One of the most illuminating moments in the debate Republican debate came 20 minutes in. The reaction to it was swift and – among some young people and scientists – angry.

Did humans contribute to climate change? That was the question by the Fox News moderators, after reading out some statistics from the deadly Hawaii wildfires and other extreme weather events.

They played a clip from a member of the Young Americans for Freedom, a conservative youth activist organisation, who said climate was young people’s number one issue.

“How will you as both president and leader of the Republican Party calm the fear that the Republican party doesn’t care about climate change?” asked Alexander Diaz.

Asked to raise their hand if they thought humans were to blame, not one hand on the stage went up. And only one candidate said it was real.

Florida Governor Ron DeSantis said it was childish to ask for a show of hands and instead laid into Joe Biden for the president’s response to Hawaii.

Former UN ambassador Nikki Haley went the furthest in agreeing with the question by saying: “Is climate change real? Yes it is.” It was up to China and India to lower emissions, she said.

Cutting the US carbon footprint has been costly to the economy and put the country at a disadvantage globally, said Tim Scott. The South Carolina senator said bringing jobs “home from China” would help ease that pain and improve the environment.

One Republican candidate said the climate change agenda was a hoax.

“The reality is that the anti-carbon agenda is the wet blanket on our economy,” said Vivek Ramaswamy, a biotech millionaire and the youngest candidate in the field. “More people are dying of bad climate change policies than they are of actual climate change.”

Boos followed. But as he prefaced his remark by saying he was the only one on stage not “bought and paid for”, it was unclear what the audience was responding to.

Instead of answering the climate question, Chris Christie attacked Mr Ramaswamy as “a guy who sounds like ChatGPT”, a line the former New Jersey governor surely had up his sleeve. The debate then descended into chaos.

Donald Trump, who chose not to attend, has in the past minimised the threat of climate change by joking about oceans rising by one hundredth of an inch over 300 years. In office, he pulled out of the Paris accord on climate change.

The issue of climate has become more talked about as extreme weather has caused havoc in the US. More than 100 people were killed on the Hawaiian island of Maui by wildfires and hundreds more remain missing, while California has been battered by its first tropical storm in 80 years.

The American Conservation Coalition, which mobilises young people around environmental action, condemned what they heard on stage in Milwaukee. Challenging Mr Ramaswamy directly, they said “No, climate change is not a hoax. Candidates with this thinking are completely out of touch with young conservatives.”

They then thanked Mrs Haley for being the only person on stage “bold enough to tackle climate change head-on”.

The Sunrise Movement, a youth activist group which campaigns on climate change, said: “Vivek calls climate change a hoax. We call bluff that he’s been outside this summer.”

This year has already seen the hottest day ever recorded, the hottest month and – very likely – will also become the hottest year on record.

The science is clear-cut – the world’s rising temperatures are the result of man-made climate change.

So it is perhaps not surprising that many climate scientists reacted with dismay at the opinions expressed during the Republican debate.

Corinne Le Quere, professor of climate change science at the University of East Anglia was unambiguous – humans are causing climate change.

“This is not a political issue, it is a scientific fact that stems from overwhelming evidence from observed climate and from understanding the Earth’s physical processes.”

Professor Ed Hawkins, a climatologist at the University of Reading, agreed that whether humans are causing climate change is not a matter of belief. “It is a scientific fact, endorsed by the climate science community, learned societies and every government around the world,” he said.

“How to respond to that fact is certainly a matter for political debate, and that is where the focus needs to be,” he continued.

The climatologist Professor Michael E Mann of the University of Pennsylvania was more pungent. The Republican Party “is not just a threat to the nation. It is a threat to the planet”, he said on X, formerly known as Twitter.

Meanwhile the meteorologist Eric Holthaus posted “Climate denial is alive and well in the year 2023.”