GoKawiilWhy are glaciers important?
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.
How have glaciers changed?
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 on the glaciers 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.
How is glacier mass balance measured?
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 other means. Large ice masses can also be monitored by dedicated scientific missions that measure the Earth’s gravitational field, such as GRACE.
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