The poor glaciers

At present, more than 90% of the glaciers on Earth are retreating. Of course, glacier retreat is not as simple as it may seem.

First, a little refresher: glaciers form when snow accumulation rates exceed melting rates over many years. Eventually the snow metamorphoses into ice as a result of increasing pressure in the snowpack. When the ice becomes thick enough, it deforms under its own weight and flows downhill. As the glacier reaches lower elevations, melting begins to exceed accumulation, until it reaches a point (the terminus) where the ice melts faster than it can flow forward. A change in terminus position can be due to a change in (1) air temperature, (2) snowfall, or (3) ice dynamics. Snowfall is related to air temperature, and ice dynamics is related to air temperature, snowfall, ice temperature, crystal size, and basal properties. It's clearly a difficult problem.

Furthermore, two glaciers located close to each other can behave very differently as a result of local variations in climate or dynamical processes. So the fact that over 90% of the glaciers are retreating is actually very alarming - most of the world's advancing glaciers are tidewater glaciers, which are glaciers that end in the sea and can behave independently of climate. I'd rather not get into the reasons for this, but I can point to the right direction if you're curious enough to learn more.

Evidence for glacier retreat is clear everywhere in Alaska; when walking in the mountains you can see recently-deposited moraines (rock piles left by the glaciers) that are now high above the ice surface. There is also plenty of photographic evidence - for example, see this photo at the USGS website.

A significant portion of this retreat can be attributed to natural cycles. Much of the Earth experienced a cooling period from about 1550-1850, dubbed the "Little Ice Age". And so some of this retreat is result of subsequent warming. What might not be attributed to natural causes is the increasing rate of retreat. Between the mid-1950s and mid-1990s, Alaska glaciers contributed about 0.14 mm/yr to sea level rise. Over the last 15 years, that rate has increased to closer to 0.30 mm/yr.

Glaciers in Greenland remained in pretty steady positions until the last 5 years or so. Now, all (?) glaciers south of about 68° N are rapidly retreating and accelerating. For example, Jakobshavns Isbræ, the glacier I will study for my Ph.D., has retreated 15 km in the last few years and ice flow has accelerated from 7 km/yr to ~15 km/yr at the terminus. Here's another common misconception, almost always screwed up the media and the anti-climate warming people - an accelerating glacier does not have to be advancing. The beds of the glaciers in Greenland are well below sea level. When the ice thins, it can become buoyant - this reduces the drag and also results in the rapid calving of large icebergs. (Yes, these are tidewater glaciers that can respond independently of climate, but that fact that so many of them are retreating simultaneously points to a climate-induced retreat. The popular theory at the moment is that the retreat is due to increasing ocean temperatures.)

Opponents of human-induced global warming would ask why didn't these changes happen earlier - we've been using fossil fuels for over 50 years. The answer to that is again related to the complex relationship between ice volume distribution and dynamics. Simply put, large ice masses respond slowly to changes in climate.