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The prime minister cannot take the train from Glasgow tomorrow because it would take too long, his spokesperson claimed.
He defended the decision on the grounds that his plane uses “sustainable” fuel – and said the emissions produced will be offset.
It comes after fierce criticism of the government for making flying and driving cheaper in last week’s Budget, widening the gulf with sky-high train fares after a decade of inflation-linked price rises.
Mr Johnson is only attending the first two days of the two-week summit, before returning to Westminster for Prime Minister’s Questions in the Commons on Wednesday.
Asked to justify the decision to fly back, his spokesman said: “The fuel we use for the flight is sustainable and the emissions are offset as well.
“It is important that the prime minister is able to move around the country and we have obviously faced significant time restraints.”
The spokesman did not set out the nature of the “time constraints” that prevent Mr Johnson from taking a four-and-a-half-hour train journey on Tuesday afternoon.
He added that the charter plane to be used emits only half the carbon dioxide of other aircraft, but was unable to say what type of plane it was.
However, critics of flying internally – in a small country such as the UK – point out that a plane still emits seven times the carbon dioxide, per person, than a train journey.
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Mr Johnson has refused to set out what personal sacrifices – if any – he is making to cut his own carbon footprint, declining to say if he is eating less meat for example.
Just hours earlier, setting out the Cop26 challenge, he told the opening ceremony: “The people who will judge us are children not yet born – and their children. We mustn’t fluff our lines or miss our cue.
“If we fail they will not forgive us. They will judge us with a bitterness and a resentment that eclipses any climate activist of today. And they will be right.”
Explaining the flight, the spokesman added: “The plane is one of the most carbon-efficient planes of its size in the world. It produces 50 per cent less CO2 emissions than, for example, the larger Voyager plane [often used by Mr Johnson].
“We use a specific type of fuel which is a blend of 35 per cent sustainable aviation fuel and 65 per cent normal fuel – which is the maximum amount allowed – and, obviously emissions, will be offset.”
Denying the charge of “hypocrisy”, the spokesman said the UK is “leading the way in the in the commitments needed and investment required in order to get to net zero”.
Meanwhile, chancellor Rishi Sunak defended his decision to cut air passenger duty on domestic flights on Monday, claiming it would be “offset” by raising the duty on long-haul flights abroad.
Grilled by MPs on the Treasury Committee, the chancellor dismissed the idea of using aviation taxes to help achieve net zero – arguing that investment in sustainable aviation fuel was more important.
“I assume people will be flying for years to come all over the place,” Mr Sunak told MPs.
“The best way to solve emissions from that problem is trying to figure out what a sustainable aviation fuel looks like. The duties are not, one way or another, going to do anything.”
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