Millions of people across the globe are already affected by natural variability in the water cycle. A multidisciplinary team of experts from the University of East Anglia and the University of Nottingham, led by Timothy Osborn, Professor of Climate Science at the world-renowned Climatic Research Unit, set out the empirical evidence - and argue the need for implementation of measured adaptation mechanisms that take into account uncertainties in the projection of future precipitation patterns

Dorling, Stephen

Gosling, S. N.

Osborn, Timothy

Wallace, Craig

English

University of East Anglia digital repository

14 Words into ActionSETTING THE SCENE157th world water forumTHE WATER CYCLE IN A CHANGING CLIMATEMillions of people across the globe are already affected by natural variability in the water cycle. A multidisciplinary team of experts from the University of East Anglia and the University of Nottingham, led by Timothy Osborn, Professor of Climate Science at the world-renowned Climatic Research Unit, set out the empirical evidence - and argue the need for implementation of measured adaptation mechanisms that take into account uncertainties in the projection of future precipitation patterns.Earth’s frEshwatEr is distributed unevenly – spatially, day by day, through the changing seasons, and between dry and wet periods lasting years to decades. Climate change, though still very uncertain in its regional details, will have a tendency to exacerbate this unevenness, drying some already arid regions, wetting some humid regions, enhancing seasonality in many areas, and concentrating rainfall into shorter but more intense spells. As a result, projections of future changes in climate and the hydrological cycle indicate that the population exposed to hydrological risks will increase significantly during the 21st century, though slowing climate change through reductions in greenhouse gas emissions could reduce this risk.Climate and water: tightly coupled partnersClimate change occurs due to disturbances to the Earth’s energy budget, driven particularly by natural and, more recently, human-related variations in greenhouse gases (GHGs) such as CO2. The response of our climate to such changes is through a complex mixture of mechanisms in which the hydrological cycle plays a central role – transporting heat energy (through evapotranspiration followed by condensation) from surface to atmosphere and from tropics to poles, affecting absorption and reflection of radiation by the influence of clouds and the water vapour content. Indeed, water vapour is the strongest component of the natural greenhouse effect, yet unlike other GHGs it condenses and rains out of the atmosphere at only moderately low temperatures [Lacis et al., 2010]1. Thus the amount of water vapour depends strongly on how warm the air is: the atmospheric water vapour concentration is enhanced because CO2 keeps the atmosphere naturally warm – and increased CO2 will warm the atmosphere further, allowing increased water vapour to further strengthen the total greenhouse warming effect. This intertwining of the hydrological cycle and the Earth’s energy budget means the hydrological cycle is both modified by climate change and interacts with it to shape all aspects of climate change.SCIENTIFIC PERSPECTIVESETTING THE SCENEof agriculture, industry and society has been shaped by this distribution of water resources.The reliability of precipitation is also key, and many regions exhibit large seasonal and interannual variability. Water storage and other adaptations provide some resilience to these large variations, but additional measures and infrastructure may be needed, especially where climate change is expected to alter average rainfall or its variability. The natural infrastructure of geological aquifers plays a key role here, but recent assessments [Gleeson et al., 2012]2 have highlighted that many aquifers are being unsustainably exploited, with annual abstractions exceeding the renewable groundwater resources by more than tenfold in some instances (Figure 2).The number of people across the globe currently living in water scarcity has been estimated at between 1.2 and 2.4 billion (Gosling and Arnell, 2013)3, with the wide range arising because the measure of scarcity differs between studies. Figure 2 shows regions of high Water Stress Index.Natural climate variabilityDespite the high profile given to climate change driven by increasing GHGs, it is the natural and mostly unpredictable variations in our weather and climate which will dominate the hydrological cycle in the coming decade or two. This is especially the case when considering local scales, where rainfall variability is large compared with the expected climate change signal [Hawkins and Sutton, 2011]4. Nevertheless, the underlying climate change signal is expected to grow and may be felt most where it alters the frequency of extreme events such as droughts and floods. There is indeed some evidence for increased precipitation extremes in recent decades, though not in all regions and our observing networks are not sufficient to provide a global picture [Hegerl et al., 2014]5. Natural variability, especially related to the El Niño–Southern Oscillation (ENSO) phenomenon, has so far prevented the identification of any clear trend in drought occurrence at the global scale [Trenberth et al., 2014]6. water scarcity in a changing climateThe effect of climate change on the hydrological cycle is expected to grow The number of people across the globe currently living in water scarcity has been estimated at between 1.2 and 2.4 billion.”TiMoTHy J. oSboRnProfessor of Climate Science, Climatic Research Unit, School of Environmental Sciences, UEA, Norwich, UKSiMon n. GoSlinGAssociate Professor in Climate RiskSchool of Geography, University of Nottingham, Nottingham, UKwater in our current climateWater evaporated from the oceans is carried by the atmosphere away from the subtropics (where evaporation peaks) both equatorward and to the mid latitudes, though also over subtropical land regions during summer monsoons. This transport, combined with the uplift of moist air flows by mountains or over warm land surfaces, and the recycling of water evaporated and transpired from the land surface, determines the complex pattern of land precipitation (Figure 1). It is usually uneconomic to transport water over long distances, so the development as we look further ahead, with increased precipitation projected at mid to high latitudes and drying on the poleward side (e.g. the Mediterranean) of the already dry subtropical regions. Changes in the equatorial and monsoon regions are very uncertain, with some climate models projecting drying in some regions and other models projecting increased precipitation, and vice versa elsewhere. This uncertainty should not be equated with an assumption of little change: on the contrary the changes in precipitation could be very large, perhaps leading to changes in average streamflow of more than +/-30%. But there is more to consider than simply changes in average climate. The vertical gradient, up through the troposphere, is expected to weaken in temperature but strengthen in water vapour, especially in the topics. The former may stabilise the atmosphere and make convection less frequent, while the latter would intensify convection when it does occur [Hegerl et al., 2014]5. This combination of less frequent but more intense rainfall is a robust prediction of how the hydrological cycle will change, and coupled with increased potential evapotranspiration (PET) resulting from warmer temperatures, may lead to widespread decreases in soil moisture during the growing season and thus an increased risk of agricultural drought [Jiménez Cisneros et al., 2014]7.Over the past decade, assessments of how water scarcity might change under different socio-economic pathways and emissions scenarios have increased in number, and they have become more comprehensive by coupling multiple climate models and hydrological models. There is considerable uncertainty in the regional patterns of change (Figure 3), but nevertheless strong scientific agreement that increasing GHG emissions will adversely affect water security at the global-scale and especially in most dry subtropical regions due to an figure 1. Climatology of annual total precipitation, also highlighting areas where precipitation is highly seasonal (defined here as regions where half the annual total falls in three months or less).figure 2. Areas of present-day water stress (grey shading) and six aquifers where groundwater resources are under particular pressure (red outlines). Water Stress index is the percentage of total water resource that is being withdrawn for human use, estimated using population in year 2000 (11-20% light grey; 21–40% medium grey; above 40% dark grey)3. Highlighted aquifers all have annual abstractions between 8 and 54 times the renewable groundwater resource.increase in variability of precipitation and surface runoff [Gerten et al., 20138; Arnell et al., 20139]. Schewe et al. (2014)10 estimated that about 8% of the global population would see a severe reduction in water resources with even a relatively modest global warming of 1 °C above present day, rising to 14% at 2°C and 17% at 3°C. Change in groundwater recharge and storage will reflect the regional interplay of future precipitation, runoff and PET. Simulations using the latest climate projections [Collins et al., 2013]11 indicate recharge and storage volume increases in the northern mid-to-high-latitudes but a mixed pattern of decreases and limited increases in lower latitudes. Although increases are projected in parts of agriculturally-intensive eastern China and India, climate projections for these areas are very uncertain and groundwater withdrawal already greatly exceeds recharge in some significant aquifers (Figure 2) such as the North China Plain and Upper SCIENTIFIC PERSPECTIVESETTING THE SCENE18 WORDS INTO ACTIONGanges (Gleeson et al., 2012)2. It might appear beneficial to increase groundwater withdrawals in response to increases in river flow variability under climate change but this is only sustainable where groundwater withdrawals remain well below groundwater recharge (Kundzewicz and Döll, 2009)12. Moreover, groundwater would not offset declines in surface freshwater availability in regions where groundwater recharge decreases due to climate change. Portmann et al. (2013)13 predict that for each degree of global temperature rise, an additional 4% of the global land area would experience at least 30% decrease in groundwater resources.a more variable future?Assessments with multiple climate and hydrological models indicate that fluvial flood hazard could increase over about half of the globe by the end of the century but there is high variability at the catchment scale and the magnitude of projected changes varies strongly between climate models (Dankers et al., 201414.1; Hirabayashi et al., 201314.2; Arnell and Gosling, 201414.3),. The direction of change is more certain, however, for increases in flood frequency in Southeast Asia, Peninsular India, eastern Africa and the northern half of the Andes (Hirabayashi et al., 2013)14.2. Approximately 600 million people currently live in flood-prone areas, yet few studies have incorporated data on figure 3. Uncertainty in the effects of climate change on exposure to water scarcity in 2050 (relative to 1961–1990 estimates) by considering climate change projections from 21 different climate models. The upper panel shows how many of these 21 projections result in a significant increase in exposure to water scarcity, while the bottom shows how many project a significant decrease in exposure to water scarcity. Regions shaded in both panels are those where uncertainty in climate change is greatest, with some models predicting significantly drier conditions but others predicting significantly wetter conditions. [Redrawn from Gosling and Arnell, 2013]3CRAiG J. WAllACESenior Research AssociateClimatic Research Unit, School of Environmental Sciences, UEA, Norwich, UKSTEvE DoRlinGSenior LecturerSchool of Environmental Sciences, UEA, Norwich, UKflood-prone populations and infrastructure to quantify global-scale fluvial flood risk. An initial assessment (Arnell and Gosling, 2014)14.3 suggests that by 2050 between 31 and 450 million people may experience an increase in exposure to flood hazard, with the large range reflecting our uncertainty in future precipitation changes.Closing remarksMillions of people across the globe are already affected by natural variability in the water cycle. In some regions, an increasing population and climate change will combine to exacerbate issues such as water scarcity and flood risk. This points towards a need for the implementation of measured adaptation mechanisms that take into account uncertainties in the projections of future precipitation patterns. While climate change presents opportunities for some regions (e.g. lowering water scarcity), such opportunities may only be realised if infrastructure is in place to manage water resources appropriately.Approximately 600 million people currently live in flood-prone areas, yet few studies have incorporated data on flood-prone populations and infrastructure to quantify global-scale fluvial flood risk.”197th world water forumreferences1 Lacis AA, Schmidt GA, Rind D and Ruedy RA (2010) Atmospheric CO2: principal control knob governing Earth’s temperature. Science, 330, 356-359, doi:10.1126/science.1190653.2 Gleeson T, Wada Y, Bierkens MFP and van Beek PH (2012) Water balance of global aquifers revealed by groundwater footprint. Nature, 488, 197-200.3 Gosling S N and Arnell N W (2013) A global assessment of the impact of climate change on water scarcity. Climatic Change (doi: 10.1007/s10584-013-0853-x).4 Hawkins E and Sutton R (2011) The potential to narrow uncertainty in projections of regional precipitation change. Climate Dynamics, 37, 407, doi: 10.1007/s00382-010-0810-65 Hegerl G C, E Black, R P Allan, W J Ingram, D Polson, K E Trenberth, R S Chadwick, P A Arkin, B Balan Sarojini, A Becker, A Dai, P Durack, D Easterling, H Fowler, E Kendon, G J Huffman, C Liu, R Marsh, M New, T J Osborn, N Skliris, P A Stott, P-L Vidale, S E Wijffels, L J Wilcox, K M Willett and X Zhang. (2014) Challenges in quantifying changes in the global water cycle. Bull. Am. Meteorol. Soc. doi: 10.1175/BAMS-D-13-00212.16 Trenberth K E, Dai A, Van Der Schrier G, Jones P D, Barichivich J, Briffa K R, Sheffield J (2014) Global warming and changes in drought. Nature Climate Change, 4, 17-22.7 Jiménez Cisneros B E, T Oki, N W Arnell, G Benito, J G Cogley, P Döll, T. Jiang, and S S Mwakalila (2014) Freshwater resources. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Field C B, et al. (eds.)]. Cambridge University Press, Cambridge, United Kingdom, 229-269.8 Gerten D, Lucht W, Ostberg S, Heinke J, Kowarsch M, Kreft H, Kundzewicz ZW, Rastgooy J, Warren R and Schellnhuber HJ (2013) Asynchronous exposure to global warming: freshwater resources and terrestrial ecosystems. Environmental Research Letters, 8, 034032.9 Arnell NW, Lowe JA, Brown S, Gosling SN, Gottschalk P, Hinkel J, Lloyd-Hughes B, Nicholls RJ, Osborn TJ, Osborne TM, Rose GA, Smith P and Warren RF (2013) A global assessment of the effects of climate policy on the impacts of climate change. Nature Climate Change, 3, 512-519.10 Schewe J, Heinke J, Gerten D, Haddeland I, Arnell NW, Clark DB, Dankers R, Eisner S, Fekete BM, Colón-González FJ, Gosling SN, Kim H, Liu X, Masaki Y, Portmann FT, Satoh Y, Stacke T, Tang Q, Wada Y, Wisser D, Albrecht T, Frieler K, Piontek F, Warszawski L and Kabat P (2014) Multimodel assessment of water scarcity under climate change. Proceedings of the National Academy of Sciences, 111, 3245-3250.11 Collins M, et al. (2013) Long-term climate change: projections, commitments and irreversibility. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker T F, et al. (eds.)]. Cambridge University Press,  Cambridge, UK.12 Kundzewicz ZW and Döll P (2009) Will groundwater ease freshwater stress under climate change? Hydrological Sciences Journal, 54, 665-675.13 Portmann FT, Döll P, Eisner S and Flörke M (2013) Impact of climate change on renewable groundwater resources: assessing the benefits of avoided greenhouse gas emissions using selected CMIP5 climate projections. Environmental Research Letters, 8, 024023.14.1 Dankers R, Arnell NW, Clark DB, Falloon PD, Fekete BM, Gosling SN, Heinke J, Kim H, Masaki Y, Satoh Y, Stacke T, Wada Y and Wisser D (2014) First look at changes in flood hazard in the Inter-Sectoral Impact Model Intercomparison Project ensemble. Proceedings of the National Academy of Sciences, 111, 3257-3261.14.2 Hirabayashi Y, Mahendran R, Koirala S, Konoshima L, Yamazaki D, Watanabe S, Kim H and Kanae S (2013) Global flood risk under climate change. Nature Clim. Change, 3, 816-821. 14.3 Arnell N and Gosling S (2014) The impacts of climate change on river flood risk at the global scale. Climatic Change, 1-15.acknowledgementsPreparation of this paper was supported by the EU HELIX project (603864), the Belmont Forum SAHEWS project (NERC NE/L008785/1) and a NERC Impact Accelerator Award.author contributionsAll authors contributed to the writing of the paper.about University of East angliaUEA is one of the world’s leading  institutions concerned with the study of climate change. Housing the Climatic Research Unit, the School of Environmental Sciences and the Tyndall Centre, its research spans climate of the past, present and future, and its impact on humanity and the natural world. SCIENTIFIC PERSPECTIVE

The water cycle in a changing climate

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