If you're looking at something but don't have anything to compare it to, it's hard to know what's really going on. Maybe you meet someone for the first time and think they look a little sick, so you think they're not doing too well. But if you had met them a year ago when they were terminally ill, you'd think that today’s health was a huge improvement. Everything's relative. It's all about your frame of reference.

That's the idea behind repeat photography. Some things change fairly slowly, like glaciers that may take thousands of years to form or melt, so comparing photos taken at many years' interval is the best way to see how things are evolving. The U.S. Geological Survey has been doing exactly that with a series of photos of glaciers from Alaska. Most were shot in the Glacier Bay National Park and Preserve, Kenai Fjords National Park and the northwestern Prince William Sound area of the Chugach National Forest.

The change between past and present is striking. See for yourself:

Muir Inlet glacier at Glacier Bay National Park in 1880 and 2005

It might not look like it, but both photos above were taken in the same location on the east shoreline of Muir Inlet, in Glacier Bay National Park. The top one was taken around 1880-1890, while the second one was taken in 2005. A century ago, the glacier was over 300 feet high, and pieces of ice more than 6 feet in diameter could be seen littering the shore. Now, well, not so much...

And even back then, tourists loved glaciers. The people that you see in the photo are likely tourists who came to the area on a steamboat.

Pedersen Glacier in Kenai Fjords National Park in the 1920s and in 2005

The change in this one is very striking, isn't it? From something that looks straight out of a documentary about emperor penguins to a grassy field that someone might guess is in the European Alps.

Both photos were taken in Kenai Fjords National Park. They show the Pedersen Glacier, or at least what remains of it now. Up top was taken sometimes around the 1920s or 1930s (the photo is actually from a postcard that was sold to tourists!), and below in 2005.

Plateau Glacier in 1961 and 2003

This one shows Plateau Glacier in 1961 and 2003, just 42 years, the blink of an eye in the life of a massive glacier. If you have sharp eye, look around the center of the top photograph and you should be able to see two people standing near the water.

Here the glacier was about 115 feet above the surface of the water, but if you include all of it, including submarine ice, the total thickness was around 650 feet! Can you imagine the weight of all that ice?

A glacier in Glacier Bay National Park in 1976 and 2003

This last one shows how fast things can change. The top photo was taken fairly recently, in 1976 (that's very recent on the geological timeframe that glaciers usually evolve in). The bottom one was taken in 2003, both from the shoreline in upper Muir Inlet, also in Glacier Bay National Park.

Another interesting thing to note is that in the 1976 photo, apart from the algae on the rocks in the forefront, there was no vegetation in the mountains. In the 2003 photo, you can clearly see vegetation starting to grow, which implies higher average temperatures.

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Michael Graham Richard ( @Michael_GR ) Michael writes for MNN and TreeHugger about science, space and technology and more.