Human Warming Reversed 2,000 Years of Growth at Barnes Ice Cap Around 1960, Study Finds

A new study in Science says the Barnes Ice Cap on Canada’s Baffin Island had been expanding for roughly 2,000 years until human-caused warming reversed that trend around 1960, pushing the ice into retreat.

The importance of the finding is not that people somehow “stopped an ice age.” Rather, the researchers say it shows modern greenhouse warming was strong enough to overpower a long natural cooling trend that had favored renewed ice growth. In the paper, published online Oct. 22, Nicolás E. Young of Columbia University’s Lamont-Doherty Earth Observatory and co-authors describe the reversal as evidence that human influence can overwhelm climate shifts that normally unfold over thousands of years.

The study focuses on the Barnes Ice Cap, a roughly 6,000-square-kilometer body of ice on Baffin Island that researchers describe as the last remnant of the Laurentide Ice Sheet. That former ice sheet once covered much of Canada and, at its peak, about 30% of North America. Today, Barnes is about the size of Delaware.

According to the study, the ice cap reached its smallest extent of the current interglacial period about 3,600 years ago. It then began to grow again around 2,000 years ago. The authors interpret that re-expansion as the first stage of the next glaciation — the slow return of larger ice sheets under natural cooling conditions. That growth continued until about 1960, when, the paper says, anthropogenic summer warming caused a negative mass balance, meaning the ice cap began losing more ice than it gained, and its margins started to recede.

Young said in a Columbia Climate School press summary that the process had barely begun before the modern warming trend interrupted it. “It was just getting started,” he said.

To reconstruct that history, the team combined two dating methods. One was cosmogenic-nuclide exposure dating, which can show when rock surfaces were uncovered after ice retreated. The other was radiocarbon dating of preserved plants, including moss, that had been buried when the ice advanced. Co-author Joerg M. Schaefer, also of Lamont-Doherty, said in the same press summary, “Our approach helped us see the first steps of ice-sheet growth preserved in the landscape.”

The broader backdrop is that Earth has cycled between glacial and interglacial periods for the past 2.6 million years, largely because changes in Earth’s orbit alter the distribution of sunlight. The present warm interval, the Holocene, began about 11,700 years ago. Climate and ice-sheet models had already suggested that elevated carbon dioxide levels could delay or suppress the next glaciation for very long periods. What this paper adds, the authors argue, is a physical regional record showing that such a reversal was already happening at one surviving fragment of a continental ice sheet.

A Science perspective by paleoclimatologist Shaun Marcott in the same issue framed the result as evidence that greenhouse-gas forcing may have forestalled a naturally beginning glaciation. The paper’s abstract makes the point more formally: “The demise of the next glaciation, despite the ongoing decline in summer insolation, illustrates the influence of anomalous greenhouse gas forcing on climate.”

The authors are careful about the timescale. The study does not mean Earth was on the brink of an imminent ice age in any human sense. Instead, it presents Barnes as an early, local sign of a much slower natural process that would otherwise have played out over many thousands of years. As Young put it, “It was just getting started.”