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68 R. A. Betts et al. 1. Introduction The Working Group I (WGI) volume of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4; [1]) assessed the global climate change projected to result from six scenarios of greenhouse-gas aerosol emissions, taken from a larger set of scenarios from the IPCC Special Report on Emissions Scenarios (SRES; [2]). These six SRES ‘marker scenarios’ are identified as A1FI, A1B, A1T, A2, B1 and B2 and are discussed in more detail in §2. The scenarios represent the emissions that would be consistent with a range of plausible future trajectories of population, economic growth and technology change, without policies to specifically reduce emissions in order to address climate change. Even though the possibility of reducing emissions through climate policy was not included in these scenarios, they still project a very wide range of emissions (figure 1). Considering emissions over the entire twenty-first century, the lowest cumulative emissions are projected by the B1 scenario and the highest by A1FI. All six scenarios were considered by the IPCC to be equally sound; no scenario was considered to be more or less likely than any others [1]. The IPCC WGI assessed climate change under these scenarios from a large number of different climate models of varying levels of complexity, including ocean–atmosphere general circulation models (GCMs) and simple climate models (SCMs), with some models also including feedbacks between climate change and the carbon cycle. The IPCC used these model projections, along with observational constraints, to inform an expert assessment of the likely range of global warming that would arise from each scenario [4]. The conclusion was that under the six SRES marker scenarios, global mean temperatures are likely to increase by between 1.1 ◦ C and 6.4 ◦ C by the end of the twenty-first century, relative to the 1980–1999 average (figure 2). To present these changes relative to the usual policy-relevant baseline of pre-industrial rather than relative to 1980– 1999, the IPCC recommended adding 0.5 ◦ C[5]. This implies that the likely range of global warming relative to pre-industrial under the SRES scenarios is 1.6 ◦ C and 6.9 ◦ C. Although the six scenarios were all considered by the IPCC to be equally sound as representations of a world that does not implement policies specifically to mitigate climate change [1], not all the scenarios were examined to the same depth with climate models. Practical reasons, such as computational costs, meant that only a subset of the scenarios (A1B, A2 and B1) could be systematically examined with complex ocean–atmosphere GCMs from all the participating modelling groups. 1 SCMs were then used to estimate the warming that would have been projected by the complex models under the other scenarios (B2, A1T and A1FI). Consequently, in the AR4, the highest emissions scenario (A1FI) was examined only with SCMs and not directly with complex ocean–atmosphere GCMs [4]. The B1, A1B and A2 projections are shown in the main part of figure 2 (reproduced from the AR4) with multi-model means represented by the coloured lines and the model spread (5–95%) illustrated by the coloured plumes. The likely range of warming for these scenarios and that for the B2, A1T and A1FI projections are represented by the grey bars on the right-hand side. The best 1 A small number of groups had previously examined A1FI. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
emissions (GtC) Review. Global warming reaching 4 ◦ C 69 40 30 20 10 0 1950 1975 2000 2025 year 2050 2075 2100 Figure 1. Emissions of CO2 from fossil fuel in the six SRES marker scenarios (black curves) and the full range of SRES scenarios (grey plume). The SRES scenarios also include emissions of non-CO2 greenhouse gases, aerosols and emissions from land-use change, which are not included in this figure. Filled circles, A1FI; open circles, A2; filled squares, A1B; open triangles, B2; filled triangles, B1; open squares, A1T. Reproduced with permission from van Vuuren & Riahi [3]. Copyright © Springer Science + Business Media B.V. 2008. global surface warming (°C) 6 5 4 3 2 1 0 –1 1900 2000 2100 year B1 A1T B2 A1B A2 A1F1 4°C above pre-industrial Figure 2. Past changes in global mean temperature (black curve), and projected future changes resulting from the IPCC SRES marker scenarios of greenhouse-gas and aerosol emissions (coloured curves and grey bars), relative to the 1980–1999 mean [4]. Climate changes under the A2, A1B and B1 scenarios were projected with GCMs (red, green and blue lines, with plumes showing 5–95% range of model projections without uncertainties in climate–carbon-cycle feedbacks). The full set of marker scenarios including a range of strengths of climate–carbon-cycle feedbacks were examined with SCMs. Grey bars show the likely range of warming at 2090–2099 for each scenario, from expert assessment based on all available evidence from GCMs, SCMs and observational constraints. The red dashed line marks warming of 3.5 ◦ C relative to 1980–1999, which represents 4 ◦ C relative to pre-industrial [5]. Red line, A2; green line, A1B; blue line, B1; orange line, year 2000 constant concentrations; black line, twentieth century. Copyright © IPCC, 2007. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
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ISSN 1364-503X volume 369 number 19
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Phil. Trans. R. Soc. A (2011) 369,
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Editorial David Garner Phil. Trans.
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Four degrees and beyond: the potent
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Preface 5 commitments might give us
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8 M. New et al. In the final plenar
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16 M. New et al. as permanent crop
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118 P. K. Thornton et al. 1. Introd
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122 P. K. Thornton et al. Table 1.
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REVIEW Phil. Trans. R. Soc. A (2011
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Review. Interactions in a 4 ◦ C w
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spread in mosquitoborne disease may
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Editorial Editorial 3 D. Garner Pre
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