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Greta Thunberg is angry, and not just about climate change.

"The haters are as active as ever", the 16-year-old Swedish climate activist posted on social media on Thursday, "going after me, my looks, my clothes, my behaviour and my differences". Anything, she says, rather than talk about the climate crisis.

It hasn't stopped her from campaigning. On Friday, she led another of her climate strikes, heading up a huge demonstration in Montreal, demanding international aviation does more to cut its carbon footprint.

But she is clearly very annoyed, accusing critics of "crossing every line to avert the focus" with, what she calls, lies and conspiracy theories.

Of course, the attention isn't surprising. Over the past year, Greta Thunberg has arguably done more to galvanise global action on climate than any other single individual.

And there is no question the unnerving power of her rhetoric comes in part from the fact that she is so young.

Skip Twitter post by @GretaThunberg Here we go again...

As you may have noticed, the haters are as active as ever - going after me, my looks, my clothes, my behaviour and my differences.

They come up with every thinkable lie and conspiracy theory. (Thread->) pic.twitter.com/5rS2VpZj00 — Greta Thunberg (@GretaThunberg) September 25, 2019 Report

A key reason her "How dare you!" message hit home so hard in the UN this week was because she seemed so jarringly out of place in the air-conditioned formality of the UN's New York HQ.

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.

Donald Trump is one. The US president appeared to mock Thunberg this week when he tweeted that she "seems like a very happy young girl looking forward to a bright and wonderful future".

She brushed off his sarcasm on Swedish television with a laugh, saying she "knew he was going to say something about me".

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Others have been more offensive, claiming she's being manipulated by sinister forces.

The UK newspaper The Sun has suggested these include "energy giants and pushy celebrity parents - including a fame-hungry mum who once appeared on Eurovision".

Or how about this: Thunberg is "the deeply disturbed messiah of the global warming movement", according to one Australian newspaper.

"I have never seen a girl so young and with so many mental disorders treated by so many adults as a guru," wrote the columnist Andrew Bolt in the Herald Sun.

So, is Greta Thunberg the dour, vulnerable child with mental health issues that her critics present her as?

She was certainly not dour when I met her in Plymouth Sound aboard the racing yacht on which she was about to sail to America. She'd just got back from a trip out in the high winds beyond the breakwater and was clearly thrilled by the experience.

And, having met her, the idea that she was somehow persuaded by someone to take up the climate issue is fanciful.

Media playback is unsupported on your device Media caption Greta Thunberg: "I feel like what I am doing is meaningful"

She explained she first heard the world's climate was changing when she was eight, and couldn't understand why so little was being done about it. By the time she was 11, she was very unhappy indeed. She stopped eating. She stopped growing, and she stopped speaking to almost everyone.

"It felt like I was the only one who cared about the climate and the ecological crisis," Thunberg told me. "My parents didn't care about it, my classmates didn't care about it, my relatives didn't care about this. I mean nobody I knew cared about this and I felt like I was the only one."

She was determined to change that and, starting with her family, she began her campaign to get the world to take action on climate change.

First, she persuaded her parents to stop eating meat and even got her mother, Malena Ernman, a famous opera singer whose career depends on travelling, to agree to stop flying.

Next came the climate strikes. One Friday just over a year ago (20 August 2018) Thunberg walked alone on to the steps of the Swedish parliament building holding her now-famous "Skolstrejk För Klimatet" - school strike for climate - sign. The rest is history. She rapidly became one of the most celebrated - and reviled - people in the world.

She has developed a network of contacts who advise her - including climate scientists and campaigners - but she's the one who calls the shots.

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Those close to her confirm that she writes all her speeches. She told me she planned to use the two-week trip across the Atlantic to figure out what she was going to say at the UN Climate Summit.

"I will try and communicate the urgency and say that they are the ones in charge now. Show leadership!" she said.

She certainly did that on Monday when she accused world leaders of "stealing my dreams and my childhood", and warned them that "the eyes of all future generations are upon you. And, if you choose to fail us, I say: 'We will never forgive you'".

So, how about her mental health? And, while we're about it, does anyone have the right to comment on it? What's so impressive about Thunberg is how clear-headed she is on the urgent need for action in the light of the science.

Understandably, Thunberg particularly resents her mental capacity being called into question. "Being different is not an illness and the current, best available science is not opinions - it's facts", she said this week.

She acknowledges that she was very unhappy in the past, but says that championing the climate issue has helped lift her out of despair.

"It makes me feel good that I'm not alone in this fight," she told me, "and it feels like my life has some kind of meaning lately and I feel that what I am doing is meaningful."

She's famously described her Asperger's as a "superpower", allowing her to cut through the noise and see to the heart of the issue.

Media playback is unsupported on your device Media caption "You are failing us" - Greta tells world leaders

Dr James Cusack, director of science at the autism charity, Autistica, says she's been a really important role model for others with the condition. "Her brave, clear-minded and science-led approach to activism is inspiring for many people," he says.

And, while Thunberg is sometimes emotional about her cause she says that's because she's appalled and frustrated that the world isn't doing enough to tackle climate change.

She has the full support of her family. Her father, Svante Thunberg, is travelling in America with her - and shared the privations on her transatlantic voyage. Her mother and younger sister, Beata, stayed in Sweden.

She's no plans to stop campaigning any time soon.

These coming months are crucial, she told me, if the world is going to manage to keep the global temperature increase below 1.5 degrees above pre-industrial levels. Emissions have to start reducing before the end of next year, she warns, or we are likely to pass tipping points leading to uncontrolled climate change.

Yet, last year global carbon emissions increased by 2.7%, hitting a record high of 37.1bn tonnes.

Thunberg was upset by the response of the world leaders in the UN HQ this week. They politely applauded her speech but had no new initiatives that would significantly reduce greenhouse gas emissions, and most also failed to acknowledge either her message or the fact that millions of people had taken to the streets to demand action just days before.

So, climate action remains very much a work in progress for Thunberg, despite her critics.

"I guess they must feel threatened by us," she wrote this week. "But the world is waking up."