Cargo ship

The most eco-friendly option would be travelling by cargo ship, according to Dr Roger Tyers, Research Fellow in Environmental Sociology at the University of Southampton.

Between 2007 and 2012, shipping accounted for about 3 per cent of the world's carbon emissions, according to the International Maritime Organisation.

But joining a ship on its scheduled journey would have a negligible effect on the vessel's emissions, Tyers said.

Dr Tristan Smith of University College, London, said several shipping companies offered berths for passengers.

"This used to be quite a common means of travel," he said, "so many lines have plenty of experience of offering this option."

Racing yacht

Thunberg travelled to New York for the United Nations General Assembly on the Malizia II, a state-of-the-art, zero-emissions racing yacht.

The vessel was fitted with solar panels and underwater turbines to produce electricity, and took nearly 14 days to reach New York.

But the crossing was planned for months, and the yacht required two experienced sailors to make the journey safely.

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It is unclear whether another crossing could be organised at such short notice. If crew were required to fly to New York to help Thunberg make the journey, it could negate the emissions savings from travelling in the boat.

Rowing boat

If the yacht is unavailable, Thunberg could consider making the 4800-kilometre journey by rowing boat.

The journey was first completed by Frank Samuelsen and George Harbo, who rowed from New York to Britain's Isles of Scilly in 1896.

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Ohio teacher Bryce Carlson completed the feat in 2018 in 38 days, 6 hours and 49 minutes, setting a new speed record for a west-to-east solo and unsupported crossing of the North Atlantic, The New York Times reported.

If she wanted some company, Thunberg could join the participants of the annual Talisker Whisky Atlantic Challenge, who will be rowing from the Canary Islands to Antigua and Barbuda in early December.

Flying

The activist vowed to make her journey without flying, but it may be the only way to arrive in time.

Thunberg could offset her flight emissions using one of several schemes offered by major airlines, but Tyers, the academic, said she should be wary. "The problem with carbon offsets is that they can create a kind of moral licence for us to maintain our polluting lifestyles," he said.

The best carbon offsetting schemes are the least photogenic, according to Tyers. Rather than donating to tree-planting initiatives, travellers should fund programs such as those capturing methane gas from landfill sites and abandoned mines.

Swimming

Numerous people claim to have swum across the Atlantic.

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In 1998 French swimmer Beniot Lecomte swam from Massachusetts to France. He claimed to be the first man to swim across the Atlantic without a kickboard.

In 2009, the American swimmer Jennifer Figge became the first woman on record to complete the feat after crossing the ocean in 24 days.

But Thunberg would need to be escorted by a boat to protect her from sharks and give her somewhere to rest. With the emissions of a full boat behind her, this option might not be as eco-friendly as it first appears.

Reuters