Europe faces even deeper political turmoil and the possibility of mass migration from Africa unless the world urgently addresses the threat of climate change, the former US secretary of state John Kerry said on Thursday.

Speaking at a Guardian Live event, at Central Hall in London, he said he was deeply disturbed at how issues such as climate change, cyber wars and the future of the oceans were not ballot box issues, admitting it was hard to translate these issues into an acceptable set of choices for voters.

John Kerry: US 'cannot afford truculent child president' Read more

In the discussion with the Observer’s chief political commentator, Andrew Rawnsley, Kerry said: “We are heading for catastrophe unless we respond to some life-threatening challenges very rapidly. We have a climate-denying president that pulls us out of the the Paris climate change agreement at a time when literally every day matters.

“Europe is already crushed under this transformation that is taking place due to migration. In Germany Angela Merkel is weakened. Italian politics is significantly impacted.

“Well, imagine what happens if water dries up and you cannot produce food in northern Africa. Imagine what happens if Nigeria hits its alleged 500 million people by the middle of the century … you are going to have hordes of people in the northern part of the Mediterranean knocking on the door. I am telling you. If you don’t believe me, just go read the literature.”

Play Video John Kerry: A life in politics – watch the discussion live now

The former secretary of state for the Obama administration reminded his audience that climate change scientists had just warned that historically unprecedented steps were needed to prevent an extra 0.5C (0.9F) increase.

He pointed to the fires in California and the repeated intensive storms – Harvey, Irma and Maria – that cost the US $265bn last year.

Kerry said Harvey dropped as much water on the Houston, Texas, area in five days as goes over Niagara Falls in an entire year. Irma, he said, had the first recorded, sustained winds of over 185 mph for 24 hours.

We have 12 years to limit climate change catastrophe, warns UN Read more

He said the Green Climate Fund set up in Paris was supposed to have $100bn, but it only had $6bn, making it very difficult for poorer countries to make the right energy decisions. China and India, he said, were still on course to build a wave of coal-fired stations. “We should not be building a single coal fired plant anywhere on the planet,” he added.

Kerry insisted the doors to the G20 summit in Argentina later this year should be locked until the fund was replenished. At the same time, he said, the private sector had to come up with the transformative breakthroughs but all voters have to make “damn sure” that politicians will take the issue seriously.

He also made an impassioned defence for the EU, saying the institution had been set up initially to prevent war, but Europe was now seeing old demons return in the form of antisemitism and racism.

Kerry, insisting he did not want to interfere in the UK elections, reminded his audience that before the 2016 referendum he and president Obama had called for the UK to stay in the EU. “To the best of my knowledge [Obama] has not changed view,” he said.

He added: “I do not think this is the moment for the EU to start falling apart.”

Kerry also defended the Iran nuclear deal that he signed in 2015, but that has been torn up by Donald Trump. He said the “deal is the tightest, most stringent, most accountable, transparent nuclear arms agreement on the planet”.

Play Video 12:53 ‘There’s always hope’: John Kerry on Donald Trump, the US midterms and climate change – video

Under the deal, if the Iranians had tried to make a bomb “we would know immediately because we have radio transmission seals on centrifuges and stockpiles. We have a right to trace, from cradle to grave, every piece of uranium”.

He described the US midterm election results as encouraging and said the Democrats should be preparing for the 2020 election. He refused to identify his chosen presidential candidate, saying he was not convinced he had identified that person yet. But he said the essential element of any ideal successful candidate “will above all be authenticity”.

• This article was amended on 19 November 2018 to correct the fahrenheit conversion of a difference in temperature of 0.5C.