The Australian couple who sailed Greta Thunberg across the Atlantic for the UN COP25 climate summit say people would be shocked to see how human and vulnerable the teen climate activist is.

Key points: Over the past year, Greta Thunberg has become a global environmental icon

Over the past year, Greta Thunberg has become a global environmental icon An Aussie family offered her a lift to Europe when she became stuck in the US

An Aussie family offered her a lift to Europe when she became stuck in the US Over the course of the trip they saw a very different side to the climate crusader

Riley Whitelum and Elayna Carausu have crossed many oceans since they began sailing the world in 2014, but never has the couple received a reception like the one that awaited them upon pulling into the port of Lisbon in Portugal on Tuesday morning (local time).

"When we arrived there was, I don't know, thousands of people waiting and then huge amounts of media," said Mr Whitelum, owner and skipper of 15-metre catamaran La Vagabonde.

Climate activist Greta Thunberg is greeted by crowds on her arrival in Lisbon aboard the sailboat La Vagabonde. ( AP: Armando Franca )

The couple themselves are no strangers to the spotlight — their YouTube account documenting their voyages has more than 1.2 million followers — but the crowd was there to see one person: Greta Thunberg.

After three weeks at sea the teen climate activist had arrived in Europe for the international climate summit, which began on Monday in the Spanish capital of Madrid.

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 Chile's capital, Santiago.

Crowds of people were there to greet Greta Thunberg when she arrived in Lisbon. ( Reuters: Rafael Marchante )

When the Santiago summit was cancelled due to political unrest, Ms Thunberg found herself on the wrong continent and in need of an environmentally-friendly way to cross the Atlantic.

'We had to say yes'

After seeing her Tweet for assistance Mr Whitelum and Ms Carausu, who sail the seas with their almost one-year-old son Lenny, said they felt compelled to answer her call.

"It was one of those things that we couldn't say no to," Ms Carausu said.

"It was like the mission was put up to us and … we had to say yes even though it wasn't maybe the best time of the year to be doing the crossing and there was a little bit more risk than usual.

"But we had a really good boat, really good crew, so we thought well, if anyone's fit and up for the task there's no better people than us right now, so let's just give it a go."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 44 seconds 44 s The UN has warned that humanity is running out of time to save itself from climate change.

Three Australians, two Swedes and a Brit

After a week of preparation, which saw British professional yachtswoman Nikki Henderson fly in to assist the journey, La Vagabonde set sail from Virginia early on November 14.

Joining Ms Thunberg was her father, Svante, taking the total number on board to six.

The three Australians, the Brit and the two Swedes may have been strangers when they left, but after three weeks at sea the group forged a strong bond.

Greta Thunberg and her five companions sailed into Portugal on Tuesday. ( Supplied: Sailing La Vagabonde )

"Honestly it could not have been better," Ms Carausu said.

"Riley and Nikki took turns with shift work during the night [working in two-hour shifts] and during the day occasionally myself or Svante would keep watch so they could catch up on some sleep.

"Greta took care of Lenny, babysat, you know would feed him his breakfast if I asked her to.

"And Svante kept our spirits high, he's just really funny and he knew how to make us all laugh, which was great."

The teen activist with a 'microscopic' ego

The Australians, who make a living out of filming their adventures and posting them to YouTube, said they looked forward to showing a different side to the teen.

"So one of the things that I really wanted to do on this trip was to film Greta with Lenny, film her stressed and worried and really show the human side of her," Mr Whitelum said.

Greta Thunberg plays Yahtzee on board La Vagabonde with her father Svante (left) and skipper Riley Whitelum. ( Supplied: Sailing La Vagabonde )

"It is striking because when you lay eyes on this girl she's very petite. I mean when I'm looking around the boat for someone that I think might be able to help pull down a sail or something it's not going to be Greta.

"But if there's a discussion to be had on climate change or politics then she's the first person you'd turn to."

In little more than a year Ms Thunberg has become a household name, amassed millions of followers on social media and inspired a generation of climate activists.

Yet the teen's explosion into the public domain has also seen her used as a verbal punching bag for those who disagree with her message or her methods — with prime ministers and presidents among her most outspoken critics.

Ms Carausu said she had been surprised at how the teenager, whose ego she described as "microscopic", responded to all the attention.

"Greta being 16 you'd think that she would let those negative comments really get to her and anger her, but honestly it is the opposite," she said.

"I think it might be because she has Asperger's, which she also talked about, it's like it doesn't seem to really worry her.

"She really didn't want all of this attention … she's not in it for herself, she's in it for the cause and she wants people to use their voices — that was the main thing she spoke about."

Much more than an 'angry kid'

In a brief media conference upon her arrival in Lisbon, Ms Thunberg was asked how she felt about politicians blaming climate activists for current environmental problems.

"I mean when people in power are criticising you like that, that is only proof that you are having an impact," she told the crowd to loud applause.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 3 seconds 1 m 3 s Greta Thunberg says she is travelling by catamaran to 'send a message' that it's possible to live sustainably

"That you among others are bringing change or trying to bring change and many people don't want that, and of course they will do everything they can to try to prevent change from happening."

And when asked about being an "angry kid" the teen took her time to formulate her response but did not shy away.

"I think people are underestimating the force of angry kids," she said.

"I mean we are angry, we are frustrated and that is because of a good reason.

Riley Whitelum, Nikki Henderson, Greta Thunberg and Elayna Carausu with son Lenny prepare to set sail. ( Facebook: Greta Thunberg )

But for those among the five who sailed alongside Ms Thunberg she was much, much more than angry.

"I think they [people] will be shocked at how human and vulnerable she is," Mr Whitelum said.

"If they see her the way that I do now, as a very nice, quiet, respectful, brave young lady, I think that it will change a lot of minds."

Strong bonds form in difficult crossing

Mr Whitelum, who spoke to the ABC after having slept for 12 hours straight, said while the crossing had not been without its challenges it had been more than worth it.

"So a crossing in November in the north Atlantic is going to always be very difficult," he said.

"There's going to be a huge amount of challenges and that's because of the weather systems that are just constantly going through, and they're going through faster than your boat.

"You need to position yourself and be looking at the weather forecast constantly and making the best guess as to the safest place to put your boat.

Both Mr Whitelum and Ms Carausu said they could not believe how well Ms Thunberg and her father had managed the trip.

The group endured high seas and strong winds during the journey. ( Supplied: Sailing La Vagabonde )

"And she was really relaxed on that trip, you could tell the difference," Ms Carausu said.

"Before we left with all the media she was like in her own little world and then as soon as we got on the boat and left she was overwhelmed with what had just happened.

"But within a few days she was at ease and doing her exercises, listening to her audio book, helping me with the cooking and she just seemed really relaxed, you could tell."

'We went through something huge together'

Both said they would miss Ms Thunberg, her father and their many games of competitive Yahtzee, but had hopes they, along with Ms Henderson, would all stay in touch.

"We just went through something huge together … it's a bit weird to never talk again after what we just experienced," Ms Carausu said.

"We'll be talking about this for the rest of their lives I'm sure — it's a story for your grandchildren."

Greta Thunberg accepted a ride across the Atlantic Ocean with Riley Whitelum and Elayna Carausu. ( Supplied: Sailing La Vagabonde )

Mr Whitelum said the trip added a new sense of meaning to their voyage.

"Elayna and I have crossed oceans before … but this particular trip injected a sense of adventure and purpose into our journey that we hadn't really had before," he said.

Ms Thunberg told the crowd on Tuesday she planned to spend a few days in Lisbon getting used to being on land again before heading to join the climate summit.

And when asked what she would be doing after that, the teenager — who plans to go back to school next year after taking a year off — simply said, "home".