Polar ice sheets melting - in pictures

Each summer, streams channel much of the melt that is produced by the warmer temperatures along lower levels of the Greenland ice sheet Photograph: Courtesy Ian Joughin/AAAS

Surface melt water rushes along the Greenland ice sheet through a glacial stream channel Photograph: Courtesy Ian Joughin/AAAS

Over the course of several years, turbulent water overflow from a large melt lake carved this 60-foot deep canyon Photograph: Courtesy Ian Joughin/AAAS

For several summers this deeply incised melt channel transported overflow from a large melt lake to a moulin (a conduit that drains the water through many hundreds of feet to the ice sheet's bed Photograph: Courtesy Ian Joughin/AAAS

A large melt lake, around 0.75 miles in diameter, that is just one of the many supraglacial lakes (liquid water on the top of a glacier) that form on the ice sheet's surface during the period of strong summer melt Photograph: Courtesy Ian Joughin/AAAS

Moulins like this one drain surface meltwater through ice more than 3,000 feet thick to reach the base of the ice sheet, where a subglacial drainage network carries it to the ocean Photograph: Courtesy Ian Joughin/AAAS

A close-up of crevasses produced by rapidly stretching ice in Antarctica's Pine Island glacier Photograph: Courtesy Ian Joughin/AAAS

Crevasses in an ice flow on Pine Island glacier Photograph: Courtesy Ian Joughin/AAAS

A view of Pine Island glacier's floating ice shelf with a train of crevasses receding toward the horizon Photograph: Courtesy Ian Joughin/AAAS