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

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34 K. Anderson and A. Bows food production), inputting the 2000–2049 cumulative total for each B6 scenario into PRIMAP will result in an underestimate of the probability of exceeding 2 ◦ C. To account for this underestimate, an alternative probability is calculated assuming the following. — The shorter-lived nature of methane compared with N2O results in a negligible impact on post-2050 warming from methane. 22 —N2O and methane each account for approximately 50 per cent of the non- CO2 greenhouse gases post-2050 (Smith, personal communication). — The amount of non-CO2 greenhouse gas emissions released per year possible post-2050 is 6 GtCO2e (in line with assumptions made by the UK CCC [8]) of which 3 GtCO2e per year is from N2O. — Thus 150 GtCO2e of cumulative emissions are added to the pre-2050 emissions to estimate an alternative probability. If 150 GtCO2e is added to the cumulative emission total for each scenario, PRIMAP estimates an approximate ten percentage point increase in probability of exceeding 2 ◦ C (see table 1, figure is in brackets). For example, B6 2 has at least a 39 per cent chance of exceeding 2 ◦ C and is potentially as high as 48 per cent. Both B6 3 and B6 4 take the IPCC’s ‘high’ cumulative budget as a constraint. B6 3 assumes non-Annex 1 nation emissions peak in 2025 following a growth of 3 per cent per year between 2010 and 2020 (figure 4). With steep emission reductions for non-Annex 1 nations post-peak of 6 per cent per year, Annex 1 nations would need to reduce emissions from 2010 onwards at more than 10 per cent per year to remain within the high IPCC cumulative budget. More gradual reductions in emissions from non-Annex 1 nations would render this scenario pathway impossible. B6 3 has the same probability of exceeding 2 ◦ CasB62. B6 4 mirrors the assumptions within C+3, with considerably slower growth of 1 per cent per year until a peak date in 2025 (figure 4). The emission reductions post-peak for non-Annex 1 nations are 4–5% per year, whereas for Annex 1 nations, following a levelling off of emissions until 2014, emissions decrease at 6 per cent per year. This scenario pathway has at least a 38 per cent probability of exceeding 2 ◦ C, and potentially as high as 47 per cent once post-2050 emissions are factored in. (e) Orthodox scenario pathways The final scenario pathways developed are unconstrained by a particular emission budget (figure 5). For both the C+ and B6 regimes, pathways are constructed that assume non-Annex 1 nations’ emissions continue to develop along their current trajectory until 2025, peaking in 2030, then reducing at 22 This is an explicitly conservative assumption and results in slightly higher probabilities of not exceeding 2 ◦ C than would be the case if some allowance were to be made for post-2050 emissions of methane. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
Table 1. Summary of scenario pathway characteristics. Annex 1 peak non-Annex 1 approximate % global 21st century date/21st peak date/21st of exceeding CO2 or century century Annex 1 % non-Annex 1 % post-peak 2◦C (based on greenhouse gas cumulative cumulative reduction on reduction on post-peak post-peak global rate of 2000–2049 budget in emissions emissions global 1990 levels 1990 levels Annex 1 non-Annex 1 reduction emissions scenario GtCO2 or in GtCO2 in GtCO2 peak by 2020 by 2020 rate of rate of (includes using pathway [GtCO2e] or [GtCO2e] or [GtCO2e] date (2050) (2050) reduction reduction deforestation) PRIMAP) Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010 Beyond dangerous climate change 35 C+1 1321 2007 2020 2012 56% (98%) +1714% (54%) 10–11% 6–7% 6–7% 36% 313 742 C+2a 1321 2007 2025 2007 100% (100%) +193% (27%) — 6–7% 6–7% 139 916 C+3 1321 2007 2025 2007 56% (98%) +143% (54%) 10–11% 7–8% 7–8% 37% 313 742 C+4 1578 2007 2020 2019 6% (84%) +186% (45%) 5–6% b 5–6% 5–6% 50% 532 780 C+5 1578 2007 2025 2020 44% (95%) +220% (32%) 8% 7–8% 7–8% 52% 363 949 C+6a 1578 2007 2025 2024 100% (100%) +220% (+38%) — 4–5% 4–5% — 153 1159 B6 1a,c [1376] 2007 2017 2017 95% (95%) 61% (61%) — — — — [265] [841] B6c 2 [2202] 2010 2020 2017 25% (82%) +135% (46%) 4–6% 5–6% 3% 39% (48%) d [639] [1293] B6c 3 [2202] 2007 2025 2013–2018 57% (95%) +154% (24%) 8–10% 6–7% 4–5% 39% (48%) d [429] [1503] B6c 4 [2202] 2007 2025 2013 34% (90%) +111% (17%) 6% 4–5% 4–5% 38% (47%) d [552] [1380] orthodox C+ 2741 2015 2030 2027 2% (60%) +223% (+163%) 3% 3% 3% 88% [729] 1747 orthodox B6 [3662] 2015 2030 2028 5% (62%) +180% (128%) 3% 3% 3% 88% (92%) d [891] [2501] aThese scenario pathways are not viable as they could not remain within the carbon budget prescribed. bThis is the reduction rate following the period of relatively stable emissions until 2016. cAll B6 scenario pathways assume an ‘emission floor’ of 6GtCO2e for food-related emissions for an approximate 9 billion population post-2050 until 2100. If a different ‘emission floor’ were to be used, emission reduction rates would be altered for the same cumulative values. dThe figure in brackets illustrates a higher probability to take into account the ongoing emissions associated with food production as opposed to greenhouse gas emissions tending to zero.
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REVIEW Phil. Trans. R. Soc. A (2011
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Editorial Editorial 3 D. Garner Pre
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