Research, Science & environment

Record-breaking heat and humidity predicted for tropics this summer

Combination of global warming and El Niño could lead to extreme humid heat stress

By Robert Sanders

young man spraying water on himself to cool off

A rickshaw driver cooling off at a roadside water pipeline during a high-temperature day in Dhaka, Bangladesh, on April 19, 2024.

Habibur Rahman/Abaca/Sipa USA (Sipa via AP Images)

A new statistical analysis of the interaction between El Niño and rising global temperatures due to climate change concludes that the approaching summer in the tropics has nearly a 7 in 10 chance of breaking records for temperature and humidity.

The prediction, by climate scientists at the University of California, Berkeley, applies to a broad swath of the world straddling the equator, including India and the bulk of Africa, Central and South America and Australia, but also includes Florida and Texas.

Long-term predictions like this can help regions prepare for extreme heat events and protect humans, livestock and crops, said William Boos, a UC Berkeley professor of earth and planetary science and an author of the study, which was led by UC Berkeley Miller Postdoctoral Fellow Yi Zhang.

"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," Boos said.

While temperatures around the globe have been setting records nearly every year, the combination of high heat and high humidity is a double whammy that can be deadly. While most healthy people can handle a dry heat, humid heat is much more stressful for the body. The more humid it is, the less sweat evaporates, which reduces sweating's cooling effect and makes it harder to keep the body's core temperature within normal range.

"If you can't cool your body to below 98.6 F, or 37°C, then you'll die," Boos said. "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."

The prediction was published this month in the journal Geophysical Research Letters. It is based on scientists' current understanding of El Niño's impact on tropical heat and humidity, in particular, that atmospheric temperatures several kilometers above the ground control how hot and humid it can get at ground level. These upper-level temperatures are at their warmest about five months after El Niño peaks. The most recent peak occurred in December 2023.

"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 said. "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."

The researchers concluded from their analysis that the "strong‐to‐very‐strong El Niño" at the end of 2023, which was rated a 2.0 on the Oceanic Niño Index, suggests a 2024 tropical land mean maximum wet bulb temperature of 26.2 C (79.2 F) and a 68% chance of breaking existing records. The wet bulb temperature — basically the temperature you can maintain when covered in sweat or a wet T-shirt in the presence of a strong wind — is a better indication than temperature alone of how humans feel under humid heat conditions. In warm-humid environments like the tropics, wet bulb temperatures above 30 C could lead to irreversible heat stress.

According to Boos, some areas that frequently suffer under humid heat stress, such as northern India, have a 50% chance of suffering record heat and humidity this summer. The Sahel region in Africa, however, has a 35% chance of record humid heat.

Effects of El Niño are ratcheted up by global warming

El Niño is a periodic weather pattern associated with a warming of the ocean surface in the eastern Pacific Ocean, which in turn pumps heat and moist air into the upper atmosphere that spreads around Earth's equator. El Niño conditions, which alternate with the cooler La Niña conditions in what is called the El Niño-Southern Oscillation (ENSO) climate pattern, are one of the major drivers of tropical weather, Boos said. The heat and humidity in the upper atmosphere reach the ground during thunderstorms via the gusts of air we associate with these events.

flattened map of Earth's equatorial region with higher predicted heat indicated by redder colors

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 El Niño alone (bottom). The reddest areas are predicted to have the most unusually warm wet bulb temperatures — a measure of the combined effects of humidity and temperature. The red boxes outline regions that were analyzed in more detail.

Yi Zhang, UC Berkeley

"The gusty, cool wind that kicks in during a thunderstorm is actually cold air coming down from higher up in the atmosphere, cooling down the surface," Boos said. "When El Niño happens, the upper atmosphere gets warmer, which means that these downdrafts won't be as cold. So your surface overall will move to a higher heat and humidity content."

For their study, Boos and his colleagues, including lead author Zhang, 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, then combined these data with the increased temperature and humidity accompanying global warming. The continual rise in global average temperature compounds the effects of El Niño, Boos said.

He noted that the statistical nature of the analysis allows a long-term prediction that is difficult to reliably make with current computer weather models, which are good at short-term predictions but bad at predicting weather several weeks out.

"What we've done in this work is come up with a simple statistical model, and we've validated it well against the observational data by training it on some part of the data while holding back other parts of the data, making sure that it performs well on the data it wasn't trained on," he said. "I think it provides a pretty good bar for the supercomputer models that are predicting climate on a seasonal time scale."

Zhang noted that the five-month lead time for the prediction is based on the current state of ENSO. If weather centers could forecast ENSO conditions six to 12 months in advance, the lead time for wet bulb temperature predictions could be extended to approximately a year, allowing even more preparation time for societies across the tropics.

Boos acknowledged that natural events could throw the prediction off. A volcanic eruption can cool Earth's climate, as happened after the eruptions of Mt. Pinatubo in 1991 and El Chicon in 1982. A sudden descent into cooler La Niña conditions in the Eastern Pacific — essentially the reverse of El Niño — could also offset temperature and humidity increases. But barring such events, the prediction could help countries prepare for a potentially deadly combination of high heat and high humidity.

"We're quantifying the combined influences of El Niño and global warming on this humid heat stress metric. That's new," Boos said. "We're also quantifying the probability of a record-breaking event. That combination of things has not been done before."

Other co-authors of the paper are Isaac Held and Stephan Fueglistaler of Princeton University, and Christopher Paciorek, an adjunct professor of statistics at UC Berkeley. The research was partially supported by the U.S. Department of Energy (DE‐AC02‐05CH11231). Zhang is now at New York University's Courant Institute of Mathematical Sciences.

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