Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 189

FIGURE 4.34 Muir Glacier photographed in 2004. (Photo by B. Molina. Courtesy National Snow and
Ice Data Center.)
A similar but more sophisticated approach to the study of glacial mass balance is the
use of the equilibrium line (or equilibrium line altitude [ ELA ]). Since the ELA is calcu-
lated from measurements of snow density, water equivalent, ablation loss, and other in-
ternal qualities, it does not always coincide with the firn limit. The ELA marks the zone
on the glacier where the mass of the glacier stays approximately the same during the
year. The area above the equilibrium line receives an excess of winter snow, resulting
in increased mass. The area below the equilibrium line loses more to ablation than is
gained by accumulation, resulting in decreased mass. If the mass gain above the line
equals the mass loss below the line, the glacier is in a (rare) steady-state condition. If
the mass gain above the equilibrium line exceeds the mass loss below the line, the res-
ult will be mass transfer to lower levels by glacial thickening of the ice and advance of
the toe, whereas if mass loss exceeds accumulation, the glacier will shrink. Positive or
negative trends will only become apparent over a period of several years (Posamentier
1977; Braithwaite 2002; Dyurgerov 2002; Barry and Gan 2011). In all cases, mass trans-
fer across the equilibrium line occurs whether the glacier is retreating or advancing,
except at the extreme of final decay, when the entire ice mass may become stagnant.
There are several problems involved in conducting formal glacial mass balance stud-
ies, as discussed by Meier (1962). In spite of these problems, numerous mass balance
observations have been conducted in many mountain and subpolar locations (Østrem
and Brugman 1991; Braithwaite 2002; Dyurgerov 2002; Kaser et al. 2003; Hubbard and
Glasser 2005). The National Snow and Ice Data Center (NSIDC) reports, “. . . about 70
percent of the observations come from the mountains of Europe, North America and
the former Soviet Union. Mass balance on more than 280 glaciers has been measured