The extreme heat and humidity broiling swathes of the United States as it hosts World Cup matches and prepares to celebrate the Fourth of July would have been "virtually impossible" without climate change, a new study showed Friday.
"On America's 250th birthday, our study gives a clear reality check," said Theodore Keeping, an extreme weather and wildfire researcher at Imperial College London who co-authored the research for the World Weather Attribution group (WWA).
"The climate the country has today is fundamentally different to the one it had when the Founding Fathers signed the Declaration of Independence."
WWA, which comprises climate researchers from several leading institutions, examined the heat wave that is being driven by a strong "heat dome" high-pressure system trapping warm moist air like a lid over much of the central and eastern parts of the country, as well as southern Canada.
Although such weather patterns are commonplace, they now cause higher temperatures as a result of climate change.
Daytime temperatures in many of these areas are topping 100 degrees Fahrenheit (38 degrees Celsius) but feel even hotter when humidity is factored in.
Such heat stress is commonly measured by an index known as Wet Bulb Globe Temperatures (WBGT), which is more telling than temperature alone, with WGBT values set to hit record highs across much of the region that was studied.
Using climate models, WWA compared versions of the world today featuring human-induced heat trapping emissions, which have caused global temperatures to rise 2.5F (1.4C) since before the industrial age, and a world without.
They found that in a world free of climate change, forecast WBGT levels would have been so rare as to be virtually impossible. At most, they would have occurred once every 5,000 years.
Even in today's climate, such conditions are estimated to be exceedingly rare -- a one in 200-year event -- though there is a high degree of uncertainty given just how extreme the event is.
To rule out natural variability as a cause, the researchers tested the impact of developing El Nino conditions in the Pacific, but found its effect over northeastern North America was minor cooling.
World Cup matches including the July 4 clash between France v Paraguay in Philadelphia are forecast to be in excess of 82F WBGT at kick-off. These are levels at which FIFPRO, the players' union, has called for games to be delayed on grounds they are unsafe for players and fans.
