Thousands of ponds are scattered like mirrors across Alaska’s coastal plain, providing nesting and feeding grounds for waterfowl. The bodies of water, each less than a hectare in area, fill depressions in the hummocky tundra landscape with meltwater from thawing permafrost. How the surface hydrology of Arctic permafrost regions—a key part of the Arctic carbon cycle—will transform in a changing climate isn’t well understood, but tundra ponds may be a powerful guide, because they are closely tied to changes in precipitation and temperature, scientists report in a study published online before print in the Journal of Geophysical Research: Biogeosciences. To gauge how the ponds have changed in the past 65 years, the researchers put high-resolution aerial photos taken across Alaska’s Barrow Peninsula in 1948 (at left) side-by-side with modern satellite images from 2002, 2008, and 2010 (at right). They also used pond data collected during the International Biological Program in the 1970s, including areal extent estimates, water depths, and pond depths, and compared those with field data collected from 2011 to 2013. In all, they found, the number of ponds had shrunk by at least 17% since 1948 and had overall shrunk in size by about 30%. Several factors influenced the change—as temperatures rise, evaporation increases, and rainfall isn’t keeping pace. But warmer temperatures, longer growing seasons, and thawing permafrost (which supplies nutrients) are also promoting the growth of aquatic plants in the ponds, shrinking the size of the basins.