Global warming: real or hoax?

October 20, 2000 -- Newspaper headlines trumpet record-breaking temperatures, dwindling sea ice, and retreating glaciers around the world. Concentrations of atmospheric carbon dioxide, one of the greenhouse gases responsible for scalding temperatures on Venus and at least 33 degrees C of normal warming here on Earth, are on the rise. Our planet seems destined for a hot future!

But is it really? Or are we simply experiencing a natural variation in Earth's climate cycles that will return to "normal" in time?

Correlations between rising CO2 levels and global surface temperatures suggest that our planet is on a one-way warming trend triggered by human activity. Indeed, studies by paleoclimatologists reveal that natural variability caused by changes in the Sun and volcanic eruptions can largely explain deviations in global temperature from 1000 AD until 1850 AD, near the beginning of the Industrial Era. After that, the best models require a human-induced greenhouse effect.

In spite of what may seem persuasive evidence, many scientists are nonetheless skeptical.

They argue that natural variations in climate are considerable and not well understood. The Earth has gone through warming periods before without human influence, they note. And not all of the evidence supports global warming. Air temperatures in the lower atmosphere have not increased appreciably, according to satellite data, and the sea ice around Antarctica has actually been growing for the last 20 years.

It may surprise many people that science -- the de facto source of dependable knowledge about the natural world -- cannot deliver an unqualified, unanimous answer about something as important as climate change.

Why is the question so thorny? The reason, say experts, is that Earth's climate is complex and chaotic. It's so unwieldy that researchers simply can't conduct experiments to check their ideas in the usual way of science. They often rely, instead, on computer models. But such models are only as good as their inputs and programming, and today's computer models are known to be imperfect.

Most scientists agree that no single piece of data will likely resolve the global warming debate. In the end, the best we can expect is a scientific consensus based on a preponderance of evidence.

The canary in the coal mine?

The recent discovery that Greenland's ice sheet is thinning is a good example of our climate's sometimes vexing ambiguity.

About 85 percent of Greenland is covered by a massive ice sheet with an area of about 1,736,000 square kilometers and an average thickness of about 1,500 meters. The volume of ice in the Greenland sheet is estimated to be about 2,600,000 cubic kilometers -- enough ice to raise sea levels by 6.4 meters if it all were to melt.

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 center but considerably lower at the edges -- particularly the southeastern edge.

The overall result: The ice sheet lost at least 51 cubic kilometers 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.

NASA