A honeycombed cave formed untold millennia ago beneath what is now southeastern Minnesota. Larry Edwards is standing in a subterranean chamber, his headlamp illuminating a series of mineral formations. From the cathedral-like ceiling dangle tubes known as soda straws. Along a waist-high ledge squats a trio of stout stalagmites, their surfaces slick with ecru-colored ooze. “Now that’s the kind of thing we might be interested in,” Edwards says, bending to peer at one.
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I hear the plink, plink, plink of falling droplets. One hits the top of a stalagmite, then spreads out, laying down a thin film of the mineral calcium carbonate, or calcite, from rainwater seeping though limestone. Drop by drop the stalagmite has grown to its present height—about 18 inches—over who knows how many centuries.
Edwards, a geochemist at the University of Minnesota and a pioneer in the use of cave formations to document ancient climate, is not planning to collect stalagmites today. But two specimens severed from their moorings when the owner of the cave complex, Spring Valley Caverns, opened a deeper passageway recently provided Edwards and his colleagues with a record of extreme rainfall events over the past 3,000 years. Edwards wonders if some of Spring Valley’s stalagmites could contain older records still, dating back to when giant glaciers covered much of the Northern Hemisphere or even to one of the distant warm periods, or interglacials, that punctuated the ice age world.
A short time later, we retrace our steps, navigating the sequence of walkways and ladders that leads to the cave’s entrance. As we step into the light, Edwards turns to me. “Do you notice all the sounds, all the smells?” he says. “When you come up, they seem so pronounced.” Edwards, renowned among paleoclimatologists for his cave findings, isn’t much of a spelunker. “It’s not that I’m claustrophobic,” he says with a shrug, “I just like it better up here.”
To Edwards, a stalagmite is more than a chunk of geology that looks like a modern sculpture: It’s a collection of climate sensors, rather like tree rings but extending often hundreds of thousands of years back in time. Perhaps the only other earthly archives that have provided such a high-resolution portrait of the past half-million years are ice cores.
But unlike ice, caves can be found all over the world. “I would go so far as to say that these are among the most important paleoclimate records we have,” David Battisti, a University of Washington atmospheric scientist, says of the cave data.
From Edwards’ lab is emerging a high-resolution picture of precipitation patterns long ago. Just as important, his work is providing the scientific community with an increasingly precise time scale, one that is bringing other records into alignment. Edwards and his colleagues have used cave formations to tighten up the timing for ancient rises in carbon dioxide locked into Antarctica’s ice. They’ve even used them to date skeletal remains that trace human migration routes.
Why should we care about what happened so long ago? A reason can be found in the mounting nervousness over the consequences of global warming. With heat-trapping carbon dioxide in the atmosphere already reaching levels not seen for at least 800,000 years, scientists like Edwards worry that weather patterns could undergo sudden, destabilizing changes.
Ice cores from Greenland, for instance, reveal a sequence of abrupt temperature oscillations over the past 140,000 years, with severe cold snaps leavened by a series of sudden, if ephemeral, warmings. Eerily similar oscillations are now showing up in the precipitation records from caves. In a landmark study, Edwards and his collaborators compared the precipitation swings captured by Hulu Cave on the outskirts of Nanjing, China, with temperatures encoded by Greenland’s ice. Plotted as graphs, and positioned side by side, the dips and valleys in both records are sharp and—for the 60,000-year period covered by the stalagmites—synchronous.