As climate change intensifies, people around the world are learning firsthand how dangerous high temperatures can be. But prolonged heat becomes even more dangerous, and deadly, when paired with high humidity — a one-two punch known as a humid heat wave.
Scientists report that humid heat waves have intensified rapidly over recent decades and are projected to worsen, raising the risk of significantly more heat-related mortalities. But quantifying the origins of these extreme weather events has remained challenging.
A new study published in Nature Geoscience has identified and quantified a likely cause. It traced a strong connection between coastal waters heated by climate change and the development of humid heat waves that spread out over large areas inland — an effect especially pronounced in the tropics.
“Compared with mid-to-high latitudes, the tropics encompass most [humid heat wave] high-risk areas and exhibit stronger land-ocean linkages, highlighting the critical role of tropical oceans,” according to the study conducted by researchers from Germany’s Potsdam Institute for Climate Impact Research (PIK), Princeton University in the US, and Sun Yat-sen University in China.
Humid heat waves — periods when high temperatures couple with high humidity — are particularly dangerous for human survival, Fenying Cai, study lead author with PIK, told Mongabay in a phone interview.
“
With global warming, detrimental conditions are going to be more widespread and last longer everywhere in the world. There is no single place where it’s not going to get worse.
Guillame Chagnaud, land-atmosphere scientist, UK Centre for Ecology & Hydrology
Previous research indicated that even young, healthy people can experience dangerous heat stress when a wet-bulb thermometer (measuring ambient temperature plus relative humidity) has readings exceeding 31° Celsius (87.8° Fahrenheit), a point at which the body can no longer effectively cool itself by sweating. Prolonged exposure to these conditions can be fatal; 100 per cent humidity matched with 35°C (95°F) temperatures are unlivable.
“We see a strong link between warming coastal waters and clustered hot, humid extreme events, especially in the tropics, where oceans supply more moisture to the atmosphere, which is then transported to the land, amplifying the heat,” Cai says.
Though the new study focuses on the pattern by which oceanic warming influences the occurrence of humid heat waves, a strong correlation can also be drawn between escalating climate change, humid heat waves and human health, Cai notes. That’s because widespread humid heat waves raise the risk of heat-related fatalities.
Land-ocean linkages to humid heat waves are most pronounced in equatorial regions, where global humidity is most severe, say the researchers. High-latitude regions currently experience fewer dangerous humid heat waves, since humidity is generally lower there.
Humid heat wave hotspots are found in the Persian Gulf, southeastern Asia, northern South America, and both the tropical Atlantic and southern North Atlantic, Dieter Gerten, a study co-author also with PIK, explained to Mongabay in an online interview.
To reach their conclusions, the scientists mapped when and where high coastal sea surface temperatures were synchronous with humid heat waves using historical data. They found that “the observed intensification of humid heatwaves is closely associated with coastal oceanic warming over the period 1982–2023,” according to the paper.
The research team studied the summer months in both the Northern and Southern Hemispheres record during this 41-year stretch, and found a strong link between warmer coastal ocean temperatures, land-moisture transport, and the increasing frequency of humid heat waves inland.
“To understand heatwaves on land requires integrating the ocean into the picture, because it explains part of the variability of the emergence of these heatwaves,” Gerten says.
The study determined that “Approximately 50 per cent and 64 per cent of the upward trends in humid heatwave frequency and spatial-aggregation strength [or concentrations] over [tropical] hotspot regions are linked to their adjacent oceans.”
Not only is this a statistical correlation, but there is also a proven mechanism by which moisture flows from sea to land, Gerten notes. Rossby waves — atmospheric fluctuations that push air masses across vast distances — help spread weather patterns. They can transport marine heat and moisture far inland, generating large-scale humid heat waves.
Tropical human populations and ecosystems are currently reaching critical physiological thresholds, warns Guillame Chagnaud, a land-atmosphere scientist at the UK Centre for Ecology & Hydrology, who was unaffiliated with the study and was interviewed by Mongabay for this story. As oceans continue warming, likely making humid heat waves more frequent and intense, tropical regions are likely to become ground zero for these climate change-driven extreme events.
Further magnifying the vulnerability of the people living in tropical regions, Chagnaud adds, is the fact that impacted areas often lack air-conditioning. He also suggests that as global warming worsens, populations could experience detrimental living conditions that devastate economies. Deadly humid heat waves could, for example, prevent agricultural labour and other outdoor work, and drastically curtail outdoor recreation and tourism.
“We know to what extent the humid heatwave trend is linked to their neighbouring oceanic warming,” Cai says. “If future ocean warming is projected to intensify in the tropics, it may pose substantial risks to populations living in adjacent tropical terrestrial regions.”
It’s important to fully understand that while humid heat waves may arise in coastal areas, their consequences aren’t localised. The researchers found that areas 1,000 kilometres (620 miles) away from a humid heat wave event have approximately a 90 per cent chance of sharing that experience with their coastal neighbours. While chances lessen the farther away an area is from a humid heat wave’s origin, there’s still a 20 per cent chance of their related occurrence up to 8,000 km (nearly 5,000 mi) away from their coastal source.
“Global warming will induce a change in temperature and humidity at a global scale,” Cai explains. “This long-time change will cause connections in [heat and humidity over] a very far distance. Because no matter at what distance — 2,000 kilometres or 7,000 kilometres — the occurrence of extremes are all increasing over these regions.”
This is one reason people in power need to limit fossil fuel emissions and curb climate change, Chagnaud says, though he adds he doesn’t expect this to happen. Instead, he stresses the need for human adaptation to global warming, noting that the tracking of sea surface temperatures could provide a potential early-warning indicator for widespread humid heat extremes.
But creating this early-warning system requires a better understanding of how warming oceans impact weather phenomena over great distances, as well as the communication of those findings to regional and local meteorological services so that potentially impacted communities can activate plans to protect public safety before a humid heat wave hits.
“With global warming, detrimental conditions are going to be more widespread and last longer everywhere in the world,” Chagnaud says. “There is no single place where it’s not going to get worse.”
This story was published with permission from Mongabay.com.
