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This proves, what?

Watching a video about global warming, and this picture was flashed up as evidence.

Retreating Glacier EO Newsroom: New Images - Retreat of the Gangotri Glacier

Hold on ... what does this prove?

Note that the area lost between 1780 and 1935 (155 years) looks to be about twice the area lost between 1935 and 2001 (66 years). Given that the amount of greenhouse gases generated by humans in the past century, what caused the larger retreat before 1935? If human generation of greenhouse gasses was the dominant influence, you might expect most of the loss would have been since 1935 rather than before.

If you were just looking at this glacier for evidence, you would have to conclude that "global warming" was already occurring (at least for this glacier) before human mass generation of greenhouse gasses. You would also have to conclude that humans have not had a big effect on the already-warming climate.

Of course, this is assuming there is a linear relation between temperature and the area lost by the glacier. At the very least, any change in slope of the valley probably could influence the rate of shrinkage.

So ... what does this prove?