You have to realize that 1982 is about 25% of the time back to the end of The Little Ice Age which is likely when the glaciers were at a maximum for the last thousand years. The glaciers have probably been shrinking since around 1850.
Also, the smaller a mass of ice gets, the faster it melts. The ratio of surface area to volume becomes greater.
Glaciers melting at “an unprecedented rate” is what one calls “Duh!”