Prince Harry burned 0.3 tons of CO2 for each minute he visited King Charles, it has been claimed.
The Duke of Sussex has returned to the US to be with his wife and children, after a whistlestop 24-hour trip to the UK.
Harry, 39, flew to London Heathrow after being informed his father, King Charles, had been diagnosed with a form of cancer.
He originally moved to the US in 2020 with his Californian wife, Meghan Markle, and their son Prince Archie.
Prince Harry came to the UK for 24 hours
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The Duke and Duchess of Sussex later welcomed a daughter, Princess Lilibet, who was born in June 2021.
On Tuesday morning, Harry arrived in the UK after boarding a Boeing 777 plane at LAX Airport.
According to a CO2 emissions calculator, a roundtrip for a flight from LAX to Heathrow would burn 13.5 tons of emissions, which would equate to 0.3 tons for each of the 45 minutes he spent with his father at Clarence House.
The 17,500 km trip takes roughly 11 hours, and Harry's first flight was reportedly delayed by 38 minutes.
King Charles and Prince Harry had a 45-minute meeting at Clarence House
PAPrince Harry was understood to have spent 45 minutes with his father when he was in the UK.
The duke travelled immediately to Clarence House where he was greeted by King Charles and Queen Camilla.
Although many royal fans had hoped for a reunion between Prince William and Prince Harry, the Prince of Wales was understood to have "no plans" to see his younger brother.
The King, 75, travelled to Sandringham via helicopter immediately after seeing his younger son, who he had not seen since the Coronation.
Prince Harry took a British Airways flight to London Heathrow
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Queen Camilla and King Charles pictured straight after meeting Prince Harry
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Meghan Markle, 42, stayed in the US with Archie and Lilibet.
Prince Harry and Meghan are scheduled to make a joint appearance next week at an event to commemorate a year until the Invictus Games in Vancouver and Whistler.
The duke founded the Invictus Games as a working member of the Royal Family and has continued to champion the sporting event despite stepping back from duties in 2020.