A series of troubling reports in recent years have suggested Antarctica is warming and shedding its ice shelves at an alarming rate.
But a new study that used a highly precise image-snapping satellite suggests at least one prominent ice sheet — the West Antarctic Ice Sheet — is in fact getting thicker.
The report, plus other work finding that desert valleys on the continent have cooled recently, appear to contradict predictions that global warming is melting the continent's massive ice reservoirs.
This may seem like good news, but scientists say: Don't count on it. They warn other ice sheets continue to shrink even as this one thickens.
"There's no question that some parts of Antarctica are warming," said Ian Joughin, a geologist with the Jet Propulsion Laboratory at the California Institute of Technology in Pasadena. "But it could be this part of the ice sheet is not necessarily sensitive to global warming."
Details of the study by Joughin and Slawek Tulaczyk, a professor of earth sciences at the University of California, Santa Cruz. are published in this week's issue of the journal Science.
Mighty Ice Rivers
The West Antarctic Ice Sheet contains enough ice to raise sea levels more than 16 feet if it were to melt. Recent concern has centered on whether all or part of this contribution might break off from the continent and flow rapidly into the ocean. Such a rise could flood coastal regions and submerge islands.
One way to measure the mass of an ice sheet is to compare how quickly ice flows into and out of it via ice streams. Ice streams are sections of ice and snow ranging 20-60 miles wide that grind along slippery, muddy beds at an average speed of about a half mile a year.
By measuring ice flow speeds at entrance and exit points of the ice sheets, Joughin and his team determined that one ice stream — Ice Stream C — has ground to a halt while another — the Whillans Ice Stream — has dramatically slowed its drainage into the sea.
If true, that would mean more ice is flowing in than out and the ice sheet's thinning or "purging" phase that has been in effect for the past 12,000 years is now in the process of reversing into a "binge" phase.
As Richard Alley, an ice scientist at Pennsylvania State University wrote in an accompanying Science column, "Perhaps … researchers turned on their instruments just in time to catch the stabilization or re-advance of the ice sheet."
But why would a 60-mile-wide ice stream suddenly grind to a halt? Joughin suspects it's part of a natural cycle.
Slipping on Mud, Stopping on Gravel
After churning large amounts of snow and ice into the sea, he explains, an ice stream thins and loses insulating layers of snow. Those insulating layers normally serve to trap heat from the Earth's core and melt water. As the layers thin, lubricating water refreezes and the flow of snow and ice slows.
"Think of it like sliding on mud versus sliding on gravel," said Joughin. "There's a big difference."
If its ice stream has indeed slowed for good, the Ross Ice Sheet will thicken as long as new ice continues flowing in — and that's exactly what appears to be happening. That would be promising news, if not for another nearby ice sheet — the Pine Island Glacier — where satellite studies revealed last February that about 7.5 cubic miles of ice had eroded in just eight years.
Results from another study — release just last week — showed that air temperatures recorded over a 14-year period ending in 1999 declined by one degree Fahrenheit. The cooling defies a global trend spanning more than 100 years in which average land surface temperatures have increased by about 1 degree Fahrenheit. The scientists could not explain why Antarctica, alone, is cooling, but they point out 14 years is barely a blink in geological time scales.
So scientists are still left wondering if Antarctica's ice is thickening or thinning, warming or cooling … or both.
"It could be that only time can tell," said Abbey. "We just haven't been watching long enough."