Although reliable figures are often missing, considerable detrimental changes due to shrinking glaciers are universally expected for water availability in river systems under the influence of ongoing global climate change. We estimate the contribution potential of seasonally delayed glacier melt water to total water availability in large river systems. We find that the seasonally delayed glacier contribution is largest where rivers enter seasonally arid regions and negligible in the lowlands of river basins governed by monsoon climates. By comparing monthly glacier melt contributions with population densities in different altitude bands within each river basin, we demonstrate that strong human dependence on glacier melt is not collocated with highest population densities in most basins. G laciers and seasonal snow cover are expected to change their water storage capacity under the ongoing warming of the global climate with major consequences for downriver water supply (1-4). Despite reliable observations and model results of projected changes in runoff from individual highly glacierized basins (5-13), a severe lack of appropriate data records and inadequately resolved model results (14-16) leave us with only vague ideas of the importance of glaciers and seasonal snow cover on regional scales.Although reliable figures are often missing, considerable detrimental changes due to shrinking glaciers and snow cover are universally expected for water availability in river systems that originate from glacierized mountain regions. Approaches that compare glacier melt water production (obtained through measurements or modeling) with measurements of discharge volume somewhere downstream (e.g., ref. 17) are problematic because of the different nature of the two observed variables: Whereas glacier melt water can be considered as raw volume input into the runoff system, the discharge further downstream has been modified by, e.g., precipitation, evaporation, irrigation, damming, or exchange with subsurface flow regimes and groundwater. With increasing distance from the glaciers, modifications of runoff volume become more important, and the remaining fractional melt water contribution decreases. In a direct comparison between glacier melt water and runoff downriver, the volume contribution from glaciers is therefore overestimated by default with increasing distance from the glaciers. At the same time, the population that may depend on glacier melt as a resource typically increases downriver. A more detailed discussion of the shortcomings in the published literature is presented in ref. 18.Here we quantify the importance of glacier melt for the availability of water in large river basins, on the basis of globally available datasets and fundamental considerations. We deliberately perform our analysis from a perspective of total water availability within the whole river basin, as opposed to estimating volume discharge rates of the main river within a basin. Approaching the ProblemGlaciers produce melt water only during warm periods...

Abstract. The El Niño/Southern Oscillation (ENSO) is a major driver of climate variability in the tropical Andes, where recent Niño and Niña events left an observable footprint on glacier mass balance. The nature and strength of the relationship between ENSO and glacier mass balance, however, varies between regions and time periods, leaving several unanswered questions about its exact mechanisms. The starting point of this study is a 4-year long time series of distributed surface energy and mass balance (SEB/SMB) calculated using a process-based model driven by observations at Shallap Glacier (Cordillera Blanca, Peru). These data are used to calibrate a regression-based downscaling model that links the local SEB/SMB fluxes to atmospheric reanalysis variables on a monthly basis, allowing an unprecedented quantification of the ENSO influence on the SEB/SMB at climatological time scales (1980-2013. We find a stronger and steadier anti-correlation between Pacific sea-surface temperature (SST) and glacier mass balance than previously reported. This relationship is most pronounced during the wet season (December-May) and at low altitudes where Niño (Niña) events are accompanied with a snowfall deficit (excess) and a higher (lower) radiation energy input. We detect a weaker but significant ENSO anticorrelation with total precipitation (Niño dry signal) and positive correlation with the sensible heat flux, but find no ENSO influence on sublimation. Sensitivity analyses comparing several downscaling methods and reanalysis data sets resulted in stable mass balance correlations with Pacific SST but also revealed large uncertainties in computing the mass balance trend of the last decades. The newly introduced open-source downscaling tool can be applied easily to other glaciers in the tropics, opening new research possibilities on even longer time scales.

No abstract

Abstract. The El Niño/Southern Oscillation (ENSO) is a major driver of climate variability in the tropical Andes, where recent Niño and Niña events left an observable footprint on glacier mass balance. The nature and strength of the relationship between ENSO and glacier mass balance, however, varies between regions and time periods, leaving several unanswered questions about its exact mechanisms. The starting point of this study is a four-year long time series of distributed surface energy and mass balance (SEB/SMB) calculated using a process-based model driven by observations at Shallap Glacier (Cordillera Blanca, Peru). These data are used to calibrate a regression-based downscaling model that links the local SEB/SMB fluxes to atmospheric reanalysis variables on a monthly basis, allowing an unprecedented quantification of the ENSO influence on the SEB/SMB at climatological time scales (1980–2013, ERA-Interim period). We find a stronger and steadier anti-correlation between pacific sea surface temperature (SST) and glacier mass balance than previously reported. This relationship is most pronounced during the wet season (December–May) and at low altitudes where Niño (Niña) events are accompanied with a snowfall deficit (excess) and a higher (lower) radiation energy input. We detect a weaker but significant ENSO anti-correlation with total precipitation (Niño dry signal) and positive correlation with the sensible heat flux, but find no ENSO influence on sublimation. Sensitivity analyses comparing several downscaling methods and reanalysis datasets resulted in stable mass balance correlations with pacific SST but also revealed large uncertainties in computing the mass balance trend of the last decades. The newly introduced open-source downscaling tool can be applied easily to other glaciers in the tropics, opening new research possibilities on even longer time scales.

No abstract