When the global climate wiggles, it shakes its hips at high elevations
by Allen Best
Jeff Pigati arrived at Snowmass, Colo., just two weeks after a bulldozer operator uncovered the first bone of a juvenile mammoth. That was in October 2010. A specialist in radiocarbon dating, Pigati remembers being immediately awed by what he saw.
“It took me about five seconds to figure out that this was something unprecedented and unique and really something that I had never seen before,” says Pigati, a geoscientist with the U.S. Geological Survey.
In time, nearly 6,000 large-animal bones, many of them of mastodons, mammoths and other now-extinct species, were eventually unearthed. What wowed him even more was the exceptional preservation of leaves, seeds and other representations of the changing climates from 140,000 to 77,000 years ago.
Those tell-tale clues have now allowed scientists to paint a picture of climates that changed in sometimes surprising ways. When it got warmer globally, it got warmer at the Snowmass site, located at 9,000 feet in elevation. And when it cooled globally, it got cooler at Snowmass.
But the lake at Snowmass swung both hotter and colder than lower-elevation sites. When the global climate wiggled, the climate at 9,000 feet threw its hips.
“The climate here doesn’t march in lockstep with global conditions. That is the absolute No. 1 take-home message,” says Pigati.
The conclusions and evidence of Pigati and the 43 other scientists involved in the research at Snowmass has been summarized in a special issue of Quaternary Research, a subscription-only journal.
Windows into the past
From the outset, scientists who gathered at the site in Snowmass, which is within a quarter-mile of the ski lifts, had speculated that the site might deliver valuable clues about the climate of the last interglacial period, especially 130,000 to 110,000 years ago.
The American West has several important windows into that period, but none at 9,000 feet. A site near Yellowstone National Park is at about 6,000 feet, as is another site at Bear Lake, along the Idaho-Utah border. Yet a third is at American Falls, Idaho, at about 3,500 feet.
These windows into the past, however, are more smudged. Snowmass is higher and superior in its clarity, what is called fidelity.
One portion of this window into the past, however, is revealing for what it doesn’t show. From about 87,000 to 100,000 yeas ago the Earth’s climate chilled again, as is clear from ice cores in Greenland and Antarctica as well as sediments off the shores of North America.
With this global trend clearly understood, scientists examining the timeline at the Snowmass site, called Ziegler Reservoir, expected to see evidence of plants and other life better suited to colder temperatures. They found much less than expected. Midges, the insects found along creeks and in lakes, were absent, as was Mormon tea. Trees were altogether absent, suggesting that treeline—today it’s at nearly 12,000 feet—had dropped to below 9,000 feet. It is possible that the lake that had entombed mastodons remained frozen even during summers.
The lesson, says Ian Miller, chairman of the Earth Sciences Department at the Denver Museum of Nature & Science, is that the climate at 9,000 feet “responds dramatically to change.”
Scientists, he says, had expected evidence of a colder climate at Snowmass, but not that much. “We have some records elsewhere in North America that says yes it’s going to get cold. But in the high mountain, it’s going gangbusters. This just drives home the point that the system responds dramatically at higher elevations.”
This is sobering to Miller. If the high mountains have an amplified response to climate change, he says, then how will they respond during the coming century as the global climate warms due to heat-trapping greenhouse gases now accumulating in the atmosphere.
“The implication is quite clear that the system does respond dramatically and that it does have an amplified response,” says Miller, a paleontologist who specializes in Cretaceous remains.
Patrick J. Bartlein, a paleoclimatologist in the geography department at the University of Oregon, says the Snowmass findings do not necessarily break ground but do emphatically confirm existing thinking about how climates change.
Bartlein talks about amplifiers. Solar radiation patterns can produce changes in climate, but at Snowmass those changes blew up into even bigger changes, both hot and cold.
Today, we have a different amplifier, the rapid accumulation of greenhouse gases in the atmosphere. Based on what has been documented at Snowmass, the Colorado Rockies might well change much more rapidly in temperatures than other, low-lying areas.
“This helps us understand better the current experiment that we are doing with the climate system by changing the concentrations of carbon dioxide and other greenhouse gases. As we have only one climate system, and it’s sort of dangerous to experiment with it, it’s useful to see these past experiments that nature did,” says Bartlein.
Greenhouse theory has long held that higher elevations and higher latitudes will see the effects of the reinforced greenhouse sooner and more dramatically than lower elevations, at least in terms of temperatures. Measurements have shown this to be the case in the American West but also in the Arctic. This, in turn, is causing other changes, such as the retreat of picas to higher elevations.
In the last interglacial period, absent the buildup of greenhouse gases, the climate at Snowmass got toasty enough. Vegetative fossils found at Ziegler Reservoir suggests that at the peak of warming 125,000 years ago, summer temperatures were comparable to our own or warmer, but winters were definitely warmer.
That was strictly based on natural variability. Now, add human intervention, in the form of elevated greenhouse gases. What temperatures might result from the combination?
Laura E. Strickland, of the Geosciences and Environmental Change Science Center at the U.S. Geological Center, says that higher levels of carbon dioxide in the atmosphere allow some plants to move into drier environments because they are able to use water more efficiently.
“Plants don’t move around the landscape in intact plant communities,” she explains. “Each plant responds to climate individually. With a warming climate we would likely see plants from warmer, lower-elevation localities moving into the Snowmass area, as they did during the last interglacial warming. I would expect changes in floral composition, but not a huge change in the overall plant community (mixed montane conifer forest).”
Strickland thinks the most interesting discovery at Snowmass is the strong similarly between the present-day montane vegetation at the site and the vegetation of the interglacial period documented by the fossil.
“It’s incredible because the existence of the current and fossil plant communities are separated in time by more than 60,000 years, with full interglacial conditions prevailing during that time,” she says. Plants species would have vanished from the Ziegler site and, for that matter, the highlands of west-central Colorado. Plants we see today would have had to immigrate back after the glaciers receded about 15,000 yeas ago.
Can the findings at Snowmass inform climate change models looking forward? Scientists involved in such modeling did not respond to requests for interviews. Pigati surmises the value of the climate record is strongest in west-central Colorado. “My guess is that the farther your get away for our study site, the less secure you can be about making these interpolations,” says Pigati.
Tools of no use here
When Pigati first arrived at Snowmass four years ago, he knew that the remains would be older than 15,000 years old. That is roughly when mammoths disappeared in North America and there was, for sure, a mammoth bone.
Unfortunately Pigati’s radiocarbon dating equipment was of no value. The technique only works on material less than 45,000 years old. Everything in the depths of what is now Ziegler Reservoir was older. For that, scientists needed a different dating technique, called optically stimulated luminescence, which measures the accumulated radiation dose of the time elapsed since material containing crystalline minerals was either heated or exposed to sunlight. It provides dating that is less precise but deeper in time than radiocarbon dating.
Those green leaves that had so captivated Pigati when he first got to Snowmass? They were 100,000 years old.
Using this same luminescence technique, the scientists discovered that the remains of tree-browsing mastodon bones that were found lower in the reservoir site had been deposited 140,000 to 90,000 years ago.
“We didn’t find a single mammoth (bone) during that time period,” says Pigati.
Mammoths bones found at Snowmass were deposited 80,000 to 55,000 years ago, before the climate veered colder once more and the Earth entered the most recent ice age, only to emerge again about 16,000 years ago.
Absolutely nothing was found from 90,000 to 80,000 years ago, the time when the curtain of ice descended.
Both mastodons and mammoths, who are both related to elephants of today, disappeared from North America about 10,000 years ago, or soon after the first known arrival of humans in the Western Hemisphere. They may not have been well-suited to the new shift in climate, although many scientists think their demise can be explained by the new presence of humans and their increasing technological prowess in the form of projectiles.
Does another site with the same riches as Snowmass exist somewhere else, say in Montana? Pigati says that researchers looked for other sites in the West where the same characteristics of glaciation could have yielded a similar circumstances and a comparable window into the past.
“We found nothing,” he says. “That’s not to say they (the sites) are not out there, but it’s not obvious. This might truly be one of a kind.”
This story was amended after its original posting to reflect additional e-mail interviews with Laura Strickland and other small changes.