During Earth's ice ages, much of North America and northern Europe were covered in massive glaciers.

About 20,000 years ago, those ice sheets began to melt rapidly, and the resulting water had to go somewhere — often, underneath the glaciers. Over time, massive valleys formed underneath the ice to drain the water away from the ice.

A new study about how glaciers melted after the last ice age could help researchers better understand how today's ice sheets might respond to extreme warmth as a result of climate change, the study's authors say.

The study, published this week in the journal Quaternary Science Reviews, helped clarify how — and how quickly — those channels were formed.

"Our results show, for the first time, that the most important mechanism is probably summer melting at the ice surface that makes its way to the bed through cracks or chimneys-like conduits and then flows under the pressure of the ice sheet to cut the channels," said Kelly Hogan, a co-author and geophysicist at the British Antarctic Survey.

Researchers found thousands of valleys under the North Sea

By analyzing 3D seismic reflection data originally collected through hazard assessments for oil and gas companies, researchers found thousands of valleys across the North Sea. Those valleys, some of them millions of years old, are now buried deep underneath the mud of the seafloor.

Some of the channels were massive — as big as 90 miles across and three miles wide ("several times larger than Loch Ness," the U.K.-based research group noted).

What surprised the researchers the most, they said, was how quickly those valleys formed. When ice melted rapidly, the water carved out the valleys in hundreds of years — lightning speed, in geologic terms.

"This is an exciting discovery," said lead author James Kirkham, a researcher with BAS and the University of Cambridge. "We know that these spectacular valleys are carved out during the death throes of ice sheets. By using a combination of state-of-the-art subsurface imaging techniques and a computer model, we have learnt that tunnel valleys can be eroded rapidly beneath ice sheets experiencing extreme warmth,"

The meltwater channels are traditionally thought to stabilize glacial melt, and by extension sea level rise, by helping to buffer the collapse of the ice sheets, researchers said.

The new findings could complicate that picture. But the fast rate at which the channels formed means including them in present-day models could help improve the accuracy of predictions about current ice sheet melt, the authors added.

Today, only two major ice sheets remain: Greenland and Antarctica. The rate at which they melt is likely to increase as the climate warms.

"The crucial question now is will this 'extra' meltwater flow in channels cause our ice sheets to flow more quickly, or more slowly, into the sea," Hogan said.