Since there is more melt than snowfall, the glacier recedes until it reaches a new balance. If warmed enough, large sections of the glacier can be lost before it stabilizes again. In Greenland, I was in the ablation zone and this zone has been expanding over time.
Meltwater also plays a role in basal sliding. Glaciers can slide because of a film of water at the ice bed interface. Naturally, just from the heat due to the pressure of ice weighing down, ice right near the base of the glacier will thaw and a thin water film will develop.
Increased surface melt due to warmer climate conditions can introduce more water to the bed and cause faster glacier flow. How does the water get from the surface to the bed? Over time melt ponds on the ice surface get big enough to cause cracks (hydrofracturing) in the ice and the water drains since it is more dense than ice. This water will eventually make it to the bed adding further lubrication for the glacier to slide.