The scorching heat blanketing much of the US this week would have been “virtually impossible” if not for the climate crisis, researchers have found, warning that the high temperatures could threaten Independence Day celebrations and World Cup matches this weekend.

“The climate the country has today is fundamentally different to the one it had when the founding fathers signed the Declaration of Independence,” said Theodore Keeping, extreme weather and wildfire researcher at Imperial College London, in a press release.

The analysis from World Weather Attribution, an international consortium of climate researchers, comes as a high-pressure system, or heat dome, brings hot and humid conditions to a large swath of the central and eastern United States, as well as southern parts of Canada.

Even amid the climate crisis, caused primarily by the burning of fossil fuels, such heatwaves are rare, the analysis says, occurring an estimated one time in every 200 years.

But if planet-warming emissions had not heated the planet by 1.4C (2.5F), such events would not have been expected to occur even once in many thousands of years, the researchers found.

The mercury soared in the US capital of Washington DC, as thousands gathered to celebrate the 250th anniversary of the country. At 1pm local time on Friday, events at the so-called Great American State Fair were postponed until 5pm, with people there being ushered to the exits.

World Cup matches will also be affected. France’s match against Paraguay in Philadelphia on Saturday is expected to see levels of extreme heat that a global players’ union has previously said should trigger the delay or postponement of games, and a Miami game between Cape Verde and Argentina scheduled the day before is also expected to be played in potentially dangerous heat and humidity.

On Thursday, in Muhlenberg Township, Pennsylvania, more than 100 people who had gathered at the Union Pacific Big Boy whistle stop event needed heat-related medical treatment, according to a statement from officials, who treated the episode as a mass casualty event involving ambulances, tents and cooling buses.

And in separate locomotive-related news, Amtrak said it canceled more than two dozen trains in the north-east part of the US due to the heat from Wednesday to Saturday. Amtrak warned travelers about potential delays for trains that would remain operational.

All of this serves as an urgent warning showing that the world must immediately begin the swift drawdown of greenhouse gas pollution, said Friederike Otto, professor of climate science at the Centre for Environmental Policy at Imperial College London, in the release.

“When a historic Fourth of July celebration is disrupted, and World Cup matches are played in conditions that are unsafe for players and fans, it shouldn’t take another scientific study to wake people up,” she said.

“Climate change is here; it’s already impacting the things we enjoy in our everyday lives, and it will continue to get worse the longer we drag out the inevitable transition to net zero emissions.”