By Poonam Narotam
Medill Reports

Veteran climate scientist George Denton calls Pine Island Bay the “weak underbelly” of the western Antarctic ice sheet. Increased melting at the bases of Pine Island and Thwaites glaciers into this bay makes them susceptible to collapse in coming decades, which would raise global sea levels by 11.5 feet (3.5 meters) and flood most major coastal cities and islands.

Global warming, driven by ever-rising carbon dioxide levels from fossil fuel emissions, is nearing a tipping point that raises the threat for massive melting of the ice sheet. United Nations officials, who organized the COP26 climate summit in Glasgow this fall, are calling for countries to reduce CO2 emissions by 45% below 2010 levels by 2030 to keep temperature rise below the tipping point. But COP26 closed for this session and no commitment was made on this safety net.

At another recent conference, Denton and other glacier scientists introduced a new theory for abrupt climate change that ended the last ice age, research that betters our understanding of glacier melt today. The powerhouse behind their theory is in the westerly winds. Today, the westerlies’ movement and an unprecedented level of human-driven CO2 emissions together accelerate warming that can collapse the western Antarctic ice sheet.

Denton gathered with climate scientists from all over the world at the annual Comer Climate Conference in October, before COP26, to present their latest research and swap solutions virtually over Zoom due to the COVID-19 pandemic.

Denton is a distinguished professor at the University of Maine’s School of Earth and Climate Sciences and Climate Change Institute. He has studied glaciers all over the world for 63 years. He nurtured a strong group of glaciologists who study past glacier behaviors, said Thomas Lowell, once Denton’s master’s student and now a geology professor at the University of Cincinnati.

“Driven by George’s example, we’re motivated to understand big-scale questions on the ice age and the interaction of climate from a glaciated world to an unglaciated world,” Lowell said.

Many of Denton’s students have mentored new generations of scientists in turn. Denton currently studies New Zealand’s Southern Alps, where he was conducting field work when the pandemic started. New Zealand’s strict border closure prohibited him from leaving but allowed him to continue his research.

“I felt like I wasn’t wasting my life here,” Denton said. “If I were home, I wouldn’t be able to do anything.”

In a paper published in April, Denton proposed the “Zealandia Switch” theory of how the last ice age started and ended. Lowell said digging into another layer of the “onion” of glacier research spurred a “big rethinking” of understanding Earth’s whole climate system.

The long-held Milankovitch theory (named for Serbian scientist Milutin Milankovitch) hypothesized a century ago that ice ages were primarily caused by fluctuations in the amount of solar heat trapped in Earth’s atmosphere (insolation), which shifts as the planet’s orbital position relative to the sun changes over long periods of time.

Brenda Hall, another University of Maine School of Earth and Climate Sciences and Climate Change Institute professor and prior Denton student, said glaciers in the Northern Hemisphere should grow as those in the Southern Hemisphere shrink, and vice versa, according to the Milankovitch theory.

Glacier records in the Northern Hemisphere support this theory. But through different studies, Denton, Hall and Lowell found evidence “the Milankovitch theory doesn’t work so well” to explain patterns in Southern Hemisphere glaciation, Lowell said.

Hall said glaciers grew in the Falkland Islands, an archipelago off the coast of Argentina, during Heinrich stadials, Northern Hemisphere cold periods established by studies of the Greenland ice core and North Atlantic marine sediments.

“What we found was that the glaciation there bears no relationship at all to insolation, which is actually rather interesting,” Hall said. She and Lowell published their study in December 2020.

Instead, Denton’s Southern Hemisphere research produced evidence suggesting the westerlies, the world’s strongest wind system, cause abrupt climate changes that trigger glaciers to form and melt.

The westerlies either move towards the poles, stimulating global warming, or toward the equator, stimulating global cooling and ice ages, according to Denton’s “Zealandia Switch” study.

Denton said the Southern Hemisphere westerlies are bound by New Zealand’s Tasman Glacier in the north and Antarctica’s Pine Island Bay glaciers in the south. His study shows the westerlies’ movement toward the poles causes the air around New Zealand to warm, melting the Tasman Glacier from above.

Poleward-shifted westerlies also cause upwelling of warm water in the Southern Ocean, “the only place in the world where upwelling water is warmer than the surface water,” Denton said. These heated waters flow underneath the Pine Island Bay glaciers, melting them from below.

Denton’s theory is the increased atmospheric carbon dioxide today forced the global climate system – and the southern westerlies – into the same poleward mode that sparked the end of the last ice age and “that’s causing renewed collapse of the glaciers in both Antarctica and New Zealand.”

According to Denton’s research, excess atmospheric CO2 caused by fossil fuel emissions made the Southern Hemisphere’s westerlies swifter, vaguer and farther south in the past few decades.

“New Zealand glaciers have collapsed and there’s increased melting around the west Antarctic ice sheet,” Denton said.

This group of glacier historians plans to dig deeper into this new way of understanding abrupt climate changes. While Denton is continuing his research in New Zealand, Lowell and Hall, both based in Maine, are laying plans for improving glacier records in the Falkland Islands.

“Climate changes rapidly,” Lowell said. “If the Zealandia concept is right … once that change is afoot, that’s it. You can debate anything you want. It’s all defensive playing at that point. There’s no offense. That scares the bejesus out of me.”

Poonam Narotam is a health, science, and environment reporter at Medill. You can follow her on Twitter at @namsorama.