Glaciers expand during glaciation due to temperature decreases, and they shrink during glacial retreat, often caused by global warming, leading to sea level rise.
Glaciers expand when the mean average global temperature decreases, and this process is called glaciation. The shrinking of glaciers is called glacial retreat. When temperatures drop, more snow remains unmelted, accumulating and compacting into ice, thus expanding glaciers. During periods of warming, like the one we are currently experiencing due to global warming, glaciers lose mass because melting outpaces snowfall, leading to retreat and contributing to sea level rise, as has been observed with Grinnell Glacier and many others globally.