Study Links Region’S Rain To Ice Caps

University of Arizona researchers use cave formations to show abrupt rainfall shifts



Pete Aleshire/Roundup

Stalagmites in Cave of the Bells have revealed abrupt Ice Age climate shifts.

Long before human beings started pumping greenhouse gases into the air, the Earth’s climate underwent alarming shifts.

Several studies focused on some of those abrupt changes in temperature and rainfall have underscored the frightening swings — which climate scientists says make the possible shifts due to human activities even more worrisome.

Researchers from the University of Arizona have recently added two key studies to the growing body of research that suggests the Earth’s climate remains stable for long periods — but can then shift dramatically in a short time.

One study discovered that during the period marked by repeated Ice Ages between 11,000 and 55,000 years ago, the climate in the American Southwest repeatedly underwent dramatic shifts.

UA geosciences researcher Julia Cole and her colleagues based their conclusions on a careful study of cave formations in Arizona’s Cave of the Bells, a limestone cavern in southeastern Arizona. They published their work in Nature Geoscience. The key lay in measuring the elements in a spear-like stalagmite that had formed on the floor of a chamber deep in the cavern from the drip-drip-drip of water bearing a load of dissolved limestone. They focused on the amount of oxygen 18 in the formation, since the proportion of this rare isotope of oxygen rises in dry years.

The scientists could also precisely date the rock layers by analyzing minute amounts of uranium picked up by the ground water as it seeped through the fractured layers of limestone. The clock-like decay of the uranium into thorium provided a way to measure the age of each layer of rock.

The researchers carefully removed the stalagmite, shaved off 1,200 hair-thin samples from a one-inch core, then carefully used epoxy to glue the formation back in place in the deep dark.

The study found that the Southwest would warm up abruptly and remain warm for a few hundred to 1,000 years at a time. During the cool periods, glaciers would advance to cover much of North America and the Southwest became a cool, grassy oak woodland — with mammoths, sloths, camels and other long-extinct creatures wandering grasslands that have since turned to desert.

Scientists still can’t fully account for the dramatic, global cooling of the periodic Ice Ages, but they suspect variations in the Earth’s orbit and tilt set in motion a runaway process that was affected by carbon dioxide and methane put into the air by volcanoes, the reflection of energy by expanding ice caps and other factors.

The UA analysis neatly matched previous work on abrupt climate shifts based on analysis of ice cores from Greenland. The research showed that whenever Greenland heated up, so did the American Southwest.

The study suggested that the climate may remain stable for long periods, but then undergo violent shifts — a pattern similar to the Ice Age cores taken from the 660,000-square-mile Greenland Ice Sheet which is two miles thick in spots.

A panel of climate scientists recently reported that the Greenland Ice Sheet has already started to shrink, including earlier this year losing a 97-square-mile iceberg. The scientists predicted that a temperature increase of as little as 2 degrees centigrade could lead to the run-away melting of the Greenland Ice Sheet.

The UA study for the first time now directly links temperatures in the Southwest to the fate of the Greenland Ice Sheet.

The UA study was funded by the National Science Foundation. Co-authors included Jennifer Wagner, J. Warren Beck, P. Jonathan Patchett and Heidi Barrnet.

A similar study of cave formations by researchers from the University of California at Davis linked droughts lasting for centuries in California to the thawing of the Arctic Ice Caps in the past 20,000 years.

That study also found a close correlation between rainfall in California and the warming of the ice caps. The link was especially pronounced in the warming period 15,000 and 13,000 years ago that signaled the end of the last Ice Age.

Other studies show that the Arctic ice cap has shrunk an average of 3 percent annually for the past 30 years. Some forecasters predict that the long-frozen Arctic Ocean will be ice-free by 2020.


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