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ON the RISK of OVERSHOOTING 2°C Paper Presented at Scientific Symposium “Avoiding Dangerous Climate Change”, Metoffice, Exeter, 1-3 February 2005

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ON the RISK of OVERSHOOTING 2°C Paper Presented at Scientific Symposium “Avoiding Dangerous Climate Change”, Metoffice, Exeter, 1-3 February 2005 MEINSHAUSEN – ON THE RISK OF OVERSHOOTING 2°C Paper presented at Scientific Symposium “Avoiding Dangerous Climate Change”, MetOffice, Exeter, 1-3 February 2005 On the Risk of Overshooting 2°C Malte Meinshausen Swiss Federal Institute of Technology (ETH Zurich), Environmental Physics, Department of Environmental Sciences, Rämistrasse 101, CH-8092 Zurich, Switzerland, Email: [email protected] Abstract This article explores different greenhouse gas stabilization levels and their implied risks of overshooting certain temperature targets, such as limiting global mean temperature rise to 2°C above pre-industrial levels. The probabilistic assessment is derived from a compilation of recent estimates of the uncertainties in climate sensitivity, which summarizes the key uncertainties in climate science for long-term temperature projections. The risk of overshooting 2°C equilibrium warming is found to lie between 68% and 99% for stabilization at 550ppm CO2 equivalence. Only at levels around 400ppm CO2 equivalence are the risks of overshooting low enough so that the achievement of a 2°C target can be termed “likely”. Based on characteristics of 54 IPCC SRES and post-SRES scenarios, multi-gas emission pathways are presented that lead to stabilization at 550, 450 and 400ppm CO2eq in order to assess the implications for global emission reductions. Sensitivity studies on delayed global action show that the next 5 to 15 years might determine whether the risk of overshooting 2°C can be limited to a reasonable range. 1. Introduction For these reasons, this study focuses on an 2 analysis of the risk of overshooting global mean In 1996, the European Council adopted a temperature levels of 2°C above pre-industrial climate target that reads “[…] the Council believes levels. In order to allow for a more comprehensive that global average temperatures should not climate impact risk assessment for different exceed 2 degrees above pre-industrial level”. This greenhouse gas stabilization levels, temperature target has since been reaffirmed by the EU on a levels between 1.5°C and 4°C are also analyzed. number of occasions, including as recently as December 20041. 2. Method However, reviews of the scientific literature on climate impacts largely conclude that a temperature Climate sensitivity is the expected equilibrium increase of 2°C above pre-industrial levels can not warming for doubled pre-industrial CO2 be regarded as ‘safe’. For example, the loss of the concentrations (2x278=~556ppm) (see Figure 1)3. Greenland ice-sheet may be triggered by a local Since the IPCC TAR, some key studies [13-18] temperature increase of approximately 2.7°C [1, 2], have published ranges and probability density which could correspond to a global mean functions (PDFs) for climate sensitivity. Using a temperature increase of less than 2°C. This loss is standard formula for the radiative forcing ∆Q likely to cause a sea level rise of 7 meters over the caused by increased CO2 concentrations C above next 1000 years or more [2]. Similarly, unique pre-industrial levels Co (∆Q =5.35ln(C/Co))[20], one ecosystems, such as coral reefs, the Arctic, and can derive equilibrium temperatures ∆Teq for any alpine regions, are increasingly under pressure and CO2 (equivalent) concentration and climate may be severely damaged by global mean sensitivity ∆T2xCO2 as ∆Teq=∆T2xCO2(∆Q/5.35ln(2)). temperature increases of 2°C or below [3-5]. Thus, the risk R(∆Tcrit,PDFi,C) of overshooting a Beyond 2°C, climate impacts are likely to increase certain warming threshold ∆Tcrit when stabilizing substantially. A sea level rise of up to 3-5 meters by 2300 is possible for a 3°C global mean warming [6] due to, among other factors, the disintegration of 2 Note that throughout this paper, the term ‘risk of overshooting’ is the West Antarctic Ice-Sheet [7]. Other large scale used for the ‘probability of exceeding a threshold’. Technically speaking, ‘risk’ is thereby used as describing the product of likelihood discontinuities are increasingly likely for higher and consequence with the consequence being sketched as a step temperatures, such as strong carbon cycle function around the threshold. feedbacks, [8-10] or potentially large, but still very 3 In this study, when necessary, climate sensitivity PDFs have been uncertain, methane releases from thawing truncated at 10°C. Furthermore, the 90% uncertainty range given by Schneider von Deimling [17] for tropical sea surface temperatures permafrost or ocean methane hydrates [11, 12]. between 2.5°C and 3°C during the last glacial maximum has been translated into a lognormal PDF in the same way as the conventional IPCC 1.5°C to 4.5°C range has been translated into a PDF by Wigley and Raper [19]. Note as well that the climate sensitivity PDF by 1 2632nd Council Meeting Environment, Luxembourg, see Andronova and Schlesinger is the one that includes solar and aerosol http://ue.eu.int/ueDocs/cms_Data/docs/pressData/en/envir/83237.pdf forcing. 1/6 MEINSHAUSEN – ON THE RISK OF OVERSHOOTING 2°C Paper presented at Scientific Symposium “Avoiding Dangerous Climate Change”, MetOffice, Exeter, 1-3 February 2005 CO2 (equivalent) concentrations at level C can be 3. The risk of overshooting 2°C in calculated as the integral ∞ equilibrium R ∆=T,, PDF C PDF x ln(2)ln C C dx where ()crit i∫ i()()() o ∆Tcrit At 550ppm CO2 equivalence (corresponding PDFi(∆T2xCO2) is the assumed probability density for approximately to a stabilization at 475ppm CO2 climate sensitivity ∆T2xCO2. only), the risk of overshooting 2°C is very high, In contrast to the parameterized calculations, ranging between 68% and 99% for the different transient probabilistic temperature evolutions were climate sensitivity PDFs with a mean of 85%. In computed for this study with a simple climate other words, the probability that warming will stay model, namely the upwelling diffusion energy below 2°C could be categorized as ‘unlikely’ using balance model MAGICC 4.1 by Wigley, Raper et al. the IPCC WGI terminology4. If greenhouse gas [21]. As this study focuses on long-term concentrations were to be stabilized at 450ppm temperature projections, the probabilistic treatment CO2eq then the risk of exceeding 2°C would be of uncertainties has been confined to the climate lower, in the range of 26% to 78% (mean 47%), but sensitivity on the basis of the above cited still significant. In other words, 7 out of the 8 studies probability density estimates. For other analyzed suggest that there is either a “medium uncertainties, this study assumes IPCC TAR ‘best likelihood” or “unlikely” chance to stay below 2°C. estimate’ parameters, such as those related to Only for a stabilization level of 400ppm CO2eq and climate system inertia and carbon cycle feedbacks. below can warming below 2°C be roughly classified Assumptions in regard to solar and volcanic forcing as ‘likely’ (risk of overshooting between 2% and are described elsewhere [22]. 57% with mean 27%). The risk of exceeding 2°C at In order to assess the emission implications of equilibrium is further reduced, 0% to 31% (mean different stabilization levels, this study presents 8%), if greenhouse gases are stabilized at 350ppm new multi-gas emission pathways that were derived CO2eq (see Figure 2). Radiative Forcing (W/m2) by the ‘Equal Quantile Walk’ method [23] on the 1.23 1.95 2.58 3.14 3.65 4.12 4.54 4.94 5.31 100% basis of 54 existing IPCC SRES and Post-SRES very scenarios. The emissions that have been adapted 90% unlikely 80% to meet the pre-defined stabilization targets include 70% unlikely those of all major greenhouse gases (fossil CO , 2 60% land use CO2, CH4, N2O, HFCs, PFCs, SF6), ozone 50% medium precursors (VOC, CO, NOx) and sulphur aerosols 40% likelihood (SO2). The basic idea behind the ‘Equal Quantile 30% Andronova and Schlesinger (2001) - with sol.&aer. forcing Forest et al. (2002) - Expert priors Terminology) (IPCC Forest et al. (2002) - Uniform priors Walk’ method is that emissions for all gases of the Gregory et al. (2002) 20% likely Knutti et al. (2003) Murphy et al. (2004) 2˚C below stay to Probability new emission pathway are in the same quantile of Schneider et al. (in prep.) - trop. SST 2.5˚C-3˚C Risk of overshooting 2˚C warming Risk of overshooting 10% Wigley and Raper (2001) - IPCC lognormal very the existing distribution of IPCC SRES and post- 0% likely 350 400 450 500 550 600 650 700 750 SRES scenarios. In other words, if fossil CO 2 CO2 equivalence stabilization level emissions are assumed in the lower 10% region of the existing SRES and Post-SRES scenario pool, Figure 2 – The risk of overshooting 2°C global mean equilibrium warming for different then methane, N2O and all other emissions are designed to also be in the pool’s respective lower CO2 equivalent stabilization levels. 10% region [23]. The CO2 equivalent (CO2eq) stabilization 4. The risk of overshooting different targets are here defined as the CO2 concentrations that would correspond to the same radiative forcing warming levels as caused by all human-induced increases in concentrations of greenhouse gases, tropospheric For a more comprehensive climate impact ozone and sulphur aerosols. assessment across different stabilization levels, it is warranted to also include the lower risk / higher 0.9 ) magnitude adverse climate impacts that can be 0.8 -1 Andronova and Schlesinger (2001) - with sol.&aer. forcing Forest et al. (2002) - Expert priors 0.7 Forest et al. (2002) - Uniform priors expected at higher temperature levels. Given that Gregory et al. (2002) 0.6 Knutti et al. (2003) Murphy et al. (2004) climate sensitivity PDFs largely differ on the 0.5 Schneider et al. (in prep.) - trop.
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