The heat we fear may already be here

Heat and humidity beyond what the human body can tolerate is emerging ahead of projections, a new study suggests. 

In a paper in the journal Science Advances, researchers present observational data showing that wet-bulb temperature – which incorporates measures of humidity – has in some places already exceeded 35 degrees Celsius, the point at which humans can no longer regulate body heat. 

“Previous studies projected that this would happen several decades from now, but this shows it’s happening right now,” says lead author Colin Raymond from Columbia University, US, who worked with Columbia’s Radley Horton and Tom Matthews from Loughborough University, UK.

As occurrences to date have tended to be brief and very localised, they have not been picked up by previous studies that looked at averages of heat and humidity measured over large areas and over several hours at a time, the researchers say. 

For their study, they looked at hourly data from 7877 individual weather stations, allowing them to pinpoint shorter bouts affecting smaller areas.

Analysing data, they found that extreme heat/humidity combinations doubled between 1979 and 2017. Repeated incidents appeared in much of India, Bangladesh and Pakistan; northwest Australia; and along the coasts of the Red Sea and Mexico’s Gulf of California. 

Incidents tended to cluster along confined seas, gulfs and straits, where evaporating seawater provides abundant moisture to be sucked up by hot air. However, moisture-laden monsoon winds or wide areas of crop irrigation appear to play the same role in some inland areas. 

As Cosmos reported four years ago, humidity could genuinely be the killer in climate change because it worsens the effects of heat.

Humans cool their bodies by sweating; water expelled through the skin removes excess body heat, and when it evaporates, it carries that heat away. The process works nicely in deserts, but less well in humid regions, where the air is already too laden with moisture to take on much more. 

Raymond and colleagues say prior studies have suggested that even the strongest, best-adapted people cannot carry out normal outdoor activities when the wet-bulb hits 32 degrees Celsius (equivalent to a US heat index of 132 degrees Fahrenheit). A reading of 35 – the peak briefly reached in some Persian Gulf cities – is the theoretical survivability limit. 

“It’s hard to exaggerate the effects of anything that gets into the 30s,” said Raymond.

The study found that worldwide, wet-bulb readings approaching or exceeding 30 on the wet bulb have doubled since 1979. There were about 1000 readings of 31 (previously believed to occur only rarely, the researchers say) and 80 of 33 (almost non-existent).

Kristina Dahl, a climatologist at the Union of Concerned Scientists, says some localities may already be seeing conditions worse than the study suggests, because weather stations do not necessarily pick up hot spots in dense city neighbourhoods

An interactive version of the map below is available here.