Changing Glaciers and Hydrology in Asia - Environmental Health at ...

Field-based measurements in remote glaciated areas such as the Himalaya are often, bynecessity, limited to a few measurements, typically from accessible glaciers at the lowerelevations. Therefore, scientists know more about lower-lying glaciers than they doabout the more abundant glaciers at higher altitudes.The location of a glacier’s terminus is not a comprehensive assessment of total glaciercondition or health. It is possible that a glacier with a retreating terminus may be gainingin total mass from one year to the next, due to increasing amounts of snow arriving atthe higher elevations by precipitation, wind deposition and avalanching. Therefore, itshould be understood that measurements showing short-term retreat only indicate thatthe recent climate does not support the extension, or even stability, of the lowermostelevation of a given glacier, and does not define the current conditions controlling thechanges in volume over the entire glacier at all elevations. Certainly, however, whenglaciers are observed to have been in retreat consistently over many decades, they arenot in balance with the recent climate.2.3 MEASUREMENTS OF GLACIER MASS BALANCESMore direct and comprehensive methods have been developed to determine the year-to-yearcondition of a total glacier system through measurements of “mass balance.” During theaccumulation season (often, but not in all locations, the winter season), a glacier gains mass fromaccumulating snow. During the following summer melt season, some or all of that winteraccumulation is lost to ablation, predominantly by melt, but perhaps by sublimation/evaporationas well as calving where a glacier enters a water body. The upper elevation zone where theglacier experiences an annual net gain in mass is called the accumulation area; the lowerelevation zone where the glacierexperiences a net mass loss iscalled the ablation area. Theelevation contour at which thesetwo zones meet is called theequilibrium line altitude (ELA)representing the point on theglacier where the annual netmass balance is zero (see Figure2.1 at right). The differencebetween the accumulation andablation for a given yeardescribes the annual net massbalance, which corresponds tothe change in total glaciervolume and mass. If the annualnet mass balance is zero, theglacier is neither growing norshrinking. Methods to monitormass balance vary but are always14Glacier Mass BalanceAccumulation AreaHeadNet AccumulationArmstrong, NSIDC/U. of ColoradoAblation Areaequilibrium linealtitude (ELA)Net AblationTerminusAnnual mass balance = net accumulation minus net ablationFigure 2.1. Glacier accumulation, and ablation areas and ELA.s