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.

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.

“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.”

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

Greta it’s Riley here from Sailing La Vagabonde. If you get in contact with me I’m sure we could organise something.

— Riley Whitelum (@Sailing_LaVaga) November 4, 2019

Ms Thunberg posted to her Instagram page earlier today 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.

“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.

Mr Whitlum and Ms Carausu have been sailing around the world since 2014, filming their adventures and posting them to YouTube full-time.

“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.

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

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

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.

-ABC