Arctic ice cap's 2,000-year expansion halted by warming, study finds

A new study suggests that the Arctic was already moving towards another glaciation nearly 2,000 years ago, before rising greenhouse gas emissions reversed the trend. Researchers found that the Barnes Ice Cap in Canada's Arctic began expanding again around that time, reaching its greatest extent in this advance around 1960.

It has since retreated, reversing centuries of ice growth.Published in Science on 22 October, the study, titled Late Holocene reexpansion of the Laurentide Ice Sheet, examines the last surviving remnant of the Laurentide Ice Sheet, which once covered much of Canada and parts of the northern United States. The researchers argue that human-caused global warming overwhelmed the natural cooling influence of changes in Earth's orbit, halting the ice cap's expansion. The findings offer physical evidence of a renewed glacial advance in the Arctic.

How the Arctic ice cap began growing again

The Barnes Ice Cap, located on Baffin Island in Nunavut, Canada, covers an area roughly the size of Delaware. It is the last remnant of the Laurentide Ice Sheet, which reached across much of North America during the last glacial period.According to the study's reconstruction, the ice cap reached its minimum interglacial dimensions around 3,600 years ago. It began expanding again approximately 2,000 years ago, as cooler summers allowed more snow and ice to survive the melting season.

The advance continued for centuries, reaching its greatest extent around 1960.The trend then reversed. Since the 1980s, the ice cap's margins have retreated by an average of about four metres a year, according to reporting on the study. The researchers attribute this reversal to rising greenhouse gas emissions, which have warmed Arctic summers and caused the ice cap to lose more ice than it gains.The study's central argument is that this was not simply a local fluctuation in a glacier's size.

The authors interpret the renewed expansion as the beginning of the next glaciation, interrupted before a much larger continental ice sheet could develop.

What drives an ?

The climate of the Earth naturally oscillates between colder glacial periods and warmer interglacial periods. We are now in the Holocene , a relatively warm period that began about 11,700 years ago , after the last glacial period .These long-term changes are driven by variations in Earth's orbit and orientation, known as Milankovitch cycles.

They alter how sunlight is distributed across the planet, including how much summer sunlight reaches high northern latitudes.Cooler summers allow more snow to survive from one year to the next. Over long periods, this snow can accumulate into ice. As ice spreads, it reflects more sunlight back into space, encouraging further cooling and potentially allowing ice sheets to grow.The researchers argue that these natural conditions were favouring renewed ice growth while human-produced greenhouse gases increasingly warmed the atmosphere.

By the 1960s, they say, the warming influence had become strong enough to reverse the advance.

How researchers reconstructed the ice cap's history

The evidence comes partly from plants preserved beneath the advancing ice. Because the Barnes Ice Cap is frozen to its bedrock, rather than sliding over it like many glaciers, it can preserve vegetation that once grew on the exposed ground.As the ice advances, it covers and kills mosses and other plants. When the ice later retreats, these remains become exposed again.

Researchers can use radiocarbon dating to estimate when the plants died, providing what scientists call a “kill date” for the advancing ice.By comparing dates from different locations, the team reconstructed the sequence of the ice cap's expansion. The researchers also used beryllium-10 exposure dating, which measures how long rocks have been exposed to cosmic radiation, to help establish when the landscape was last uncovered by ice.Historical aerial photographs and mapped ice margins provided additional evidence of the ice cap's more recent retreat.Taken together, these records indicate that the ice cap had already begun to expand again well before the modern reversal.The timing of that reversal is attributed by the researchers to anthropogenic warming, but they say more research is required to understand the precise mechanisms behind glacial inception.

What does this mean for the next ice age?

The findings revisit a debate from the early 1970s, when scientists discussed the possibility of an approaching glaciation. A meeting at Brown University reportedly prompted a warning letter to US President Richard Nixon. Later media coverage of the discussion, alongside a period of cooler weather, was frequently cited by climate-change skeptics as evidence against global warming.The new research does not suggest that global warming is a myth.

Instead, its authors argue that natural conditions may have been favouring renewed ice growth, only for human-caused warming to reverse the trend.There are, however, important limits to what the study establishes. Evidence from the Barnes Ice Cap provides a detailed record of one region, but does not independently demonstrate that a new continental-scale ice age had begun across the planet. Other researchers have called for comparisons with records from additional locations to better understand the wider pattern.Nor does the study establish precisely when the next glaciation would otherwise have developed, or whether future glacial cycles have been permanently prevented. Elevated greenhouse gas concentrations make such predictions more difficult.The researchers project that the Barnes Ice Cap could disappear within the next few centuries. If it does, the last remnant of the Laurentide Ice Sheet will be gone.The study offers a striking illustration of how human-caused warming can alter a long-running natural climate process. But the broader claim that humanity stopped the next ice age remains an interpretation of the evidence, rather than proof that a global glaciation was already imminent.