Extreme heat stress now reaches one billion more people, study says
Research in Nature Climate Change finds heat stress days, hot nights and “feels-like” extremes have grown longer and more common since the 1970s.
By Hana Yoshida · Markets Reporter
3 min read
One billion more people now face at least one day of extreme heat stress each year than in the 1970s, according to a study published Monday in Nature Climate Change. The research says the rise is widening health risks because dangerous heat depends on humidity, wind and other conditions, not air temperature alone.
The study found that extreme “feels-like” temperatures, heat stress days and hot nights have become more frequent, longer-lasting and more severe over the past six decades. The Associated Press reported that the study links the trend to global warming driven by burning coal, oil and gas.
Researchers measured how heat affects the body
The research team used the Universal Thermal Climate Index, a measure that combines temperature, humidity, wind speed and other factors to estimate how the human body responds to outdoor conditions. That approach differs from studies that look mainly at thermometer readings.
Humidity can make heat more dangerous because it limits the evaporation of sweat, a key way the body cools itself, according to the Associated Press. Humid heat waves can therefore pose greater health risks than dry heat waves at the same temperature.
The study grouped heat stress into three thresholds. Strong heat stress begins at an index reading of 32 degrees Celsius, or 89.6 degrees Fahrenheit; very strong heat stress begins at 38 degrees Celsius, or 100.4 Fahrenheit; and extreme heat stress begins at 46 degrees Celsius, or 114.8 Fahrenheit.
Heat stress seasons are expanding
Parts of Mexico, Central America, Southern Africa and Eastern Africa now face about 50 more days a year of at least strong heat stress compared with the 1970s, according to the study. The affected areas include parts of Namibia, Angola, Tanzania, Kenya and Uganda.
In Southern Europe, the study found that parts of southern Spain, Italy, Greece and Turkey are seeing up to 40 additional days of strong heat stress. Much of the region has gained nearly a month of such conditions compared with several decades ago.
In the United States, much of the country now experiences at least 15 additional days of strong heat stress, according to the study. Southern areas including Texas and Florida are seeing close to 25 or more added days of very strong heat stress.
Rebecca Emerton, the study’s lead author and a senior scientist at the European Centre for Medium-Range Weather Forecasts in the United Kingdom, told the Associated Press that heat stress is intensifying in places already known for hot conditions and spreading into regions where it was historically rare or absent.
Nights are warming fast
The study also found that the feels-like temperatures on the 10 warmest nights of each year have risen by 0.32 degrees Celsius, or 0.58 Fahrenheit, per decade. That is faster than the 10 warmest days, which rose by 0.27 Celsius, or 0.49 Fahrenheit, per decade.
Researchers defined tropical nights as nights with minimum temperatures of 20 degrees Celsius, or 68 Fahrenheit. The Associated Press reported that warmer nights can reduce the body’s chance to recover after daytime heat.
Jennifer Francis, a climate scientist at the Woodwell Climate Research Center who was not involved in the study, told the Associated Press that the work adds detail on rising dangers to billions of people. She said increasing humidity makes high temperatures more deadly because sweating becomes less effective.
Emerton said the findings show the need to limit future warming and put in place adaptation measures, including heat-health plans, early warning systems and climate risk assessments, according to the Associated Press.
This story draws on original reporting from Fortune.