Ancient Cave Formations Reveal History of Abrupt Climate Changes

Ancient rock spires found in caves in the southwestern United States are providing strong evidence that the region is headed for longer droughts as the global climate warms, according to two new studies.

The calcite layers of the spires, known as speleothems, record changes in precipitation just as the rings of a tree trunk do. Two research teams, working independently, recently discovered and analyzed 53,000- and 56,000-year-old speleothems in Arizona and New Mexico. They concluded that rainfall patterns shifted abruptly as climate fluctuated during the last Ice Age.

The tower-like speleothems — which are common features in many caves — are formed over thousands of years by mineral-rich drips of water. Those that grow up from the cave floor are known as stalagmites while those that grow down from the ceiling are called stalactites.

The researchers compared these subterranean moisture records to temperature data from Greenland ice cores, another natural record of ancient climate. Both research teams found that climate in the Southwest oscillated rapidly between wet and dry as the North Atlantic cooled and warmed between 50,000 and 10,000 years ago. Their findings reinforce computer model predictions of similarly abrupt climate change during the coming century, as emissions of greenhouse gases, such as carbon dioxide, heat the atmosphere. The studies were published in the February 2010 issue of the journal Nature Geoscience.

Researchers from both teams said they were surprised to see the near-duplicate studies but agreed this strengthens their results.

“It means that what we’re looking at isn’t just some quirk of one of our caves,” said Yemane Asmerom, a geochemist at the University of New Mexico and the lead author of one of the papers. He’s confident that “when Greenland warms up, that really doesn’t bode well for places like the Southwest.”

Climatologists have been studying speleothems as climate archives since the 1980s, when geologists refined the techniques for dating them precisely. Since then, they’ve used speleothems to reconstruct past climate change in more than a dozen places around the globe, including China, Brazil and the Middle East.

“Cave records have been really important because they’ve allowed us to cover huge portions of the Earth that we haven’t been able to access in anywhere near the same way in the past,” said Larry Edwards of the University of Minnesota, who studied some of the most well known speleothem records in Hulu Cave near Shanghai, China.

Climatologists have long relied on corals, lakebed sediments, tree rings and ice cores to reconstruct climate history. But corals and tree rings date back only a few centuries or millennia, and sediments are difficult to date because they’re constantly being mixed up and burrowed into. Even ice cores, despite their iconic status among paleoclimatologists, have limitations.

“Ice cores only exist where there’s ice,” explained Jess Adkins, a geochemist at the California Institute of Technology who studies speleothem records in Borneo. “And we know that most of climate variability comes from the tropics. So we need an ice core that can be the ice core of the tropics.”

Speleothems are at least partially filling this role. Scientists can track when a climate flip-flopped between wet and dry periods by measuring the composition of oxygen atoms in a speolothem’s calcite layers. Speleothems also contains traces of the chemical element uranium, which slowly decay into thorium over time. Scientists can therefore assign each layer a specific date depending on its uranium-to-thorium ratio.

Because speleothems provide such a reliable record of climate change, said Richard Alley of Pennsylvania State University, who has analyzed ice core data from Greenland for 25 years, they offer “yet another opportunity to test climate models.” According to these models, warming in the North Atlantic causes fast-flowing air currents, such as the jet stream, to migrate northward, bringing monsoons to China and drought to the southwestern United States.

Speleothems tell the same story.

“They’re telling us a consistent story, absolutely,” said Kim Cobb, a climatologist at the Georgia Institute of Technology. “As global warming continues, the position of the jet stream is going to change, and it’s going to bring dire conditions to the American Southwest.”