Boulders reveal past climate change record and accelerating pace now

Climate researchers may hike uphill for hours or wade knee-deep through a bog just to reach the boulders tossed aside by glaciers. Through the time machines in these rocks, they hope to reconstruct the events of the last great ice age and reveal clues to climate switches now.

By Louise Kim
Medill Reports, March 8, 2024

Near Lake Ohau, New Zealand – Long before humans shaped the earth with bulldozers and cranes, glaciers shifted the landscape with massive chunks of ice and gravity. As glaciers advanced, they carried off rocks with them and, as they retreated, they tossed the rocks behind, sculpting landforms called moraines.

It’s these boulders that today’s researchers seek out, sometimes hiking uphill for hours or wading knee-deep through a bog just to reach them. Through the time machines in these rocks, they hope to reconstruct the events of the last great ice age, roughly 26,000 to 19,000 years ago, and reveal clues to climate switches now.

“The biggest questions in earth science are still the biggest questions today” for climate change, said Aaron Putnam, associate professor of earth and climate sciences at the University of Maine. Joining his research team near Lake Ohau in New Zealand this winter, we searched out the record in moraine boulders revealing past climate change and the accelerating pace of climate change now. Putnam plans to compare the record in the rocks in New Zealand to the record in boulders of the Northern Hemisphere to determine to what degree glacial retreat moved in sync across the globe. 

Aaron Putnam, a professor at the University of Maine, gazes out at the natural fence of boulders near Lake Ōhau in New Zealand. These moraines form as giant glaciers shed material, tossed aside like a handful of pebbles. (Louise Kim/MEDILL REPORTS)

Nature has subtle ways of pointing out its connections to antiquity: tree rings reveal a year-by-year story of floods and droughts in the size of each ring, radiocarbon dating points to the age of buried teeth  from past eras and beryllium-10 reveals the movement of glaciers in the last ice age. 

That means researchers like Putnam use this isotope to find out how old moraines are, since beryllium-10 collects at predictable rates once a rock is freed from the ice and exposed to air. To get the approximate age of a moraine, researchers drill for the isotope in the boulders that lie on it.

By retrieving a sample of a granite or sandstone boulder and extracting its quartz, they can find the amount of beryllium-10 that has accumulated near its surface in the quartz as cosmic rays bombarded it. The more beryllium-10, the longer the time span since the boulder was tossed from the ice. And dating correlates to predictable rates of beryllium-10 deposits over time 

During glacial advance, ice cover protects boulders from cosmic rays and prevents the production of beryllium-10, which is formed as the rays collide with molecules of oxygen in the quartz. Previous buildup of beryllium-10 is scoured away when a rock is devoured by a glacier. When the glacier drops the boulder onto the Earth’s surface as it retreats, what happens is like starting a stopwatch. This deposition becomes the starting point as beryllium-10 production begins.

“Our method depends upon a surface that has not been previously exposed to cosmic ray bombardment,” said Tricia Collins, a graduate student at the University of Maine.

Putnam holds up a rock sample retrieved from the lateral moraines of New Zealand. This sample, among others, will eventually be shipped back to the United States where the scientists will analyze the amount of beryllium-10 to date the retreat of the last ice age.  (Louise Kim/MEDILL REPORTS)

As researchers obtain moraine ages, they’re able to get a picture of how glaciers from the Last Glacial Period advanced to the maximum some 20,000 years ago and retreated throughout time ever since. The age difference between the moraines shed early and later in the retreat tells scientists about the speed of the retreat in the past compared to how fast ice is retreating now. 

“Glaciers are a physical manifestation of the climate state,” said Putnam, and the rapid movements of the Last Glacial Period show us that climate change isn’t gradual. It’s more like a switch where the climate system will cling to its current state until it crosses a threshold and is forced into a new state, he said. The abrupt warming that terminated the Last Glacial Period is an example of this. And as abrupt as change was then, it’s accelerating now. 

During the past 12 months, we’ve surpassed the notorious 1.5 degrees Celsius threshold. Questions about how abrupt climate change is moving now continue to circulate as the ice continues to retreat.

Records from the past will be an essential reference for what our world could look like in the future. An understanding of abrupt climate change from the past can inform predictive models, and these models will need all the information they can get to analyze the extremes today’s warming world is already experiencing.

 There are the human  forces heating up the planet to unprecedented levels that researchers have to take into account on top of what they know from the past.

“Most of the climate information you’re looking at is pertinent to equilibrium states,” said Putnam. “We’re not in equilibrium. We won’t be for a long time.”

Photo at top: Tricia Collins and Alexzander Roman, graduate students at the University of Maine, hike through the Ruataniwha Conservation Park in New Zealand in search of boulders deposited by glaciers from the Last Glacial Period. (Louise Kim / MEDILL REPORTS)

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