HYDROLOGIC SENSITIVITY OF GLOBAL RIVERSTO CLIMATE CHANGEBART NIJSSEN, GREG M. O’DONNELL, ALAN F. HAMLET andDENNIS P. LETTENMAIERDepartment of Civil and Environmental Engineering, Box 352700, University of Washington,Seattle, WA 98195-2700, U.S.A.E-mails: [email protected], [email protected], [email protected],and [email protected]. Climate predictions from four state-of-the-art general circulation models (GCMs) wereused to assess the hydrologic sensitivity to climate change of nine large, continental river basins(Amazon, Amur, Mackenzie, Mekong, Mississippi, Severnaya Dvina, Xi, Yellow, Yenisei). The fourclimate models (HCCPR-CM2, HCCPR-CM3, MPI-ECHAM4, and DOE-PCM3) all predicted tran-sient climate response to changing greenhouse gas concentrations, and incorporated modern landsurface parameterizations. Model-predicted monthly average precipitation and temperature changeswere downscaled to the river basin level using model increments (transient minus control) to adjustfor GCM bias. The variable infiltration capacity (VIC) macroscale hydrological model (MHM) wasused to calculate the corresponding changes in hydrologic fluxes (especially streamflow and evapo-transpiration) and moisture storages. Hydrologic model simulations were performed for decadescentered on 2025 and 2045. In addition, a sensitivity study was performed in which temperatureand precipitation were increased independently by 2◦C and 10%, respectively, during each of fourseasons. All GCMs predict a warming for all nine basins, with the greatest warming predicted tooccur during the winter months in the highest latitudes. Precipitation generally increases, but themonthly precipitation signal varies more between the models than does temperature. The largestchanges in the hydrological cycle are predicted for the snow-dominated basins of mid to higherlatitudes. This results in part from the greater amount of warming predicted for these regions, butmore importantly, because of the important role of snow in the water balance. Because the snowpack integrates the effects of climate change over a period of months, the largest changes occur inearly to mid spring when snow melt occurs. The climate change responses are somewhat differentfor the coldest snow dominated basins than for those with more transitional snow regimes. In thecoldest basins, the response to warming is an increase of the spring streamflow peak, whereas forthe transitional basins spring runoff decreases. Instead, the transitional basins have large increasesin winter streamflows. The hydrological response of most tropical and mid-latitude basins to thewarmer and somewhat wetter conditions predicted by the GCMs is a reduction in annual streamflow,although again, considerable disagreement exists among the different GCMs. In contrast, for thehigh-latitude basins increases in annual flow volume are predicted in most cases.1. IntroductionThere is a growing consensus in the geoscience community that the Earth willexperience a gradual warming in the coming decades, the major cause of which iscontinuing increases in global concentrations of so-called greenhouse gases. Burn-Climatic Change 50: 143–175, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.144 BART NIJSSEN ET AL.ing of fossil fuels, in particular, is likely to continue to increase atmospheric CO2concentrations to more than double their pre-industrial levels within the next 100years (IPCC, 1996). In the past few years, the focus of research related to climatechange has shifted from the question whether global warming will occur to under-standing where and how much change is likely to occur. In the United States, forexample, the U.S. National Assessment of Climate Variability and Change (here-after USNA) is assessing the potential impacts of climate variability and change on20 regions and five sectors throughout the U.S. One of the five sectors addressedby the USNA is water resources.Changes in atmospheric circulation, as evidenced by fluxes of moisture andenergy at the land surface, have immediate as well as long-term effects on riversystems. At short time scales, from days to months, changes in weather patternscan lead to changes in the incidence of floods. At longer time scales, from seasonsto years, changes in drought characteristics are the main hydrologic manifestationof climate change. At annual to decadal time scales, teleconnections in global at-mospheric circulation patterns, caused primarily by ocean-atmosphere interactions,strongly affect the hydrology of certain regions, especially in the tropics, but also insome extra-tropical regions (e.g., Battisti and Sarachik, 1995; Glanz et al., 1991).For example, the El Niño/Southern Oscillation (ENSO) has been linked to floodsand drought in Southern Africa (Thiaw et al., 1999), and precipitation anomaliesin eastern Australia (Simpson et al., 1993).Global increases in temperatures directly affect the hydrology of the land sur-face through changes in the accumulation and ablation of snow, as well as inevapotranspiration. Changes in atmospheric circulation are also predicted to re-sult in changes in precipitation amounts, intensities and patterns (e.g., Felzer andHeard, 1999). Although most general circulation models (GCMs) predict increasesin global average precipitation, there is little consensus on the amount or evendirection of regional changes. On the other hand, almost all climate models showincreases in temperature in most regions and for most seasons. Discrepancies inGCM predictions of temperature change are more in magnitude than in direction.Changes in land surface hydrology due to changing climate, such as changesin the discharge of large, continental rivers, have potentially far reaching implica-tions both for human populations and for regional-scale physical and ecologicalprocesses. The geographic and topographic characteristics of large river basins andthe climatic variations that determine their hydrologic characteristics often consti-tute the defining features of the regions they occupy. They govern to a considerableextent the development of ecosystems, as well as human communities and theiractivities. These regional ecosystems and human activities are usually reasonablywell adapted to the current climate conditions, but may be vulnerable to large orrapid changes in climate.In industrialized nations, food supplies and human health are at least partiallyinsulated from natural