CU study finds volcanoes likely cause of 'Little Ice Ages'

Ice cap growth coincides with major eruptions

By Bill Scanlon, Rocky Mountain News (Contact)

Monday, February 4, 2008

Aerosols from tropical volcanoes likely caused the "Little Ice Ages" that kept the Northern Hemisphere cool for 600 years until 1850, a study by the University of Colorado found.

Nothing seems to be helping the ice caps of Baffin Island, west of Greenland, though, say the researchers who found the ice caps are smaller now than at any time in at least 1,600 years. They've shrunk by more than half in the last half-century.

"Even with no additional warming, our study indicates these ice caps will be gone in 50 years or less," said Gifford Miller, of CU's Institute of Arctic and Alpine Research.

Earlier studies by CU have found that temperatures on the Greenland ice sheet have climbed 7 degrees Fahrenheit in the past 17 years because of buildup of greenhouse gases in Earth's atmosphere.

Miller and his colleagues used radiocarbon dating of dead plant material to measure the highs and lows of the Baffin Island ice caps over the past several thousand years.

They found two distinct bursts of ice cap growth, one starting about A.D. 1280 and the other about A.D. 1450. Both of those dates coincide with increases in stratospheric aerosols tied to major volcanic eruptions in the tropics.

The unexpected findings "provide tantalizing evidence that the eruptions were the trigger for the Little Ice Age," Miller said.

A paper on the subject was published online in Geophysical Research Letters. Authors included Miller, graduate students Rebecca Anderson and Stephen DeVogel of the arctic research institute, Jason Briner of the State University of New York at Buffalo and Nathaniel Lifton of the University of Arizona.

Baffin Island is the fifth-largest island in the world, at just under 200,000 square miles. Most of it lies above the Arctic Circle.

The researchers used aerial photos dating back to 1949, and more recent satellite data, to document the shrinkage of 20 ice caps. The caps are up to four miles long, usually less than 100 yards thick and are frozen to their beds.

They also extracted carbon 14 in quartz crystals that formed inside the island's rocks from cosmic radiation bombardment. The carbon 14 reveals the amount of time the rocks have been exposed.

They found that for most of the last several thousand years, there has been more ice on Baffin Island than in earlier times.

The large amount of ice in the Arctic over most of the past several millennia probably is due to less summer solar radiation because of a long-term cyclic wobble in Earth's axis.

"This makes the recent ice cap reduction on Baffin Island even more striking," Miller said.

The study was funded primarily by the National Science Foundation.

scanlon@RockyMountainNews.com or 303-954-2897