Why do glaciers retreat?

I only have 1 point.. They're melting. As the ends break off and melt, the glacier gets shorter.

Answer #1

Glaciers melt because of climate warming.

And because of the "Albedo effect" That is: If there is ice, there is white surface. The white surface reflects sunlight --> place cold. If there is no ice, sunlight shines on darker surfaces, such as Rock --> place warms up --> more ice melts --> more dark surface --> place warm up more --> and so on.
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Answer #2

The global warming.