Two thousand years ago, the Arctic remnants of a once-vast ice sheet started to grow again. It was the beginning of a new glacial period, more conversationally known as the next ice age—but by the 1960s, summer temperatures were already so high that the ice sheet started to shrink. In other words, global warming ended a period of glaciation that had already begun.
"It was just getting started, everything was normal, the next glaciation was about to happen—and yet it stopped," says Nicolás Young , a geochemist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School. "It's a very powerful example of how we have jolted the system."
In a new study published in the journal Science, Young, fellow Lamont geochemist Joerg Schaefer and their colleagues describe their research on the Laurentide Ice Sheet, and his colleagues describe their research on the Laurentide Ice Sheet, which covered much of Canada and the northern United States from about 95,000 to 20,000 years ago. At its largest, it stretched from above the Arctic Circle to Illinois, and in places the ice was over two miles thick.
The last vestige of the Laurentide still survives as the Barnes Ice Cap, now a Delaware-sized ice cap located on central Baffin Island in the Canadian territory of Nunavut. "People always say that it disappeared, but that's not true," says Young. And while it's known that the Laurentide has repeatedly expanded and retreated across North America since widespread, continental-scale glaciation began about 2.6 million years ago, Young's team wanted to know whether it had ever shrunk even smaller than it is now and then regrown.
(A note on terminology: Techincally we are still living in an ice age–the Late Cenozoic Ice Age–which started about 34 million years ago. Within it, colder glacial periods alternate with warmer interglacial ones, including the one we live in now. What people casually call an "ice age" is usually a glacial period. The last glacial period ended about 11,700 years ago.)
Reconstructing glacial histories is always difficult, and especially so when trying to reconstruct moments of maximum contraction and initial re-expansion: Apart from the challenge of identifying the origin point, whatever geological clues are left on the landscape tend to be scoured away as glaciers regrow. The cradle of the Laurentide, however, is a special case.
The Barnes Ice Cap is frozen directly to its underlying bedrock. Unlike most glaciers it does not slide, and thus preserves the vegetation that grew at earlier times when the glacier was small and was subsequently entombed by its expansion. Vegetation exposed since the glacier started retreating in 1960 can be sampled, allowing scientists to determine its age by analyzing traces of carbon-14, a harmlessly radioactive form of atmospheric carbon that collects in organisms while they live and breaks down at predictable rates after they die.
"If you do this enough times, you can actually develop a timeline of that ice margin systematically advancing over plants. You're actually tracking the expansion of that ice margin," says Young.
By combining analyses of vegetation preserved beneath the Barnes Ice Cap and smaller recently-melted ice caps in the region, the researchers pieced together a climate history. The Laurentide reached its minimum dimensions about 3,000 years ago, then started re-expanding about 2,000 years ago when the climate became cold enough for ice to accumulate.
The researchers argue that this inflection point should be considered the beginning of Earth's next glacial period. This interpretation challenges prevailing ideas about when glacial periods begin. Previous model-based studies have suggested that the next glaciation would not begin for thousands or even tens of thousands of years. Young and his colleagues argue that the first renewed growth of an ice sheet should count as the beginning, rather than waiting until enormous continental ice sheets have formed.
The findings are also a coda for climate scientists who in the 1970s made the controversial claim that a new ice age was imminent. "It actually was happening," Young says. "They couldn't quite prove it, but now we sort of have the evidence." It would have taken thousands of years for the cooling to be noticeable, and about 30,000 years for the Laurentide to regain its continent-sprawling mass, but the next glaciation had started, says Young.
By the 1960s Earth's climate had warmed enough that the Barnes Ice Cap lost more ice in the summer than it added in winter. The next glaciation fizzled out. According to models of the glacier's future, it will melt completely within the next few centuries. "That will be the true very last remains of the Laurentide, disappearing for good," Young says.
Its disappearance could change the dynamics of future glaciations and ice ages. The Laurentide served as a glacial nucleation point when Earth's climate cooled, as its frigid mass created self-sustaining feedbacks that caused more ice to grow on it. Scientists can't say when the next glaciation might occur in a world with elevated freenhouse gas concentrations. But if continental ice sheets eventually begin growing again, they may have to do so without the last surviving remnant of the Laurentide.
"What has been an extremely natural and predictable phenomenon is now gone," says Young.
