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

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106 F. Fung et al. under 1700 m 3 per capita per year are classified as being water stressed [38] and the colour scale of figure 1c has been devised accordingly whereby any areas showing a value of WSI less than 1700 m 3 per capita per year are coloured from pink to red and those showing a greater value, i.e. no stress, from pale to dark blue. According to the Falkenmark stress indicator, the WSI shows that many of the world’s large river basins are currently showing signs of extreme water stress, including those in South Asia, Southeast Asia, the Nile and eastern USA, i.e. in regions of the world where there are large populations. (b) Future water availability (i) Global mean annual surface run-off and model consensus We now analyse the spatial distribution of the change in MAR (DMAR), change in WSI (DWSI) and the consensus among the MME and PPE ensembles for a +2◦C and +4◦C world. As the CMIP3 ensemble only contains one member that reaches a 4◦C warming, we do not include an analysis of this ensemble at 4◦C. For MAR, we only show the mean of ensemble members that show changes greater than natural variability: this has been defined as twice the standard deviation of the 30 year average annual run-off in the baseline period. If we compare the +2◦C and +4◦C worlds of the CPDN ensemble, it is clear that DMAR (figure 2a) becomes larger in the latter, as areas previously showing changes smaller than natural variability become more noticeable. We also evaluate consensus across the ensembles, defined as the percentage of models that show the same direction of change (figure 2b). The consensus is an indication of the uncertainty of the projected MAR where a smaller percentage value represents areas where there is little agreement on the direction of change in the ensemble and vice versa. Note that we only show areas of consensus where DMAR is larger than natural variability. In the +4◦C world, there is stronger model consensus in many parts of the world and the spatial extent of these areas becomes larger. This result agrees partly with that of Milly et al. [5] who showed that regions experiencing a decrease in run-off are projected to grow in magnitude and spatial extent. For each ensemble member, we calculated the correlation between DMAR at corresponding gridpoints at +2◦C and +4◦C. Across the CPDN ensemble, there is a high correlation1 between areas that get drier/wetter in the two temperature regimes, i.e. the pattern of changes in MAR at +2◦C and +4◦C is consistent in the direction of change, but amplified at +4◦C. Thus, the ensemble shows linearity of hydrological response in relation to temperature: an assumption that underlies many of the pattern-scaling-based downscaling procedures for impact assessments [21]. A comparison of the CMIP3 and CPDN ensembles at +2◦C shows that there is reasonable agreement in most parts of the world for the direction of DMAR apart from an area encompassing eastern Africa and the Middle East. For the CPDN ensemble, the positive direction of change in MAR in eastern Africa agrees with HadCM3, the GCM on which the CPDN is based (not shown). However, the positive changes in the Middle East, which are greater than that of natural variability, are not seen in HadCM3 where changes are either smaller than natural variability or negative. Figure 2 shows that there is much stronger consensus in 1 Ninety per cent of the ensemble members have a correlation between 0.87 and 0.97. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
Water availability in +2 ◦ C and +4 ◦ C worlds 107 many regions of the world for the CPDN ensemble compared with the CMIP3 ensemble, most likely owing to the fact that the CPDN ensemble is based only on one GCM, albeit with a wide range of physics parameter values. We also explored the relative importance of changes in precipitation and evaporation on MAR, although completely disentangling the two effects is difficult without additional experiments. In our simulations, changes in potential evaporation (PE) are a function of temperature change only, as we assume other variables are unchanged in the future. Actual evaporation (AE) is dependent on both PE and soil water availability, so that in areas where there is soil water stress, increases in PE will not translate into equivalent changes in AE. For global land areas, the average ratio of MAR to MAP decreases in all simulations as one moves from the present day to +2 ◦ C and +4 ◦ C worlds (not shown), indicating that AE increases as a proportion of the overall water balance. However, this relative proportion also varies according to the direction and extent of precipitation change, with the ensemble members with the greatest reductions in precipitation showing the smallest decreases in MAR : MAP. This effect is amplified in a +4 ◦ C world, so that ensemble members that are particularly dry do not show as large a reduction in MAR : MAP going from +2 ◦ Cto+4 ◦ C as they do going from the present to +2 ◦ C. (ii) Global water stress index and model consensus The mean percentage changes in WSI compared with the baseline WSI for the major river basins of the world across each of the climate model ensembles are also shown in figure 2, for the 2060s population scenario. Similar patterns to those found for DMAR emerge as one moves from the present day to +2◦C and +4◦C worlds, with 71 and 74 per cent of the basins showing an ensemble mean increase in stress (decrease in WSI) at +2◦C and +4◦C, respectively. For river basins that show an increase in stress, the stressed areas generally grow in spatial extent and magnitude as one moves from +2◦Cto+4◦C, with 71 per cent of basins becoming more stressed. A number of rivers show a different sign of change for WSI at +2◦C and +4◦C. The Godavari, Tapti and Yangtze basins show a decrease in WSI in a +2◦C world and an increase in WSI in a +4◦C world; the opposite trend occurs for Dniester, Garonne, Oder, Vistula, Rio Grande de Santiago, Sacramento and Chubut basins. The change in population and the directions of change in runoff in these basins for both temperature regimes are in fact the same: it is the magnitude of the changes in run-off and population that results in the opposite signs of change in WSI for the two temperature regimes. For the first set of basins, the changes in run-off are small and close to natural variability in a +2◦C world. An increase in the population is observed for the 2060s, hence there is an increase in stress. However, in the +4◦C world, the increase in run-off is much larger, thereby showing a decrease in stress. The reverse is observed for the second set of basins. The CMIP3 and CPDN ensembles for the +2◦C world are in general agreement that water stress will increase in all river basins in Africa, India, eastern USA and southern Europe; these areas are regions that are already indicated as experiencing some water stress for the baseline period according to the Falkenmark stress indicator (figure 1c). 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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10 M. New et al. supports the messa
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12 M. New et al. Table 1. Impacts a
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14 M. New et al. actors (NNSAs)—r
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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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48 N. H. A. Bowerman et al. a likel
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50 N. H. A. Bowerman et al. (b) Mod
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52 N. H. A. Bowerman et al. In most
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54 N. H. A. Bowerman et al. rates o
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56 N. H. A. Bowerman et al. (a) pea
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Humid tropical forests on a warmer
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Sea-level rise and its possible imp
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162 R. J. Nicholls et al. expected.
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164 R. J. Nicholls et al. sea-level
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166 R. J. Nicholls et al. mean temp
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168 R. J. Nicholls et al. interglac
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170 R. J. Nicholls et al. discussed
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172 R. J. Nicholls et al. land loss
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174 R. J. Nicholls et al. global ad
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176 R. J. Nicholls et al. Hence, th
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178 R. J. Nicholls et al. to climat
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180 R. J. Nicholls et al. 43 Pardae
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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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