Like rivers on land, melting water erodes Greenland’s ice

Streams and rivers that form on top of the Greenland ice sheet during spring and summer are the main agent transporting melt runoff from the ice sheet to the ocean. The image above was captured on July 19, 2015. (Credit: NASA's Goddard Space Flight Center/Maria-José Viñas)

To get a clearer idea of how Greenland’s ice sheet is responding to climate change, scientists are taking a closer look at the drainage channels of water as it melts during the summer.

The work suggests the resulting erosion on the ice sheet shapes landscapes similarly to, but much faster than, rivers do on land.

meltwater pond
Refrozen meltwater ponds in northeast Greenland seen during an April 2013 IceBridge flight. (Credit: NASA/Michael Studinger)

“How fast is the ice sheet melting, and how much the melt will contribute to rising sea levels are important questions,” says Leif Karlstrom, a professor in geological sciences at the University of Oregon. “It is important to quantify the melt rate, but that is not easy.

“Our study allows us to use geometric characteristics of the channel network—their patterns on the landscape—as a diagnostic tool.”

Projections on sea-level rise, such as those done with remote sensing or satellite observations, he says, have been difficult to determine accurately because melt rates vary widely each year, based on such factors as summer temperatures and elevations across the ice sheet.

Like rivers on land

In the study, Karlstrom and Kang Yang of the University of California, Los Angeles, analyzed high-resolution satellite imagery from NASA digital elevation models that let them see the slope of the ice sheet and underlying bedrock. They focused on stream channels at four levels of the ice sheet, from 1,000 meters (3,280 feet) to 1,600 meters (5,249 feet), of southwest Greenland.

Geometrical characteristics of these streams—called supraglacial channels because they occur on the ice surface—mimic features often found for rivers on land. Such similarities of erosion patterns on ice and land, despite having different mechanisms, came as a surprise, Karlstrom says.

Listen to Karlstom describe the similarities


On the ice sheet surface, erosion occurs as meltwater streams carve drainage channels by melting underlying ice. On land, rivers carve drainage channels by pushing and plucking sediment as they flow toward the sea, cutting down as the land surface uplifts due to tectonic activity.

Geologists who study geomorphology—how landscapes form—now have a virtual real-time model to test theories of landscape evolution, Karlstrom says. River erosion on land occurs over millions of years, but streams on the ice sheet carve their routes much more rapidly. In the study, researchers documented daily incision by flowing meltwater of up to 10 centimeters (4 inches).

“It’s lower elevations at the margins of the ice sheet that experience the most melt,” Karlstrom says.

River erosion stops each year when freezing temperatures return. Frozen channels from previous years remain visible, providing a yearly history of erosion patterns much like tree rings reflect age, he says.

In addition to using glacial melt to test theories of land-based geological processes, the researchers suggest an application to studies of other planets. The study, which NASA supported, appears in the journal Geophysical Research Letters.

Source: University of Oregon