Green policies can win votes, London Mayor Sadiq Khan said Thursday, after winning a record third term despite the war he has waged on polluting vehicles.

"This year, more than half the world's population will be voting, whether it's the USA, whether it's India, whether it's the UK," Khan told AFP on the sidelines of a three-day climate change conference at the Vatican.

"I think the results (in London) two weeks ago show that green policies are popular," he said.

The 53-year-old won London's May 2 mayoral race by a wide margin despite anger in some quarters over the expansion last year of an Ultra-Low Emission Zone into the largest pollution-charging scheme in the world.

Khan said the key was explaining to voters that green policies -- such as targeting polluting cars or inefficient homes -- "are also policies that tackle social injustice, racial injustice and health inequalities as well".

His election shows that "rather than backing down when it comes to green policies, you should double down", he added.

Khan is the co-chair of C40 Cities, a global network of nearly 100 mayors dedicated to fighting climate change.

He said 75 percent of C40 cities were "going faster when it comes to reducing emissions than our national governments".

The mayor, who credits an adult diagnosis of asthma for fuelling his pollution fight, said London now has the "world's biggest clean air zone".

He also said his administration has planted over half a million trees, increased fourfold the amount of cycling in the capital city and aims to have a fully electric bus network by 2030.

The annual UN climate meetings known as Conferences of the Parties (COP) should not just work with national governments "but also cities, mayors, governors, and states, because we can be the drivers for change", he added.

- Adapt or die -

Khan was one of several mayors and governors from across the world addressed Thursday by Pope Francis, who has made protecting the environment one of the major themes of his papacy.

"The effects of climate change loom over every aspect of our lives, threatening water, air, food and energy systems," the 87-year-old pontiff told the conference.

"The refusal to act quickly to protect the most vulnerable who are exposed to climate change caused by human activity is a serious offence," he said, blaming "the greedy pursuit of short-term gains by polluting industries" as well as fake news.

California Governor Gavin Newsom, another conference speaker, echoed the pope's message, saying that "the polluted heart of the climate crisis are these fossil fuel companies that have been lying to us".

"They've known the science. They've denied the science. We have the tools, we have the technology, we have the capacity to address this issue at a global level, and they've been fighting every single advancement," he said.

The Pontifical Academies of Science, the Vatican institution which organised the summit, was set to adopt a climate resilience protocol co-signed by all those present.

"We've got to make sure we give people hope," Khan said.

But he added: "We've got to adapt or die."