In some ways, drilling into Antarctica’s ancient ice is easier than interpreting it. Today, more than 2 years after presenting the discovery of the world’s oldest ice core, scientists have published an analysis of the 2.7-million-year-old sample. One surprising finding: Air bubbles from 1.5 million years ago—from a time before the planet’s ice age cycles suddenly doubled in length—contain lower than expected levels of carbon dioxide (CO2), a possible clue to the shift in the ice age cycle.
The CO2 levels are “amazingly low,” says Yige Zhang, a paleoclimatologist at Texas A&M University in College Station. He adds that the study, published today in Nature, is “quite interesting” because it reports the first direct measurements of atmospheric gases from that mysterious time.
Some 2.6 million years ago, Earth entered a time known as the Pleistocene, which saw the planet swing in and out of deep periods of glaciation at regular 40,000-year intervals. About 1 million years ago, during what’s called the Mid-Pleistocene transition, these ice age cycles went from occurring every 40,000 years to 100,000 years. (The most recent ice age ended 11,000 years ago.) Scientists have long known that tiny changes in Earth’s orbit, called Milankovitch cycles, drive the planet in and out of these ice ages. But nothing changed in orbital patterns 1 million years ago that would have driven the “flip.”
Some scientists suspect that overall CO2 levels were higher in the 40,000-year world, but declined over time and cooled the planet, eventually reaching a point where Earth transitioned into deeper, longer freezes every 100,000 years. One way to check that theory would be to examine samples of Earth’s atmosphere from before the flip. But before the discovery of the new ice core, the oldest greenhouse gases one could measure were in trapped bubbles in ice dating to about 800,000 years ago.
To reach further back in time, a team of scientists targeted so-called “blue ice” near Antarctica’s surface in the Allan Hills. Here, ancient ice flows have exhumed the oldest ice from the deep. Old ice layers are driven up from below, while wind strips away snow and younger ice. Paul Mayewski, a glaciologist at the University of Maine in Orono, suspected such ice could be ancient, and Michael Bender, a geochemist at Princeton University, developed a way to date chunks of ice directly from trace amounts of argon and potassium gases they contain. In 2015, a team led by John Higgins, a Princeton geochemist, excavated the record-setting core.
At first, the oldest ice seemed to contain startling levels of CO2, several times the 407 parts per million (ppm) we see today, says Yuzhen Yan, the Princeton geochemist who led the new study. Further analysis, however, revealed the bubbles had been contaminated by CO2 percolating from beneath the ice, likely released by microbes. That meant the team had to toss out data from many of the oldest samples—a reflection of their conscientiousness, says Bärbel Hönisch, a geochemist at Columbia University. “The authors had to do a lot of work to convince themselves of what they’re actually seeing.”
When the team looked at CO2 levels from 1.5 million years ago, they found them on average quite similar to the postflip world, swinging between 204 and 289 ppm, depending on whether the world was in an ice age or not. “It’s surprising,” Yan says, given broad evidence that the world was warmer in the early Pleistocene, before the ice ages grew deeper. “The educated guess is you’d have higher CO2 to achieve that. But that’s not something we see.”
That means that something other than a long-term CO2 decline was likely driving the cooling, says Peter Clark, a glaciologist at Oregon State University in Corvallis. One such driver could be the cumulative buildup of ice across the Northern Hemisphere; more ice would leave the world more arid, for example, allowing iron-rich dust to fertilize ocean microbes, encouraging them to absorb more CO2 from the atmosphere during glacial times. Clark has long advanced a hypothesis that repeated glaciations gradually scoured away soil and other loose grit that would have prevented ice from “sticking” to bedrock. Once that grit was gone, the anchored ice sheets could thicken and grow to a tipping point, sending the planet into 100,000-year cycles.
Other subtleties in the ice’s CO2 levels point to other possible mechanisms. When the planet transitioned to 100,000-year ice ages, for example, levels of CO2 dropped on average 24 ppm lower during glacial periods compared with similar events in the previous era. That suggests the world was quite sensitive to CO2 and could have “flipped” thanks to something like a small disruption in the currents that drive carbon storage in the ocean—perhaps caused by ice sheet growth or something else, Hönisch says.
Given its limitations, including a small amount of material collected by a narrow drill, the Allan Hills core is unlikely to settle debate on the ice age transition. However, its data are helping calibrate other, indirect methods of measuring ancient CO2, like using isotopic shifts in single-celled foraminifera fossils. As those methods have improved, their estimates have lined up with the new findings. “It’s a wonderful confirmation that the proxies are really working,” Hönisch says.
Meanwhile, the team hasn’t stopped its exploration of the blue ice. “It’s conceivable that there’s ice as old, or even older, out there,” Yan says. Next month, a team led by Higgins will arrive in Antarctica to hunt for it. And this time, they’re bringing a bigger drill.
*Correction, 31 October, 5:15 p.m.: An earlier version of this story implied that Clark’s hypothesis suggested ice reflecting sunlight into space drove ancient cooling.