The phrase "glacier extent" refers to the size or area covered by a glacier. It indicates how far and wide the ice stretches across the land. Full definition
Eventually the resulting decline in glacier extent reduces the area available for melting causing a decrease in glacier runoff. (nichols.edu)
In the last few decades, satellite imagery has provided a means to monitor glacier extent changes worldwide, greatly expanding the areas of ice that scientists can examine. (nsidc.org)
Then the loss of ice could be calculated by reference to the current glacier extent. (earthtimes.org)