By Sheryl Zhang
Findings on the past, presented at this year’s annual Comer Climate Conference in Wisconsin, challenge a long-standing climate theory and suggest that Earth’s last great warming unfolded simultaneously across the planet, a pattern we are seeing now — not in alternating hemispheres.
Because Earth’s climate record extends over hundreds of millions of years, the 150 years of direct meteorological data available to scientists represent only a snapshot in time. Research on ancient climate shifts helps reveal how greenhouse gas–driven changes may shape the planet’s future, according to Aaron Putnam, an associate professor at Maine’s Climate Change Institute.
Led by Putnam, graduate students Katie Westbrook and Ph.D Tricia Hall Collins are building glacial chronologies — timelines showing when glaciers advanced and retreated near the end of the last Ice Age. That brief period, known as the Late-Glacial Climate Reversal, saw global warming suddenly stall and reverse.
“That’s an exciting mystery,” said Westbrook. “To try our hardest to understand that complicated mystery, we’re retracing where ice sheets used to be in the past, since they record what temperature was doing.”
Together, the team is testing whether this “mystery” happened in unison across the planet or in a hemispheric relay.
“It’s still perplexing,” Putnam said, “whether the process with which the Earth emerges from an Ice Age is globally synchronous or if they were kind of ping-ponging between the hemispheres. That’s called the bipolar seesaw hypothesis.”
The bipolar seesaw hypothesis proposes that ocean currents redistribute heat between hemispheres, causing one to warm as the other cools. While ocean currents do redistribute heat, they do so continually. Putnam’s data reflects that and points toward a different story — one of synchronized global change.
Westbrook’s fieldwork in Norway focuses on moraines — rocky ridges left behind by retreating glaciers that tossed aside the ridges of boulders as they retreated. Extracting samples to the time when the glaciers left the moraines ice free can take months, however.
“We collect small pieces from large boulders embedded in moraines,” Westbrook said. “We drill a little hole, insert a charge like a bullet without the gunpowder, cover up the rock, and set it off. It pops a chunk of rock off the top.”
Back in the lab, her samples — initially a few hundred grams — are ground down to the size of a pencil tip. Those tiny fragments are then shot by a laser in a mass spectrometer, which counts atoms of an isotope – beryllium-10 – that determine exposure age. Beryllium-10 is like a time machine because it collects in the rock in predictable amounts once the surface is ice cree.
Her results show that glaciers in both hemispheres advanced and retreated at nearly the same time, challenging the seesaw theory. Westbrook now plans to apply the same dating methods to a site in New Zealand to build a direct hemispheric comparison.
While Westbrook focuses on cross-hemispheric timelines, Collins is reconstructing local histories in Lysefjord, Norway, and Wyoming’s Wind River Range to understand the drivers of abrupt climate change.
Her findings in Norway challenge another accepted idea — that during a cold spell about 16,000 years ago, glaciers were expanding. Instead, Collins’s data show they were retreating, suggesting the North Atlantic may not have been as cold as once thought. The period coincides with the retreat of glaciers after they reached their farthest southward extent in the Northern Hemisphere about 18,000 years ago.
“Understanding those mechanisms in the past is what lets us set the parameters for our current situation,” Collins said. “The same processes that drove abrupt warming then are probably still at work today.”
For Putnam, the evidence from both hemispheres carries profound implications.
“The initial warming event that caused that ice to retreat into that fjord by 16,000 years ago,” Putnam said. “That being simultaneous between the hemispheres, that to me says there’s energy being put in both hemispheres that was available to melt ice, and I can’t see any way around it.”
The project, he added, is both a scientific and human challenge. From scaling cliffs to hauling heavy rock samples across glaciers, the team’s work demands patience and precision.
“It’s a lot of effort,” he said. “You have to be really careful and methodical every step along the way, or the data quality can suffer. Westbrook and Collins have been meticulous from start to finish — and now they’re making interpretations that deepen our understanding of the climate system. That’s the really fun part.”