Teen climate activist Greta Thunberg will make the COP25 UN climate summit after hitching a ride back across the Atlantic with an Australian couple travelling the world.

Key points: Greta Thunberg is one of the world's highest profile environmental activists

Greta Thunberg is one of the world's highest profile environmental activists She was stranded in the US after travelling there to attend a UN climate summit

She was stranded in the US after travelling there to attend a UN climate summit She will travel to the new summit location in Spain with an Australian couple

The 16-year-old had travelled from her native Sweden to California by boat, train and electric car and was planning to continue on to the next round of climate negotiations, originally scheduled to be held in Santiago, Chile in December.

The Santiago summit was cancelled by Chile's government because of political unrest in the South American country and moved to the Spanish capital of Madrid.

Ms Thunberg refuses to fly because of the carbon emissions involved and had been searching for an environmentally-friendly way to travel back to Europe to attend the summit.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 32 seconds 32 s Swedish teen climate activist Greta Thunberg lands in New York City following 14-day trans-Atlantic trip

"As #COP25 has officially been moved from Santiago to Madrid I'll need some help," Ms Thunberg tweeted earlier this month.

"Now I need to find a way to cross the Atlantic in November … If anyone could help me find transport I would be so grateful."

Australian couple to the rescue

A week ago, South Australian Riley Whitelum, who travels around the world in a 48-foot catamaran called La Vagabonde with his West Australian partner Elayna Carausu and their son Lenny, responded to the tweet with an offer of help.

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Ms Thunberg posted to her Instagram page on Wednesday saying she had accepted the offer from the couple, who will take her from Virginia to Spain with the help of professional yachtswoman Nikki Henderson.

"So happy to say that I'll hopefully make it to COP25 in Madrid," Ms Thunberg said.

Riley Whitelum, Nikki Henderson, Greta Thunberg and Elayna Carausu with her son Lenny on their yacht preparing to set sail. ( Facebook: Greta Thunberg )

"I've been offered a ride from Virginia on the 48ft catamaran La Vagabonde. We sail for Europe tomorrow morning!"

Ms Thunberg said she expected the voyage to take three weeks.

"If I get to the COP25 in time then I will participate in that because I received an invitation to do so. And then I will go home," she said.

A voyage funded by YouTube

Mr Whitelum and Ms Carausu have been sailing around the world since 2014, filming their adventures and posting them to YouTube where they have amassed more than 1 million followers.

"We've suffered terrifying storms, pirate scares, financial breakdowns, equipment failures, water shortages and other interesting mishaps, but we wouldn't trade living on the sea and traveling wherever the wind takes us for anything," their website states.

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Ms Carausu posted to Instagram that the crew would set sail from 7:30am US eastern standard time on Thursday.

Ms Thunberg rose to prominence last year after she started spending her Fridays protesting outside Sweden's parliament.

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 angrily denounced world leaders for failing to tackle climate change.

Her action has grown into a global campaign, with an estimated 300,000 Australians taking part in climate change rallies around the country in September.

Ms Thunberg's zero-emissions expedition has been highly publicised and has included travelling by boat, train and an electric car borrowed from Arnold Schwarzenegger.

Final preparations underway

Speaking to ABC Radio Perth from aboard La Vagabonde docked in Virginia, Mr Whitelum said he was surprised when he heard back from Ms Thunberg, but he thought they were suitable travel companions.

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"Elayna and I responded [to Ms Thunberg's tweet], and about a week later here we are about to sail 3,000 nautical miles across to Europe," he said.

"It sort of came back through to us through a back channel and they just said, 'are you available, do you think you can do it? We've been let down a couple of times, are you sure you would be able to do it?'"

Mr Whitelum said after confirming they were up for the challenge, he and Ms Carausu had been busy preparing for the trip and getting to know Ms Thunberg.

Sorry, this audio has expired Teen climate activist Greta Thunberg hitches ride with Australian sailing couple

"When I met her I wasn't ready for quite how young and short she would be," he said.

"We've been hanging out basically for the last five or six days getting the boat prepared.

"She's smart, kind of powerful and independent, she's a good person to have around."

Mr Whitelum said while he and Ms Carausu had crossed the Atlantic twice before as well as the Pacific Ocean, and had about 70,000 nautical miles of sailing under their belts, getting to Madrid would be no mean feat.

"The journey is long and fairly complicated. The northern Atlantic at this time of year is fairly treacherous," he said.

"We're working with some of the best weather routers in the world.

"On this particular trip the first 500 or 600 nautical miles are super important, so it's good that we've been able to choose our window, which is tomorrow morning."