ice

an introduction to glaciers by j. lebert

Glaciers are a dynamic body of ice that, together with ice sheets (their enormous, continent-spanning relatives), represent a significant portion of Earth's water and a volatile element in the changing climate. Although sea level rise is an important consequence of glacial change, glacial instability can pose particular hazards, though the impacts are not as wide spread.

Mountain glaciers are formed in locations where the snow that falls year to year outpaces the rate of melt. The snow turns into firn, or year-old snow, which begins to compress under the weight of even more snow building up, becoming glacier ice. Glacier ice thickness can be 10s to 100s of meters thick; Alaska has glaciers up to 1500 meters thick. The ice eventually reaches a thickness and mass sufficient to deform the glacier.

Ice starts to act like a thick, viscous fluid and flows under the stress caused by gravity. The flow rate is a combination of the internal deformation of the ice and the sliding on the underlying ground layer beneath. A glacier moves fastest in the middle and slowest along the edges. It moves fastest also at mid-elevations – near its Equilibrium Line Altitude (the boundary between the zone of snow accumulation and the zone of ablation (melting, calving, and other ice loss). Add in topography, precipitation patterns, and predicting glacier flow gets... complicated. That being said, glaciers move on a scale measured in meters per year. Some creep along, moving at an optimistic five or ten meters per year, others can clock in at several kilometers.

The temperature of the ice plays a significant role, and we can look at glaciers as having two temperature themes. Temperate, or isothermal, glaciers have ice at or near the pressure adjusted melting point for the ice, though the near surface ice can be quite cold from winter temperatures but generally have average surface temperatures near 0 degrees C. Cold glaciers, polythermal glaciers, are cold at the surface, comprised mostly of high elevation and polar glaciers where air temperatures are mostly well below freezing. Their beds, however, can be either be cold and frozen or warm enough to melt. If they are thick enough, heat from the Earth can keep the base warm. The conditions at the base, as you might imagine, are key to understanding glacier movement. The base itself can slide from the stress, and pressurized meltwater building up under the glaciers can lift the glaciers enough to promote movement down the slope.

Glaciers come to an end at some point determined by the environments ability to chip away at the glacier's mass. A glacier is a balancing act between snow build-up and loss. As the ice moves down the slope, it passes a point where snowfall no longer accumulates, and it starts wasting. Eventually the glacier ends; either at some point on land where the mass of ice cannot move forward any further, or into the water as a tidewater glacier. These glaciers generally lose a lot of their mass by calving; chunks breaking off in the water. Alaska tidewater glaciers are sitting on bedrock when they push out into the water, but glacier ice can float above the bedrock in some places forming an ice shelf. This usually results in a fairly fast moving glacier, and really large calving events.

Although a small portion of the total surface ice, glaciers hold a significant amount of water, and are particularly fragile with short term climate changes when compared to ice sheets. Ice sheets are large, continuous heat-sinks that should take a while to feel any impact from rising annual temperatures – although ice sheets have their own potential instabilities. Glaciers make up a fraction of the total ice mass and it is divided up into a multitude of glaciers, so is going to react to changing temperatures on a shorter time frame than the ice sheets. As such, up to about 0.4 meters sea level equivalent of water might be entering the oceans sooner than later, although the inventory of world-wide glaciers is limited so the exact amount of ice is not known.

A change in the seasonal flux of water into the local environments is going to be felt by a good number of people. Retreating glaciers is the norm, and the IPCC (international panel on climate change) is predicting that a reduction in available fresh water from glaciers is going to impact somewhere between 500 million and 2 billion people, with the majority of that is melt water no longer making its way into the Ganges river, which feeds hundreds of millions of people. The actual impact on communities so far removed from the glacial systems is not well known, but there are closer to 50 million people living in communities along the Himalayas and Andes that rely on glaciers to get them through the drier parts of the year.

Too little water might be a welcome alternative to an immediate over abundance of the
stuff. Glaciers can form reservoirs of water, whether by forming an ice-damn or accumulating a pressurized water layer near the base. Warming conditions are going to result in more melt, and not necessarily at the mouth of the glaciers. Lakes that already exist may become overloaded, putting enough stress on the dams to burst. A sudden glacier outburst will flood downriver, possibly destroying homes, infrastructure, livestock... and lives. Whatever is in the path of the outflow.

See Jeremy’s presentation to learn more about outburst floods.