Record-breaking heat and humidity warning for the tropics in 2024

New statistical analysis in the journal Geophysical Research Letters predicts the combined influences of rising global temperatures and El Niño could lead to extreme humid heat stress throughout tropical latitudes this summer. 

The researchers concluded that the “strong‐to‐very‐strong El Niño” at the end of 2023 suggests an annual average maximum wet bulb temperature of 26.2°C, and a 68% chance of breaking existing records, in the tropics in 2024. 

Their prediction is pitched at people living in the tropics, including the Pacific, Asia and Australia, as an “advanced warning.”  

Study co-author William Boos, professor of earth and planetary science at the University of California, Berkeley, says: “We’re quantifying the combined influences of El Niño and global warming on this humid heat stress metric. That’s new. 

“We’re also quantifying the probability of a record-breaking event. That combination of things has not been done before.” 

The combination of high heat and high humidity can have deadly consequences for people living in the tropics. The more humid the air is, the less sweat evaporates, which reduces the cooling effect of sweating and makes it harder for the body to keep its core temperature within the normal range. 

“Sweat is the main way we have to cool ourselves when it gets hot. So, if sweating will not allow you to cool below your core body temperature, that’s the survivability limit,” says Boos. 

This combination of heat and moisture is measured by the wet-bulb temperature.  

Twmax2024 predictionvselnino 850
Maps of the tropics showing the annual maximum wet-bulb temperatures predicted by the authors for the upcoming summer, taking into account current El Niño conditions and global warming (top) and the incremental effect due to El Niño alone (bottom). The reddest areas are predicted to have the highest wet bulb temperatures — a measure of the combined effects of humidity and temperature. The black boxes outline regions that were analysed in more detail. Credit: Yi Zhang, UC Berkeley

“It’s commonly known that the Earth is warming, and El Niño is a warm episode of a natural climate oscillation, so we expect the two to constructively interfere — that El Niño will compound the effects of global warming,” Boos says. 

“Over the long term, global warming brings increased temperature, as well as increased humidity — that is, increased water vapor content of the air. Together with El Niño, this allows the heat and humidity to build up to greater levels at a given location in the tropics.” 

For the study, Boos and colleagues took data on heat and humidity extremes throughout the tropics over the past 45 years and correlated them with El Niño warming in the Pacific Ocean. They then combined these data with the increased temperature and humidity accompanying global warming.  

According to the authors of the paper, they found that “extremely humid and hot conditions on land can be predicted about 5 months in advance”. 

The forecast is possible because “the peak of El Niño comes before the peak in the warmest sea surface temperatures, which affects the maximum [wet-bulb temperature] on land”. 

Boos adds that long-term predictions like this can help regions prepare for extreme heat events and protect humans, livestock, and crops. 

“Humanitarian aid and outreach, preparation for medical care and advising and distribution of crops and agricultural equipment can all be adjusted in ways that can account for that prediction.” 

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