The thermometer that governs the world’s weather this winter is a stretch of water most of humanity will never see: the equatorial Pacific, in the band forecasters call the Niño 3.4 region. There, the sea has been running hot since summer, and it is not finished. The Boston Globe reports that most forecasters and climate models expect this El Niño to strengthen further, peaking at nearly 4 degrees Celsius above normal by December — enough to make it the strongest event in a century and a half of observations, overtaking the monster of 1877-78.
Daniel Swain, a climate scientist at the University of California, Los Angeles, put it without ornament: “The warning lights are flashing. ... This extreme El Niño event will have widespread global impacts.” The unsettling part, he added, is not only the size. “We’ve never seen an event this big before in the context of our rapidly warming climate.”
The rolling rule of three months
El Niño is a naturally occurring warming pattern that swings back and forth every two to seven years, rearranging weather across the United States and the world. It is not proclaimed because one hot day arrives in the Pacific. Sea surface temperatures in the Niño 3.4 region must climb at least 0.5 degrees Celsius above the norm and stay there for at least five consecutive months, counted in overlapping three-month periods — October through December, then November through January, and so on, a rolling average that shuffles forward one month at a time.
From there, strength is measured in half-degree increments. The summit of the scale — “Very Strong,” or, in the currency of the headlines this year, “super,” “mammoth,” even “Godzilla-like” — begins at 2.0 degrees C (3.6 degrees F) above normal. This event passed that threshold in the summer and kept climbing.
Four titans before this one
The National Oceanic and Atmospheric Administration has counted 28 El Niño events, at every intensity, since its classification standard took hold in 1950. Only four reached the very strongest tier: 1982-83, 1991-92, 1997-98 and 2015-16. In its advisory Thursday, NOAA said there is now at least an 83% chance — up from 75% only a month ago — of “a historic event that would exceed the strength of previous El Niño events dating back to 1950” by the end of the year.
And the true predecessor is older still. The greatest El Niño on record, in 1877-78, is known to us through ship records — readings taken by sailors in an age of sail — showing sea-surface temperatures that peaked as much as 3.5 degrees C above normal. That is the mark the present ocean now aims at, with forecasters predicting a near 4-degree peak.
The weather archives keep the receipt for what such events do. In 1982-83, a parade of Pacific storms battered California: flooding, mudslides, 25 inches of rain in 36 hours in the Santa Cruz Mountains, more than 8 feet of snow near Lake Tahoe, and a January nor’easter that buried Albany, New York, under 24.5 inches of snow while parts of the East Coast got rain instead. In 1991-92, the same class of El Niño produced an unusually mild winter across much of North America — proof that even titans deal different hands to different places. In 1997-98, a devastating January ice storm coated Northern New England in up to 3 inches of ice, knocking out power and ravaging millions of acres of forest and farmland. In 2015-16, an exceptionally warm, nearly snowless start to winter flipped abruptly, and the January 2016 blizzard stacked as much as 3 feet of snow on parts of the East Coast.
The reservoir below
The surface numbers are not what gives forecasters their confidence; the deep is. Waters below the El Niño region are also at record levels of heat — in places nearly 20 degrees F above normal. Think of it as a great reservoir of heat waiting beneath the sea, one that can feed this El Niño through the winter and well into early 2027 whatever the atmosphere does. The consequences are already being tallied in the places that can least afford them: researchers at the University of Chicago have projected 451,000 extra heat deaths by February, most of them falling on the Sahel, while the same anomalous warmth has pushed the Pacific’s rains east — drying Indonesia and flooding Peru.
A new ruler for an old sea
There is a quiet argument among the measurers, and it matters. The traditional gauge — the Oceanic Niño Index — compares today’s ocean against a static 30-year block of recorded temperatures. But a warming planet shifts the baseline beneath the comparison, and saying the sea is over 3 degrees “above average” no longer tells the whole story. Earlier this year NOAA adopted the Relative Niño Indices, or RONI-3.4, as its official metric for monitoring sea surface temperatures, though forecasters still run both in their analyses. Updated monthly on the same rolling three-month intervals, RONI adjusts for the warming state of the planet — and by that corrected measure, this El Niño recently reached 2.0 degrees C (3.6 degrees F) above average.
Emily Becker, a research associate at the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science, who writes a blog for the nonprofit Climate.us, explained the stakes: amid climate change, “we need a way to isolate the strength of El Niño itself. ... Our old yardstick was telling us whether the El Niño region was warmer than average, but not if it was significantly warmer than other parts of the tropical oceans.”
A yardstick, then, must be held honest as the sea it measures changes. The warmest months of this event still lie ahead. What a 4-degree ocean means for anyone’s particular winter — snow in Albany or rain on the coast, ice in Maine or mud in the Santa Cruz hills — will be learned, as the Globe’s Ken Mahan notes, firsthand. The ocean, as always, is not required to explain itself. It only sets the terms.