Seeking the Source of ‘Big Freeze,’ Trigger of Abrupt Climate Change in Ancient Arctic of 12,000 Years Ago

UMass Amherst scientists use climate-modeling experiments to study past changes
Thick, multi-year “paleocrystic” Arctic sea ice.

AMHERST, Mass. – Climate scientists at the University of Massachusetts Amherst recently received a three-year, $595,922 grant from the National Science Foundation to study an extremely thick, immobile area of ice that may once have covered much of the Arctic Ocean during glacial periods, providing new insights into its possible role in, and mechanisms of, abrupt climate change.

Geoscientist and climate modeler Alan Condron, a research assistant professor and the project leader, is working with Distinguished Professor Raymond Bradley on this research. 

As they explain, British naval officer and Arctic explorer Sir George Nares in the 1870s encountered ice 50 to 60 feet (15 to 18 meters) thick extending into the Arctic Ocean north of Ellesmere Island. Because this ice was very different from the seven to 13-foot thick (two- to three-m) ice normally floating in the Arctic Ocean, Nares called it “palaeocrystic,” or “ancient ice.”

Besides Nares’ eyewitness accounts, additional evidence suggests that parts of the central and western Arctic Ocean were also covered by very thick, perennial ice at that time. Condron notes, “We want to explore whether paleocrystic ice could have covered much of the Arctic Ocean during glacial periods, and if so, how thick was it? Could its moving and melting have produced a freshwater flow into the North Atlantic large enough to weaken North Atlantic Deep Water formation and triggered abrupt climate cooling?”

This project has the potential to establish a link between the physical arctic system and factors such as circulation, sea ice and icebergs, with global climate. Condron adds, “An interesting aspect of this project is that if this was what the ice was like only about 150 years ago, then it must have been considerably thicker during full glacial periods.” 

The most recent cold episode in Earth’s history is known as the Younger Dryas, or “Big Freeze,” that began about 11,500 years ago and lasted around 1,300 years. It is widely believed that it came about because of a massive influx of fresh water associated with a sharp, rapid reduction in North Atlantic thermohaline circulation, that is, ocean density gradients related to temperature and salt content. But the source of the fresh water is not clear.

With this NSF grant, the UMass Amherst researchers plan climate-modeling experiments to explore whether paleocrystic ice melt might be that source. Condron and Bradley will use a suite of sophisticated, high-resolution numerical model experiments to address these questions and highlight the connection between changes in the Arctic hydrological cycle and global climate. 

They say recent numerical models show that freshwater from Arctic sources is twice as effective at disrupting climate than freshwater released from more southerly sources, but to the present the subject of freshwater and abrupt climate change has been dominated by discussion of meltwater floods emanating from glacial lake outbursts. Bradley and Condron hope to determine whether sea ice and other Arctic ice could supply enough freshwater to contribute to the climate disruption.

“By quantifying the sensitivity of Atlantic meridional overturning circulation to arctic freshwater forcing, we should be able to better examine whether changes in the arctic hydrological cycle in the near future from sea ice melt and freshwater export from the Beaufort Gyre pose a threat to the stability of modern-day climate and human society,” they write.

The numerical model they will use resolves ocean circulation, sea ice and iceberg transport at a resolution approximately five to 10 times higher than that of existing paleoclimate models. It will also be able to simulate narrow coastal boundary currents, shelf-breaks and frontal zones important for freshwater transport along the continental margins, plus other features not captured by the current generation of paleoclimate models.

This NSF grant will support one doctoral student, the researchers say, and they plan to contribute to ongoing middle- and high-school science teacher training activities at UMass Amherst, plus development of an interactive sea-ice learning tool to teach school children about the mechanisms behind sea ice formation and how ice will change as the Arctic climate warms.

Condron says, “Ray and I are both seriously committed to education and public outreach. We have both been involved with local primary and secondary schools, either by making presentations, hiring students for the summer, mentoring science fair projects or contributing to curriculum development. The idea of thick sea ice in the Arctic has a remarkable ability to capture the public imagination in much the same way as it did in the Victorian period when explorers returned with tales of thick impenetrable ice. It helps to convey the concept of the important role that the Arctic plays in global climate.”