Glaciers are formed from snow that has accumulated and compacted to ice. The ice can deform and flow downhill. At lower elevations, where the air is warmer, the ice melts. If the glacier terminates in the ocean, it is melted by warm water, and chunks of ice may then break off to become icebergs. Some glaciers are large, measuring hundreds of kilometres in length, but others are relatively small, just 100m or so in size.
Glaciers are an important store of water. Their melt off during the dry season provides water for human use. Loss of glaciers could therefore lead to reduced seasonal water availability affecting the lives of many. If glaciers melt, as they have been doing over the past 30 years for which we have detailed measurements, the water ends up in the oceans, raising sea level, which can have impacts on coastal communities.
The balance between the accumulation of mass from snow falling on a glacier and ice loss, also knowns as the mass balance, controls how a glacier changes over time. Melting of the lower glacier can cause the terminus to retreat to higher and colder elevation. Increased snowfall can make the glacier advance downhill until it is halted by increased melting at the terminus. If the terminus is in the ocean, warmer water can melt the glacier, it then flows faster producing more icebergs. Glaciers are not only sensitive to changes in temperature and snow fall, but also to other elements like sea water temperature, humidity, cloudiness and whether precipitation falls as rain or snow.
There are thousands of glaciers world-wide and it is not practical to measure them all in detail. The World Glacier Monitoring Service monitors a set of reference glaciers across 19 mountain zones. The mass balance of these glaciers has been negative, i.e. they have been shrinking overall, for the past 31 years.
The cumulative loss of mass since 1976 amounts to around 20m of water equivalent. “Water equivalent” is the depth of water that we’d get if all the lost ice and snow were melted. Snow and ice can have very different densities depending on how compact the snow is, and how much air is trapped in the ice. Using “water equivalent” allows for consistency between measurements.
Longer records show that glaciers have been in long-term retreat since the 19th Century.
Mass balance is measured using a variety of techniques. Ice loss can be measured using stakes driven deep into the surface of the glacier. As the ice melts, it reveals more of the stake allowing the ice loss to be quantified. Accumulation can be measured using stakes too, by digging pits that show how much fresh snow has accumulated, by probing through freshly fallen snow to the hard surface of ice beneath or even by analysing the annual layers of snow revealed by crevasses. Combinations of the different techniques provide information about mass gain and loss across the surface of a glacier.
Glaciers’ heights and shapes can be mapped from space, using radars, visible light sensors and others. Large ice masses can also be monitored by dedicated scientific missions that measure the Earth’s gravitational field, such as GRACE.
Glaciers respond to changes in temperature, the strength of sunlight reaching the glacier through changes in cloud cover and albedo, and precipitation. Rate of flow of glaciers can also be affected by a change in surface gradient. This may occur through downstream changes such as the loss of buttressing ice shelves, changes in basal lubrication due to meltwater, rainfall or infiltration of sea water. Each glacier is subject to a unique combination of the above-mentioned factors so there is variation in the mass balance between glaciers.
The IPCC AR5 concluded that “the robustness of the estimates of observed mass loss since the 1960s, the confidence we have in estimates of natural variations and internal variability from long-term glacier records, and our understanding of glacier response to climatic drivers provides robust evidence and, therefore, high confidence that a substantial part of the mass loss of glaciers is likely due to human influence.”