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The Un-Ice Age
Earth’s remaining ice sheets head for the ocean.
8 MIN READ
Jane Beitler
Dec. 28, 2020
Feature Article
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Six hundred and fifty thousand years ago, mammoths and mastadons cavorted on the plains of North America, on the fringes of a massive sheet of ice almost two miles thick in places and as big as a continent, covering most of what are now Canada and the upper United States.
Earth warmed, and the ice sheets receded, so people can now live in places once buried by ice like Quebec and Chicago. Correctly speaking, Earth remains in an ice age. Ice still sits thick atop Greenland and Antarctica, holding enough water to raise sea levels by hundreds of feet; and in recent decades, the ice sheets have begun to melt more rapidly.
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Is global warming pushing Earth into an ice-free age? Scientists doubt a total melt this century, but think there could be enough to raise sea levels up to three feet. Only a foot or two could drive millions of coastal dwellers to higher ground. A closer estimate of the potential for sea level rise from ice sheet melting would give low-lying communities around the world more time to figure out how to adapt.
Icing up
Water on Earth resides in liquid form in the ocean, lakes, rivers, and underground, in moisture form in the air and soil, and in solid form as ice and snow. The total amount of water on Earth is more or less constant. Water can redistribute within the Earth; the water contained in the massive ice sheets on land during the ice ages originated from the oceans. As a result, at the peak of the ice ages, sea levels were 400 feet lower than they are today.
Today Earth is in an interglacial period, a relatively warmer period of the current ice age, but in recent decades Earth’s climate has been warming. While past shifts took hundreds or thousands of years, today people may be able to see changes in their lifetimes. Only a few degrees of cooling or warming can alter the balance between ice age and ice melting, and the beginnings of that melt are evident. Ted Scambos, a glaciologist at the National Snow and Ice Data Center (NSIDC) said, “Greenland mass loss started in the most likely place—at the southern glacier outlets—then spread to the rest of the island. In Antarctica, it started at the northernmost point on the Antarctic Peninsula, and on the coast of the West Antarctic Ice Sheet, at Pine Island Bay.”
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Isabella Velicogna, an assistant professor at the University of California, Irvine, said, “It’s hard to make long-term predictions, but we see that things are changing fast.” The questions pile up like the ice once did: How fast is it melting? How much will sea levels rise, and when? What will this mean to people along low-lying coasts?
Melting down
What scientists know about how these ice sheets grow and shrink has been hard won. They have dragged sled-mounted radars across the vast Antarctic continent to probe its ice cover. They planted gauges on glaciers to monitor their shedding of ice towards the sea. They camped on Greenland’s ice sheet to watch melt water pour down deep drain holes that pock the ice sheet in summer. They combed satellite imagery for changes to the outlines of the ice sheets, and saw the sudden collapses of ice shelves that fringe Antarctica’s edges. In recent decades, they have witnessed telltale signs of warming unlike any in the last thousand or more years.
But putting absolute numbers on the cubic feet of ice on land, and the melt water flowing into the oceans, is a slippery problem. It meant figuring the amount of ice being added as snowfall, and subtracting ice and melt water flowing into to the ocean. Scambos said, “We didn’t have enough measurements of snow accumulation in Antarctica, so when earlier models estimated the mass input to the ice sheet, they could be slightly off in the middle of the ice sheet. That can add up to a lot over a huge area.”
Glaciers on Greenland and Antarctica have also changed rapidly in recent decades. Large shelves of ice floating on the ocean in front of outlet valleys, where glaciers shed their ice into the ocean, help slow glaciers down. The warming of recent decades has resulted in the break up and rapid collapse of several ice shelves. “When the glaciers were suddenly freed from the gate, they began galloping along,” Scambos said.
New information came from NASA's Ice, Cloud, and Land Elevation Satellite (ICESat), which used a laser to measure the height of the ice sheet. Launched in 2003, it was providing a clearer picture of ice thickness when it ceased operations in 2010....