Estimating future global per capita water availability based on changes in climate and population

Abstract

Human populations are profoundly affected by water stress, or the lack of sufficient per capita available freshwater. Water stress can result from overuse of available freshwater resources or from a reduction in the amount of available water due to decreases in rainfall and stored water supplies. Analyzing the interrelationship between human populations and water availability is complicated by the uncertainties associated with climate change projections and population projections. We present a simple methodology developed to integrate disparate climate and population data sources and develop first-order per capita water availability projections at the global scale. Simulations from the coupled land–ocean–atmosphere Community Climate System Model version 3 (CCSM3) forced with a range of hypothetical greenhouse gas emissions scenarios are used to project grid-based changes in precipitation minus evapotranspiration as proxies for changes in runoff, or fresh water supply. Population growth changes, according to Intergovernmental Panel on Climate Change (IPCC) storylines, are used as proxies for changes in fresh water demand by 2025, 2050 and 2100. These freshwater supply and demand projections are then combined to yield estimates of per capita water availability aggregated by watershed and political unit. Results suggest that important insights might be extracted from the use of the process developed here, notably including the identification of the globe's most vulnerable regions in need of more detailed analysis and the relative importance of population growth versus climate change in altering future freshwater supplies. However, these are only exemplary insights and, as such, could be considered hypotheses that should be rigorously tested with multiple climate models, multiple observational climate datasets, and more comprehensive population change storylines.

Introduction

One recent work has concluded that almost 80% of the world's population is exposed to significant fresh water security threats via multiple stressors (Vörösmarty et al., 2010), two of which are population and climate change. Water stress can be qualitatively defined as the lack of sufficient fresh water for domestic, agricultural, and/or industrial needs. Water stress may result from overuse of water supplies due to population increase, industrialization, and/or lack of conservation practices, as well as from decreased rainfall due to changes in climate and/or lack of storage capacity in areas that receive variable amounts of water throughout the year. Building on work by Falkenmark (e.g., Falkenmark, 1986), world relief agencies commonly define conditions of “water stress” as less than 1700 m³ of available water per person per year and “severe water stress” as less than 1000 m³ per person per year; we also adopt those definitions for this work. This widely accepted method of estimating “water stress” is based on the fact that lack of freshwater availability imposes constraints on food production and industrial development that tend to scale with population growth (Edwards et al., 2005). A 2003 study by the World Resources Institute (WRI, 2003) concluded that 48 percent of the world's projected population (∼3.5 billion people) will live in water-stressed river basins by 2025. According to the World Meteorological Organization (Shiklomanov, 1997), water availability will be one of the major challenges facing human society in the 21^st century, and lack of water will be one of the key factors limiting development in many areas of the world. Determination of future water availability is further complicated by water quality, which may be adversely affected by competition resulting from increases in population (Vörösmarty et al., 2000) as well as consequences of climate change such as sea level rise (Kundzewicz et al., 2008).

Analyzing the complex interrelationship between growing human populations and available water supplies is complicated by climate change projections and non-stationarity, or the traditional water-resource engineering concept that the hydrologic system fluctuates within an unchanging, manageable envelope of variability. Water stress is anticipated to combine with other stressors to exacerbate problems in areas already prone to regional instability, as indicated in recent publications such as the United Kingdom's Ministry of Defense Publication (DCDC, 2007).

Several studies quantified future water availability and pinpointed areas of potential water scarcity prior to the conception of the Intergovernmental Panel on Climate Change (IPCC) “storylines” of carbon emissions and population change (IPCC, 2007). A landmark study by Vörösmarty et al. (2000) derived global projections of water availability in 2025 compared to 1985 and concluded that changes in population and economic development will cause more changes in water stress than changes in climate. Alcamo and Henrichs (2002) compared global projected water availability in 2032 to a baseline scenario of 1995 using the WaterGAP model to relate changes in national income to changes in water use per person and per unit of generated electricity under four different social/economic scenarios. They calculated water stress as the average annual withdrawals-to-availability ratio 0.4 or greater and found that severe water stress would be most likely in parts of central Mexico, the Middle East, large parts of the Indian sub-continent, and stretches of the North African coast (Alcamo and Henrichs, 2002). An exemplary analysis following the definition of Falkenmark (1986) (see introduction) is contained in the 2003 WRI “Water Resources eAtlas” produced by WRI, the International Water Management Institute (IWMI)'s “Watersheds of the World Map 15,” showing the annual renewable water supply (m³/person/year) per major basin for 1995 and for 2025 based upon 1995 census data, UN lower-end projections for population growth (such that global population peaks at 7.2 billion in 2025), and global runoff results developed by the University of New Hampshire and the WMO/Global Runoff Data Center. This 2003 WRI study concluded that 48% of the world's population will live in water-stressed river basins by 2025.

Following the development of the 2000 IPCC SRES socioeconomic storylines, Arnell (2004) used 30-year-mean climate projections from six global climate models (GCMs) in conjunction with the Gridded Population of the World (GPW) version 2 datset (Deichmann et al., 2001) to evaluate potential areas of water stress in terms of Falkenmark's water scarcity index. Arnell (2004) found that the Mediterranean, parts of Europe, the central and southern United States, and southern Africa will most likely experience increased water stress as a consequence of climate change.

In this work, we develop a data integration, analysis, and visualization process to estimate future per capita freshwater water availability and potential areas of water stress across the globe using updated global climate change projections and concurrent population change projections. We have developed this toolkit using the Oak Ridge National Laboratory's state-of-the-art high-resolution gridded population dataset, Global LandScan (Dobson et al., 2000) in conjunction with the regionally disaggregated IPCC (2007) socioeconomic storylines. We have derived our freshwater availability from a fully integrated earth system model, the Community Climate System Model, version 3 (CCSM3) (e.g., Drake et al., 2005), where the disadvantages of lower resolution are balanced by the ability to consider the land-atmosphere feedback effects. The methodology and toolkit that we have developed is flexible enough to enable incorporation of other population and climate projections as they become available.

Exemplary use cases of this tool that are explored here include: (1) determine potential human impacts at global and regional scales; (2) determine if there might be any new “hot spots” of water scarcity under a changing climate regime that might require higher resolution analyses for planning and mitigation purposes; and (3) assess the relative (proportional) roles of global and regional population versus climate change as drivers of water availability, thus ultimately informing where adaptation and/or mitigation efforts ought to be directed. Section 2 details the choices of data and methodology, including a brief discussion on population and climate change uncertainty as it relates to these choices; Section 3 details exemplary results from the analysis; Section 4 summarizes the objectives and significance of this work and suggests possible future direction for this line of research.