Boris Johnson has acknowledged that he has altered his views about the climate crisis over recent years, saying, “the facts change and people change their minds”.

As he travelled to the US in a bid to accelerate progress towards an agreement at the Cop26 climate summit, the prime minister was asked about the views of his new international trade secretary, Ann-Marie Trevelyan.

The shadow international trade secretary, Emily Thornberry, had highlighted a series of tweets sent by Trevelyan between 2010 and 2012 that explicitly rejected the science of global heating. “Clear evidence that the ice caps aren’t melting after all, to counter those doom-mongers and global warming fanatics,” read one.

Another, sent in support of a campaign against windfarms, said: “We aren’t getting hotter, global warming isn’t actually happening.” A third approvingly shared an article by an explicitly climate emergency-rejecting Twitter account Climate Realists.

Challenged about Trevelyan’s record, the prime minister said: “Ann-Marie will do an outstanding job as secretary of state for international trade.” He then went on to raise his own past record as a climate sceptic.

“I don’t want to encourage you, but if you were to excavate some of my articles from 20 years ago you might find comments I made, obiter dicta, about climate change that weren’t entirely supportive of the current struggle, but the facts change and people change their minds and change their views and that’s very important too,” he said, adding: “The fact is the UK is leading the world and you should be proud of it.”

As recently as 2015, Johnson claimed “global leaders were driven by a primitive fear that the present ambient warm weather is somehow caused by humanity; and that fear – as far as I understand the science – is equally without foundation”.

He also wrote an article in 2013 suggesting the government should consider preparing for a mini-ice age caused by solar activity, drawing on a discredited theory by the climate denier Piers Corbyn – brother of the former Labour leader. And in the same year, Johnson said windfarms – now a key part of the government’s plan to transition to net zero – couldn’t “pull the skin off a rice pudding”.

Johnson was expressing scepticism at a time when there was already a strong scientific consensus about the science of global heating.

Some friends attribute the prime minister’s newfound enthusiasm for the cause of tackling the climate crisis partly to the influence of his wife, Carrie Johnson, who is an enthusiastic advocate of conservation.