Heatwaves amplified by high humidity can reach above 40°C and may occur as often as every two years, leading to serious risks for human health. If global temperatures rise with 4°C, a new super heatwave of 55°C can hit regularly many parts of the world.

Warm air combined with high humidity can be very dangerous as it prevents the human body from cooling down through sweating, leading to hyperthermia. As a result, if global warming trends continue, many more people are expected to suffer sun strokes, especially in densely populated areas of India, China and the US.

If global temperatures increase up to 2 C above pre-industrial levels the combined effect of heat and humidity (known as apparent temperature or Heat Index) will likely exceed 40°C every year in many parts of Asia, Australia, Northern Africa, South and North America. Europe will be least affected with up to 30% chance of having such strong heat wave annually.

However, if temperatures rise to 4°C a severe scenario is on the horizon. Scientists predict that a new super-heatwave will appear with apparent temperature peaking at above 55°C- a level critical for human survival. It will affect densely populated areas such as USA's East coast, coastal China, large parts of India and South America. Under this global warming scenario Europe is likely to suffer annual heatwaves with apparent temperature of above 40°C regularly while some regions of Eastern Europe may be hit by heatwaves of above 55°C.

Although some urban areas such as Chicago and Shanghai are not considered to have high risk for heatwaves based on temperature only, the probability of extreme weather strongly increased when considering relative humidity.

The effect of relative humidity on heatwave's magnitude and peak might be underestimated. There is a need for urgent mitigation and adaptation action to address the impacts of heatwaves and indicate regions where new adaptation measures might be necessary to cope with heat stress.