After atmospheric scientist Ivana Cvijanovic began pushing a computerized climate simulation to its limits, she noticed a disturbing result: as Arctic sea ice nearly disappeared, massive high-pressure systems built up thousands of miles away, off the west coast of the United States.

The atmospheric ridge blocked major storms bound for California, cutting off rainfall. Cvijanovic’s model shows that as the North Pole’s summer sea ice vanishes, as expected in the next few decades, it could turn down the tap for Central Valley farmers, Sierra Nevada ski resorts, and cities throughout the nation’s most populous state (see “The Year Climate Change Began to Spin Out of Control”).

Climate scientist Ivana Cvijanovic conducted field work in the Alaska North Slope as part of her PhD research.

The results, published in Nature Communications in December, also suggest that shrinking sea ice may have played a role in the extreme, costly drought that plagued California for most of this decade. That was driven by a “ridiculously resilient ridge” closely resembling the one that Cvijanovic’s simulation predicts.

Climate models are often derided as unreliable approximations of Earth’s complex systems, particularly among climate change deniers. But thanks to advances in computational power, the inclusion of more planetary components, and other technical strides, these simulations are becoming incredibly powerful.

They can predict with growing certainty how global warming is already altering the planet, and how it’s likely to in the future. In particular, the improvements are helping scientists to untangle the complex mechanisms driving extreme weather events.

Cvijanovic had tried to study the puzzling connections between melting ice and faraway precipitation changes for years. But standard climate models didn’t offer a way to simulate the processes in a realistic way, until a recent breakthrough arrived from an unlikely source: nuclear weapons research.

Flipping the coin again

Cvijanovic’s curiosity had taken on a new urgency by the time she landed as a postdoctoral researcher at California’s Lawrence Livermore National Laboratory in 2014. Two summers earlier, Arctic sea ice had melted away far faster than scientists expected. Nearly five million square miles of ice disappeared from the peak period in late March, by far the biggest loss ever recorded.