Uncertainty in climate change projections of discharge for the Mekong River Basin

摘要

The Mekong River Basin is a key regional resource in Southeast Asia forsectors that include agriculture, fisheries and electricity production. Herewe explore the potential impacts of climate change on freshwater resourceswithin the river basin. We quantify uncertainty in these projectionsassociated with GCM structure and climate sensitivity, as well as fromhydrological model parameter specification. This is achieved by runningpattern-scaled GCM scenarios through a semi-distributed hydrological model(SLURP) of the basin. Pattern-scaling allows investigation of specificthresholds of global climate change including the postulated 2 °Cthreshold of "dangerous" climate change. Impacts of a 2 °C rise inglobal mean temperature are investigated using seven different GCMs,providing an implicit analysis of uncertainty associated with GCM structure.Analysis of progressive changes in global mean temperature from 0.5 to6 °C above the 1961–1990 baseline (using the HadCM3 GCM) reveals a relativelysmall but non-linear response of annual river discharge to increasing globalmean temperature, ranging from a 5.4 % decrease to 4.5 % increase.Changes in mean monthly river discharge are greater (from ?16 % to+55 %, with greatest decreases in July and August, greatest increases inMay and June) and result from complex and contrasting intra-basin changes inprecipitation, evaporation and snow storage/melt. Whilst overall results arehighly GCM dependent (in both direction and magnitude), this uncertainty isprimarily driven by differences in GCM projections of future precipitation.In contrast, there is strong consistency between GCMs in terms of bothincreased potential evapotranspiration and a shift to an earlier and lesssubstantial snowmelt season. Indeed, in the upper Mekong (Lancangsub-basin), the temperature-related signal in discharge is strong enough tooverwhelm the precipitation-related uncertainty in the direction of changein discharge, with scenarios from all GCMs leading to increased river flowfrom April–June and decreased flow from July–August.