Greta, Aussie adventurers sail for Spain

Peter Mitchell, AAP US Correspondent
An Australian family is giving climate activist Greta Thunberg a lift to Spain on their catamaran

Teenage environmental crusader Greta Thunberg and the Australian family she hitched a ride with on a catamaran to Spain have overcome a rough first day on the Atlantic Ocean.

Australian couple Riley Whitelum and Elayna Carausu, along with their 11-month-old son Lenny, offered to take 16-year-old Ms Thunberg to next month's United Nations Climate Change Conference in Madrid.

They departed Virginia on Wednesday for the two-week voyage across the Atlantic.

"Yesterday was quite rough conditions in the shallow waters off the coast," Ms Thunberg wrote on Twitter to her 2.97 million followers on Thursday.

"Today much nicer.

"Slept really well.

"Great to be back on the ocean!"

The Swedish teenager travels the globe by boat, electric cars, trains or other methods to limit producing carbon emissions.

With plane travel ruled out, Ms Thunberg needed help travelling from the US to the UN conference in Spain.

Mr Whitelum and Ms Carausu, who've spent the last four years sailing around the world on their 15-metre catamaran, La Vagabonde, saw Ms Thunderg's predicament on social media and reached out to her.

The couple has documented their years of travel on YouTube, have more than 1.172 million subscribers and offer live tracking of their journey on sailing-lavagabonde.com.

Ms Thunberg's father Svante and English sailor Nikki Henderson are also on the La Vagabonde.

"I decided to help @Sailing_LaVaga and support Greta because she is changing the world - simply by standing up for what she believes is right and staying true to her values," Ms Henderson wrote on Twitter.

"I so admire that, and hope to draw strength from her."