some circumstances. We use radiative forcings and lifetimes from the literature to derive GWPs and GTPs for the main transport-related emissions, and discuss the uncertainties in these estimates. We find large variations in metric (GWP and GTP) values for NO x , mainly due to the dependence on location of emissions but also because of inter-model differences and differences in experimental design. For aerosols we give only global-mean values due to an inconsistent picture amongst available studies regarding regional dependence. The uncertainty in the presented metric values reflects the current state of understanding; the ranking of the various components with respect to our confidence in the given metric values is also given. While the focus is mostly on metrics for comparing the climate impact of emissions, many of the issues are equally relevant for stratospheric ozone depletion metrics, which are also discussed. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Transportation is an important contributor to global emissions of many different gases and aerosols that can have an impact on climate and stratospheric ozone, either directly or indirectly (e.g. Eyring et al., 2005; Fuglestvedt et al., 2008; Kahn Ribeiro et al., * Corresponding author. Tel.: þ44 118 378 8405; fax: þ44 118 378 8905. E-mail address: k.p.shine@reading.ac.uk (K.P. Shine). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.04.044 Atmospheric Environment 44 (2010) 4648–4677