The Earth is expected to shift from a cooler La Niña state into the heat of an El Niño event as early as autumn, with researchers urging policy-makers to prepare for further warming—and the possibility of breaching 1.5°C as early as 2024.

The World Meteorological Organization’s (WMO) latest update on the El Niño/La Niña Southern Oscillation (ENSO) states that the “triple dip” La Niña that began in 2020 “is gradually weakening”, leading to a 90% chance the ENSO will shift to a temporary neutral state between March and May. Those neutral conditions become less likely between May and July, which “can be seen as a potential precursor for El Niño to develop.”

During a La Niña year, “winter temperatures are warmer than normal in the South and cooler than normal in the North,” explains the U.S. National Oceanic and Atmospheric Administration (NOAA). After an El Niño shift, areas in the northern U.S. and Canada are dryer and warmer than usual. “But in the U.S. Gulf Coast and Southeast, these periods are wetter than usual and have increased flooding.”

NOAA forecasts an 85% chance that the February–April period will be neutral, putting the odds of an El Niño by fall at about 60%.

Scientists at the Potsdam Institute for Climate Impact Research see greater likelihood of El Niño’s arrival in 2023, putting the odds at 89% by year’s end. They warn that it may be a strong event, with knock-on effects on global heating.

Both the WMO and NOAA are careful to stress that ENSO forecasts at this time of year are uncertain because of a phenomenon known as the “spring predictability barrier.” Early summer is expected to bring clearer forecasts.

The biggest driver of year-to-year differences in the global climate and regional weather, ENSO is affected by east-to-west equatorial Pacific trade winds on sea surface waters. In La Niña years, these winds are stronger, pushing warm surface waters ahead of them, in turn causing upwellings of colder water in the east that help keep a lid on global heating. By contrast, the trade winds wane in El Niño years, which reduces the upwelling of planet-cooling colder waters. While the two parts of ENSO typically last from nine to months, they can also stretch from two to seven years, like the latest La Niña—the first triple dip of the 21st century. Regularity is not part of the ENSO phenomenon, notes NOAA.

El Niño/La Niña events also have disparate impacts on regional rainfall patterns. Even a moderate El Niño could bring misery to hundreds of millions, with parts of Asia and Australia left parched and sweltering, while other regions, like the Yangtze basin in China, are hammered by torrential rains. More droughts will also be in the cards for the already dangerously dry Amazon, Southern Africa, and India, where El Niño tends to suppress monsoon rainfall.

It is already shaping up to be a sweltering year on the Indian subcontinent, which recorded its hottest February since 1901, with average maximum temperatures of 29.5°C,  reports BBC.

The India Meteorological Department forecasts “enhanced probability” of heat waves between March and May, and the unseasonable late winter heat could herald yet another searing spring. At least 90 people died from heat stress last year across India and Pakistan, while the wheat crop withered and power grids buckled under surging demand.

“India saw a 55% rise in deaths due to extreme heat between 2000-2004 and 2017-2021,” writes the BBC, citing a Lancet study published last year.

The approach of El Niño also raises acute concerns for the Horn of Africa, where a multi-year drought was prolonged partly by the persistent La Niña. The WMO is urging the international community to respond with “multi-sectoral” humanitarian aid as Ethiopia, Kenya, and Somalia are likely to see their growing seasons fail for the sixth consecutive year. The higher temperatures of an El Niño event will deliver more drought and further suffering, especially in pastoral communities.

Even with La Niña’s cooling impacts, 2022 was the fifth-warmest year on record, so the coming El Niño is expected to have major repercussions for global heating.

“It’s very likely that the next big El Niño could take us over 1.5°C,” Professor Adam Scaife, head of long-range prediction at the United Kingdom Met Office, told the Guardian in January.

That would be a “temporary breaching,” Potsdam Institute scientist Josef Ludescher told Grist, but while “one year might be survivable,” it’s a different story for vulnerable species like coral if such heat breaches become common. The last two coral mass bleaching events occurred in 2020 and 2022, during the relative cool of La Niña, so the coming El Niño bodes further harm.