Heat and humidity as severe, prolonged and far-reaching as this week’s would have been “virtually impossible” in the Northeast and eastern Canada before humans began warming the planet, a team of scientists said on Friday.
Emissions of carbon dioxide and other greenhouse gases from the burning of oil, gas and coal have trapped more of the sun’s heat at Earth’s surface, raising temperatures worldwide for more than a century. Summer hot spells are nothing new, but because of the excess heat around the planet caused by global warming, they can produce higher temperatures today than they once did.
To estimate how much climate change increased the likelihood of this week’s sweltering conditions, the scientists analyzed records of a measurement of heat stress called “wet bulb globe temperature,” a figure that accounts for humidity, wind and direct sunlight. Given that the heat wave is still unfolding, the researchers combined weather observations and forecast data.
They found that the highest five-day average this week of wet bulb globe temperature over the northeastern United States and eastern Canada was still rare in today’s climate, with a roughly 0.5 percent chance of occurring in any given year. But it would have been so rare as to be effectively impossible in the cooler world before the Industrial Revolution, they found.
The scientists are affiliated with World Weather Attribution, a collaboration that examines extreme weather events to pinpoint the degree to which they were influenced by global warming. Their findings have not yet been published in a peer-reviewed academic journal.
Last month, the same group of researchers analyzed the recent searing heat in Western Europe and concluded that climate change had also fueled that hot spell.
“On America’s 250th birthday, our study gives a clear reality check,” said Theodore Keeping, a climate scientist at Imperial College London who worked on the analysis. “The climate the country has today is fundamentally different to the one it had when the founding fathers signed the Declaration of Independence.”
