Glacier mass balance is a measure of glacier health and describes the increase or decrease of mass in the glacier system based on measurements of snowfall accumulation and ice melt (this is surface mass balance). The following graph shows the mass balance record for White Glacier between 1960-2014.
On the y-axis, B indicates the glacier-wide mass change in millimeters of water equivalent (mm w.e.) per year. This means that, in 1960 when B = -408 mm w.e., White Glacier lost the equivalent of approximately 40 cm of water across its entire surface. Considering that the area of the glacier was approximately 41 km^2 in 1960, we calculate that in this year the glacier lost around 17.5 mega-tonnes of water. That’s enough water to fill 7000 Olympic-sized swimming pools!
Today, we monitor snow accumulation and ice melt at approximately 40 glaciers stakes, which gives a spatial coverage of roughly one stake per square kilometer. At high elevations, we measure how much snow accumulates in the winter, and compare this with how much ice we observe has melted at lower elevations. The difference between the two sets of observations tells us how much the glacier mass has changed over one year.