Trump announced last year his decision to pull his country out of the Paris accord, arguing that sticking with it would cost the United States trillions of dollars in lost jobs and damage to its industries - but he left the door open to renegotiating the pact.
"We still maintain hope that there could be a reconsideration," said Patricia Espinosa, executive secretary of the United Nations Framework Convention on Climate Change.
"In the meantime, they have been participating in the negotiations and deliberations," she told the Thomson Reuters Foundation in an interview.
U.N. rules mean the world's second-biggest greenhouse gas emitter cannot leave the Paris climate deal until November 2020. Most governments do not want to reopen discussions on its terms.
Espinosa said the Paris Agreement had now been ratified by 175 nations.
The climate deal set a goal of keeping the rise in average global temperatures to "well below" 2 degrees Celsius above pre-industrial times, and ideally to 1.5 degrees. The world has already warmed by about 1 degree.
Countries need to speed up measures to slash their heat-trapping emissions to meet the Paris targets, said Espinosa, adding that impetus could come from an "operating manual" for the deal due to be finalised at December talks in Poland.
"We are seeing very mixed signals. On the one side, we are seeing incredible examples of innovation, and even passion and commitment to address climate change," said the former Mexican diplomat, noting that efforts by U.S. cities and states to push forward on climate action have been a bright spot.
"On the other hand, we are also hearing ... and seeing in front of our eyes signals that the effects of climate change may be worse than we had foreseen or scientists had foreseen," she added.
Putting the trend of rising global emissions into reverse by 2020 is the "ideal scenario", but science suggests there is a "very small window" of opportunity to do that, she said.
"What needs to happen is that each and every person, every company, every government at all levels, every institution really includes climate change in their assessments of risk, in their decisions about how they operate," she said. "That would bring the transformation."
The meeting will bring together mayors and business leaders on Monday to discuss the role women can play in curbing climate change.
Trillions Of Dollars Needed
One major concern is nailing down an annual $100 billion that rich countries have promised to raise by 2020 to help poorer nations ramp up efforts to deal with climate change, Espinosa said. Even that money would be only one step towards meeting the vast costs involved, she added.
Private-sector investment is crucial given the high cost of measures to switch from polluting fossil fuels to renewable energy sources or make heavy industry greener, she said.
"The $100 billion - and having a clear picture on how we're going to get there - is a very fundamental, trust-building measure in the whole regime we are trying to build," she said.
"The truth is the transformation we need requires not $100 billion - it requires trillions, and it requires a huge mobilisation of resources," she added.
Attracting investment for national plans to better withstand extreme weather - including hurricanes and droughts - remains tricky, said Espinosa. But climate change is increasingly being recognised as a security risk, she noted.
"Building resilience should also be seen as building peace, and building better conditions towards peaceful environments and prevention of crises," she said.
(Reporting by Sophie Hares; editing by Megan Rowling and Laurie Goering. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, climate change, resilience, women's rights, trafficking and property rights. Visit http://news.trust.org/)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)