Is That Glacier Growing or Shrinking? Adding Up Snow Against Melt

A glacier is a budget. Snow falling in the upper part compacts into ice and adds mass, which is accumulation, and melting, evaporation and the breaking off of icebergs remove mass, which is ablation. The difference over a year is the mass balance, and a glacier with a persistently positive balance thickens and eventually advances while one with a negative balance thins and retreats. The two zones are separated by a line where the year's gains and losses exactly cancel, which can be identified at the end of the melt season as the boundary between exposed ice below and retained snow above, and the position of that line is itself a useful indicator tracked from year to year.

Several methods are used and each has different strengths:

• •Stakes drilled into the ice, with the exposed length measured at intervals, which gives melt directly at a point
• •Snow pits and cores in the upper zone, measuring the depth and density of the year's accumulation
• •Extrapolating those point measurements over the whole surface, which is where most of the error enters
• •Repeat surveys of the surface by aircraft or satellite, giving volume change over the whole glacier
• •Satellite gravity measurements, which detect mass change over whole regions
• •Modelling from weather data, which is the only option for the great majority of glaciers

A glacier does not respond immediately to a change in climate, and the lag explains a great deal about what glaciers are doing now. Mass lost or gained in the upper zone takes time to travel down as the ice flows, so the terminus reflects conditions from years or decades earlier depending on the glacier's size and speed, with small steep glaciers responding within a decade and large ones taking a century or more. That means a retreating terminus today may reflect warming that occurred well before anybody was watching, and it means that even if conditions stabilised, glaciers would continue adjusting for decades. It also means the length of a glacier is a poor measure of current conditions while mass balance is a direct one.

A minority of glaciers do things the simple picture does not predict and they are worth knowing about. Surging glaciers advance rapidly for months or a few years and then stagnate for decades, in a cycle driven by conditions at the bed rather than by climate, and a surging glacier can advance kilometres while losing mass overall. Debris-covered glaciers carry a layer of rock that insulates the ice beneath, so they melt far more slowly than a clean glacier in the same conditions and respond differently to warming. Glaciers ending in water lose mass by calving as well as melting, and that loss depends on the depth of water at the terminus rather than on air temperature, which is why several are retreating rapidly for reasons that are only indirectly climatic.

A small number of glaciers have been measured directly for long periods and the picture is consistent. The reference glaciers monitored continuously since the 1950s show negative balance in nearly every year of recent decades, with the losses accelerating, and the cumulative thinning at several sites exceeds twenty metres of water equivalent. Regional and global estimates combining direct measurement with satellite methods find glaciers outside the two ice sheets losing mass at rates that make them a substantial contributor to sea level rise. Some glaciers advance while the regional trend is negative, generally because of local precipitation or surging behaviour, and those exceptions are quoted selectively in public argument more often than they are explained.

Snow adds mass and melt removes it, with the difference over a year deciding whether a glacier thickens or thins, and a line where the two cancel separating the zones. Stakes, snow pits, repeat surveys and satellite gravity each measure part of it. A terminus reflects conditions from years or decades earlier, so length is a poor current indicator while mass balance is direct.