Ice sheet expansion may have started 2,000 years ago, but we canceled that

What happened

A team led by Nicolás Young of Columbia University's Lamont-Doherty Earth Observatory studied Barnes Ice Cap, a small ice mass left over from a giant ice sheet. Their study in the journal Science says the ice sheet shrank to its smallest size about 3,600 years ago. Then it began to grow again about 2,000 years ago. The team thinks this was the start of the next ice age.

Key numbers and facts

• The Laurentide Ice Sheet at its peak covered most of Canada under about two miles (over 3 km) of ice.
• At its peak, it held around 80 meters of sea level water locked up as ice.
• The ice reached its smallest size about 3,600 years ago.
• Regrowth started about 2,000 years ago.
• The growth was likely on track until about 1960, when human-caused warming reversed it.

Why Barnes Ice Cap is special

Most glaciers are warm at the base. Meltwater there helps them slide, and sliding ice scrapes up the ground under it, which erases old evidence. Barnes is cold at the base. It is frozen onto the rock below, so it does not slide. Because of this, it keeps plants and ground that were trapped under it, like a time capsule. Tundra plants that grew when the ice retreated were later covered and preserved for about 2,000 years.

Why it matters

This is direct physical evidence, not just computer models, that ice sheets can turn around and start growing again. It helps scientists understand how ice ages begin. It also shows that human warming may have stopped a natural cooling trend that was already underway.

What to watch

• Whether other cold-based ice areas show the same turnaround around 2,000 years ago.
• How scientists use computer models to test if the ice was really growing before 1960.
• How future warming will affect ice sheets and sea levels.