Some of the world’s largest cities, including London, Paris and New York, have revealed they are no longer increasing their climate-harming carbon emissions — and pledged to cut them further.

Greenhouse gas pollution from 27 of the world’s largest cities has been on a downward trajectory for five years thanks to a switch to renewable energy and cleaner transportation systems, it emerged at the Global Climate Action Summit, an event being held in San Francisco to encourage “deeper worldwide commitments” to save the environment.

The pledges fly in the face of scepticism from US president Donald Trump, who pulled the US out of the Paris agreement and has cut environmental protections while encouraging coal and fossil fuel production. California governor Jerry Brown referred to the president as a “liar, criminal [and] fool” when it came to climate change, after announcing his own state would aim for 100 per cent clean energy by 2045.

The conference, which is hosting thousands of leaders, climate activists and business representatives, has already seen several other major commitments to cut carbon emissions.

City leaders representing 650 million urban dwellers around the world are committing to leading the fight against climate change as part of a coalition called C40.

Scientists have calculated global emissions need to peak by 2020 and then decline steeply if the ambitious targets set by the Paris climate agreement are to be met.

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As globally emissions are still rising, mayor of Paris and C40 chair Anne Hidalgo welcomed the “incredible achievement” the cities had made in leading the way towards this target.

Within the C40 group, 73 cities have now committed to become carbon neutral by 2050.

The UN secretary-general’s special envoy for climate action, Michael Bloomberg, said: “To prevent the worst impacts of climate change, we have to cut greenhouse gas emissions even as the population grows.

“Cities are showing that it can be done – and that the same steps they’re taking to reduce their carbon footprint are also strengthening their local economies, creating jobs and improving public health.”

In a speech, Mr Bloomberg called the conference a way to broadcast that the US is still committed to the cause.

“Climate change is a global challenge and Washington ought to be leading from the front,” he said.

Mr Trump announced last year that he would be walking away from the Paris agreement, which commits nations to set ambitious plans to cut emissions. He has also stated he thinks global warming is a “hoax” perpetrated by China.

Many people around the world wrongly concluded that America was “walking away from climate action” when Mr Trump pulled the country out of the Paris agreement, Bloomberg said, stressing that “nothing could be further from the truth”.

Besides the commitment by cities, the summit has seen a pledge by business and political leaders in the US and Europe to increase the capacity for electric vehicles by making massive investments in charging points.

The summit has not been without controversy, with hundreds of protesters gathering outside to protest to the California governor about “his failure to rein in the state’s oil extraction”.

Protestors also stormed the stage as Mr Bloomberg delivered his speech, prompting him to quip that “only in America could you have environmentalists protesting an environmental conference”.