How long can Greenland ice hold out? Scientists track climate switches

by Stephanie Novakand Rachel E. Gross
Oct 23, 2012

Greenland Ice1

Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

NASA calls it "unprecedented Greenland ice sheet surface melt." According to NASA: "Extent of surface melt over Greenland’s ice sheet on July 8 [top] and July 12 [bottom]. Measurements from three satellites showed that on July 8, about 40 percent of the ice sheet had undergone thawing at or near the surface. In just a few days, the melting had dramatically accelerated and an estimated 97 percent of the ice sheet surface had thawed by July 12. In the image, the areas classified as “probable melt” (light pink) correspond to those sites where at least one satellite detected surface melting. The areas classified as “melt” (dark pink) correspond to sites where two or three satellites detected surface melting." 

Greenland Ice2

Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

Richard Alley

Rachel Gross/MEDILL

At the conference, Pennsylvania State geoscientist Richard Alley demonstrates the effect of glacial melt on sea level.

As news of record sea ice loss in the Arctic made headlines this fall, some of the world’s top climate scientists met in Wisconsin to talk about the latest in climate change research and the accelerating pace of melting glaciers and global warming.  

Helicopters whirred onto the private runway of a rural estate, bringing scientists working in New York, New Zealand, Antarctica and around the globe back to the heartland. From September 16-19, the family of the late Gary Comer—the founder of Lands’ End and a philanthropist who invested a fortune funding environmental research—opened the estate to the annual Comer Conference on abrupt climate change.

The researchers looked at abrupt climate switches and retreats of glaciers from the past to shed light on Greenland's dwindling ice today and what it means for the planet. Yet they remain hopeful that it will be "centuries, not decades,” before the ice cap melts. But no one knows for sure as the melting accelerates. 

To make solid predictions, the scientists rely on surface dating techniques, reconstruct snowlines and analyze the composition of polar ice and sediment cores. But no matter how nuanced the method, their guiding goal remains the same: to understand the planet’s past and—in doing so—better predict the future.

“We’ve got to understand enough so that we know what is, what was, and what will be,” said Richard Alley, one of the world’s premiere climate change researchers who worked extensively on the mile-long ice cores from Greenland, time machines trapping a record of hundreds of thousands of years of climate.

More than 30 climate scientists shared their findings and discussed the evolving role of key planetary markers that reveal changes in the earth’s climate. Here are some of the different markers scientists depend on to understand past climatic upheavals, and what they mean for the planet going forward.

Sediment Cores

At 4 meters, or 13 feet long, a clear cylindrical tube was the most striking way for climate scientists to display how they know when the earth’s surface was covered with glaciers and when it wasn’t.

Blue-grey layers of clay filled half the tube, showing periods of glaciation. The other half, layers full of sandy brown dust and dirt, was where scientists plucked out bits of vegetation—a small flower or scrap of moss—to show that the glaciers had retreated.

Sediment cores are physical geological records documenting where sediments accumulated over time, explained Gordon Bromley, a paleoclimateologist at the University of Maine. These sediments act as proxies of climate and environmental change because - as the earth’s climate shifts from hot to cold -  vegetation changes as well.

“It’s geology in your hand, you pull them out and you have like a little time machine in your hand,” Bromley said.

Scientists at Bromley’s lab recently used these sediment cores to challenge a longstanding hypothesis. For decades, climate scientists have postulated that the Younger Dryas, the ice age that occurred between 12,900 and 11,500 years ago, was an anomaly of Northern Hemisphere cooling during an overall period of warming, especially in the Southern Hemisphere. 

But Bromley’s research with sediment cores from Scotland questioned this idea, using bits of newly vegetated landscape to show that, during the Younger Dryas, Scotland warmed too.

“Our ages came back earlier than the end of the Younger Dryas, which made us say that Scotland had become deglaciated by then,” Bromley said. “The vegetation can only grow when the ice is gone and so radiocarbon dating is a solid example of when the ice was gone,” he added.

But despite the fact that some climate scientists found Bromley’s results surprising, he said that his work adds to a growing body of research rather than being a completely new finding.

“We can see this theory of the Younger Dryas cooling being eroded all over the place,” with cooling in some areas and warming in others. 

Sediment coring, however, is both an extremely accurate and imperfect method of dating the earth’s past, according to Bromley. In cores, the brown dirt clearly indicates when rock was exposed to air and vegetation grew. But since vegetation can be sparse - a bit of moss at one location and more a mile away - samples from all over the same rock formations do not always yield the same information.

“It’s like you’re drilling down into this unknown world which you can’t see and trying to find the bottom, trying to find the treasure,” Bromley said.

Ice Cores

Like sediment cores, ice cores from Greenland and Antarctica record time in their clearly-defined stripes. Winter snow is tightly packed, while summer snow falls more loosely, with pockets of air bubbles trapped within.  

Looking at a core drilled from one of the earth’s two ice caps, it’s easy to determine what happened when, said Alley, a prominent geoscientist at Pennsylvania State University.

“If you want to know what the temperature in Greenland or the temperature in Antarctica was, there are a number of indicators in an ice core that, taken together, give you a clearer picture of the the history of temperature than anywhere else,” said Alley. He served as the conference’s unofficial emcee, kinetic with an unflappable enthusiasm for the science and casual, wearing silver-rimmed glasses and a polar-blue polo shirt.

Alley, who authored the book "The Two Mile Time Machine" about the ice cores, explained how they continue to be one of the most versatile markers of past climate switches. The Greenland cores show scientists that the world’s climate has always been in flux, alternating between deep freezes, melts and mild spells.

Ancient air pockets in ice cores track carbon dioxide levels back as much as 800,000 years, for instance. But nowhere in this 800,000-year history do they show CO2 levels in the atmosphere as high as they are now.

Today carbon dioxide in the atmosphere hovers at a global average of 394 parts per million. Nearly 40 percent of that has accumulated in the past century and a half, from a relatively stable 280 parts per million after the last ice age and prior to the Industrial Revolution. The ice cores tell a sobering story - before humans started burning fossil fuels for energy and creating greenhose gas emissions, levels hadn't topped 300 parts per million in 800,000 years. And CO2 is a thermostat for global warming.

Ice cores are sliced into sections and stored at the National Ice Core Laboratory in Colorado, some 800,000 years of climate history stored in its layers. But those layers reveal more than just time.

Trapped in the ice is dust showing periods of dryness in Asia; ash from fires that came downwind from Canada; and lead deposited during the First Industrial Revolution. The air bubbles in the ice capture earth’s atmospheric past, revealing changes in greenhouse gases like carbon dioxide and methane across years and poles.

“The ice core is sampling all different places and it’s putting it in the same record. It’s the same ledger, the same memory box, at the same time,” said Alley. “So you can go ask the ice core, did a lot of the world’s climate change at the same time? And it says, yes it did.”

Researchers are looking forward to results from cores drilled through the West Antarctic Ice Sheet, a relatively unstable part of Antarctica that NASA experts estimate could hike up sea levels by 16-20 feet if it were to melt completely.

Thanks to new techniques and ideal snowfall conditions this year, the new cores will be the first clear record in the region going back 68,000 years, said Joseph McConnell, who helped analyze the new cores at his laboratory at the Desert Research Institute in Nevada. McConnell attended the conference, but could not report publicly on his results because they aren't published as yet.

“We got very lucky with the timing,” he said via email. He added that the data revealed by the new cores “will be much broader in scope than other deep Antarctic ice cores and measured at ultra-high depth resolution so the climate record will be very detailed."

The results will also strengthen climate scientists’ understanding of large switches in the earth’s past and allow them to better match up past climates in the Northern and Southern hemispheres, said Alley, who helped work on the forthcoming core.

“They are recording this coupled oscillation of the whole earth system, and it really does all work together,” he said.

Tree Rings and Radiocarbon

Researchers at the conference are making strides using a powerful combination of older and newer dating techniques: radiocarbon dating and counting tree rings.

John Southon, a radiocarbon dating researcher at the University of California at Irvine, reported on work with a windfall of fossilized trees from Northern New Zealand that will help climate researchers more accurately date a crucial but poorly understood transition in the earth’s past—the rapid switch into the Younger Dryas, the Earth's most recent ice age that covered the Great Lakes in glaciers.

“The Younger Dryas epitomizes abrupt climate change,” said Bromley, the paleoclimatologist studying sediment cores in Scotland. “Understanding it means understanding fully the implications of climate change.”

Along with New Zealand researchers, Southon is working to improve radiocarbon dating techniques for the Younger Dryas interval. He has dated the growth rings from trees going back nearly 13,000 years to better calibrate the accuracy of radiocarbon dating during that time. The tree rings -- fatter in warm and wet periods and stringy during cold and dry years -- offer a remarkably accurate climate almanac. 

The partially fossilized New Zealand Kauri trees are especially important, because few trees in the Northern Hemisphere survived the frigid weather conditions of the Younger Dryas. Finding a batch from that had lived during this time period “was a big deal,” Southon said. “It was a way of filling in that gap.” 

The radiocarbon-tree ring dating combination has also been a boon for Aaron Putnam, a glacial geologist and paleoclimateologist at Columbia University’s Lamont-Doherty Earth Observatory in New York.  At the conference, he presented work on a “graveyard” of 42 long-dead poplar trees that have been left standing in the Taklamakan desert of Southern China.

Poplar can’t survive in the desert heat. That tells Putnam the Taklamakan must have once been a thriving river ecosystem, until the change to desert conditions dried these trees out. But he needed evidence of the specific timing to test his hypothesis.

So he sent samples of the 600-year-old wood—with growth rings intact—to Southon for radiocarbon dating. Southon ran the dates.

The trees had lived from 1389 to 1411 AD, he reported back to Putnam—precisely.  


In order to gain a greater understanding of climate changes across the planet, climate scientists are increasingly using an isotope called beryllium-10, which allows them to date rocks left behind by glaciers from all over the Earth.

“Beryllium-10 has special cosmogenic nuclei that’s created in a rocks surface when the surface is exposed to the sky,” said Putnam.

When glaciers retreat, the surfaces of of rocks they leave behind is exposed to the air. Once this exposure happens, cosmic rays hurling across the galaxy strike the rock, blasting atoms apart creating beryllium-10. This particle builds up on the rock’s surface the longer it is exposed but not when it is covered by a glacier. This means that scientists can measure the amount of beryllium-10 on a rock’s surface and the concentrations tell them when the surface was covered in ice and when the ice retreated. 

Beneath the science of cosmic rays and atoms blasted apart, Putnam said that charting when glaciers waxed and waned is critical to climate science.

“Glaciers are the earth’s best thermometers because they’re physical geologic formations that respond to temperature,” he said. “What it allows us to do is reconstruct, on a global basis, anywhere there have been glaciers on the planet and how climate has changed in the past.”

Understanding how climate changed in the past, said Putnam, allows climate scientist to study the way the climate changed during the 20th century and put modern global warming into a larger, historical context. This perspective lets them analyze the impact that humans have on climate today.

Putnam said that Beryllium-10’s greatest value is that it allows climate scientist to test the hypothesis that current global warming is not human produced, but rather a natural expression of climate variation.

“That’s a completely valid hypothesis, because it can be tested,” Putnam said. “The requirement for that hypothesis to be correct is that you have to go on opposite sides of the planet and see if there was warming on both,” he added.

By charting past climate change on the planet—wherever glaciers left rocks behind—scientists can paint a more accurate picture of the climate footprint that humans are leaving here now.