Former United States President Barack Obama described the young climate activist Greta Thunberg as “one of our planet's greatest advocates”.
The 16-year-old Swedish activist rose to prominence in recent months after her school strike to call for more government action on climate change inspired thousands of young people around the world to follow suit in protests called "Fridays for Future". She has been nominated for a Nobel Prize.
Greta is in the United States for a month-long tour, having arrived before the United Nations General Assembly next week in an 18-metre carbon-neutral sailboat.
On Wednesday afternoon, she met with the former US president to discuss her activism and her goals.
“Recognising that her generation will bear the brunt of climate change, she’s unafraid to push for real action,” Mr Obama tweeted alongside a picture of the pair fist-bumping.
"That's the power of young people – unafraid to believe that change is possible and willing to challenge conventional wisdom, Greta and her generation are making their voices heard, even at a young age," Mr Obama.
In a video of their meeting, Greta said: "My message to young people who want to have an impact on the world is to be creative. There's so incredibly much you can do and do not underestimate yourself."
While president, Mr Obama's administration helped pass the Paris Agreement in 2016.
But his successor, Donald Trump, promptly cut the US commitment to the climate action deal and vowed to revitalise America’s coal industry.
While Mr Obama was vocal about the need for action on climate change, he also presided over a boom in the US shale gas and gas industry that has propelled America to its current place as one of the largest petrochemical producers.
Greta also met with American lawmakers on Tuesday and offered stark advice.
"Please save your praise. We don't want it," Greta told the Senate Climate Change Task Force. "I know you are trying but just not hard enough. Sorry."
Representatives have passed little legislation to cut America’s greenhouse gas emissions with many Republican and some Democratic politicians refusing to back such legislation.
When drafting the Paris Agreement, Mr Obama lobbied the international community not to include a requirement to pass legislation backing the deal as he knew he would not be able to get it through the Republican-controlled house and senate.
Ed Markey, a Democratic senator and chairman of the task force, said the passion of young people could play a significant role in next year's US presidential elections.
"2020 is going to be a referendum on climate change," Mr Markey told a news conference on Tuesday. "It will be a referendum between Donald Trump and a whole new Green New Deal direction."
The Green New Deal is a non-binding congressional resolution Mr Markey co-sponsored with Alexandria Ocasio-Cortez, a Democratic congresswoman, that aims to transform the US economy away from fossil fuels within a decade.
Republican lawmakers and Trump administration officials have pilloried the plan as fantasy.
President Trump, one of the few world leaders who openly question the science on climate change, has made a priority of rolling back Obama-era climate protections he says are not necessary and hurt the US economy.
Anaiah Thomas, a 17-year-old climate activist and member of US youth movement Zero Hour, told the senators they needed to take an urgent approach to climate change and support proposals like the Green New Deal.
"What's most important is to decide to take action today. Not in five years. Not gradually. Not tomorrow," she said.