Regardless of the cause, the change can be seen.

“We can monitor it in real time,” said Theresa Damiani, a research geologist at the National Oceanic & Atmospheric Administration’s National Geodetic Survey in Camp Springs, Maryland.

Earth Rebound

The weight of the ice sheets, which were several miles thick in places, pushed down on the Earth’s surface. When the ice melted and retreated, the ground began to spring back.

NOAA describes it as similar to what happens when an aging mattress returns to its original shape after a person gets up in the morning. When the ice sheets first retreated, there was a quick rebound in many areas that slowed through the centuries, Damiani said.

“It has been rebounding and it still is,” Damiani said. “It takes a very long time.”

The Earth’s surface rises about 1 millimeter a month in Canada and about the same amount over the course of a year in the U.S., she said. For a place like Canada’s Hudson Bay, the changes would be apparent over a person’s lifetime, she said.

“If you have a dock that you built, the ocean appears to be going down,” Damiani said.

It isn’t that the sea level is dropping -- it’s that the land is springing back up.

When researchers calculate the rise of the world’s oceans because of the current melting of glaciers and ice sheets, they need to keep the rebound rates in mind, according to a December 2012 NOAA report.

Area Covered

Of course, the land is only rising in those areas covered by the ice sheet. In the Western Hemisphere, that’s pretty much all of Canada down through New England, across the Great Lakes and into the Midwest, the northern Great Plains and Pacific Northwest.

Beyond the limit of where the ice pushed the ground down, there are areas where the surface is actually settling lower. What happened here is that the land at the edges of the ice sheets was like jelly oozing out of a squashed sandwich.

The changes can lead to small earthquakes, although those are very weak and probably go unnoticed by most people.

The motion is tracked using global positioning satellites and is used in making sure maps stay accurate, Damiani said.

“My agency, the NGS, is tasked with monitoring the shape of the planet,” Damiani said.

The Scandinavian nations also track the rate of how the surface has been slowly rebounding over the past 14,000 years, give or take a century or two, since the ice sheets peaked in the last Ice Age.

Patience is definitely a job requirement.

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