Four degrees and beyond: the potential for a global ... - Amper

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36 K. Anderson and A. Bows (a) (b) GtCO 2e yr –1 60 40 20 0 2000 2020 2040 2060 year 2080 2100 2000 2020 2040 2060 2080 2100 year Figure 5. Emission scenarios for approximately 88–92% chance of not exceeding 2 ◦ C. Both plots illustrate ‘orthodox’ mitigation pathways with (a) C+ for CO2 only (twenty-first century cumulative emissions: 2741 GtCO2) and (b) B6 for Kyoto gases (twenty-first century cumulative emissions: 3662 GtCO2e). Blue line, Annex 1; red line, non-Annex 1; dotted line, total including deforestation. a rate considered politically and economically acceptable 23 (3% reduction per year). For the Annex 1 nations, emissions are assumed to be relatively stable with a peak in 2016 and subsequent emission reductions of 3 per cent per year. These scenario pathways both result in an 88 per cent chance of exceeding the 2 ◦ C threshold (potentially 92% for orthodox B6). Furthermore, their twenty-first century cumulative budgets suggest the future temperature increase compared with pre-industrial times is more likely to be of the order of 4 ◦ C rather than 2 ◦ C. For orthodox C+, cumulative emissions of CO2 alone are 2,741 GtCO2. Cumulative emissions of approximately 2700 GtCO2 are associated with stabilization of 550 ppmv CO2. Figures in excess of 3500 GtCO2e can be assumed to be closer to the 750 ppmv range. Orthodox B6 has cumulative emissions of greenhouse gases of 3662 GtCO2e, thus a reasonable probability of exceeding 4 ◦ C. 5. Discussion (a) CO2 plus (C+) Although six C+ pathways were developed in the previous section, two were rejected for exceeding the constraints on cumulative CO2 budgets related to the 2◦C temperature threshold. One exceeded the lower of the two chosen 23The pathway of the CCC’s [8] most challenging scenario, ‘2016:4% low’, has post-peak emissions reducing at 3.5 per cent per year from all sources. Stern [6] states ‘there is likely to be a maximum practical rate at which global emissions can be reduced’ pointing to ‘examples of sustained emissions cuts of up to 1 per cent per year associated with structural change in energy systems’ and that ‘cuts in emissions greater than this (1%) have historically been associated only with economic recession or upheaval’. Stern concludes ‘it is likely to be difficult to reduce emissions faster than around 3 per cent per year’ [6, pp. 201–204]. The most stringent of the ADAM scenarios assumed emission reduction rates of approximately 3 per cent per year between a 2015 peak and 2050 [47 fig. 2, p. 32]. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
Beyond dangerous climate change 37 budgets (approx. 37% of not exceeding 2 ◦ C) owing to non-Annex 1 emissions both continuing with recent growth rates out to 2020 and peaking in 2025. The other surpassed the higher of the two cumulative budgets (approx. 50% of not exceeding 2 ◦ C) because, in addition to the emissions growth and peak years, post-peak emission reductions of 4–5% per year were insufficient to stay within the cumulative budget. Those pathways remaining within the lower budget required an immediate and rapid decline in Annex 1 emissions and an early peak in non-Annex 1 emissions, unless the latter’s emission growth was constrained to rates much lower than historical trends (i.e. to 1%, compared with current growth of 3–4% per year [48]). In all cases, Annex 1 emissions continue to decline following the current economic downturn at rates in excess of 5 per cent per year. Given these pathways explicitly exclude non-CO2 greenhouse gas emissions, the rates of reduction for CO2 presented in table 1 illustrate the change necessary within the energy system primarily. Furthermore, figures 1 and 2 illustrate the need for complete decarbonization of the Annex 1 energy system by around 2050. For the higher (approx. 50%) chance of not exceeding 2 ◦ C, figure 2 and table 1 illustrate that a 5 year delay in the peak year for non-Annex 1 nations (from 2020 to 2025) forces a 2 per cent increase in reduction rates globally in addition to an immediate emission reduction for Annex 1 nations. (b) Basket of six (B6) When developing the B6 pathways (figures 3 and 4), it became apparent immediately that the ‘low’ IPCC cumulative emission value was not viable if non-Annex 1 emissions peak as late as 2020. Consequently, the minimum probability of not exceeding the 2◦C threshold achievable in this scenario pathway set is 38 per cent. Moreover, given a significant portion of emissions are attributable to food production post-2050, this probability is likely to be a significant underestimate, with a more probable figure closer to 48 per cent. This result is in line with the scenario pathway analysis within Anderson & Bows [2]. For the IPCC’s ‘high’ end of the range, emission reductions of 6 per cent per year for both Annex 1 and non-Annex 1 nations are necessary if non- Annex 1 emissions continue at recent growth rates and peak in 2020. However, if these rates are sustained for a further five years (i.e. to 2025), and non- Annex 1 post-peak reductions are 6 per cent per year or less, no emissions space remains for Annex 1 nations. Even if non-Annex 1 emissions grow at much slower rates to a 2025 peak, post-peak emission reductions of 4–5 per cent per year are still needed from the aggregate of all nations. Therefore, under the IPCC’s higher budget, all viable scenario pathways exhibit emission reduction rates well in excess of those typically considered to be politically and economically feasible. (c) Orthodox In light of the recent Copenhagen negotiations, there continues to be an absence of any meaningful global action to mitigate emissions or set binding targets. Even if Annex 1 nations agree on the scale of necessary emission reductions, it is more Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
Page 1 and 2: ISSN 1364-503X volume 369 number 19
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Water availability in +2 ◦ C and
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(ii) Water stress index Water avail
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(a) number of models, black line nu
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change in run-off (%) change in run
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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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