Deep, cold ocean currents from the North Atlantic blunt the effect of global warming on Antarctica and slow the rise of sea levels, according to a study published on Monday. This icy insulation of the snowy continent - covered by a sheath of ice up to four kilometres (2.5 miles) thick - could last for centuries, the research published in Nature Geoscience said.
That's good news to hundreds of millions of people in low-lying regions who are threatened by seas set to rise up to a metre by the end of the century, according to the latest report by the UN climate science panel. Newer studies suggest the ocean waterline could go up even more, pushed by surface water that expands as it warms, along with runoff from glaciers and two huge ice sheets.
One of those ice sheets sits atop Greenland. The other is on West Antarctica, a sliver of the larger continent that is warming faster than the rest. If East Antarctica were melting at the same rate, the impact on human settlements along coastlines world-wide would be truly catastrophic.
Scientists have long known that climate change has affected Antarctica's Southern Ocean far more slowly over the last half-century than oceans elsewhere.
They also know why: the sheer vastness of the continent's ice sheet and the reflective sea ice that surrounds it, along with the winds and ocean currents that circle the continent like a buffer zone.
But the new study assigns the key role to a conveyer belt of deep ocean currents that transports icy water - barely one degree Celsius (34 degrees Fahrenheit) - from the Arctic region.
"The primary source of delayed Southern Ocean warming is the background ocean circulation," lead author Kyle Armour, a researcher at the University of Washington in Seattle, and her colleagues wrote in Nature Geoscience.
It doesn't move quickly: the water welling up today around Antarctica began its north-to-south trans-Atlantic voyage before the dawn of the Industrial Era, the study said. But the impact is lasting.
Only over a "timescale of multiple centuries" will the heat-trapping greenhouse gases that are fuelling droughts, superstorms and erratic weather on the rest of the planet have a profound impact on the Southern Ocean, they conclude. This "good news", however, is tempered by the fact that even slight warming can do damage - and not just in West Antarctica.
A glacier the size of France atop East Antarctica is rapidly shedding water and could raise oceans some two metres (six feet) in a few centuries, another study reported last week.
Totten Glacier - most of which sits below sea level - is being eroded from below by sea water flowing hundreds of kilometres inland, increasing the portion resting on ocean water rather than rock.
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