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A new study from Chloe Brashear, Tyler Jones and others suggests abrupt warming events were preceded by periods of unusually stable temperatures during the last ice age. The researchers point toward shifting sea ice as a potential driver of the phenomenon.

On July 21, 2019, Chloe Brashear carried another disc of ice through the underground ice cave at the East Greenland Ice-Core project. The cave lay a few meters below the surface of the sprawling Greenland ice sheet, more than 200 miles inland from the coast. Brashear loaded the disc onto a hot aluminum plate and then stepped into the sampling room, where the melt water was pumped through an array of equipment that would filter it, vaporize it and produce a readout of its chemical contents.

Despite the sub-freezing temperatures in the cave, space heaters and an array of whirring instruments kept the sampling room hot. Brashear cast off her parka and got to work.

In most ways, it was a typical day of late-summer field work, but this day was also special. Brashear and her colleagues were analyzing samples extracted from deep within the ice sheet—more than 2,000 meters below the surface. The scientists estimated that the ice was more than 40,000 years old. Later that night, they would celebrate over drinks and grub.

Chloe Brashear poses in the drill trench at the East Greenland Ice-Core Project. Photo courtesy of Chloe Brashear.

New Insights

Five years later, Brashear—now a PhD candidate at Utrecht University in the Netherlands—has teamed up with her former mentor,��INSTAAR fellow Tyler Jones, and others��. Their new study takes a fresh look at some of the most dramatic climate upheavals in Earth’s history: abrupt warming events that punctuated the last ice age, between 11,000 and 50,000 years ago.

The data revealed something unexpected. On average, the colder periods between warming events displayed variable temperatures—it might be very cold one decade and much warmer the next. But, during the few hundred years before an abrupt warming event, this volatility flattened out. Each rapid warm-up was preceded by centuries of unusually stable temperatures.

“Variability would start to decrease first at decadal and multi-year scales,” Jones said. “Then, a few hundred years later, on average, there would be an abrupt warming event.”

It was as if the climate system was holding its breath before suddenly exhaling in a burst of warmth. But why?

The new paper proposes that shifting sea ice conditions in the North Atlantic may be the missing puzzle piece. If their hypothesis is correct, it could reshape our understanding of Earth's climate system—especially in times of abrupt change.

Ice age heat

If the phrase "abrupt warming event" makes you think of modern climate change, you're not wrong. But, the events that Brashear and Jones focused on in their latest paper, known as Dansgaard–Oeschger events, were actually much more intense.��Researchers estimate that, in the most extreme version of their projections, temperatures in Greenland may have risen by as much as 29 degrees Fahrenheit in less than a decade.

“As an analogy, imagine you live in Northern Maine when you start college, and by the time you finish college it feels like you’re living in Southern Arizona,” Jones said.��

Climatic variability—basically the volatility of temperature fluctuations—. But, previous research lacked the precision needed to parse out the timing between changes in variability and these extreme warming events.

Freshly-drilled ice cores are stored in the ice cave, where they await processing and analysis. Photo courtesy of Tyler Jones.

That changed when Jones and his colleagues, including INSTAAR faculty Bruce Vaughn, Valerie Morris and James White, developed a new methodology for analyzing ice cores: continuous flow analysis. Instead of chopping an ice core into chunks and analyzing each separately, continuous flow analysis melts the core tip to tail, extracting a near-unbroken record of past temperatures. This allows scientists to study changes in climate on a millimeter-by-millimeter scale. In the case of this project, continuous flow analysis allowed Brashear to interpret temperature data for distinct intervals of 7 to 15 years of ancient history.��

“If you continuously sample the ice core, you capture all this detail that you are losing with discrete sampling,” she said.

This technique provided the new paper’s biggest insight: the stable temperatures that preceded each of the Dansgaard–Oeschger events. It also provided Brashear with a powerful dataset to compare to sea ice models.��

The comparison once again produced an intriguing result. The changes in temperature variability were highly correlated with modeled changes in sea ice variability. In the new paper, Brashear provides a hypothesis: the leading edge of North Atlantic sea ice may have become more stable, which would have decreased its influence on short-term temperature fluctuations in Greenland.

If true, the finding could influence scientists seeking to refine models of Earth’s climate and gain insights into the modern era.

“This result doesn’t directly apply to the modern changes we’re seeing, because they are unprecedented,” Jones said. “But, our hope is that we can shed light on the mechanisms that gave rise to this lead-lag relationship in variability and temperature, and then pass those results on to the modeling community.”

The next chapter

The researchers are cautious to not overstate their results. After all, the sea ice hypothesis is just one of several possible explanations. More evidence is needed.

Some of that evidence may come soon. Jones’ lab has secured funding to reanalyze an ice core��extracted in the late 1980s and early 1990s from a site 200 miles south of the East Greenland Ice-Core Project. Using continuous flow analysis, they hope to confirm the patterns Brashear identified and gain further insight into these ancient climate shifts.

“We’re hoping we can replicate the result and push further into modeling,” he said.

The final chapter of Brashear’s research at INSTAAR is now over, but the experience of working in the remote scientific encampment atop the Greenland ice sheet remains vivid. She looks back with fondness on long days in the underground lab, neverending Arctic sun and nights spent celebrating new discoveries with international collaborators.

“It’s awesome to be able to look at a dataset and then have these memories associated with it,” she said. “It helps you stay motivated… I’m still pursuing a career in science, so you could say it had a positive impact.”

A line of national flags waves in the arctic wind. 15 Institutes from 14 different countries participate in research at the East Greenland Ice-Core project.��Photo courtesy of Tyler Jones.

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If you have questions about this story, or would like to reach out to INSTAAR for further comment, you can contact Senior Communications Specialist Gabe Allen at gabriel.allen@colorado.edu.