The trend over recent years of increasing Antarctic sea ice extent — seemingly at odds with climate model projections — can be mostly explained by a natural long-term climate fluctuation, according to a paper published this week in Nature Geoscience.

The study, led by Colorado’s National Center for Atmospheric Research with co-authors at the University of Washington and other institutions, may resolve a longstanding mystery: Why is Antarctic sea ice expanding despite climate-related global warming?

The study offers evidence that the current negative phase of the Interdecadal Pacific Oscillation, which brings cooler-than-average sea surface temperatures in the tropical eastern Pacific, has created favorable conditions for additional Antarctic sea ice growth since 2000.

“Compared to the Arctic, global warming causes only weak Antarctic sea ice loss, which is why the IPO can have such a striking effect in the Antarctic,” said co-author Cecilia Bitz, a UW professor of atmospheric sciences who studies sea ice. “There is no comparable natural variability in the Arctic that competes with global warming.”