Over the past 25 years or so, global warming, melting and thinning of the ice have caused Columbia Glacier to lose its grip on its bed—about 1,500 feet below the water surface—and to accelerate and stretch. That stretching, and the crevassing that accompanies it, causes an increase in calving (breaking off of icebergs). The faster flow is roughly balanced by the rate of calving, and so the terminus during this three-month sequence stays in about the same position in spite of the fast-moving ice. The calving rate varies, however, and the over the course of a year, the calving exceeds the flow and the terminus retreats. To give a sense of scale, the mountain range in the background is about three miles (five kilometers) away and the ice cliff in the foreground is about 300 feet (91 meters) high.

Next: AK-03, Columbia Glacier, Alaska