Teenage climate activist Greta Thunberg has already caused a sensation on her trip to North America, tearing into world leaders yesterday in an emotional speech at the United Nations and giving a death stare to Donald Trump.

What is not yet clear is how the 16-year-old will get back to Sweden, because she refuses to travel on fuel-guzzling planes and the 60ft racing boat she travelled on is already back in Europe.

Two crew members have flown to the U.S. to sail the Malizia II back across the Atlantic and it is not yet clear exactly when Thunberg will be going home.

When she set off from Britain last month she admitted: 'I don't know yet how I will get home'.

Teenage climate activist Greta Thunberg, pictured during her impassioned address at the UN yesterday, does not yet know how she will get back to Sweden

The 16-year-old Swedish teenager sails aboard the racing yacht Malizia II as it arrives in New York after a two-week journey across the Atlantic last month

Non-stop flights from London to New York City take between eight to ten hours for a 3,470-mile journey.

It equates to about 183,910 pounds of emissions per flight between the two cities and Thunberg refuses to add to the environmental cost.

She has also persuaded her parents to stop flying, hampering her mother's career as an international opera singer.

'My generation won’t be able to fly other than for emergencies, in a foreseeable future if we are to be the least bit serious' about limiting global warming, she has previously said.

Thunberg also refuses to travel aboard a cruise ship, as they are also notoriously big polluters.

The Malizia II is now back in France after sailing back from New York without Thunberg.

Unless the Malizia II returns to North America she will have to find a different mode of transport back across the Atlantic when she eventually leaves.

In the meantime, she plans to promote her climate agenda across North America.

This week she will travel from New York to Montreal to join a school climate strike in Canada on what is expected to be a day of global protests.

She also hopes to visit Mexico, and is due in Chile for a climate summit in December, travelling across the Americas by bus and train.

Environment activist Greta Thunberg made a fiery appearance at the United Nations Climate Summit on Monday where she glared at Donald Trump when he walked into the UN headquarters in front of her

The 16-year-old is taking a year out of school to lobby for climate action after starting the school strike movement in Sweden last August.

She skipped school for the first three weeks after term began and then took Fridays off to spend the day outside Sweden's parliament with a sign labelled 'School strike for climate'.

Her protests have inspired wealthy donors to contribute to a Climate Emergency Fund to support the protests and the simultaneous Extinction Rebellion movement.

Its donors include heiress Aileen Getty, American philanthropist Trevor Neilson, and filmmaker Rory Kennedy, a member of the political Kennedy clan.

The Swedish teenager named this week's UN Climate Action Summit and the Chile conference in December as her main reasons for crossing the Atlantic.

She said last year she had been invited to speak around the world but refused to make the necessary flights.

But in June she said 'it looks like a large part of our future will probably be decided in New York and Santiago' and agreed to travel to North America on the yacht.

Pierre Casiraghi, the grandson of Monaco's late Prince Rainier III and American actress Grace Kelly, and fellow yachtsman Boris Herrmann offered her passage on the Malizia II.

She arrived last month after a two-week journey aboard the racing yacht.

Thunberg speaks at the UN yesterday. She hopes to travel to Chile for a climate summit in December

Thunberg is seen last Friday during the Global Climate Strike demonstration in New York where protesters took to the streets demanding more aggressive steps against global warming

Thunberg documented parts of her journey on Twitter, where she included videos of choppy weather conditions, photos of herself and the crew, and commentaries about climate change.

Last week she joined a school strike in New York City where school officials allowed pupils the day off to attend the protest.

In her impassioned address yesterday she said: 'This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean.

'Yet you have come to us young people for hope. How dare you? 'You have stolen my dreams, my childhood with your empty words.

'Yet I'm one of the lucky ones. People are suffering, people are dying, entire ecosystems are collapsing, and all you can talk about is money and fairy tales of eternal economic growth. How dare you?'

'The eyes of future generations are on you and if you choose to fail us I say we will never forgive you. We will not let you get away with this.'

Trump sent a mocking tweet, saying: 'She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!'

Fewer than half the 136 heads of government or state in New York this week to attend the UN General Assembly attended on Monday.

Ahead of the conference, the United Nations issued a release saying 66 countries vowed to achieve carbon neutrality by 2050, along with 10 regions, 102 cities, and scores of businesses.