By Morgan Levey
We think of rising fossil fuel use and the resulting carbon dioxide emissions as key catalysts of climate change in today’s warming world. And changes in climate throughout Earth’s history had many drivers, including geological forces, orbital cycles, ocean circulation and a litany of planetary mechanisms that happen on a microcosmic scale. Now imagine that there is another way for Earth’s temperatures to rise rapidly – one we’re not aware of as yet beyond a few clues and one with the potential to amplify today’s global warming.
A little over 17,000 years ago, Earth was well past the peak of the last great ice age and the Southern Hemisphere had begun shifting into the warmer climate we know today. Globally, however, this was not the case. The North Atlantic was experiencing one of the coldest periods of time in the region’s history. Yet scientists are beginning to believe that, although winters were extraordinarily cold, summer temperatures might have been fairly mild in the North Atlantic during this time, warm enough that the ice sheet covering the region may have begun retreating. If that were true, new research suggests that a new mechanism exists, one with the capacity to quickly warm the entire globe at once, at least on a seasonal basis.
“This is still under study, but our work indicates the end of the ice age may have been rapid,” says Brenda Hall, a glacial geologist at the University of Maine. Climate scientists such as Brenda Hall are urgently trying to put all the pieces together from the past to better predict where human activities may be taking climate now. She’s attempting to chronologically map the retreat of the North American glacial mass through Maine roughly 17,000 years ago in an effort to understand the mechanism that might have triggered warming in this area.
During the last ice age carbon dioxide was trapped in the depths of the Southern Ocean. When deglaciation began, normal ocean circulation resumed and the trapped carbon dioxide was slowly released into the atmosphere. But Hall thinks warming might have occurred so quickly that temperatures may have risen prior to much rise in carbon dioxide levels.
“This may indicate a reduced role for carbon dioxide in causing the warming [then], and it also indicates that another mechanism is present that can cause rapid global warming,” Hall says.
That doesn’t mean that carbon dioxide – driven by fossil fuel emissions to the highest levels in at least a million years – isn’t forcing global warming now. But this other mechanism Hall is investigating could turn up the thermostat even more.
A lineage of research
The impetus for Hall’s research in Maine started with her mentor, George Denton, her Ph.D. advisor in the 1990s. Denton is a glaciologist whose research for tracking glacial retreat has been so fundamental to the field that he has not one, but two glaciers in Antarctica named after him.
Denton, with fellow climate scientists Richard Alley and Wallace Broecker, and philanthropist Gary Comer, who supported widespread climate research, published a paper in 2005 based on fieldwork they had conducted in Greenland. They had discovered an inconsistency in the annual temperature records preserved in physical characteristics of the past during the Younger Dryas, a period of moderate glaciation that started roughly 13,000 years ago.
Glacial geological fieldwork involved dating beryllium-10 isotopes found in samples of rocks taken from glacial moraines, lines of boulders that trace what was once the edge of glaciers and ice sheets, as well as boring ice core samples and dating the pockets of air trapped inside. From the moraines that Denton and the others were dating, the team learned that temperatures during the Younger Dryas were only shifting a few degrees. However, previously published data from ice cores bored recorded a much larger change of 15 degrees Celsius from the same area.
The Younger Dryas marks an era when the North Atlantic region was suddenly thrown back into a near-glacial state as the rest of the world thawed almost completely. Like the era Hall is studying, it was thought to be bitterly cold. The small shift in temperature recorded in the moraines led to the belief that varying temperatures between seasons could account for the ice core shift. While the winters in the region were frigidly cold, the summers may have been quite moderate.
Hall began to wonder if seasonality was having an effect on ice sheets during other periods of extreme cold in the North Atlantic. Around 17,000 years ago Earth was warming and glaciers were melting from the Southern Hemisphere to the Swiss Alps. “And so that made us wonder, what about the ice sheets?” says Hall.
Hall recently presented her findings at the Comer Climate Conference in southwestern Wisconsin, an annual meeting of climate scientists who have received research funding from the Comer Family Foundation.
Hall’s work is in its infancy, as she gets the dates back from the lab and beginning to analyze her results. But her initial data is proving consistent with her hypothesis. The ice sheets were likely retreating.
Traditionally it is believed that the climate of the hemispheres is out of phase, driven by a bipolar see-saw mechanism, a roughly 1,500 year cycle based on ocean circulation where one pole warms and the other cools. The recorded prevalence of seasonality suggests another mechanism might be at play, one that also “would have to be able to affect the entire globe at once – at least in summer,” Hall says.
She acknowledges that this suggestion is highly controversial, but her initial findings are further confirmation of this. “The bipolar seesaw – if it exists – would only occur in the winter,” Hall says.
Climate scientists, suhc as Hall, map the warming and cooling trends that have occurred throughout our planet’s lifespan in an effort to track the causality of individual factors on climate changes. Earth is large, so a lot of this happens at the level of regions or continents. When a force affects the whole planet, it’s significant. And in today’s warming world, where we are driving carbon dioxide levels to unprecedented heights, it means that warming could be amplified by forces we are only beginning to discover, let alone understand.
The lineage continues
On the other side of the North Atlantic, Gordon Bromley, a glacial geologist and former student of Hall’s, is conducting similar studies – trying to chronologically map the retreat of the ice sheet – but through Scotland.
“We’re using exactly the same techniques, same philosophy as we approach it,” says Bromley who also presented his latest findings at the Comer Climate Conference. His results so far have matched Hall’s. “I was pleased to see the same signal, same recession, during the periods that we think our conceptual model says it should be retreating, that is happening on the west side of the Atlantic and on the east.”
Like Hall’s research, Bromley’s Scottish work is building upon previous evidence of global ice sheet retreats around the same period of time. “I’d say that Brenda’s New England work and our Scottish work both grew organically and quite separately from the same line of inquiry. That they are converging attests to the global nature of abrupt climate change,” says Bromley.
For Bromley and Hall, the similarity in results is encouraging, but the work is also tied to a legacy that both are proud to be a part of.
“The way that George trained Brenda and Brenda trained me – hopefully I can train my students that well. It’s like being part of the family business,” says Bromley.
Photo at top: Elevation map of the coastline of Maine. (From Brenda Hall’s presentation at the Comer Abrupt Climate Change Conference)
Note: Morgan Levey is a Comer Scholar, a Medill scholarship program supported by the Comer Family Foundation to promote graduate studies in environmental journalism.