Record-strength El Niño may leave behind climate shifts lasting decades

The record is set in one patch of ocean. An El Niño is declared when sea-surface temperature in a region along the equator runs 0.5C above average ^[2], so the current reading is 6.8 times that bar ^[20]. The warmth starts in the western Pacific, where warm water builds up and then spills east across the equator ^[3].

Ending one takes several processes working at once. The warm water slowly disperses, partly by flowing away from the equator, and it cools as it hands vast amounts of heat to the atmosphere. That handover produces the sudden jump in global surface temperatures ^[6]. Below the surface, warm water arriving in the east sets off Rossby waves that travel west ^[7]. Months later they hit the western continental shelf and bounce back east as faster, cold Kelvin waves ^[7]. When those arrive weeks after that, they thin the warm layer and let deeper, colder water reach the surface ^[7].

"Many different processes are involved in the onset, maturation and demise of El Niño events," said Michael McPhaden of the US National Oceanic and Atmospheric Administration ^[8]. Those processes did not line up the same way in the last two strong events ^[9]. "So, at this point, it is difficult to know precisely how the current El Niño will wind down," he said ^[10]. "Nature loves to confound us with new wrinkles in the epic saga of El Niño, so be prepared for surprises." ^[11]

Jeff Masters, a hurricane specialist, expects the effects to outlast the event. "I expect that this super El Niño event will drive multiple climate regime shifts like we've seen after past super El Niño events," he said ^[5]. Jong-Seong Kug at Seoul National University and colleagues published a study last year showing that extreme El Niños combined with global warming can produce such shifts ^[13]. According to New Scientist, the stronger event now under way is more likely to trigger them ^[19]. "We are experiencing a very strong El Niño event," Kug said ^[15].

The thing this evidence does not tell you is how much of any one shift belongs to the El Niño. The named precedents are two events, 1997 and 2015 ^[9]. In the documented case, the Gulf of Mexico, Kug said the 2015 El Niño contributed but was not the only factor ^[14]. A step change that follows an event and then persists is consistent with a trigger. One case cannot separate the El Niño's push from the warming trend underneath it. The Amazon outlook is less tested still, since New Scientist states it as an expectation ^[16].

Part of the heat shows up in the global average ^[6]. This year is on track to tie 2024 at about 1.6C above the pre-industrial level, and 2027 is expected to reach 1.7C or more ^[17]. By New Scientist's reckoning, the world is on the brink of permanently crossing the 1.5C threshold it was meant to avoid ^[18].