Discovering the Surface of Greenland

A new technique provides clearer pictures of massive ice sheets--and better insight into future sea-level increases.

Greenland holds enough water to raise global sea levels seven meters, and southern Greenland is already showing accelerated melting. But the rate of this melting and other ice dynamics are poorly understood, partly because Greenland's surface is so inscrutably white and featureless in ordinary satellite images. Now, a new image-processing approach gives a clearer view of subtle inland features, providing sharper clues into glacial movements--and better insight into future sea-level increases.

A new approach to processing satellite data of ice sheets allows images such as this one: the first detailed picture of a 600-by-50-kilometer eyedropper-shaped ice formation known informally as NEGIS (for Northeast Greenland Ice Stream). NEGIS wasn’t even known to science until 1991. The new processing approach shows structures and features that give clues into how this and other parts of Greenland and Antarctica are melting. NEGIS is sliding toward the sea at a few hundred meters per year. Credit: National Snow and Ice Data Center, University of Colorado, Boulder

The technology starts with as many as 94 red and infrared images of the same region, taken by two NASA satellites, called Terra and Aqua, that have polar orbits and cross Greenland several times a day. Each raw image--a measure of light from the surface--has a resolution of 250 meters per pixel. But by aligning and averaging values within areas of pixel overlap among multiple images of the same area, researchers at the National Snow and Ice Data Center at the University of Colorado at Boulder tightened the resolution to as little as 100 meters per pixel and roughly quadrupled contrast sensitivity.

As one example of a payoff, researchers are finally getting a clear picture of a 600-by-50-kilometer eyedropper-shaped ice formation informally known as NEGIS (for Northeast Greenland Ice Stream). This massive feature--which is sliding toward the sea at a few hundred meters per year--wasn't even known to science until 1991. And it hasn't been imaged in detail until recent months. "What we've done now is see how far upstream it goes, how close it comes to the summit of Greenland, and see some structures at the edges, to get an idea [of] how fast ice flows" and what directions it flows in, says Ted Scambos, lead scientist and glaciologist at the Boulder center, who codeveloped the image-processing approach.

Scambos says that such insights are everything when it comes to finding out how fast Greenland's ice will pour into the ocean and begin inundating the world's coastlines. The same technology is being applied to images of Antarctica, whose ice sheet contains enough water to raise sea levels 65 meters if all of it melts. The rate of such melting is one of the most poorly understood yet most high-impact effects of global warming.

"This gives us better resolution of subtle structures in the interior of the ice sheet," says Scambos. "To the naked eye it looks like a smooth white plain. But there are hills, bumps, and ridges that show us how the ice is flowing, and how it will drain out from glaciers. Once we get away from the coast, the features that are important have to do with how the ice flows. They can be very subtle--hills and valleys that show you how the ice is moving off the continent. What we've got is a map that shows details much further inland, much further than before. Other images just show the interior of the ice sheet as a blank white surface without any features."