The study was published June 19 in the journal Nature Climate Change.
The study of nearly 2,000 heat waves between 1980 and 2014 found that around 30 per cent of the world’s population is now exposed to heat that could kill them for at least 20 days a year.
“We found that killer heat waves around the world are becoming more common, and that this trend already seems unavoidable”, says Camilo Mora, a professor at the University of Hawaii and lead author of the study. “For heatwaves, our options are now between bad or awful”.
“Many people around the world are already paying the ultimate price of heat waves, and while models suggest that this is likely to continue to be bad, it could be much worse if emissions are not considerably reduced”. For the past two weeks, dozens of people have died in India and Pakistan’s current heatwave, where temperatures have spiked to a record 128 degrees Fahrenheit (53.5 degrees Celsius).
The global group of researchers and students coordinated by the University of Hawaii at Manoa set out to answer that question. We were also able to identify a threshold in these two conditions, beyond which the combination and temperature has been lethal in the past. Although it may seem like a very small change, personal decisions like how much to drive can have a large effect if enough people make those small decisions to limit their impact. “The scary thing is how common those deadly conditions are already”.
Not surprisingly, the threshold is driven not only by the air temperature, but also the relative humidity. By analyzing the climatic conditions of 783 lethal heat episodes for which dates were obtained, researchers identified a threshold beyond which temperatures and humidities become deadly. They found that almost one in three people now experience 20 days a year when the heat reaches deadly levels. For example, by 2100 NY is projected to have around 50 days with temperatures and humidities exceeding the threshold in which people have previously died.
Currently, about 30 percent of the world’s human population is exposed to deadly heatwave conditions each year, Mora said. By 2100, that number could reach 74 percent if greenhouse gas emissions continue to rise, or 48 percent with drastic cuts to global emissions.
Aggressive cutbacks now on emissions could help, though only slightly.
“Our attitude towards the environment has been so reckless that we are running out of good choices for the future”, says Mora of the University of Hawaii. The analysis sounds grim when it says that despite reductions, 50% still face the risk of extreme heat that is capable of killing for at least 20 days a year or more.
“Unfortunately there isn’t much we can do to make the situation better”, Mora said while speaking of the Paris Agreement. This is because the tropics are hot and humid year round, whereas for higher latitudes the risk of deadly heat is restricted to summer. In 2010, the Russian capital became engulfed by poisonous smog from wildfires and a sweltering heat wave that killed some 55,000 people across western Russia.
Dr. Marshall Shepherd, director of the University of Georgia’s Atmospheric Sciences Program, was not involved in the study but agreed with it’s findings. The higher the humidity, the lower the temperature needs to be in order to cause heat stroke.
Runners get out early in the morning to beat the heat in Phoenix, Arizona. Deadly heat wave reports are also in from Washington, D.C., New York City, Chicago, Los Angeles, Toronto, London, Sydney, Beijing, Tokyo, and São Paulo.
“We have overlooked that some of the most risky warming is associated with increasingly intense heat waves plus urban heat”, Shepherd said.
Even under the most optimistic emissions scenario (RCP 2.6) – which roughly corresponds to the Paris goal of capping warming at 2 C (3.6 degrees Fahrenheit) – mega-cities such as Jakarta, Lagos, Caracas, and Manila would surpass the “lethal heat” threshold half the year, the study concluded.