For months, the world battled droughts, heat waves, floods and cyclones as one of the most powerful El Niño events ever wreaked havoc on global weather systems.
This past summer, India's lack of rain was so severe that the government had to ban the export of certain varieties of rice. In addition, floods in East Africa claimed hundreds of lives and wildfires ravaged Australia.
All of these events were linked to a natural climate pattern caused by fluctuating surface water temperatures in the tropical Pacific Ocean. This pattern - on top of long-term global warming caused by greenhouse gases - has helped to break global heat records in recent months.
El Niño is now over and the tide is turning again, say meteorological experts, who predict that El Niña is about to begin.
What you can expect from the swing depends on where you are in the world, but chances are there's more chaos to come.
Here's everything you need to know about the climate phenomenon that will bring extreme weather conditions this year.
What are El Niño and La Niña?
El Niño and La Niña are the two opposing states of a natural climate phenomenon known as the El Niño-Southern Oscillation (ENSO).
The cycle was first discovered by Peruvian fishermen in the 17th century, who noticed that the waters off the west coast of America sometimes became unusually warm in the winter.
They nicknamed the phenomenon "El Niño de Navidad," or the Christ Child, because it usually occurred around Christmas.
Generally, El Niño is defined as when Pacific Ocean water temperatures rise by at least 0.5 degrees Celsius above historical average, for a period of three months or more. La Niña is typically defined as when temperatures fall by the same amount.
What causes El Niño/La Niña?
Under normal conditions, the trade winds in the Pacific Ocean blow westward along the equator, bringing warm water from South America to Asia and cold water up from the depths. This process is called upwelling.
But during El Niño, the trade winds weaken or reverse, pushing the warm water back east.
During La Niña periods, trade winds become stronger, pushing warmer water further west and bringing more cold water to the surface in the eastern Pacific.
In both cases, fluctuating temperatures at the ocean surface - no matter how small - have a domino effect on the atmosphere, with dramatic and far-reaching consequences for global weather systems.
"The devil is in the detail," said Dr. Neven Fuckar, a climate scientist at the University of Oxford. "The atmosphere responds very quickly - a matter of hours, days, weeks, even a month or two. But the ocean has slower dynamics."
While oceans evolve on time scales of hours or days, they also follow patterns that last decades and centuries or even longer, he said.
The complex way in which the tropical Pacific Ocean and the atmosphere interact makes the fluctuations between the two opposing states very difficult to predict.
"One of the biggest problems is that it is very difficult to predict," said Dr Simon Boxall, senior lecturer in oceanography at the University of Southampton.
"We know from satellite data, from history and from very detailed measurements that we can make of the oceans, that there is no real debate about when El Niño and La Niña occur," he said.
It's a phenomenon that scientists still haven't fully understood.
"I wouldn't say it's random," Dr. Boxall said. "There's got to be something behind it, but I don't think we've gotten to the bottom of what turns an El Niño on or off."
How often does El Niño/La Niña occur?
The oscillation between El Niño and La Niña periods occurs at regular intervals of every three to seven years, with 'neutral' periods between the two extremes.
The shift disrupts seasons around the world and typically lasts 12 to 18 months, but can sometimes last years.
Since the turn of the century, seven El Niño events have been recorded, while at least 30 are thought to have occurred since 1900.
The El Niño periods that began in 1982, 1997, 2014 and 2023 were among the strongest ever observed.
The 1997-1998 El Niño event brought record rainfall to Kenya and contributed to a devastating extreme weather event in Florida known as the Night of the Tornadoes, which produced 15 tornadoes, killing 42 people, injuring hundreds, and causing hundreds of millions of dollars in damage.
There have been 24 La Niña periods since 1900.
Starting in 2020, the world experienced an unusually long La Niña phase that lasted until early 2023.
While this period helped keep global temperature rise in check, it also produced a record-breaking hurricane season.
The 2020 hurricane season was the most active on record, while 2021 also saw numerous catastrophic tropical cyclones make landfall in the United States and the Caribbean, killing hundreds of people and causing billions of dollars in damage.
What stage are we in now?
We are currently in a neutral phase - neither El Niño nor La Niña.
"Right now the atmosphere is receiving a steady amount of heat from the tropical Pacific Ocean," Dr. Fuckar said.
But it looks like La Niña is coming. The U.S. National Oceanic and Atmospheric Administration (NOAA) estimates there is a 70 percent chance of a La Niña developing between now and October.
What effect will La Niña have in 2024?
Normally, air rises over the Amazon and Indonesia because moisture from the tropical forests makes the air less dense, allowing it to flow over East Africa and the eastern Pacific Ocean.
During La Niña, these arcs become more intense and stormier conditions develop, often causing wetter than normal weather in Indonesia, northern Australia and Malaysia.
Dr Boxall said: "La Niña is a stable event, but if it is too extreme it can cause problems."
The consequences are being felt all over the world.
Historically, La Niña-like conditions have caused heavier rainfall and flooding during the monsoon season in India. They have also been associated with catastrophic flooding in northern Australia.
The Queensland town of Gympie is testament to the dangerous impact La Niña can have, with four of the worst floods on record occurring during La Niña periods.
In 1955, the city was hit by a cyclone, which brought a deluge of rain, flooding the main street to a height of three meters.
And in 1974, the surrounding state received some 900,000 million tons of rain, making it one of the wettest years on record.
La Niña is also bad news for East Africa, where many vulnerable communities are already struggling with prolonged drought.
In May, NOAA warned that an "exceptional" Atlantic hurricane season could loom between June and November, fueled by exceptionally high sea surface temperatures that could allow storms to generate more energy and thus higher wind speeds.
Last month, Hurricane Beryl became the earliest Category 5 Atlantic hurricane on record, as it tore through parts of the Caribbean.
But it's not all doom and gloom.
"In fact, La Niña is good news for most populations," said Dr. Boxall.
For example, the transition has had a hugely positive impact on the fishing industry in western South America: the upwelling brings cold, nutrient-rich water to the surface, allowing phytoplankton, fish and other marine life to thrive.
The warmer waters may also attract tropical species to areas that are normally too cold for them.
Does La Niña affect the UK?
Although La Niña originates in the depths of the Pacific Ocean, its effects are felt even in Britain.
Fortunately, the impact is usually subtle.
El Niño could increase the likelihood of colder winters in the UK and made the winter of 2009 the coldest since 1979.
In contrast, the effects of La Niña on the UK's summer weather are "less pronounced", according to Malcolm Mistry, a senior lecturer at the London School of Hygiene & Tropical Medicine.
The effects of La Niña, he said, "typically take a while to play out" and can lead to wetter, milder summers - much like the one we are experiencing now.
But he added that the effects in the UK "are not consistent across every El Niño or La Niña phase".
"This remains an active area of research in climate science."
Can it help keep rising temperatures in check?
El Niño and La Niña are now adding to the effects of global warming, making 2023 the warmest year on record.
Since the summer of 2023, the world has had 13 consecutive months of record temperatures.
This has led to extreme droughts in certain areas.
Food production is extremely sensitive to climate conditions and can therefore jeopardize food security. Some of the worst food crises are due to Enso.
Some experts are optimistic that a La Niña phase could cool temperatures. But while Enso's wobble may provide some short-term relief from global warming, temperatures are likely to continue to rise.
"I hope that La Niña will develop quickly and dampen global warming," Dr. Fuckar said. "But that is not certain, it is a very noisy system."
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