The United Nations Climate Action Summit that same day turned out to be a dud and frustrations in the climate movement have built since.
It hasn't stopped her from campaigning.
Of course, the attention isn't surprising.
And there is no question the unnerving power of her rhetoric comes in part from the fact that she is so young.
She sailed to NY from Europe in August in a racing yacht to avoid air travel, although the boat crew's flight back and forth from Europe to the USA has been cited as a bigger source of pollution than her flying on her own.
It is unusual for young people to hold the adult world to account so forcefully and so publicly and some people clearly don't like it.
From school pupils to local government representatives and MPs, about 100 people from across Southland gathered today to call for "action" on climate change at the Queens Park Feldwick Memorial Gates, in Invercargill.
Donald Trump is one.
Or how about this: Thunberg is "the deeply disturbed messiah of the global warming movement", according to one Australian newspaper.
So, is Greta Thunberg the dour, vulnerable child with mental health issues that her critics present her as?
"I can't even imagine the pressure she's under". The two finally met in May 2019 at a climate conference in Austria. He told his students they should stand up for what they believe in and change will only happen if it's started by young people.
"It is very moving to see everyone, everyone who is so passionate". "My parents didn't care about it, my classmates didn't care about it, my relatives didn't care about this". And yet I'm one of the lucky ones.
Next came the climate strikes.
Last Friday, more than four million youths - and adults - rallied in "climate strikes" around the world. The rest is history.
Those close to her confirm that she writes all her speeches.
Sharing the video to her own Twitter account, Thunberg joked, "I have moved on from this climate thing". Police and firefighters tried to negotiate with them. "Show leadership!" she said.
"There is no question that Greta has had an impact that is greater than all of us environmentalists put together", Suzuki said. And, while we're about it, does anyone have the right to comment on it?
"For more than 30 years the science has been crystal clear".
Understandably, Thunberg particularly resents her mental capacity being called into question. "Just listen and act on the current best available science", she said. With just weeks until the federal election, the group is calling for a Green New Deal and demanding climate policies from all candidates. "From now on I will be doing death metal only!"
Experts say the proposal falls far short of what's needed if the world's sixth-biggest emitter is to meet the goals of the landmark 2015 Paris climate accord.
She has the full support of her family. Schwarzenegger shared a photo of himself with Thunberg, saying he was "starstruck." Her mother and younger sister, Beata, stayed in Sweden.
She told CNN: "My diagnosis has definitely helped me keep this focus".
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Commercial flying accounts for 2.5% of carbon emissions.
"This week, world leaders from all around the world gathered in NY".
The actor had also acknowledged the activist's enthusiasm and resilience for the cause in an Instagram post as well as she stated: "How dare we fail you".
In a characteristically abrasive manner, Thunberg claimed that world leaders' "world view or their interests or whatever.is threatened by us". "But the world is waking up".