This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 Science, Young, fellow Lamont geochemist Joerg Schaefer and their 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 (3.2 kilometers) 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: Technically, 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.
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