Hot and humid heatwaves predicted

You may already have already been through a heatwave. Relentlessly hot temperatures lasting over several days, with nights too hot to get any rest may sound all too familiar.

For many it's exhausting, and for the young, old or sick, heatwaves can easily become dangerous.

What makes a heatwave so dangerous is the combination of heat and humidity. The heat raises the body temperature, the humidity makes it impossible to cool down by sweating.

This combination of heat and humidity is shown by something called the wet bulb temperature. It is literally measured by wrapping a thermometer in a wet cloth.

The wet bulb temperature of 35 °C (95 °F) is the limit for human endurance. After that point, our bodies cannot cool down. In a few more hours body temperatures rise to 40 °C (104 °F), and the heat becomes unsurvivable. An adaptability limit to climate change due to heat stress — research article

It is these kind of temperatures that are forecast for the later part of this century. By 2100, scientists are predicting heatwaves that will reach and exceed survivable wet bulb temperatures, and that it may affect billions of people. Deadly heatwaves could affect 74 percent of the world’s population — press release

75% of the world population affected

Professor Camilo Mora, associate professor at University of Hawaii and his team of researchers looked at past records of heatwaves around the world, and discovered what they call "the deadly threshold". This is the point at which temperatures and humidity levels become too high for people to survive. They then forecast when and where that would occur as the world gets hotter over the rest of the century. Global risk of deadly heat — research article

The results are grim. The researchers say, if we continue to emit greenhouse gases at current levels, "By 2100, almost three-quarters of the world’s human population could be exposed to deadly climatic conditions."

The good news is still bad. Even if we found a way to remove greenhouse gas, the same scientists found that, by 2100, about a quarter of the world’s land area will exceed "the deadly threshold by more than 20 days per year" and about half the world's population will be exposed to "deadly climates".

According to Professor Camilo Mora, "we are running out of choices for the future". Deadly heatwaves could affect 74 percent of the world’s population — press release

"The scary thing is how common those deadly conditions are already", says co-author Farrah Powell.

Deadly heat forecast for 2100, Camilo Mora and colleagues – Global risk of deadly heat

Unhabitable regions

Dr Mora is not alone in his forecast for deadly heatwaves this century. A number of studies return similar results, with each study forecasting more heatwaves that exceed "the deadly threshold" — the wet bulb temperature of 35 °C (95 °F).

Of all regions, Africa will be the worst hit by heatwaves in the coming years. Scientists estimate one third of the African urban population could be exposed to lethal temperatures by 2090. One third of the African urban population exposed to extreme heat by 2090 — press release For the larger cities in Western Africa, such as Lagos, scientists predict deadly heatwaves lasting several months, even if carbon emissions were brought under control. Projections of human exposure to dangerous heat in African cities under multiple socioeconomic and climate scenarios - research article

Deadly heatwaves for Africa, Asia and the Persian Gulf, jozuadouglas, Pixabay

In China's highly populated heartland, home to 400 million people, in an area extending from Beijing to the north to Shanghai to the south, scientists forecast that by 2100, the region is likely to have experienced a number of "deadly heatwaves", all exceeding the lethal wet bulb temperature of 35 °C (95 °F). North China Plain threatened by deadly heatwaves due to climate change and irrigation — research article

The South Asian nations of Pakistan, India and Bangladesh are already experiencing major heatwaves, which will only get worse. Researchers predict temperatures approaching the 35 °C (95 °F) threshold wet bulb temperature "by the end of the century over most of South Asia." By 2100, 4% of India's massive population will have experienced deadly temperatures. Deadly heat waves projected in the densely populated agricultural regions of South Asia — research article

Always hot, it's no surprise that for the Persian Gulf, scientists forecast "temperature levels that are intolerable to humans". Future temperature in southwest Asia projected to exceed a threshold for human adaptability — research article By the end of the century, the wet bulb temperature will have exceeded the deadly 35 °C (95 °F) threshold several times in the major cities of Abu Dhabi, Dubai and Doha. Study: Persian Gulf could experience deadly heat — press release

Heatwaves already deadly

With each decade, heatwaves have been getting more and more lethal.

The first deadly heatwave of the century was across Europe in 2003. Unprepared for the heat, more than 70,000 lost their lives. Death toll exceeded 70,000 in Europe during the summer of 2003 — research article

Just a few years later in Russia, in 2010, the worst heatwave on record was to take place. Russian summer tops 'universal' heatwave index — Nature 56,000 lives were lost in heat lasting more than 40 days without a break, across a region spanning 2 million square kilometres (772,204 square miles). The hot summer of 2010: Redrawing the temperature record map of Europe — research article

In other countries heatwaves are becoming a common occurrence.

Over the period 2010-2019, Japan struggled through 11 heatwaves, more than one each year on average. International Disaster Database While casualties have not been as high as other countries, one heatwave in 2018 left 71,266 hospitalised. Record 70,000 people rushed to hospitals since April 30 amid scorching Japan heat wave — Japan Times

In the space of 30 years, heatwaves have become deadlier, more frequent, and more widespread.

Mega-heatwaves of the future

In the future, heatwaves are likely only to worsen. Mega-heatwaves more severe than the worst experienced so far, worse even than the Russian heatwave of 2010, are predicted to become commonplace in the future, according to an EU study. Humid heat waves at diferent warming levels — research article

If the world warms by an average of 4 °C (6 °F), heatwaves more extreme than the Russian mega-heatwave of 2010 are predicted for every other year, or five times a decade. This prediction covers the eastern U.S, northern Latin America and China.

For Europe, India, and across Africa, the same mega-heatwaves should take place just once a decade, according to EU scientists.

The scientists also looked at levels of humidity during the heatwaves. They found that at 4 °C (6 °F) global warming, the combination of heat and humidity in eastern U.S., India and China is expected to reach "extreme danger" levels, every one or two years. In these conditions, sunstroke is imminent and often fatal.

The scientists involved in the study questioned whether anyone could continue to live in these regions. Humid heat waves at diferent warming levels — research article