Climate change and the norther hemisphere

Alaska baked, Detroit froze, and England flooded this past winter, which was one of the coldest on record in the American Midwest. Meanwhile western states, notably drought-stricken California, saw record warmth.

Shivering or sodden people had to endure not only the unusual winter but also repeated references from meteorologists to the polar vortex—a frigid low-pressure system that caps the Arctic and is normally contained there by the polar jet stream. Last winter that ring of high-speed winds developed pronounced north-south meanders, allowing Arctic air to flow deep into the United States. (See "What Is the Polar Vortex?")

According to one theory, endorsed this year by White House science adviser John Holdren and some other scientists, the change in the jet stream—and the rotten winter—were ultimately triggered by the warming of the Arctic and the melting of sea ice there. But climate expert Tim Palmer of the University of Oxford says the culprit was the western Pacific Ocean: "What goes on in the tropical Pacific Ocean is of almost global impact."

Compared with the Arctic theory, Palmer's analysis has an upside: It foresees fewer repeats of last winter in our future.

Thunderstorms and Typhoons

Here is Palmer's explanation in a nutshell. During this past winter abnormally warm Pacific Ocean waters stretched roughly from Fiji to Indonesia. They spawned tremendous thunderstorms—and it was the energy of those storms, reaching high into the atmosphere, that rerouted the jet stream. The loopy jet stream sent warm air north toward Alaska and allowed cold air to drop south and freeze the rest of the continent.

Those same warm waters help explain the power of supertyphoon Haiyan, which devastated the Philippines in November. And they're the precursor of what's now forecast to be a large El Niño event later this year. During an El Niño, warm water that's been pushed into the western Pacific by the trade winds comes sloshing back east along the Equator.

Palmer's hypothesis is attracting both endorsement and criticism from other climate scientists.

"I think it is basically right," says climate data expert Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado. Last winter the tropical Pacific saw "incredible amounts of rain," he says, "which likely played a role in setting up the [jet stream] wave patterns across North America."

Mark Serreze, director of the National Snow and Ice Data Center (NSIDC) at the University of Colorado Boulder, agrees. "There is strong evidence of links between the behavior of the jet stream and sea-surface temperature in the tropical Pacific," he says, "compared with much more speculative links with Arctic warming."

The Arctic camp is less impressed with Palmer's paper. "I think it proposes a new mechanism, but there is still a long way to prove the argument," says climate scientist Qiuhong Tang of the Chinese Academy of Sciences in Bejing. "I can hardly find any observation-based evidence in the essay which can support the argument."

Tang and some other scientists, notably Jennifer Francis of Rutgers University, have argued that the warming Arctic and shrinking sea ice have diminished the north-south temperature contrast that drives the jet stream, robbing it of the energy that normally keeps it tightly routed around the Poles.

But, says climate scientist Katharine Hayhoe of Texas Tech University in Lubbock, "the two ideas are not necessarily competitors. They may be complementary."

Global Weirding

In Palmer's view, the past wild winter can be attributed to natural climate variability amplified by man-made climate change. A warm pool in the western Pacific is a natural phenomenon, he says, but global warming kicked the sea-surface temperature up a notch.

"Even a tenth-of-a-degree temperature change in tropical waters can have tremendous weather effects," he adds. (Related: "U.S. Cold Snap Inspires Climate Change Denial, While Scientists See Little Room for Doubt.")

Other scientists remain cautious. "The link to global warming is not obvious to me," says climatologist Gavin Schmidt of NASA's Goddard Institute for Space Studies in New York.

The Arctic explanation for the weird winter points the finger more directly at man-made climate change—and if it's right, we should expect a string of such winters in the future. But we should see fewer of them if Palmer is right that the weirdness came primarily from an abnormally warm western Pacific. Next winter will test his idea: With an El Niño likely on the way, it should be a bit more normal than the last one.