American climate researchers warn that global climate change is leading to a much faster increase in heat deaths than expected. Previous studies expected that a deadly combination of heat and moisture could occur in tropical countries by the end of the century. However, it appears to be already occurring.

That writes a team led by the California Institute of Technology in Science Advances . The researchers examined data from weather extremes and concluded that the combination of extreme heat with extremely high humidity has doubled in 38 years. In recent years, there have been thousands of cases of extremely humid heat waves worldwide creating potentially deadly conditions for humans.

For example, along the edges of the Persian Gulf, they reconstructed dozens of heat waves that briefly passed the limits that humans can theoretically survive, as they do in India, Pakistan, and Bangladesh. Coastal cities in tropical and subtropical regions of Asia, Africa and Australia are the most vulnerable. Cities along the Gulf of Mexico in the US are also susceptible to a deadly combination of heat and high humidity, according to the researchers.

Wet bulb temperature indicates deadly heat

As long as the human body can evaporate water, we can handle high temperatures - even temperatures that rise above our body temperature, such as last summer in the Netherlands, when it locally exceeded 40 degrees. When evaporating sweat, the skin cools down. Therefore also the advice to drink a lot in heat.

However, if the air is saturated with water vapor, a brake is put on this. Sweat no longer evaporates on the skin, so the cooling effect stops, so that the body temperature even exceeds that of the outside temperature. Therefore, the combination of high temperatures and very high humidity is dangerous.

This is measured with the so-called wet bulb temperature, the lowest temperature that a soaked object can get in blowing outdoor air through evaporation. As soon as this wet bulb temperature exceeds 30 degrees, a dangerous situation arises for people, because natural cooling is no longer possible and the body temperature threatens to rise too high. A wet bulb temperature of 35 degrees quickly kills.

Air conditioning and cold shower

According to the study, there have been approximately one thousand heat waves with the wet bulb temperature above 31 degrees, and eighty above 33. In July 2015, the critical wet bulb temperature of 35 degrees was briefly recorded in Iran. People reportedly survived these conditions by taking refuge in air-conditioned buildings and cars and by taking a cold shower.

Europe is also vulnerable to fatalities from heat and high humidity, according to the researchers, because air conditioning is not common there. Humid heat waves have killed tens of thousands of people in Europe and Russia in record-breaking summers like 2003, 2010, 2018 and 2019.

See also: Huge number of solar panels needed to keep up with demand for air conditioners

See also: KNMI: 'Maybe the door is open, but the chance of heat waves is increasing'