Earth's Fidgeting Climate

Other natural processes could account for the thinning as well. Ocean currents might have caused part of the change. Or the flux of warm water into the North Atlantic caused by the 1990-1996 positive phase of the slow-moving North Atlantic Oscillation could have had an influence. The ice sheet could also be thinning in response to the long-term warming of the planet since the transition from the last glacial period about 10,000 years ago. Krabill, Dr. Ron Kwok of NASA's Jet Propulsion Laboratory, and Abdalati mentioned these scenarios during interviews with http://science.nasa.gov/.

Scientists often refer to these alternate explanations under the umbrella term of "natural variability."

The ant on the hour hand

"For the ordinary person, it's a common misperception that weather is not changing ... that last winter is about as cold as this winter and last summer is about as warm ... and the world is pretty much constant," Krabill said. "That's not true. The Earth has gone through and continues to go through cycles of warming and cooling. It's just natural."

This natural variability often shows an astounding degree of complexity, much of which remains poorly understood.

"We've only begun making (large scale) measurements in the last 100 to 150 years," Abdalati said. "And climatic processes happen on very different time scales. There are some, like ice ages, that are in the tens of thousands or hundreds of thousands of years long. An then there are atmospheric processes like weather, which happen on the scales of hours and days."

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Other climate cycles fall in between, such as the North Atlantic Oscillation mentioned above, which is thought to complete one cycle roughly every 20 to 30 years.

"And so you have all these processes mixed together that have been going on for thousands of years, and you're in the difficult position of trying to separate something very recent from the natural cycle without fully understanding what that natural cycle is," Abdalati said.

Observing a system like climate that varies on several time scales -- some of which approach geological slowness -- could be likened to an ant watching the hands of a clock, "perhaps with the ant sitting on the hour hand," Abdalati added.

Seen in this context, scientists don't give much weight to the five-year snapshot of the ice on Greenland.

"You know, five years is a pretty short amount of time in glaciological terms," Krabill said. "To try to make inferences about 'Global Climate Change' in capital letters from a five-year period of time is a pretty risky business."

Other modern data sets are not much longer. The era of satellite observation is only about 30 to 40 years old -- a mere blink in climatological terms. And the widespread network of weather-measurement stations in the developed world is about 150 years old.

The Ghost of Climates Past