The US Secretary of State, former oil industry mogul Rex Tillerson, has not responded to a request from the United Nations’ most senior climate change official for a face-to-face meeting amid concern about the Trump administration’s stance on global warming.

Patricia Espinosa, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), told a conference about the issue in Chicago that progress towards a low-carbon economy was now “unstoppable” with many businesses already making the transition.

However she admitted the UNFCCC was worried about “rumours” that the US could withdraw from the Paris Agreement on climate change.

Ms Espinosa also smiled about the prospect of a Mexican woman trying to talk about global warming with the United States’ new leadership – given Donald Trump’s talk about building a wall between his and her country to prevent migration, dismissal of climate science as a Chinese hoax and remarks about being able to sexually assault women because he was a celebrity that helped spark the worldwide Women’s March protests.

After her speech in Chicago, she struck a conciliatory tone as she revealed her request for a meeting with Mr Tillerson had not produced a response.

“I have not heard back. It is understandable at the beginning of an administration,” Ms Espinosa said, according to Bloomberg.

“They are a very important partner to us, and I’m looking forward to working together.”

She made clear the importance of the United States to the world’s efforts to limit global warming to below two degrees Celsius above pre-industrial levels and as close to 1.5C as possible – the target agreed at the Paris Summit.

“We are of course worried about rumours that the possibility of the US pulling out of the Paris Agreement and the convention on climate change,” she said.

“It would be very bad if there were a change of position in the US. That’s why I’m looking forwards to engaging with the US as a partner.”

But she suggested the momentum behind efforts to reduce fossil fuel emissions was such that they would carry on regardless of the views of political leaders.

“A lot of US businesses are really going into the agenda of sustainability and some are making their own commitments in emissions reductions in their own operations,” Ms Espinosa said.

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“An incredible amount of cities have embarked on ambitious goals; some states like California have been for many years in the forefront of this agenda.

“In International Petroleum Week, I was very encouraged to hear how much some of the oil and gas companies are realising that the future of their industries is in a transformation into clean energy companies – and they have embraced this in their own interest.

“The transformation has started. I think it’s unstoppable.”