Cop26 must be a “turning point for humanity” in just 40 days’ time, Boris Johnson has urged in a call to arms to fellow global leaders ahead of the climate summit in Glasgow.

Addressing the UN general assembly in New York on Wednesday evening, Johnson was expected to warn it was time for humanity to “grow up”.

“It is time for us to listen to the warnings of the scientists – and look at Covid, if you want an example of gloomy scientists being proved right – and to understand who we are and what we are doing.”

“The world – this precious blue sphere with its eggshell crust and wisp of an atmosphere – is not some indestructible toy, some bouncy plastic romper room against which we can hurl ourselves to our heart’s content,” he added.

Johnson claimed the Muppets character Kermit the Frog was wrong when he sang: “It’s not easy being green.” He also offered a lengthy explanation of the Greek word deinos.

“Sophocles is often quoted as saying that there are many terrifying things in the world, but none is more terrifying than man,” he said. “But what Sophocles actually said was that man is deinos and that means not just scary but awesome – and he was right.

“We are awesome in our power to change things and awesome in our power to save ourselves, and in the next 40 days we must choose what kind of awesome we are going to be.”

He highlighted technological developments that have made green energy cheaper and more accessible, and warned that unless action is taken urgently, “we will have made this beautiful planet effectively uninhabitable – not just for us but for many other species.”

The speech was set to be delivered at 9pm New York time, shortly before Johnson prepared to leave the US after a three-day visit.

The prime minister has been buoyed up by progress on climate during the trip, including President Biden’s commitment to double climate financing to $11.2bn – and China’s promise to halt the funding of coal-fired power stations overseas.

With just weeks to go until the summit, the UK is urging more countries to come forward with climate financing, to help meet the $100bn target set more than a decade ago.

Cop26 chair Alok Sharma has also been in New York holding bilateral meetings with a string of countries, urging them to come forward both with cash, and more ambitious climate reduction targets, in the next few weeks.

Biden has confirmed that he will attend the summit in person – but it is unclear whether president Xi Jinping will do so.

En route to the US, the prime minister suggested he believed there was only a six in ten chance of the $100bn target being reached, but the odds appear to have shortened significantly after the US announcement.

Johnson said it was critical that the outcome of Cop26 kept within reach the target of restraining global heating to within 1.5 degrees.

“We are not talking about stopping the rise in temperatures – it is alas too late for that – but to restrain that growth to 1.5 degrees. And that means we need to pledge collectively to achieve carbon neutrality – net zero – by the middle of the century. And that will be an amazing moment if we can do it,” he said.

At the start of his visit to the US, Johnson conceded that he had changed his mind on the climate crisis in recent years, because “the facts change and people change their minds”.

He told reporters that he had looked deeply into the science while mayor of London.

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.