El Niño is a climate pattern that is defined as anomalous warming of surface and sub-surface waters in the eastern and central tropical Pacific Ocean. More specifically, El Niño is the name given to the warm phase of a larger phenomenon called the “El Niño-Southern Oscillation” (ENSO). El Niño’s greatest impact is in the winter, and it typically lasts for about a year, but it can last for much longer, which we recently saw with the recent La Niña (the cool phase of ENSO), which lasted for three years. It occurs irregularly, roughly every 2-7 years. El Niño acts as a giant heat source in the tropics, in turn affecting atmospheric circulation, which then influences global climate and weather patterns worldwide.

El Niño tends to peak in the winter months, so we expect to see El Niño continue to grow through the summer until at least the autumn. The World Meteorological Organisation outlook published in April gave a 60% chance of transition to El Nino in May-July, rising to about 80% before the end of October. Last week, NOAA – the National Oceanic and Atmospheric Administration in the US – officially declared the onset of El Niño.

We know that El Niño can warm the planet significantly (up to about 0.2°C averaged over a given year). Previous global mean temperature records have been broken in the year following big El Niños (because they take time to warm the atmosphere): namely 1998 and 2016 – which still holds the record. Given that global warming has resulted in the ten hottest years on record all occurring since 2010, I think it’s very likely that even a moderate El Niño would result in 2024 breaking the record.

El Niño tends to suppress rainfall over most tropical land. This means, all other things being equal, that the Asian, African, and South American monsoons tend to be drier than usual. It is also associated with increased rainfall, and occasionally flooding, in the southern United States, Peru, Argentina, southern Europe, Kenya and Uganda. As we have already seen this year, El Niño can also exacerbate heatwaves in the tropics, as in India in February and March.

Typically negative, and it will mostly be felt in the developing countries in the tropics that depend heavily on agriculture, and where reduced rainfall tends to drive up food prices. Estimates for the 1982-3 and 1997-8 El Niño events were growth reduction of about $5 trillion globally. For comparison, the impact on US agriculture alone is estimated to be a reduction of about $1 billion. Fishing industries in the eastern Pacific are also negatively affected by El Niño.

This is a complex question given the highly nonlinear response of human health and disease vectors to changes in weather and climate. By weakening monsoons, El Niño is often associated with food shortages in developing countries, which can lead to impaired immune responses via malnutrition. Similarly, it can lead to a scarcity of drinking water in some areas, increasing the risk of diarrhoeal diseases (e.g., El Niño has been shown to cause a significant increase in cholera risk in Bangladesh). The relationship with malaria varies from country to country. In most countries it is not statistically significant, whereas in others El Niño reduces the risk. Studies have found a significant increase in dengue risk in South America, the South Pacific and southeast Asia during El Niño.

There can indeed be benefits. For example, the southern United States and Peru can experience increased rainfall, which is potentially beneficial for agriculture. Additionally, El Niño can lead to milder winters in parts of the northern United States, Canada, and central Europe, reducing heating costs and making conditions safer for travel. El Niño is associated with reduced hurricane frequency in the North Atlantic, but an increased frequency in the East Pacific. As mentioned above, one of the potential benefits of drier monsoons is reduced malaria risk.

The role of climate change on El Niño remains unclear. In climate model studies, the ENSO response to global warming differs considerably from model to model, with some having increased El Niño frequency and intensity, some having decreases, and some having no significant change. Statistical studies that have looked at the historical relationship between ENSO and climate change agree that the frequency of El Niño has increased in recent years, but the consensus is that this is caused by natural variability, rather than global warming.