Global Warming

Scientists reconcile growth in Antarctic sea ice with global warming models

The small minority of climate change models accurately predicted the expansion of Antarctic sea ice, and now scientists think they know why.

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 Centre 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.

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.

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, NASA and the NOAA set forward another explanation for the gains in Antarctic ice coverage. 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.