Metrics for Comparison of Climate Impacts from Well Mixed
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1 ACCRI Theme 7 2 3 Metrics for comparison of climate impacts from well mixed greenhouse gases and 4 inhomogeneous forcing such as those from UT/LS ozone, contrails and contrail- 5 cirrus 6 7 Piers Forster & Helen Rogers 8 9 Acknowledgement: The numbers in Table 5 and a many of the ideas are derived from a 10 unpublished manuscript led by Keith Shine that Piers Forster and Helen Rogers were co- 11 author of. 12 13 Executive Summary.......................................................................................................... 2 14 1. Introduction and Background ................................................................................. 5 15 2. Review ........................................................................................................................ 8 16 2.1. Current state of science....................................................................................... 8 17 2.1.1. Air travel – its emissions and its trends ...................................................... 8 18 2.1.2. Aviation’s climate impact......................................................................... 10 19 2.1.3. Review of the RF characteristics and uncertainties of mechanisms ......... 12 20 2.1.3.1. Chemistry of importance to aviation..................................................... 12 21 2.1.3.2. Modelling the impact of aviation.......................................................... 14 22 2.1.4. Regional and timescale issues................................................................... 16 23 2.2. Critical role of the specific theme..................................................................... 18 24 2.2.1. Advancements since the IPCC 1999 report .............................................. 18 25 2.2.2. What is a metric? ...................................................................................... 19 26 2.2.2.1. Non-emission based metrics ................................................................. 20 27 2.2.2.2. Emission based metrics......................................................................... 21 28 2.2.3. Uncertainties of metric approaches........................................................... 25 29 2.2.4. Incorrect application of metrics – Radiative Forcing Index, an example . 28 30 2.3. Present state of measurements and data analysis.............................................. 30 31 2.4. Present state of modeling capability/best approaches....................................... 31 32 2.5. Interconnectivity with other SSWP theme areas .............................................. 37 33 3. Outstanding limitations, gaps and issues that need improvement ..................... 38 34 3.1. Science .............................................................................................................. 38 35 3.2. Measurements, analysis and modelling capability............................................ 39 36 3.3. Interconnectivity with other SSWP theme areas .............................................. 41 37 3.4. Interconnectivity with comprehensive transport policy.................................... 41 38 3.4.1. Policy interface issues............................................................................... 41 39 3.4.2. Interface with air-quality........................................................................... 44 40 3.4.3. Comparison to other sectors...................................................................... 44 41 4. Prioritization for tackling outstanding issues....................................................... 47 42 5. Recommendations for best use of current tools for modeling and data analysis 43 49 44 5.1. Options.............................................................................................................. 49 45 5.2. Supporting rationale.......................................................................................... 50 46 5.3. How to best integrate best available options?................................................... 51 1 1 6. References................................................................................................................ 52 2 3 4 5 Executive Summary 6 7 Issues of Contention 8 9 The United Nations Framework Convention on Climate Change (UNFCC) entered into 10 force in 1994 with the objective for ‘stabilization of greenhouse gas concentrations in the 11 atmosphere at a level that would prevent dangerous anthropogenic interference with the 12 climate system’. The Kyoto Protocol (1997) set out to reduce emissions of most long- 13 lived greenhouse gases in developed countries to below their 1990 levels. Probably as a 14 result of convenience and simplicity, the chosen metric to compare the climate impact of 15 these greenhouse gases was the 100-year Global Warming Potential (GWP), as calculated 16 by the Intergovernmental Panel of Climate Change Second Assessment Report (IPCC, 17 1995). 18 19 As an integral and growing part of the global economy and transportation sector, aviation 20 has the potential to significantly contribute to changes in the Earth’s climate. However, 21 the impact of short-lived species (e.g. nitrogen oxides (NOx), an ozone precursor which in 22 turn impacts on methane) and effects (e.g. aviation induced contrails) on the climate 23 system depends upon geographical and altitudinal location, season, time of the day and 24 the background meteorology and chemistry during their release (Rogers et al., 2000; 25 Sausen et al., 2005). Such short-lived species therefore require an appropriate metric 26 which takes into consideration these dependencies (Rogers et al., 2002a). For the aviation 27 sector the potential climate impact is dependent upon both long-lived and short-lived 28 emissions and effects, making the choice of a suitable metric that integrates over all 29 effects more difficult. 30 31 Gaps 32 33 In 1999, the Intergovernmental Panel on Climate Change published a landmark report, 34 ‘Aviation and the Global Atmosphere’ (IPCC, 1999) which saw the first sectoral 35 examination by the IPCC and estimates of the potential impact resulting from aircraft 36 emissions and their effects. The IPCC (1999) report identified the factors that influence 37 climate. Using radiative forcing as the chosen metric, it found that aviation gives a small 38 but significant climate forcing that is somewhat uncertain in overall magnitude. However, 39 the IPCC (1999) report came out strongly against the use of GWPs in the context of 40 aircraft emissions. In contrast, the most recent IPCC (2007) report presented a range of 41 possible GWPs for aviation NOx emissions, although not for other aviation effects 42 (Forster et al., 2007). 43 44 Due to a pressing need to provide policy-relevant answers to regulatory bodies and 45 industry, many researchers have developed their own metrics to assess the impact of 46 these short-lived species. Unfortunately, these approaches are often scientifically flawed. 47 2 1 The strong statements of IPCC (1999) have certainly affected the landscape of metric 2 design not only for aviation but also for other sectors. With climate change very much on 3 the agenda of international policy and with a need to quantify the climate impact of 4 human emissions, metric evaluation and metric design literature has flourished. Metric 5 design is no longer solely undertaken by physical scientists, but social scientists, 6 economists and industry are developing a plethora of metrics to suit individual needs. 7 8 Limitations 9 10 There is considerable controversy about the application of emission metrics to assess the 11 effect of aviation non-CO2 emissions. IPCC (1999) stated that the global warming 12 potential “has flaws that make its use questionable for aviation emissions” and that “there 13 is a basic impossibility of defining a GWP for aircraft NOx”. Wit et al. (2005) echo these 14 sentiments, concluding that “GWPs are not a useful tool for calculating the complete 15 suite of aircraft effects”. An undesirable side effect of the negative stance is that it has led 16 some policymakers and other groups to apply a Radiative Forcing Index (RFI) as if it is 17 some kind of alternative to the GWP (see Forster et al., 2006). 18 19 It is certainly true that major caveats are required in the presentation and application of 20 any currently proposed emissions metric. However, it needs to be clearly recognised that 21 some difficulties are not a function of the metric design but are due to more fundamental 22 limitations of our understanding of atmospheric processes. One example is the impact of 23 persistent contrails on cirrus clouds; these certainly do preclude confident evaluation of 24 values of GWPs, but the problem is much deeper than the evaluation of metrics – any 25 attempt to quantify their impact, using even the most sophisticated climate models, would 26 face similar limitations. Other limitations are more structural, such as the problem in 27 using global-mean values for NOx emissions, when compensation between negative 28 forcings at a global level may not apply at the hemispheric level. 29 30 Priorities 31 32 A list of recommended priorities for tackling the outstanding issues related to the 33 development and implementation of an appropriate metric for determining aviation’s 34 climate impact are given below: All of the tasks listed are achievable and will 35 significantly improve our understanding of climate impacts whilst reducing scientific 36 uncertainty 37 38 • Understand that metric choice is not solely a science issue