Earth's Fidgeting Climate

While it is only about one-seventh the size of the Antarctic ice sheet, some scientists think that watching the ice on Greenland provides better clues about global warming.

"Even though Antarctica is seven times the size of Greenland, because (Antarctica is) kind of symmetrically positioned around the South Pole, it doesn't really interact with climate up in the more temperate regions the way Greenland does," said Dr. William Krabill at NASA's Wallops Flight Facility. Krabill is the project scientist for the team that discovered the thinning. "Greenland ... is likely to be a better indicator of global climate change than Antarctica," he noted.

Krabill's team used an airborne laser to survey the altitude of the ice sheet's surface during 1993 and 1994. They repeated their survey in 1998 and 1999, making certain to retrace their flight paths from the first survey as closely as possible.

After incorporating some assumptions that let them extend their measurements to the sheet's edges, the scientists compared the second survey to the first. They found that the ice sheet's surface was slightly higher at the centre but considerably lower at the edges -- particularly the southeastern edge.

The overall result: The ice sheet lost at least 51 cubic kilometres of volume during that five-year period. Greenland appeared to be melting!

Many newspaper headlines cried the discovery as a sign of global warming -- which most readers presumably took to mean "anthropogenic," or human-caused, global warming.

But is that the right conclusion?

"What you can say is, yes, carbon dioxide (in the atmosphere) is at levels higher than ever before, and carbon dioxide is a greenhouse gas, so it's reasonable to say that there's warming associated with the increase of carbon dioxide," said Dr. Waleed Abdalati, co-author of the paper that announced the Greenland discovery.

"But you can't make the leap yet that all the cars in the world have led to what we're observing in the thinning of the Greenland ice sheet," Abdalati said.

If there's one lesson to be learned from science, it's that things are usually much more complex than they at first appear. The warming trend of the last century may seem to be the obvious explanation for the thinning seen on Greenland, but scientists are considering other possibilities.

"That's what science is about," said Dr. Ellen Mosley-Thompson, a research scientist at the Byrd Polar Research Centre at The Ohio State University.

"Just because you have an hypothesis and immediately your experiment produces support for it, you can't simply accept those results (without a degree of scepticism)," Mosley-Thompson said. "The whole idea is to play devil's advocate on your own research before your colleagues do."

Last century's warming trend is not the only possible explanation for the thinning that Krabill's team saw on Greenland.

In fact, ice cores taken as part of another NASA-funded study suggest that natural variation in snowfall may be partly to blame, Mosley-Thompson said.

"The ice core data provide evidence -- not necessarily conclusive -- that Bill's results may in part reflect variability in snow accumulation over his five-year observational window," said Mosley-Thompson, who co-authored the paper reporting these results with Dr. Joseph McConnell, an associate research professor at The Desert Research Institute in Reno, Nevada. The results of the study were published in the August 24 edition of the journal Nature.