Earth’s persistently frozen poles mean the last ice age never really came to an end. What we think of as the mammoth-romping, Neanderthal-stomping chapter of geological history was technically a glacial maximum, one our planet would now be returning to if not for our love of burning coal, according to a recently published study.
An investigation led by Columbia University geochemist Nicolás Young has uncovered strong evidence suggesting that glaciers were bouncing back by the time Rome was on the march.
“It was just getting started, everything was normal, the next glaciation was about to happen – and yet it stopped,” says Young. “It’s a very powerful example of how we have jolted the system.”
While the full return of any continental-scale ice sheets would still lie thousands of years in the future, the study’s findings are a striking interpretation of a landscape now losing its iconic white coat.
Young’s team investigated Barnes Ice Cap on Canada’s Baffin Island: the last surviving remnant of the Laurentide Ice Sheet that dominated North America’s landscape during the last glacial period.
As that enormous blanket of frozen water vanished, plant life took advantage of the exposed soil, only to be buried again when the ice advanced once more. Frozen in place, with little movement in the mass of ice above, the organic material was preserved relatively well.
Retreating ice is now giving up that ancient plant material, allowing researchers to reconstruct the chronology of fluctuating ice in the area.
The team obtained 88 new radiocarbon ages from previously buried vegetation. Because the ratio of radioactive to decayed carbon starts to change once an organism dies, the measurements inform estimates on when advancing ice killed the plants at each location.
Mineral samples provided another clue on the timeline. Beryllium-10 accumulates in exposed rock through interactions with cosmic rays, giving researchers vital information on the initial glacial retreat.
Together, the evidence indicates that the Laurentide’s edges had withdrawn inside Barnes Ice Cap’s recent boundaries by roughly 3,600 years ago, before expanding again.
The plant material documents the expansion to roughly a thousand years ago, with 186 previously published radiocarbon dates from neighboring ice caps providing critical information that suggests the glacier started its regrowth around a thousand years before that.
The interpretation of the evidence neatly fits with a familiar feature of Earth’s climate. Slow changes in our planet’s orbit and axial orientation redistribute sunlight, altering the impact of summer temperatures. Cooler summers allow more winter snow to survive, building ice that reflects additional sunlight, reinforcing the cooling.
Earth’s meandering has seen ice ages come and go throughout its history, each defined by a waxing and waning of glaciers and frozen sheets.
Young and his colleagues argue that the renewed growth identified in their study may count as evidence of a glacial inception: a point when an ice sheet begins to wax once more after an interglacial minimum.
Looking around now, it’s clear there’s no sign of a return of the mighty glaciers. Rising levels of carbon dioxide generated by human activity have dramatically accelerated the warming that defined the ice age lull.
The question of when this lull would have come to an end in a non-industrial world has been heavily debated. A 2016 modeling study in Nature suggested that an interglacial inception was “narrowly missed” before the Industrial Revolution, and that without human interference the current interglacial could have persisted for another 50,000 years.
Confirming a period of glacial regrowth based on clues gathered from a single frozen corner of the globe doesn’t mean expanding glaciers would have necessarily merged into continent-spanning ice sheets once again, of course. The new reconstruction simply provides evidence that models must explain, while leaving the subsequent trajectory open.
What is happening today is crystal clear, however. Barnes Ice Cap reached its historical maximum extent in the mid-20th century, before retreat exposed the vegetation used in the study. Rising summer temperatures now remove more ice than winter snowfall replaces.
The consequences extend beyond its shrinking margins. Its thickness once lifted part of its surface high enough to maintain an accumulation zone. Since around 2010, that elevated zone has disappeared. Projections suggest this remnant of a once great ice sheet will have melted entirely in the next few centuries.
Whether its earlier growth marks the formal beginning of another glaciation remains a question of interpretation and scale. Its present retreat records a more immediate change: a remnant that survived the last great warming is now losing the conditions that sustained it.
The research is published in Science.
Source: Columbia University
