Barnes Ice Cap warming halted a new ice age

About two thousand years ago, the surviving remnant of a vast North American ice sheet began to grow again on central Baffin Island. Barnes Ice Cap warming later reversed that growth, according to researchers at Columbia University, who report the finding in the journal Science. The reversal matters because the ice cap is the last remnant of the Laurentide Ice Sheet, a sheet that once covered much of Canada and the northern United States. The study argues that the renewed growth marked the start of a new glacial period, the kind of cold interval people usually mean when they talk about an ice age. Barnes Ice Cap warming is the mechanism the paper identifies for that reversal.

At its largest, the Laurentide Ice Sheet stretched from above the Arctic Circle to Illinois, and in places the ice stood more than two miles thick. Today only scattered remnants remain, and the Barnes Ice Cap is the largest of them in that region, roughly the size of Delaware. It sits on central Baffin Island in Nunavut, in the Canadian Arctic. Lead author Nicolas Young, a geochemist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School, led the work with fellow Lamont geochemist Joerg Schaefer and colleagues. Barnes Ice Cap warming gave the team a direct test: had the ice sheet ever shrunk smaller than its present size and then grown again. According to a Columbia Climate School report on the study, the ice sheet repeatedly expanded and retreated across North America over millions of years.

Plants buried under ice gave researchers a timeline

The answer came from plants. The Barnes Ice Cap is frozen directly to the rock beneath it, and unlike most glaciers it does not slide. That stillness preserved vegetation that grew when the ice cap was small and was later buried as the ice expanded over it. As the margin has retreated, that old vegetation has been exposed again, and the researchers dated it by measuring carbon-14, a form of carbon that collects in living organisms and breaks down at a predictable rate after death. By dating many samples, the team tracked the ice margin advancing over plants step by step, building a timeline of growth and retreat. Barnes Ice Cap warming later exposed those buried plants as the margin pulled back.

Combined with evidence from smaller ice caps nearby that have recently melted, the record shows the Laurentide reached its smallest extent about three thousand years ago. Roughly a thousand years later, as the climate cooled enough for ice to accumulate, it began to expand again. The researchers argue that this turning point should count as the beginning of a glacial period, even though a full continental ice sheet would have taken far longer to form. Earlier model-based studies had placed the next glaciation thousands or tens of thousands of years in the future, because they waited for enormous ice sheets to appear before calling the interval a glaciation. Barnes Ice Cap warming therefore sits at the center of the argument about when a glaciation begins.

Warming in the nineteen-sixties ended the expansion

Barnes Ice Cap warming ended the expansion in the nineteen-sixties, when summer temperatures rose enough that the ice cap lost more ice in summer than it gained in winter. The change fits a wider pattern: the researchers note that the retreat reversed about two thousand years of net expansion at the margin. A visible trimline around the ice cap still marks its extent at that time, with dark ground carrying mature vegetation and lichen on one side and pale, vegetation-free rock on the other. That line is a record of where the ice stood before the retreat began. The journal Phys.org also reported the Science paper, noting that the ice cap is now projected to disappear within the next few centuries if warming continues.

The finding also revisits a debate from the nineteen-seventies, when some climate scientists argued that a new ice age was imminent. Young says the evidence now supports the core of that claim, with an important qualification. The cooling would have taken thousands of years to become noticeable, and the Laurentide would have needed about thirty thousand years to regain its continent-spanning mass. The glaciation had started, in the sense the authors define it, but it was only at its very beginning when warming stopped it. Barnes Ice Cap warming stopped that slow process before it could gather force, and the paper presents the halt as a powerful example of how sharply human-driven warming has jolted the climate system.

The terminology needs care. Technically, Earth is still in an ice age, the Late Cenozoic Ice Age, which began about thirty-four million years ago. Within that long interval, colder glacial periods alternate with warmer interglacial periods, and the present warm interval is one of the latter. The last glacial period ended about eleven thousand seven hundred years ago. What the new study describes is the start of the next glacial period inside that larger ice age, not the start of an ice age from scratch. That distinction is why the authors frame their claim around renewed ice-sheet growth rather than around continental ice. Barnes Ice Cap warming does not alter that terminology, but it does change the date attached to the next glacial period in the authors' account.

What happens next is less certain. Models of the glacier future cited in the study project that the Barnes Ice Cap will melt completely within the next few centuries. Young describes that loss as the true disappearance of the last Laurentide remains. The disappearance could also change how future glaciations unfold, because the Laurentide acted as a nucleation point: its cold mass created feedbacks that helped more ice grow on it. Scientists cannot say when the next glaciation might occur in a world with elevated greenhouse gas concentrations, and if continental ice sheets do begin growing again, they may have to do so without that starting point. For now, Barnes Ice Cap warming is the record of a glaciation that began, and then stopped, long before the modern era of ice-sheet monitoring.