Earth is under an “El Niño watch” as scientists eye signs that the climate pattern is developing.

Its arrival could mean significant impacts worldwide, including a push toward levels of global warming that climate scientists have warned could be devastating.

Since March, a rapid increase in average ocean temperatures has been helping to fuel speculation that El Niño is imminent. The pattern could mark a quick departure from an unusually extended spell of El Niño’s inverse counterpart, La Niña, which scientists say ended in February.

Before it materializes, here is what you need to know about it, and what it could mean for your community and planet.

How do we know El Niño may be brewing?

El Niño is associated with warmer-than-normal waters in the Pacific Ocean along the equator — especially in eastern Pacific waters close to South America. That area has shown dramatic warming in recent months, according to satellite data that allows scientists to measure water temperatures in the uppermost layers of the ocean.

At the same time, climate scientists also look at wind patterns to detect El Niño. Typically, in the absence of El Niño, Pacific trade winds push warm surface waters to the west toward Asia. During El Niño, those winds weaken.

As of early April, National Weather Service forecasters in their latest El Niño forecast analysis described “striking” warm waters near coastal Peru and Ecuador, but noted that trade wind patterns were relatively normal and do not suggest El Niño has arrived.

Climate forecasting models predict a 62% chance of El Niño by July and a nearly 90% chance of it developing by the end of the year.

How does El Niño affect the weather?

Whether El Niño is wet or dry or hot or cold depends on where you live. It all relates to domino effects that result from El Niño’s warm waters and weakened winds in the Pacific.

The ocean warmth means increased evaporation and a rising motion in the lower atmosphere over the eastern Pacific, creating towering clouds, rain and storms. That moisture flows to the east and creates above-normal precipitation along the southern tier of the United States.

Some of the most severe El Niños have delivered heavy rainfall and mudslides to Southern California, for example. Impacts can vary depending on the strength of an El Niño, however. Larger temperature and wind anomalies mean a stronger El Niño.

On the other side of the Pacific, it has the opposite effect: Drought often plagues Indonesia, Southeast Asia and northern Australia during El Niño. That can lead to wildfires like those that burned across Indonesia in 2015.

The rising motion over the Pacific triggers changes in air circulation patterns that have far more widespread effects around the world, too.

In the Atlantic Ocean, El Niño is known for stunting tropical cyclone development and can lead to quieter-than-average hurricane seasons. That’s because the areas where Atlantic hurricanes form and strengthen tend to experience more high pressure from a sinking motion in the atmosphere. During El Niño, those areas also tend to see increased wind shear, a variation in wind speed and direction at different altitudes that makes it hard for tropical storms to organize and strengthen.

How long does El Niño last?

El Niño typically lasts nine months to a year, centered around the Northern Hemisphere’s winter months, when the pattern is known to be the strongest and most influential. It often peaks in December and January.

La Niña, on the other hand, can last for a year or more. The most recent La Niña persisted for three years, nearly uninterrupted.

What is the difference between El Niño and La Niña?

La Niña is the inverse counterpart to El Niño, known instead for cooler-than-normal waters along the equatorial Pacific and stronger-than-normal westerly winds across the Pacific.

In the United States, La Niña is known for drier and milder winters across much of the southern and eastern parts of the country, and wet, cold and snowy winters in the Pacific Northwest and Great Lakes regions.

La Niña also encourages active Atlantic hurricane seasons.

Why is the pattern called El Niño?

The explanation ties back to the warm eastern Pacific waters.

Historically, fishermen off the western coast of South America would notice that in some years, currents would change and waters would warm, driving their catch away. They most often noticed the problem in December, when we now know the El Niño pattern often reaches its peak intensity.

El Niño is a reference to that time of year, close to Christmas. It translates to “the Christ child.”

How does climate change affect El Niño?

El Niño has long been part of natural climate oscillations. But more recently, “likely due to the influence of climate change, the effects of El Niño have become more extreme and less predictable,” David Costalago, a marine scientist with the advocacy group Oceana, said in an email.

Scientists believe extreme El Niño (and La Niña) patterns may develop more frequently as greenhouse gas emissions cause the planet to continue warming. Research suggests that trend could make El Niño’s own warming influence more pronounced and dramatic.

By one estimate, if greenhouse gas emissions continue rising dramatically, extreme El Niños could occur once a decade instead of about once every other decade. That could fuel more intense storms, droughts, heat waves and wildfires.

What does El Niño mean for global warming?

El Niño has long been known to boost global temperatures. Against the backdrop of greenhouse gas-driven planetary warming, there are fears that El Niño’s influence could push Earth toward destabilizing climate change thresholds.

The warm Pacific waters and increased evaporation mean the ocean loses heat that is then trapped in the atmosphere. In addition, the increased cloud cover from that evaporation means more of the sun’s heat is absorbed into the atmosphere rather than the oceans.

Scientists estimate that trend could contribute a couple of tenths of a degree Celsius of warming to average global temperatures.

Robert Rohde of Berkeley Earth estimates that 2023 is on pace to be Earth’s second-, third- or fourth-warmest year on record, but as of mid-April the planet also had a 38 percent chance of setting a new record annual average temperature.

“If El Niño develops, it is likely to moderately boost global average temperatures during the rest of 2023 and into 2024,” he wrote.

Such a ramp-up toward record global heat has been observed during previous transitions from La Niña to El Niño. After a lengthy period of La Niña in which the oceans absorbed large amounts of heat, a strong El Niño developed in 2015 and 2016 and contributed to the hottest average global temperatures ever observed in 2016.

“We’re likely going to see the same kind of sequence play out,” said Michael McPhaden, a senior scientist at the National Oceanic and Atmospheric Administration. “We’re going to see again this big ramp-up in global mean surface temperatures.”

The Washington Post’s Brady Dennis contributed to this report.

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