About 2,000 years ago, Earth’s ice sheets were building again, signaling the possible start of a colder glacial period. By the 20th century, however, climate change had warmed the planet enough to melt ice and halt that advance.

Earth is technically still within an ice age that began roughly 34 million years ago. The planet retains continuous, year round continental ice sheets in Antarctica and Arctic Greenland.

Within this immense ice age, Earth alternates between colder glacial periods and warmer interglacial periods. The current interglacial period began about 11,700 years ago.

In everyday language, the term “ice age” generally means a glacial period. Around 2,000 years ago, the shrinking ice sheets of the Arctic began growing again as the climate became slightly colder.

New research traces that change through the Laurentide Ice Sheet in North America. From 95,000 to 20,000 years ago, the enormous sheet covered vast areas of the continent, reaching from the Arctic Circle to Illinois.

Most of the Laurentide Ice Sheet has since disappeared. Its remaining ice survives as the Barnes Ice Cap on Baffin Island in the Canadian territory of Nunavut, but researchers found its diminished condition unexpectedly revealing.

Climate scientists at Columbia University, the University of Colorado Boulder, and other institutions investigated the sheet’s history by examining vegetation trapped within the Barnes Ice Cap. That preserved material helped them study the fate of the ancient ice.

The team also measured beryllium-10, a radioactive isotope that accumulates in rocks after exposure to sunlight. Those levels can show how long ago retreating ice uncovered the underlying land.

Their findings indicate that the ice sheet reached its smallest extent about 3,000 years ago. It then began expanding again roughly 2,000 years ago as temperatures cooled slightly.

The researchers argue that this renewed expansion marked the beginning of Earth’s next glacial period. Even as recently as 60 years ago, the ice sheet was continuing to increase in size.

That growth then came to an abrupt stop as temperatures rose. The finding presents a striking picture of a natural climate cycle beginning to move toward colder conditions before warming interrupted it.

“It was just getting started, everything was normal, the next glaciation was about to happen – and yet it stopped,” Nicolás Young, study author and geochemist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School, said in a statement.

Young called the reversal a potent illustration of humanity’s impact on the climate system. “It’s a very powerful example of how we have jolted the system.”

The chances of another ice age now appear remote. In a “Perspective” article accompanying the study, Professor Shaun Marcott argues that climate change driven by humans could postpone the next ice age for hundreds of thousands of years.

“Will another ice age ever transpire? The next glacial inception is generally considered unlikely to occur in the next predicted cycle, but ultimately how many skipped glacial cycles occur is left to choices made by humanity and tied to the future carbon emissions,” wrote Marcott, a glaciologist at the University of Wisconsin-Madison.

Marcott said moving the world away from fossil fuels might allow the natural cycles to regain balance over time, although the outcome remains uncertain. Future carbon emissions and choices made by humanity will help determine how many glacial cycles are skipped.

“Humans are now geologic agents capable of thwarting millions of years of natural glacial cycles and offsetting Earth’s climate for hundreds of thousands of years to come. The absence of future glacial cycles will be profound in the context of the geologic history of the past 2 million years and will have ramifications that will extend to other biological and physical systems that coevolved with the ice ages,” Marcott concluded.

The study and its accompanying “Perspective” were published in the journal Science. Together, they describe a glacial expansion that had already begun, only to be stopped as rising temperatures transformed the planet’s expected climate trajectory.