El Niño is here. A climate pattern that cyclically but unpredictably develops in the Eastern Tropical Pacific Ocean every two to seven years, El Niño brings with it far-reaching extreme weather events such as floods, heatwaves, and thunderstorms. It is defined as the unusual warming of surface waters, often during the winter, and increases ocean temperatures and changes the speed and strength of ocean currents, causing knock-on effects around the globe. El Niño causes droughts in India, Central Africa, and Australia as well as rains ranging in strength from unseasonably wet weather in America’s Gulf Coast and East Africa to strong winds and heavy rains on the American West Coast. South America is particularly affected, with the Amazon Basin drier and hotter than usual and the continent’s west coast prone to flooding.
It is well known that these meteorological disturbances have profound human consequences. National Geographic’s report on El Niño highlighted general effects such as power grid failures and malaria epidemics, heat-related public health emergencies and droughts, and flooding causing crop losses and property damage. Its economic effects seem wide-ranging as well: El Niño events have contributed to global inflation and dampened economic growth in vulnerable countries. However, many of its subtler effects are difficult to predict, with the climate and economy together forming an exceedingly complex system. El Niño has been accused of damaging Indian rice production; however, this appears only to be true during some El Niño events. It has also been accused of making stagflation more likely, despite research suggesting it creates stronger GDP growth in the US. While many of its consequences are predictable, the entire profile of its effects is difficult to foretell.
The challenge for our forecasters was to understand the severity of some of the specific, measurable claims, many of which could have momentous implications for the global population. During the last El Niño (2014–2016), we registered the world’s hottest year on record, and this one looks like it will have brought about another. Media reports on this El Niño have predicted a wide array of effects, from flooding in South America to wildfires in Indonesia. Unfortunately, many of these effects are difficult to investigate: they are highly uncertain claims about complex systems, there is a paucity of high-quality data available, and our forecasters are, of course, not subject matter experts. As a result, we focused on a subset of the claims where we have sufficient data to feel confident making predictions that are strongly associated with the strength of El Niño.
How big will El Niño be?
El Niño was officially declared in July, when surface sea temperatures in the Niño 3.4 region of the Eastern Tropical Pacific rose over 0.5°C above the long-term average. In that same month, we asked our forecasters how large the 2023 El Niño will be this winter.
El Niño events vary in scale; the forecasters’ first task was estimating how large this year’s event would be. Forecasters focused on two scenarios: a sea surface temperature rise of less than 1°C (a more modest El Niño) and one of more than 1.5°C (what meteorologists generally regard a "strong" El Niño).
In July, drawing on existing predictions, past data, and their own modelling, our forecasters concluded that the likeliest outcome was a 1.5°C anomaly over October–December, with a 50% chance that the temperature increase relative to the long-term average will fall between 1.2°C and 1.7°C.
Several months later, as we enter October, NOAA have updated estimates on the expected strength of El Niño this year. The chance of a strong El Niño (>1.5°C anomaly) over the next 3 months now stands at 73% chance, and a weak one (<1.0°C) at just 3%.
Forecasting the Global Effects of El Niño
Along with our forecasts on El Niño's strength, we forecasted the effects El Niño will have on various parts of the world, starting with global temperatures.
The World’s Hottest Year on Record
The stronger El Niño is, the more significant its effects on humanity. The first conditional forecast asked whether (as a result) 2023 would surpass 2016 as the planet’s hottest year on record. At the time, forecasters estimated an 82% chance of 2023 breaking previous records, with the size of El Niño being a crucial factor.
There was some disagreement over the extent to which El Niño temperatures are correlated with global temperatures, but forecasters together estimated a 14% chance of record-breaking conditional on a weaker El Niño and a 91% chance conditional on an anomaly greater than 1.5°C. Different forecasters used different methods — referring to betting markets, official predictions, and other temperature data — to reach similar estimates.
Since forecasting, as El Niño has become stronger, the planet's temperature has warmed commensurately. According to the National Oceanic and Atmospheric Administration (NOAA), the likelihood of 2023 being the hottest year on record is now 95%. The NOAA notes that these forecasts do not take into account the effects of El Niño yet, so the best estimate is likely even higher.
We also considered more specific and tangible effects of El Niño's strength. We looked at four regional phenomena: famines in Ethiopia, riots in India, flood damage in the US, and bushfires in Australia.
El Niño is likely to make famine in Ethiopia worse, but conflict is the main determinant of the death toll
Located in the Sahel region, Northern Ethiopia often suffers droughts when El Niño strikes. Our forecasters considered whether this would translate into famine and how many lives might be lost as a result, using “deaths due to protein-energy malnutrition in 2024” as their proxy. They generally agreed that while El Niño might play a part in deepening malnutrition, other factors, such as domestic and international military conflicts, play a more significant role.
Northern Ethiopia was ravaged by war for two years until a cessation of hostilities between government forces and Tigray rebels in November 2022. This created widespread famine in the region, with estimates of people in need of food aid as high as 13 million. Additionally, Eritrean forces embroiled in the conflict were not party to the cessation of hostilities and still occupy border regions as of 2023. This conflict serves to complicate the picture. One forecaster said
“This is a very difficult question to forecast due to the enormous variables of war, locusts, and drought, as well as the lack of information.”
Still, in the baseline scenario — the event of a milder El Niño — their predictions were fairly similar, averaging 19,000 deaths with a relatively narrow probability distribution. One forecaster wrote
“I'm generally seeing non-Niño factors as the primary risks.”
A stronger El Niño pushed the aggregate forecast up to 23,000 deaths. This was a more dispersed distribution, with a lingering possibility of a death toll of 40,000 estimated to be approximately 20% likely. Factoring in circumstances such as the collapse of the Russia-Ukraine grain export deal, one of the more pessimistic forecasters said:
“Progress on funding relief efforts made over the last few decades seems to be reflected in the number of deaths recorded by the IHME (Institute for Health Metrics and Evaluation). But it doesn't currently have any data from when the Tigray war was ongoing. I'd expect deaths to be above the average of the last 10 years.”
These estimates are significantly worse than climate-related death tolls in the past few decades, but note that they are at least an order of magnitude lower than the death toll of the 1984 famine faced by Ethiopia, in which estimates ranged from 300,000 to 1.2 million deaths.
Rioting in India is unlikely to be substantially exacerbated by El Niño
In 2019, India was roiled by unrest prompted by a bill that excluded Muslims from a route to citizenship. The country suffered an estimated 4,500 riots that year — a number that has fallen dramatically since. But El Niño could cause hardship on the subcontinent, with a reduced monsoon likely to affect the production of key crops such as rice, cotton, corn, and soybeans. Our forecasters considered whether that hardship could translate into rioting.
One forecaster pointed out that El Niño can, according to the IMF, lead to higher food prices and inflation in India. Other research suggests a link between El Niño and civil conflict, arguing that it's fair to assume a 10% increase in riots against the baseline. On aggregate, our forecasters predicted 2,800 riots in the event of a strong El Niño and 2,400 in the case of a modest one. An important consideration was that records suggest a much weaker correlation between El Niño and Indian rioting than they do between political events and rioting, which suggests that rioting isn’t a particularly strong indicator of hardship caused by El Niño.
El Niño is likely to increase flood damage in the US
El Niño disrupts atmospheric currents, causing the Pacific jet stream to move south and spread east. A strong El Niño this winter is likely to create wetter conditions in the Southern US and drier conditions in the North. As a result, its effects on the US as a whole are complicated. As one forecaster put it:
“It may be that in some regions of the US, it causes more flooding, and in others less, which might tend to balance out.”
The aggregate of forecasters’ estimates of damage do increase when conditioning on a stronger El Niño, however. The aggregate estimate of (agricultural and property) damage from a stronger El Niño was $4.2 billion, approximately $1 billion higher than from a weaker El Niño and with a lot of uncertainty.
In the context of the total global damages being estimated at around $3 trillion, the estimated effect of a stronger El Niño on flooding in the US looks relatively gentle. However, there was a fat tail on our forecasters’ predictions: they estimated a non-trivial chance that the damage could reach as much as $10bn, ranging from 8% to 16% depending on the strength of El Niño.
Potential peculiarities with past data lowered the overall estimate relative to the outside view. One forecaster observed that three of the six big years for Americans claiming insurance for flood damage came just after the Great Recession, suggesting the possibility that some claims were overstated as a result of background economic hardship.
El Niño increases the risk of extremely large Australian bushfires
As the Australian Government Bureau of Meteorology outlines, nine of the ten driest winter-to-spring periods on record for eastern Australia occurred during El Niño years. This is a recipe for bushfires. Of the past 19 years, 15 have seen bushfires burn well under a standard threshold of a million hectares (approximately the size of Latvia). The remaining four years, however, exhibit massive outliers, burning away two to five million hectares in each of these years.
As a result, in the baseline scenario, the aggregate prediction was a burnt area of 550,000 hectares. Given that extreme El Niño events can bring severe droughts and bushfire events to Australia, forecasters increased estimates conditioned on a stronger El Niño by 20%, nudging this forecast up from 550,000 hectares to 647,000. In both cases, the probability distributions exhibited extremely long tails, which means there is a decent chance that the burnt area could be far bigger.
Some of the numbers in the long tail are cause for serious concern: the 85th percentile of prediction was 5 million hectares burned. In part, estimates of the long tail were driven by record-high levels of rainfall in many parts of Australia. Analysis suggests that increased rainfall in one year creates more bushfires in the next, adding more fuel to the flame. One forecaster wrote:
“Large parts of the Northern Territories look vulnerable to bushfires due to previous wet seasons providing the resources for growth in the bush and a lack of significant fires in these regions in recent years. Due to the size of the northern territory and the lack of population, we could see massive wildfires (exceeding 5 million hectares) with little-to-no property damage or direct death.”
It is difficult to disentangle the effects of El Niño from the myriad of other influences — geographical, social, political — on events like famine, rioting, and flooding. For most of the claims considered, our forecasters’ estimates were uncertain, and changes conditional resulting from a strong El Niño were relatively small. While media coverage is correct that El Niño may bring about more intense regional disasters, the limited predictive power of El Niño's strength suggests its effects on humanity are often overstated.
Here is a compilation of our forecasters' outlook on El Niño's impacts:
Illustration by Laurel Molly ©