As Greta Thunberg enters the United Nations General Assembly chamber, there is a collective intake of breath in the packed media booth looking on.

Key points: Inspired by Greta Thunberg and the threat of climate change, a network of young people around the world are fighting for change

Inspired by Greta Thunberg and the threat of climate change, a network of young people around the world are fighting for change Thrown into the limelight, some young activists are getting professional branding advice

Thrown into the limelight, some young activists are getting professional branding advice The activists say they will return to their old lives when political leaders take action

The 16-year-old's emotionally charged address, where she accuses world leaders of stealing her dreams and her childhood, is a remarkable climax to an extraordinary year of youth activism.

The hashtag #howdareyou goes viral on social media.

Around the world, Ms Thunberg has inspired a grassroots movement which is pushing for tougher action to curb global warming.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 25 seconds 2 m 25 s Greta Thunberg delivers scathing speech at UN climate summit (Photo: AP)

Two weeks ago, on September 20, this new green army of young activists organised the global climate strikes, drawing an estimated 4 million people into the streets.

Some of those campaigners are now becoming stars in their own right.

Over the past six months, Foreign Correspondent has been following three young campaigners in three continents as they learn the art of activism.

Brand management lessons in New York

It's a balmy Friday evening in late May in New York, but school's not yet out for 14-year-old climate activist Alexandria Villaseñor

Alexandria and her mum, Kristin Hogue, make their way through rush-hour traffic to a hip office building in Brooklyn, for an appointment with "creative strategist" Cristian Fleming.

They're launching a not-for-profit climate education organisation and Mr Fleming is advising them on the dos and don'ts of starting up in the activist space.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 3 minutes 1 second 3 m 1 s Young climate activists are working to manage their brand and message.

Lesson number one: Don't look too slick

"There is something to be said about being careful about how good you are at branding and activism," Mr Fleming tells them.

The branding consultant cautions them against falling into the trap of getting mass-produced protest signs "because that's easy for someone who wants to discredit an entire movement to say somebody's funding this," he says.

It's been six months since Alexandria began her own weekly strike outside the United Nations headquarters in midtown Manhattan, calling for more action on combatting climate change.

Alexandria Villaseñor strikes outside the UN headquarters in New York. ( Supplied )

She has already spent several hours on a bench today, bearing two handmade cardboard signs, and she's starting to flag.

"You're tired," Ms Hogue tells her daughter.

"I am, yes, I'm very tired," she agrees, yawning.

Throughout the punishing cold served to New Yorkers during the polar vortex back in February, Alexandria carried out her protest at the UN in a sleeping bag.

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She continued her weekly protests in the pouring rain and kept up her strike through the crippling heat of summer.

The photos Alexandria posts each week on social media have taken off and she's starting to develop celebrity status: she's hounded by media and is a regular speaker at climate and youth events.

Her mother is a PhD student in climate and cultural studies and the other half of what's become a savvy and successful double act.

Now Alexandria and her mum are stepping their climate action up a notch with their not-for-profit organisation, Earth Uprising.

Lesson number two: Don't hog the limelight

"Always do your best to promote other people so, you know, you can always … say it's not about me," advises brand manager Mr Fleming.

"That's such great advice," Ms Hogue says, nodding enthusiastically and taking notes.

Navigating the politics of climate activism is just one of the challenges facing Alexandria right now.

Like most high-profile campaigners, she faces harsh criticism from those who think they're naive, misguided and exploited by adults pushing their own agendas.

In response to the accusation the kids are being controlled by their parents, Ms Hogue is quite clear: "The students are being steered by the science … [they] are capable of reading the science and it's actually very simple."

Lesson number three: Ignore the trolls

"Don't worry about them," Mr Fleming tells Alexandria. "They're a sign of your success."

A life changed beyond recognition in Berlin

Luisa Neubauer takes part in a rally outside the European Parliament in Brussels. ( Foreign Correspondent: Ron Ekkel )

In Berlin, 23-year-old university student Luisa Neubauer is on a wild ride.

When Foreign Correspondent first met up with her in March, she'd been organising weekly school strikes for three months. Luisa was inspired to start the protests after a meeting with Ms Thunberg last year at the COP24 climate conference in Poland.

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Since then, her life has changed beyond recognition.

She went from being a geography student living in a small German town to a full-time climate activist, spending most of her time in the capital Berlin, constantly on the go and always in demand.

"I guess I'm in quite a few media outlets these days," she says, gesturing at the prestigious German weekly Die Zeit, which featured an article about her.

"I don't find myself that interesting — but apparently it's a thing."

It's a thing Luisa wrestles with because she says she'd rather promote other members of the youth strike organisation, Fridays for Future. However, she's fast learning that's not what the media wants.

"They want to pick on faces," she says. And right now, Luisa's face is everywhere.

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Avoiding the limelight is harder once you've become an established brand, and it comes at a cost.

"Jealousy, yes, it's a huge thing," she says.

When we next catch up with Luisa a couple of months later, she tells us she's cancelled 90 per cent of the events she's been invited to, but still feels like she's working constantly: chairing meetings, liaising with other activists, organising events and giving interviews.

As a student, she's been able to put her studies on hold while she carries out the campaign, but there are days when she longs to return to her old life.

Luisa says she'll only be able to do that when the German Government comes up with a plan to ensure the ambitions of the 2015 Paris Agreement are met — that's the UN-brokered deal under which member states agreed to take measures to limit global warming to well under 2 degrees Celsius.

"I wish that every person who asks me how long this movement can survive would go to the politicians and ask them when they plan to start acting," she says.

"It's really up to them. Once they act, we're good. I can get back to my thesis."

In late May, Luisa is once again front and centre of a major demonstration. This one is the Berlin leg of a global day of strike actions.

Backstage she's multi-tasking: carrying out media interviews, signing autographs, posting on social media, dealing with audio problems, late guests, uninvited guests — all while trying to rehearse her speech.

She admits to experiencing some nerves but tells us she's getting better at it.

Up on stage, the technical challenges now forgotten, she delivers a rousing speech to a huge crowd.

"As long as emissions don't go down," she declares, "our future is at risk! And do you know what? It's all we have, and we won't let anyone take it from us!"

Keeping mum in the loop in Sydney

In Sydney, Jean Hinchliffe's mum is trying to nail down where her daughter will be staying when the 15-year-old flies to New York on her own for the UN Climate Summit.

"Are you going to be sharing a room, and who with?" Lisa Healy asks.

Jean is vague on the details. "Yeah, yeah," she replies.

"It's with a few other strikers … but I'm not 100 per cent sure about some of the others who will be there."

Jean Hinchliffe, 15, has suddenly become a central figure in the environmental movement. ( Foreign Correspondent: Aaron Hollett )

A Year 10 student at an inner-west school, Jean's life has also been transformed by her involvement in the youth climate strikes.

She fell into organising the school strikes by accident after emailing some students in Victoria and offering to help out.

"Suddenly I got a response saying, 'oh we'd love to help you achieve this goal', and I'm like 'oh my gosh, I'm the organiser'."

The speed at which her daughter became a central part of the movement took her mum by surprise too.

"I just thought it's a bit pie in the sky and who knows what she's up to, and she just kept going from there," Ms Healy says.

One year ago, Jean says she didn't even know how to embed her signature in an email. Today, she's a key player in the Australian and international youth climate movement, as well as the master of ceremonies at the Sydney climate strikes.

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Hooking up with activists in different time zones can result in late nights, which means getting to school on time is a challenge.

Jean argues the life lessons she's learning outside of school are just as valuable.

"We are learning through this activism," she says. "I've been forced to learn so much and adapt in so many ways. And I'm so happy I did."

She says no-one should be surprised her generation is taking a strong stance.

"I mean I've spent my life growing up surrounded by this constant news of polar ice caps melting and the Great Barrier Reef dying and animals losing their homes and bushfires and floods," she says.

"And that's been really scary for me growing up, knowing that this is my future."

In New York for climate week

Unable to enter the UN headquarters for the Climate Action Summit, Jean Hinchliffe got as close as she could. ( Foreign Correspondent: Ron Ekkel )

Jean is miles from the action when Ms Thunberg delivers her address to the UN Climate Action Summit in New York. The closest she can get is one block away, at a heavily guarded police barrier.

"I think it's kind of ridiculous that so many young people aren't allowed inside for this event," Jean tells us.

The 15-year-old is also annoyed Australia's Prime Minister Scott Morrison has been a no-show.

"He's in the city, he should be going … I feel like his avoidance of the summit is just another way that he's sweeping this issue under the rug," she says.

Despite her frustrations, Jean feels invigorated by her trip to New York.

She's been networking with other activists and has carried out an impromptu strike.

Jean says she'll continue to pursue her activism but thinks the movement might need to take a pause and work on new strategies.

"I think there's definitely a future for the strike movement, but we need to find different forms of action," she tells us from the back of a New York cab.

"As long as we're smart about it, we should be OK."

Watch Foreign Correspondent's 'Climate Kids' 8pm tonight on ABC TV and iview