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

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100 F. Fung et al. Accord. Clearly, while 2 ◦ C remains an important policy target, larger warming is highly likely, and it is important to evaluate the difference in impact of warming between a world that meets its policy objective and one where there is significant overshoot of this target. A large number of studies have assessed the impact of climate change on global water resources and have been reported in the series of assessment reports conducted by the Intergovernmental Panel on Climate Change (IPCC) [4] and in a number of other publications [5–11]. These analyses have typically used ensembles of global climate models (GCMs) developed by a number of modelling centres across the globe, the so-called ensembles of opportunity or multi-model ensembles (MMEs). These evaluations have assessed the time-evolving impacts under different emissions scenarios and the uncertainty that arises in future projections owing to different GCM structures [12,13], rather than the impacts that occur at particular global temperature change thresholds. In this paper, we contrast water availability and water stress in a world where warming is limited to 2 ◦ C and one where policy fails and warming reaches 4 ◦ C. We make use of a new GCM dataset, from the ClimatePrediction.net (CPDN) experiment [14], in combination with data from the World Climate Research Programme’s Coupled Model Intercomparison Project Phase 3 (CMIP3) multi- GCM archive [15]. These provide us with a wide range of realizations of a world that warms by 4 ◦ C, enabling us to identify differences in climate, surface runoff and population that determine water stress in ‘+2 ◦ C’ and ‘+4 ◦ C’ worlds. We begin the paper by introducing the climate and hydrological models used, and how the data are processed to define the +2 ◦ C and +4 ◦ C worlds. This is followed by a description of the scenarios of future population growth we developed and how these are used to define water stress. We then present results showing differences in water availability and stress in +0 ◦ C (present day), +2 ◦ C and +4 ◦ C worlds. Finally, the implications for global and regional water resources are discussed. 2. Models and data (a) The climate models The CPDN perturbed-physics ensemble (PPE; available from www.climatepre diction.net) comprises a large number of runs of a state-of-the-art GCM, HADCM3L. This is a version of the UK Met Office’s (UKMO) Unified Model comprising a 3.75 ◦ longitude by 2.5 ◦ latitude resolution atmospheric model coupled to an ocean model with the same resolution; this is a lower spatial resolution than the ocean model (1.25 ◦ longitude by 1.25 ◦ latitude) used in the UKMO’s standard version of this model, HadCM3. Each individual run uses a configuration of the GCM, with parameters representing various physical processes set to different values within their acceptable range, as defined by the experts in the relevant parametrization scheme [16,17]; see www.climateprediction.net for more details. For each combination of parameter values, an initial condition ensemble is used, enabling separation of internal and parameter variability. PPEs provide an approach for exploring a wide range of future climates, and this information can be used to assess potential impacts of climate change under that range of plausible futures. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
Water availability in +2 ◦ C and +4 ◦ C worlds 101 For this study, we have selected a subset of 1557 GCM runs from the CPDN experiment that provides global climate data from 1930 to 2079. This subset has been driven by the SRES A1B greenhouse gas emissions scenario, a ‘middle of the road’ scenario, representing a world in the twenty-first century with very rapid economic growth, rapid introduction of new and more efficient technologies and a world that does not rely too heavily on one particular energy source; total CO2 emissions increase to just over 16 Gt carbon in the 2050s, and then decline slowly to 13.5 Gt by 2100 [18]. We also use an MME of 22 GCMs from the CMIP3 multi-model dataset [15] that have completed at least one model run forced by the SRES A1B scenario, and have archived precipitation and temperature for the period 1961–2079. This enables us to compare the hydrological response across different sources of uncertainty: parameter uncertainty in the case of the PPE, and combined parameter and structural uncertainty in the MME. The climate models used in this analysis include BCCR-BCM2.0, CGCM3.1, CNRM-CM3, CSIRO-MK3.5, CSIRO-MK3.0, GFDL-CM2.0, GFDL-CM2.1, GISS-AOM, GISS-EH, GISS-ER, FGOALS-g1.0, INGV-ECHAM4, INM-CM3.0, IPSL-CM4, MIROC3.2 (hires), ECHO-G, ECHAM5/MPI-OM, CCSM3, PCM, MRI-CGCM2.3.2, UKMO- HadCM3 and UKMO-HadGEM; see Randall et al. [19] for a full description of the climate models. For both ensembles, each GCM is assumed to be an equally plausible projection and no model weighting or transformation of the ensemble outputs into a probability distribution has been applied. A +4 ◦ C world is defined here as a one where a GCM’s decadal global mean temperature is +4 ◦ C warmer than the 1961–1990 average in at least one season. This is different from the standard UNFCCC definitions for global warming, which are relative to pre-industrial temperatures; assuming a global warming of ca 0.7 ◦ C since 1850, our +4 ◦ C world is about 4.7 ◦ C warmer than pre-industrial. We select models that show an increase in global mean temperature of 2 ◦ C and 4 ◦ C at some point in the twenty-first century. For the CPDN simulations, this results in an ensemble of 1518 and 399 members for the +2 ◦ C and +4 ◦ C worlds, respectively. For the CMIP3 ensemble, all models exhibit a 2 ◦ C warming at some point in the twenty-first century, but only one model, namely MIROC3.2 (hires), warms by 4 ◦ C by 2100. (b) The climate variables The global hydrological model we use requires monthly inputs of precipitation and number of rain days, as well as several variables needed to determine potential evapotranspiration: temperature, vapour pressure, radiation and wind speed. These are available for the present day from the CRU TS2.1, 0.5 ◦ latitude/longitude observed climate dataset [20], where we use the time period 1961–1990 as our ‘baseline’ against which future changes are assessed, but regridded to 1 ◦ latitude/longitude, the resolution of the hydrological model. As the CPDN experiment relies on distributed computing, where members of the public run a GCM on their personal computers [14], bandwidth limitations mean that only limited numbers of diagnostics are returned to the CPDN servers for archiving. Of the input variables needed for the 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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0 retraction risk 17 120 80 40 (a)
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Humid tropical forests on a warmer
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0 retraction risk 17 120 80 40 0
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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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172 R. J. Nicholls et al. land loss
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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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Climate-induced population displace
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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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