NEW YORK/WASHINGTON (Reuters) - Former U.S. Vice President Al Gore, a leading voice in the fight against climate change, and Donald Trump, who at one point called it a hoax, met on Monday in what Gore called a “productive” session.

Gore, a Democrat, spent about 90 minutes in meetings at the Republican president-elect’s Trump Tower apartment and office building in Manhattan. He also met briefly with Trump’s daughter Ivanka, who has attended a series of high-level meetings since her father won the Nov. 8 election.

Gore, who lost the 2000 presidential election to Republican George W. Bush, has been devoted for years to lowering carbon emissions blamed for climate change.

As he campaigned for Democratic presidential candidate Hillary Clinton earlier this year, Gore, who was vice president under Clinton’s husband, former President Bill Clinton, warned that Trump would steer the world toward “climate catastrophe” if elected.

That warning came as Trump gave speeches calling on the United States to drop out of last year’s global climate accord, signed in Paris to lower carbon emissions blamed for a warming planet.

Trump also referred to human-induced climate change as a hoax and had tweeted that “the concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.”

Since then, however, in an interview with the New York Times, Trump has indicated he might have an open mind to joining the effort to battle climate change.

‘EXTREMELY INTERESTING CONVERSATION’

Gore had a more upbeat take on Trump after huddling with the wealthy New York real estate developer.

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“It was a sincere search for areas of common ground,” Gore said. “I found it an extremely interesting conversation and, to be continued.”

Gore said on MSNBC later on Monday that Ivanka Trump was “very committed to having a climate policy that makes sense.” He said he would not disclose details out of respect for the privacy of the conversations and out of hopes to keep discussing the issue with the president-elect.

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Aides to Trump gave no further insight into Monday’s meeting. Vice President-elect Mike Pence, asked about the meeting as he left Trump Tower, declined to comment. He said Monday was “another productive day on the transition.”

After a day of meetings, Trump left his home on Monday evening to have dinner with his family at 21 Club, a New York restaurant.

Gore, who shared the Nobel Peace Prize in 2007 for his work urging action against the risks of global warming, was kicking off on Monday a 24-hour talk-a-thon to urge action in the battle against climate change, which threatens to raise sea levels, spread diseases and increase droughts in more regions around the world.

Trump was scheduled to meet on Tuesday with Rex Tillerson, the chief executive of Exxon Mobil Corp, who is being considered for the job of secretary of state. Trump takes office on Jan. 20.

Exxon Mobil has embraced the 2015 Paris Agreement that would lower global greenhouse gas emissions by between 26 percent and 28 percent from 2005 levels by 2025.

An aide to Gore said Trump’s team reached out to him after the former vice president said he intended to do everything he could to work with the president-elect to ensure the United States remained a leader in the effort to address climate change.