“I think it’s a mystery,” Bitz said.

One leading idea to explain what’s happening with Antarctic sea ice, advanced in a paper by Bitz and Gerald Meehl of the National Center for Atmospheric Research (and several colleagues), targets changes in the Pacific Ocean to explain the unexpected expansion of Antarctic sea ice between 2000 and 2014 — a trend that ran contrary to climate model projections.

This study suggests the expansion was a fluke of natural variability in the world’s largest ocean, which changed the atmosphere in ways that affected winds all the way down in the Antarctica (if winds blow floating sea ice outward from the continent, more ice can then fill the empty space, and Antarctic ice as a whole expands).

But Meehl has cautioned that the cycle in question, the Interdecadal Pacific Oscillation, plays out over decades, making it hard to be sure yet whether a turn in this cycle will correspond to Antarctic ice shrinkage. Bitz, too, said she is cautious for now about seizing on this explanation, noting that the sharp loss in Antarctic ice lagged behind a positive peak in the IPO by six to nine months, raising doubts about the connection.

But you can bet that scientists are going to spend plenty of time trying to take this question apart — and that the analysis has just begun.