He then pulled apart the famous saying, “it’s not easy being green” from the Muppets’ character, Kermit the Frog.
However, his past scepticism towards climate change was not forgotten by his Twitter critics.
The Independent’s Tom Peck pointed out that Johnson himself did not support the idea of climate change for ten years after Al Gore’s 2016 film, An Inconvenient Truth which looks at how humanity have damaged the Earth.
Peck added: “And now, apparently, we’re meant to take this cartoon clown seriously.”
Satire website The Poke did not hesitate to take aim at Johnson either, noting that he told the world to “grow up” just hours after he had flown to the US and told France to “get a grip” of its anger over the new AUKUS pact.
Even Brexiteer and former MEP Patrick O’Flynn tweeted: “Perhaps someone on the team could have look at the draft [of the speech] and told the PM: ‘Some people may react to this passage by saying ‘speak for yourself mate’.”
This was then retweeted by Alastair Campbell, former prime minister Tony Blair’s spin doctor.
Johnson wrote several columns for The Telegraph over the last two decades which undermine the idea of climate change.
In 2012 he described fracking as a “miracle”, in 2013, he backed climate change denier Piers Corbyn and questioned the phrase “climate warming”, and in 2015, he rejected the idea that warm weather in December was down to a change in the climate.
This checkered past did not escape other Twitter accounts either last night as they mocked his speech.
Some accounts noted that just the other day, he appointed former climate change denier Ann-Marie Trevelyan as his new international trade secretary.
Not everyone thought the prime minister’s words should be mocked, though.
Times Radio’s Matt Chorley tweeted that it was a “bad take” to criticise Johnson’s accurate remarks based on his past climate denialism.
Former leader of the Green Party Caroline Lucas also took note of his speech – and demanded that the prime minister’s actions now reflected his words.
She tweeted: “Good to hear PM’s new enthusiasm for climate action.
“Since Govt failing its own climate targets, I look forward to imminent cancellation of Cumbrian coal mine, Cambo oil field, £27 billion road building plan, reversal of aid cut and a #GreenNewDeal.”
She added the hashtag #WalkYourTalk.
Former London MEP Lance Forman appeared exasperated just at the idea Johnson had referred to a character from the Muppets in a serious speech on the world stage – an opinion shared by plenty of others too.