The planet is being hit with a double whammy of global heating in 2023. On top of the inexorable rise in global temperature caused by greenhouse gas emissions is an emerging El Niño. This sporadic event is the biggest natural influence on year-to-year weather and adds a further spurt of warmth to an already overheating world. The result is supercharged extreme weather, hitting lives and livelihoods. The last major El Niño from 2014 to 2016 led to each of those years successively breaking the global temperature record and 2016 remains the hottest year ever recorded. However, El Niño has now begun and may already be driving new temperature records, with record heatwaves on land from Puerto Rico to China and record heatwaves in the seas around the UK. What is the El Niño-La Niña cycle? Variations in wind strength and ocean temperatures in the vast Pacific Ocean lead to two distinct climate patterns, El Niño and La Niña. The switch between them happens irregularly, every three to seven years, usually with neutral years in between. El Niños tend to last about a year but the La Niña phase can be longer, and 2023 has brought the end of an unusual run of three successive La Niña years. What drives the cycle? Easterly winds normally push warm surface waters in the equatorial Pacific towards Australia and Indonesia and away from South America. As a result, warm water piles up in the west Pacific and cool water is drawn up from depth in the east Pacific. This is the neutral state. But at the onset of El Niño, the easterly winds weaken and the warm water spreads back across the whole Pacific. In contrast, at the onset of La Niña, the easterly winds are even stronger than normal, leading to further cooling of the east Pacific waters. The erratic timing of the switches between neutral, El Niño and La Niña conditions are the result of complex interactions between different climate system phenomena, from ocean current dynamics to thunderstorm cloud formation. How does El Niño increase global temperatures? The ocean absorbs more than 90% of the heat trapped by greenhouse gases released by fossil fuel burning and other human activities. The ocean is particularly effective at absorbing the heat during a La Niña event, when east Pacific temperatures are especially cold. However, during an El Niño, some of this heat is released to the atmosphere because warm water is spread right across the Pacific, smothering cooler waters. El Niño can add up to 0.2C to annual global surface temperatures. How does El Niño affect extreme weather? The El Niño-La Niña cycle switches the position of warm ocean waters and the damp, rain-laden air above it, meaning the cycle brings increased heatwaves, droughts, wildfires and floods to different regions. The places closest to the Pacific are most strongly affected. In Peru and Ecuador, El Niño brings heavy rains and flooding. The event’s full name – El Niño de Navidad, or the Christ Child – comes from the region and was coined because the biggest impacts occur at Christmas time. In the Amazon, the weather gets hotter and drier during an El Niño, meaning less growth and greater risk of fires in a forest already approaching a tipping point. Heat and drought also increase in Colombia and Central America. On the other side of the Pacific, Australia can be hit hard by the higher temperatures brought by El Niño. It raises the risk of heatwaves, drought, and bushfires in the east of the country and also increases the chances of mass coral bleaching of the Great Barrier Reef. The “black summer” of 2019-20 occurred during a small El Niño. Drought risk also increases in Indonesia and the last big El Niño from 2014-2016 fuelled huge forest fires, which sent a smoke plume halfway around the world. Countries further from the equator are still significantly affected by El Niño, which shifts the position of the high-altitude jet stream wind. As a result, the southern US gets wetter weather and increased flood risks, while the northern US and Canada get warmer and drier. The situation is similar In China, wetter in the south and hotter and drier in the north. Does El Niño affect places farther from the Pacific? Yes – its impacts reverberate throughout the global climate system. Perhaps the biggest impact is a tendency for reduced rainfall in the Indian monsoon, which provides 70% of the country’s water and is vital for growing food in the world’s most populous nation. However, El Niño could bring increased rain to the drought-stricken Horn of Africa, where dry conditions brought by the three consecutive La Niñas exacerbated a long-term drought in parts of Ethiopia, Kenya and Somalia. El Niño also affects the risk of hurricanes and typhoons, usually suppressing those that affect the Caribbean and US, India and Bangladesh, and Japan and Korea. However, these storms are powered by ocean heat, and record-high sea temperatures in the Atlantic in 2023 have led the UK Met Office to forecast an above-average number of tropical storms in the North Atlantic. Europe is less affected, but in winter El Niño can shift the jet stream and bring more rain to the south of the continent and drier, colder conditions in the north. El Niño’s impact on rainfall, temperature and plant growth also has knock-on effects, such as increased infectious disease including dengue fever in south-east Asia and Brazil. One study has even linked lower food production in El Niño years to civil wars. What is the situation now? Weak El Niño conditions arrived in May and are expected to strengthen in the coming months, with an 84% chance of a moderate event at its peak from November to January, and a 56% chance of a strong event. Global average temperatures in early June were nearly 1C above levels previously recorded for the same month, leading to record heatwaves from Puerto Rico to Siberia to China. Some scientists said the heating suggested 2023 could become the hottest on record, although most of El Niño’s heat will appear in 2024. Does human-caused climate change increase the likelihood of El Niño or La Niña, or both? Scientists are not sure. This is partly because there are not very many El Niño and La Niña events in the observational record, which goes back about 150 years. That makes it hard to be sure that trends in the data, which suggest a tendency towards more La Niña events, are not simply the result of chance. Furthermore, climate models do not agree on the question. This is because climate models have a relatively coarse resolution of 100km and cannot represent well either the ocean dynamics of the tropical Pacific or the thunderstorm clouds that drive large-scale circulation patterns. Prof Tim Palmer, at the University of Oxford, UK, hopes that a new generation of kilometre-scale global climate models, run on the latest “exascale” supercomputers, will provide more robust answers to this vitally important question. One thing is certain. Rising global temperatures, boosted in El Niño years and bringing worse extreme weather, will not end until carbon emissions are reduced to net zero.