Alaska’s southeast Columbia Glacier, just below the middle of the image, flows down the snow-covered slopes of the Chugach Mountains into the Prince William Sound. Over the last three decades this tidewater glacier has retreated more than 20 kilometers (12.5 miles) and lost about half of its total thickness and volume. The changing climate is thought to have nudged it into retreat beginning around 1980, when its tip broke free. The Columbia is one of the most rapidly changing glaciers in the world. August 5, 2017.