Warming ocean waters are responsible for 50-64% rise in global land heatwaves.
Coastal oceanic warming and intensified humid heatwaves are linked, particularly in tropical regions.
Rising sea surface temperatures could serve as early warning indicators for extreme heat events.
Warming ocean waters or higher sea surface temperatures is driving 50-64 per cent of increase in land heatwaves globally, according to a joint study by multiple agencies.
“We use a complex network approach applied to climate reanalysis data to show that the observed intensification of humid heatwaves is closely associated with coastal oceanic warming over the period 1982-2023,” the researchers wrote in the report Large-scale aggregation of humid heatwaves exacerbated by coastal oceanic warming.
In a warming world, heatwaves have become a permanent phenomenon, raising the risk of heat-related mortality. The risk is measured using the heat-humidity 'wet bulb temperature' threshold of about 31.5 degrees Celsius that is considered close to the physiological limit beyond which sweating becomes ineffective, increasing the risk of heatstroke.
However, this rise in heatwave-like situation, as Asian countries experienced in 2023, remains poorly understood. “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 land, amplifying the heat,” said lead author and Potsdam Institute for Climate Impact Research scientist Fenying Cai.
In regions further from the equator, combined land-ocean warming linked to large-scale atmospheric wave patterns also plays an important role, Cai explained.
By using a complex network approach, instead of looking at heatwaves at individual locations or silos, the scientists were able to map connections between land-ocean linkages.
According to the study, rising temperatures in the Indian Ocean is a trigger point for widespread heatwaves across South Asia and the West Asia. Another case in point was the record-breaking sea surface temperature in 2023 that the North Atlantic region experienced and which led to widespread heatwaves in the southern region of South America.
According to Jürgen Kurths, co-author of the study, the land-ocean linkage will help in employing better adaptation strategies. “Crucially, coastal sea surface temperatures emerge as a potential early warning indicator for widespread humid heat extremes,” Kurths said.
Down To Earth had reported on how Indians are already “experiencing temperatures close to limits of human survivability without even being aware”.
While the India Meteorological Department (IMD) has various conditions before declaring a heatwave-like situation in a region, it does not take into account relative humidity which is a measure of the moisture levels in the atmosphere.
Hence, a lot of Indians might actually be experiencing heatwaves without even knowing it — and that’s where the wet bulb temperature measurement becomes relevant.
According to IMD, the average duration of heatwaves has increased by 6.5 days in seven decades (1961-2021), while the maximum duration has also increased by about two days in the same period.