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

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112 F. Fung et al. become amplified, both the consensus and spatial coherence of these changes strengthen. By investigating the changes in water stress in 112 of the world’s major river basins, we have also found that the majority of these river basins are projected to suffer greater water stress in a +4 ◦ C world than in a +2 ◦ C world. However, as we move from a +2 ◦ Ctoa+4 ◦ C world, there are also a small but increasing number of basins that may experience less water stress, as they are located in regions where rainfall is projected to increase. By using population growth scenarios for the 2030s and 2060s, we find that in a +2 ◦ C world, water stress is dominated by the change in population. However, as we move to +4 ◦ C world and the climate change signal becomes stronger, climate change can play a more dominant role in determining water stress in a river basin. The timing of the global warming will be important in determining water stress across the world. Population growth is projected to increase through the twentyfirst century, peaking in the middle of the century [41]. Should a rapid warming occur, with a plausible +4 ◦ C warming by the 2060s as shown by our CPDN ensemble and Betts et al. [42], the drier areas of the world may be affected by a combination of peak demand for water and larger decreases in run-off. However, if a more gradual warming is experienced and +4 ◦ C is reached in 2100 or beyond, when world population is projected to decline, water stress will be lower, and there will be more time to adapt to climate change. We have also compared the PPE from the CPDN experiment and the MME from CMIP3 for a +2 ◦ C world and have shown that while there is considerable agreement between the two ensembles, there are significant regional differences as well. Thus, in order to capture the range of possible projections, both sets of climate model ensembles are required. By examining a subset of the world’s major river basins, we have shown that the picture for water stress in each river basin is dependent on the magnitude of the climate change and the nature of the population growth. For some river basins, the effects of climate change become large enough to offset the large increases in demand in a +4 ◦ C world, e.g. in the Ganges; in most basins, however, climate and population growth combine to increase stress or climate change is insufficient to offset increased demand. We have also found that seasonality in run-off may be more pronounced in a +4 ◦ C world compared with a +2 ◦ C world; thus, even where annual average runoff increases, dry seasons can become more stressed. This could mean that more sophisticated infrastructure projects may be required in a +4 ◦ C world compared with a +2 ◦ C world in order to prevent flooding and droughts. There are a number of caveats when interpreting our results. We have used an MME and a PPE (of one GCM) that, albeit covering a large range of climate projections, only represents a sample of the possible uncertainty range. The GCM ensembles have not been weighted or screened, recognizing that attempting to weight models is fraught with difficulty [12,13]. The bias-correction procedure used is also simplistic and does not include any information on climate variability, either inter-annual or intra-annual. Our results are therefore appropriate for assessing changes in mean run-off and stress, but not the possibility of changes in shorter term stress. Some care must also be taken in the interpretation of the +2 ◦ C and +4 ◦ C worlds that have been used in our analysis, as the baseline used in our analysis is for the time period 1961–1990 rather than a ‘pre-industrial’ value or the time Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
Water availability in +2 ◦ C and +4 ◦ C worlds 113 period of 1980–1999 used in IPCC [43]. Mean global temperatures for 1980–1999 are warmer than for 1961–1990 by 0.16 ◦ C and may mean larger changes in both run-off and water stress for some river basins. However, if a pre-industrialized value of 1861–1890 is used, the difference is about 0.31 ◦ C cooler, with changes in future run-off and water stress smaller in some river basins than those presented in our analysis. MacPDM has been run offline and therefore any feedbacks into the climate system have not been included. It has also not been calibrated and its parametrizations have not been explored, although previous studies on small river catchments [44,45] find that climate model uncertainty is greater than hydrological modelling uncertainty. In fact, we have found that as we move from the PPE model data (temperature and precipitation) to MacPDM output (runoff), the spatial correlation appears to increase pointing to the importance of the model input parameters, such as soil type and land cover, in controlling the resulting run-off, which may themselves change in the future owing to direct impacts of climate change, adaptation responses and CO2 effects on plant water-use efficiency. The calculation of PE is included within MacPDM and is based on the Penman–Monteith equation; as shown in recent studies [46,47], this could be another source of uncertainty. This issue may need to be explored in future studies to determine the sensitivity of the CPDN ensemble to the PE calculations. The population statistics are based on only one of the UN population growth scenarios (median variant) and exploration of the uncertainty in these projections has not been carried out. These population statistics have been calculated offline and therefore, once again, no feedback effects can be carried through to the climate and hydrological models, which may be important if the spatial distribution of populations depends on the availability of water. The WSI used in this analysis is based solely on the annual surface run-off and river basin population. It does not incorporate many of the intricacies that may exist between water-use habits, water resources management, water infrastructure and adaptative responses, to name a few, that affect on-the-ground water stress. Although simple, the WSI used in this analysis has highlighted the need to look at river basins where there is great uncertainty in the direction of change in water stress as well as the areas that show an increase in water stress. However, to start unravelling the actual change in water stress in the region, a more detailed local assessment would be required. It must also be noted here that given the GCM data that are at best decadal seasonal means, a highly detailed assessment of water resources management options for water storage in reservoirs and groundwater replenishment is beyond the scope of this analysis. The authors would like to acknowledge Nigel Arnell for the use of the MacPDM model, the use of the UK National Grid Service for the computational resources and the funding provided by RCUK, via the Tyndall Centre for Climate Change Research. We acknowledge the modelling groups, the Programme for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP’s Working Group on Coupled Modelling (WGCM) for their roles in making available the WCRP CMIP3 multi-model dataset. Support of this dataset is provided by the Office of Science, US Department of Energy. Finally, we thank the climateprediction.net project, and the many thousands of members of the public who have donated time on their personal computers, for the climateprediction.net simulations. 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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Sea-level rise and its possible imp
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162 R. J. Nicholls et al. expected.
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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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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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