Future freshwater supply, human water demand, and people's exposure to water stress are subject to multiple sources of uncertainty, including unknown future pathways of fossil fuel and water consumption, and 'irreducible' uncertainty arising from internal climate system variability. Such internal variability can conceal forced hydroclimatic changes on multi-decadal timescales and near-continental spatial-scales. Using three projections of population growth, a large ensemble from a single Earth system model, and assuming stationary per capita water consumption, we quantify the likelihoods of future population exposure to increased hydroclimatic deficits, which we define as the average duration and magnitude by which evapotranspiration exceeds precipitation in a basin. We calculate that by 2060, 31%–35% of the global population will be exposed to >50% probability of hydroclimatic deficit increases that exceed existing hydrological storage, with up to 9% of people exposed to >90% probability. However, internal variability, which is an irreducible uncertainty in climate model predictions that is under-sampled in water resource projections, creates substantial uncertainty in predicted exposure: 86%–91% of people will reside where irreducible uncertainty spans the potential for both increases and decreases in sub-annual water deficits. In one population scenario, changes in exposure to large hydroclimate deficits vary from −3% to +6% of global population, a range arising entirely from internal variability. The uncertainty in risk arising from irreducible uncertainty in the precise pattern of hydroclimatic change, which is typically conflated with other uncertainties in projections, is critical for climate risk management that seeks to optimize adaptations that are robust to the full set of potential real-world outcomes.
Lamont-Doherty Earth Observatory of Columbia University, New York, NY, United States of America;NASA Goddard Institute for Space Studies, New York, NY, United States of America;Emmett Interdisciplinary Program in Environment & Resources, Stanford University, Stanford, CA, United States of America;Author to whom any correspondence should be addressed.;Department of Geography, University of Zurich, Zurich, Switzerland;Department of Water Engineering & Management, University of Twente, Enschede, The Netherlands;Center for Climate Systems Research, Columbia University, New York, NY, United States of America;Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore, 259770, Singapore;Center for Climate Systems Research, Columbia University, New York, NY, United States of America;Lamont-Doherty Earth Observatory of Columbia University, New York, NY, United States of America;Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore, 259770, Singapore;Department of Earth System Science, Stanford University, Stanford, CA, United States of America
Recommended Citation:
Justin S Mankin,Daniel Viviroli,Mesfin M Mekonnen,et al. Influence of internal variability on population exposure to hydroclimatic changes[J]. Environmental Research Letters,2017-01-01,12(4)
31%–35% of the global population will be exposed to >50% probability of hydroclimatic deficit increases that exceed existing hydrological storage, with up to 9% of people exposed to >90% probability. However, internal variability, which is an irreducible uncertainty in climate model predictions that is under-sampled in water resource projections, creates substantial uncertainty in predicted exposure: