An Icy Paradox

The thundering collapse of ice from towering glaciers off the Antarctic Peninsula highlights their vulnerability to the warming trend there in recent decades. Does this presage the ultimate fate of Antarctica's major ice shelves, which lie in colder climes farther south? Not according to a report in today's issue of Nature. It suggests that initially, global warming may actually cause these massive ice shelves to thicken.

In Antarctica, thousands of years of snowfall has been compacted into glaciers that flow slowly out to sea, where they float as ice shelves attached to the shoreline. When the ocean surface beyond these ice shelves freezes each winter, it leaves denser, saltier water behind. Oceanographers believe that at the major ice shelves, this salty water sinks and flows along the sea floor under the shelf toward the land, where it comes in contact with the underside of the ice shelf.

Oceanographer Keith Nicholls and his group from the British Antarctic Survey have now tracked this process at the Ronne Ice Shelf (which is about the size of Spain). They drilled through the shelf at three locations and poked oceanographic probes below the ice. Nicholls and his colleagues found that the volume of high-salinity water flowing into the area beneath the shelf rises during winter, when sea-ice formation is greatest, and decreases in the spring and summer, as sea-ice melts.

Paradoxically, this pattern means that winter conditions melt more of the ice shelf than does summer. The reason: the pulse of high-salinity water is relatively warm, because its temperature is set by the freezing point of seawater at the surface. It is warm enough to melt the base of the ice shelf--which is about 1500 meters beneath sea level--because at that depth, the pressure raises the melting point of seawater by about 1 degree.

This means, in turn, that global warming could actually cause the ice shelf to thicken, Nicholls says, because it would result in milder winters, less sea-ice formation, and a smaller influx of warm salty water beneath the shelf. The Ronne shelf "seems to have little to fear from possible climate warming over the next century or two," Nicholls says. He adds that the same principles should apply to most of the major Antarctic ice shelves, which play important roles in global ocean circulation. Oceanographer Lawrence Padman of Oregon State University in Corvallis says the counter-intuitive finding shows that "your first reaction may not be right and in fact might be 180 degrees wrong."