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

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Sea-level rise and possible impacts 163 forced displacement 1 of a large proportion of the coastal population and economy. However, humans also adapt proactively to these changes via a range of measures, which can be characterized as protection, accommodation or (planned) retreat [2,8]. Such adaptation can greatly reduce the possible impacts. Most analyses have contrasted the simplest case of protection versus retreat (or land abandonment). While protection has significant costs, the available analyses suggest that in densely populated coastal areas, protection costs are generally much less than the avoided impacts, and protection generally makes economic sense (e.g. [9,10]). However, this does not mean that protection will take place, and a question remains about its practicality—and proactive adaptation in general, especially in the world’s poorest countries, such as most small-island states or sub-Saharan Africa [11]. Looking at the literature, two distinct views concerning protection emerge [12]. The pessimists assume that protection is unaffordable and/or largely fails, and that most potential impacts are realized with sea-level rise leading to large-scale forced displacements of population on an unprecedented scale. This leads to an argument for stringent and immediate climate mitigation and preparation for environmental refugees. The optimists assume that protection will be widespread and largely succeed, and residual impacts will only be a fraction of the potential impacts. Hence, the main consequence of sea-level rise is the diversion of investment into new and upgraded coastal defences and other forms of adaptation (e.g. flood-warning systems). As we consider larger rises in sea level, hence concern that protection and proactive adaptation, in general, may fail, increases, and the potential for the pessimist’s view to be realized grows. This paper explores these issues, with a focus of trying to provide indicative outcomes given a beyond 4 ◦ C world. In §2, the science of sea-level rise is reviewed, with special consideration of post-AR4 sea-level-rise scenarios. These are synthesized to develop a potential range of rise by 2100 that is broadly consistent with a beyond 4 ◦ C scenario. In §3, the paper develops indicative estimates of the impacts both with and without adaptation. It uses the framework of the Dynamic Interactive Vulnerability Assessment (DIVA) model [13] for this purpose and creates scenarios consistent with the pessimistic and optimistic views that have been defined above. In §4, these results are reviewed in the light of the new synthesis of sea-level rise. Particular attention is addressed to key issues such as vulnerable hotspots in small islands, deltas and coastal cities. Section 5 is a conclusion. 2. Sea-level-rise scenarios for the twenty-first century (a) What does the Intergovernmental Panel on Climate Change Fourth Assessment Report tell us? In the IPCC AR4, the global surface temperatures at the end of the twenty-first century (2090–2099) are projected to reach higher than 4 ◦ C relative to 1980–1999 2 in three of the Special Report on Emission Scenarios (SRES) emission scenarios: 1 Forced displacement is a (reactive) last-resort retreat response. However, more proactive approaches to adaptation would be preferred which would avoid the large costs and potential conflicts that such forced displacement would engender. 2 This is a 4.5 ◦ C rise above pre-industrial temperatures. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
164 R. J. Nicholls et al. sea-level rise (m) 2.5 2.0 1.5 1.0 0.5 0 A2. Meehl et al. [5] A1B. Meehl et al. [5] Vellinga et al. [31] A1F1. Meehl et al. [5] Rahmstorf [28] Rohling et al. [27] Pfeffer et al. [22] Kopp et al. [26] Vermeer & Rahmstorf [6] Grinsted et al. [7] Figure 1. A graphical summary of the range of IPCC AR4 [5] sea-level-rise scenarios (for 2090– 2099) and post-AR4 projections (see table 1) possible in a 4 ◦ C world. The dotted lines represent the minimum (0.5 m) and maximum (2.0 m) bounds considered in terms of impacts in this study. A1B, A2 and A1FI [14]. The upper bounds of the temperature in these scenarios are within the range of 4.4◦C (A1B scenario) to 6.4◦C (A1FI scenario). The upper bounds of projected sea-level rise for these same emission scenarios ranged from 48 to 59 cm (figure 1). 3 In those models, a significant portion of this sea-level rise (around 66%) is attributable to thermal expansion, with the contribution from glaciers and small ice caps being the next biggest term. For the A1B scenario, the contribution of the Greenland ice sheet is estimated to be 8 cm, while the contribution from Antarctica is negative at −2 cm owing to the accumulation of extra precipitation on the ice sheet. Thus, the sea-level-rise contributions from ice sheets are considered to be small in these AR4 model projections. However, prior to the publication of IPCC AR4, some rapid changes were observed on the Greenland and Antarctic continental ice sheets ([14], later published by Rignot et al. [15] andvandeWalet al. [16]), but in Greenland these high rates were not sustained from 2006 to 2008 [17]. In 2007, these observations of rapid change could not be reproduced by state-of-the-art ice sheet models, forced by outputs from climate models (temperature, precipitation), because there was limited understanding of some key processes and feedbacks between the local climate and the ice sheets. The response of the IPCC was to include an oftenoverlooked statement that the ‘understanding of some important effects driving sea level rise is too limited ... [to] provide a best estimate or an upper bound for sea level rise’ during the twenty-first century ([18], p. 45). They also provided an illustrative scenario such that if the discharge term was to increase linearly with 3This is based on the 95th percentile from table 10.7 in Meehl et al. [5]. 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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12 M. New et al. Table 1. Impacts a
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16 M. New et al. as permanent crop
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18 M. New et al. 24 Boe, J. L., Hal
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Beyond 'dangerous' climate change:
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Beyond dangerous climate change 21
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(i) Energy and industrial process e
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(a) 50 (b) GtCO 2e yr -1 40 30 20 1
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Table 1. Summary of scenario pathwa
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Cumulative carbon emissions, emissi
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46 N. H. A. Bowerman et al. althoug
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emissions (GtC) Review. Global warm
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Review. Global warming reaching 4
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global surface warming (°C) 6 5 4
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temperature (relative to 1861-1890
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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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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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[89,94,95] further acidification by
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Editorial Editorial 3 D. Garner Pre
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