If a glacier receives less precipitation, or experiences higher temperatures than in preceeding years, it will retreat. That is, more melting will occur near its mouth than is replenished by flow from above; the glacier will simply melt back from where it used to be.
If a glacier melts back intermittently, so that its retreat is punctuated by short periods of stability, it may leave behind a series of recessional moraines. The recessional moraines in the photo below are at the mouth of the Athabasca Glacier in Jasper National Park, Alberta. The Athabasca Glacier has retreated well over 0.5 km in the last 60 years.
To see a photo of the whole glacier, click here.
Before and after photos of a retreating glacier: Angel Glacier, Jasper National Park.
These certainly aren't the only examples we can find of retreating glaciers. Alpine glaciers are retreating all over the world.