On Baffin Island, in the Canadian Arctic, the Barnes Ice Cap is the last remnant of an immense ice sheet that once covered a large part of North America.
Scientists studied small plants found on rocks exposed by the retreat of the ice. These mosses lived at a time when the ground was uncovered. Then the ice advanced and covered them, preventing their decomposition. By measuring their age through carbon-14 dating, scientists can determine when the ice arrived.
Artistic representation of a fully frozen 'Snowball Earth', with no liquid water at the surface.
Wikimedia image
The method works particularly well here. Unlike many glaciers that slide over their rocky beds, the Barnes Ice Cap remains largely frozen to the ground. It has therefore preserved the plants covered over the centuries, rather than tearing them away as it advanced.
The results tell an astonishing story of a return to the past. After shrinking considerably, this ancient ice cap reached a minimum size about 3,600 years ago. It then appears to have begun reclaiming ground some 2,000 years ago. Its growth continued in stages until around 1960.
For the researchers, this recovery could correspond to the very first steps of a new ice age.
Then the trend reversed. Around 1960, summer heat became sufficient to melt more ice than formed in winter. Since then, the ice cap has retreated. The researchers link this change to warming caused by human activities, despite natural conditions that had previously favored its growth.
Today, rocks are visible where ice was still present in the middle of the 20th century. According to projections, the last remnant of this immense ice cap could even disappear in the coming centuries if warming continues.
This study cannot say exactly when a new glaciation would have spread across all of North America without human influence. It does show, however, through the traces left by the plants, that an ice cap that had been advancing for a long time began to retreat very recently.