By the end of the century, the combined effects of rising heat and humidity will affect India's northeast region.
Although humidity can greatly magnify the effects of heat, most climate projections tend to leave out this major factor that could worsen things.A new study, published in the journal Environmental Research Letters, project that in coming decades the effects of high humidity in many areas will dramatically increase.
While hundreds of millions of people would suffer worldwide, the hardest-hit area in terms of human impact, will probably be densely populated northeastern India, the research team said.
Detecting Future Temperature
Using global climate models, the researchers mapped current and projected future "wet bulb" temperatures, which reflect the combined effects of heat and humidity.
The measurement is made by draping a water-saturated cloth over the bulb of a conventional thermometer. The study found that by the 2070s, high wet-bulb readings that now occur maybe only once a year could prevail 100 to 250 days of the year in some parts of the tropics.
Lab experiments have shown wet-bulb readings of 32 degrees Celsius are the threshold beyond which many people would have trouble carrying out normal activities outside.
"Lots of people would crumble well before you reach wet-bulb temperatures of 32 C, or anything close," said coauthor Radley Horton, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory.
The study projects that some parts of the southern Mideast and northern India may even sometimes hit 35 wet-bulb degrees Celsius by late century - equal to the human skin temperature, and the theoretical limit at which people will die within hours without artificial cooling.
Other areas which are likely to bear the brunt of the crusing combination of heat and humidity include the southeastern US, the Amazon, western and central Africa, the Arabian peninsula and eastern China.
According to background information in the report: "Human health impacts depend on both temperature and humidity. The human body is efficient at shedding heat through evaporative cooling [sweating], even in high air temperatures, if moisture levels are low.
"However, in hot and humid conditions, the efficiency of evaporative cooling slows and the body may become unable to maintain a stable core temperature," the study authors wrote. This can cause heat stress, which can lead to heat stroke, heat exhaustion, heat cramps or heat rashes.
Study co-author Alex deSherbinin, of Columbia's Center for International Earth Science Information Network, added that "it's not just about the heat, or the number of people. It's about how many people are poor, how many are old, who has to go outside to work, who has air conditioning."