Glaciers are dynamic ice bodies, constantly gaining ice in some areas and losing it in others. A key region within this system is the “zone of wastage,” an area where glaciers experience a net reduction in their ice mass. Understanding this area’s characteristics, mechanisms of ice loss, and broader implications for glacier health is important.
Understanding the Zone of Wastage
The zone of wastage, also known as the ablation zone, is the section of a glacier where annual ice loss surpasses annual gain. This loss, called ablation, occurs when ice melts or dissipates faster than new snow and ice accumulate. It typically occupies the lower, warmer parts of a glacier, extending to its terminus or edge. The equilibrium line separates the zone of wastage from the accumulation zone, where ice gain exceeds loss. This line marks the altitude where accumulation and ablation are balanced over a year, with accumulation usually greater at higher elevations and ablation dominating lower reaches.
Processes Driving Ice Loss
Ice loss within the zone of wastage occurs through several distinct physical processes.
Surface Melting
Surface melting is a primary mechanism, where ice transforms into water due to solar radiation or warm air temperatures. This meltwater often flows across the glacier’s surface and can penetrate into the ice.
Sublimation
Sublimation is another process, where ice directly converts into water vapor without first turning into liquid. This phenomenon is more prevalent in cold, dry, and windy conditions, particularly at higher altitudes. Sublimation can remove significant amounts of ice from the snowpack.
Calving
Calving occurs when large chunks of ice break off from a glacier’s edge, typically into bodies of water. This process is common in tidewater glaciers, which terminate in marine environments. Stresses within the ice as the glacier flows, combined with water pressure, can lead to these break-offs.
Significance for Glacier Health
The zone of wastage serves as a key indicator of a glacier’s overall health and mass balance.
Mass Balance
Mass balance refers to the difference between total ice gained through accumulation and total ice lost through ablation, usually over a year. An expanding zone of wastage signals a negative mass balance, meaning the glacier loses more ice than it gains.
Temperature and Retreat
An enlarging zone of wastage links to rising global temperatures, as warmer conditions intensify melting and other ablation processes. The continuous retreat and thinning of glaciers worldwide reflect this expansion. Since 1970, reference glaciers tracked by the World Glacier Monitoring Service have experienced substantial ice loss, indicating a widespread decline.
Broader Implications
The implications of an expanding zone of wastage extend beyond glaciers. Glacier melt contributes to global sea level rise, accounting for approximately 21% of observed sea level rise over the past two decades. The reduction in glacial ice also impacts freshwater resources, as communities rely on meltwater for drinking water, agriculture, and hydropower during dry seasons.