Scientists trying to solve the mystery of why Antarctic sea ice is growing amid rising global temperatures may have found a new lead.
Even as sea ice was disappearing globally at an average rate of 13,500 square miles, or about an area the size of Maryland, every year, Antarctic sea ice went on a record streak beginning in 2012, expanding annually until reaching a new record high extent of 7.78 million square miles in fall of 2014.
A new study, published Monday in Nature Geoscience, suggests that the explanation of the phenomenon lies not in the Southern Ocean itself, but in the Pacific.
Researchers with the National Center for Atmospheric Research (NCAR) in Boulder, Colo., together with colleagues in Seattle and Australia, identified that the expansion in Antarctic ice began to accelerate around the turn of the 21st century. At approximately the same time, the Interdecadal Pacific Oscillation (IPO), a prolonged fluctuation in atmospheric pressure that affects sea temperatures, shifted into a negative phase, cooling the ocean surface in the tropical Pacific, with global ramifications.
“When you get changes in [sea surface temperatures] in some areas of the tropics, you affect precipitation, that affects the amount of energy released in the atmosphere,” Gerald Meehl, the study’s lead author and a climate scientist with NCAR, told The Washington Post. “That starts affecting, through this kind of chain reaction process, circulation at great distances away.”
The few climate change models that take the IPO into account accurately predict the growth in Antarctic sea ice – and the global warming slowdown in the early 2000s.
Out of “262 realizations of 20th century climate, 10 of those got this observed slowdown of global warming happening at about the same time as in the observations, at the same magnitude,” Dr. Meehl told The Washington Post. “And for those 10, there was the negative phase of the IPO, and it also has the signature of Antarctic sea ice.”
Looking forward, the scientists behind the study predict that the IPO has turned back, so the Antarctic ice won’t continue to expand. Notably, measurements of the extent of Antarctic sea ice in 2015 were only the 16th highest on record.
But scientists have suggested other possible drivers of Antarctic ice expansion, including the possibility that the Antarctic ozone hole changed the circulation of winds around the continent.
In the end of May, the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) set forward another explanation for the gains in Antarctic ice coverage, as The Christian Science Monitor reported. Geophysical characteristics, including local ocean depth and continental surface features, influence the region’s wind and ocean currents in such a way as to produce and protect sea ice. Winds push building ice out and around the continent in the summer months, when the ice is growing, creating a “Great Shield” zone that shelters young ice in the interior, allowing it to grow quickly.