G-7 in Cornwall aims to be first carbon-neutral summit. What will it take to offset all the jet fuel?

Or at least that's the idea.

Johnson's green messaging might have been undercut a bit when he arrived in Cornwall, England, via jet plane, rather than taking the slow train slog from London. It was also jarring that an aircraft carrier - a vessel that uses two large gas-turbine and four diesel engines - was brought in to serve as a backdrop for the seaside event.

Pledging to go carbon-neutral taps into the emerging trend of "sustainable events," which try to limit carbon dioxide emissions and then compensate for the overages by supporting energy-efficiency projects in the developing world.

Going big on "net zero" also plays into Johnson's pitch to make this week's G-7 in Cornwall a steppingstone toward November's huge COP26 climate conference in Glasgow, Scotland, which will seek to set tougher goals and firmer commitments to curb planetary warming.

Johnson promised the Cornwall meeting itself "will be completely carbon neutral" and "more significantly, it will be the first G-7 at which every member has committed to hitting net zero by 2050."

But the devil is the details, and the British government and its consultants have released scant information on how they will tabulate the total carbon emissions of the three-day event.

The Cabinet Office said Thursday that the British government will be responsible for mitigating the excess emissions for "official staff, leadership and delegations."

It has been silent on whether it will be addressing the carbon footprint of the Royal Navy, for example, which has not one but three warships - including its new state-of-the-art aircraft carrier HMS Prince of Wales - idling off the Cornish coast in front of the luxury Carbis Bay Hotel. The carrier needs 109 megawatts of power, enough energy to run a town the size of Swindon, England (population 155,432).

Nor does the government commit to covering the carbon tab for the 6,500 police officers standing guard, with many of them housed on a cruise ship anchored off Falmouth.

Nor, apparently, will the British government be offsetting the carbon emissions of the hundreds of journalists covering the event, and dashing about in taxis, rentals, buses and trains.

But we do know a few things about the Carbon Management Plan for the G-7 summit. According to the government, priority has been given to commissioning Cornish companies "to provide local and sustainable products for use by leaders and delegates," including recycled wooden fountain pens and reusable coffee cups.

Also, the meals served - incorporating foraged mushrooms and seaweed from the beach - will be sourced, as much as possible, from within a 100-mile radius, adhering to the popular "think globally, eat locally" diktat.

"We're using whole animals and breaking them down, using veg from farms nearby, and seafood from the harbors there," the event's guest chef, Adam Handling, told iNews.

"The menu is designed to be sustainable and zero-waste, but it's also supposed to be fun. We're using amazing ingredients, like meadowsweet and pineapple weed," he said. "We're doing an awesome velvet crab starter with roast cauliflower puree and herbs foraged from the seashore."

The extra emissions of carbon dioxide produced by the summit will be mitigated by investments in certified projects in the developing world, according to the government, including less-smoky cookstoves in Uganda, a composting facility in Vietnam and a biogas energy plant in Thailand.

"Our global projects such as hydropower in Laos will help to offset emissions generated by the gathering of world leaders and every coffee cup, pen and notepad used at the summit will be recyclable or made entirely from recycled materials," said Alok Sharma, government minister and president-designate of COP26, in a statement.

Offsetting all these extra emissions can be done, essentially, by buying carbon credits, through markets and programs run by the United Nations and other international agencies and nonprofits.

Typically, the greatest energy use for an international gathering is getting the participants there by jets.

At the G-7, Britain is hosting leaders flying in from the United States, Canada, France, Germany, Italy and Japan, for their first face-to-face discussions in almost two years, as well as delegations from the European Union, South Korea, Australia and South Africa. India's leader, Narendra Modi, is not joining in person, as he attends to a coronavirus surge in his country.

While we do not have carbon calculation for a seat on Air Force One, as an example, the German nonprofit Atmosfair estimates that a business-class, round-trip flight from Washington Dulles to London Heathrow would emit approximately 4,127 kilograms (or 4 tonnes) of carbon dioxide equivalents.

In comparison, Atmosfair calculates that driving a car for a year emits 2,000 kilograms, while the annual emissions per capita in Ethiopia is 560 kilograms. Another consultancy roundly estimates that one night in a five-star hotel equals 35 kilograms of carbon gas equivalents.

To offset that plane ride from Washington to London would set a passenger back 95 euros in carbon credits, or about $115, which would be used for one of those composing or biogas or small hydropower projects the British government is eyeballing for its mitigation.

"Calling it the most sustainable event ever? That might be a little bold and I find it unlikely," said Owen Hewlett, chief technical officer for Gold Standard, a climate mitigation think tank, who said that robust science-based emission standards are still evolving for the event sector.

But Hewlett said it was good that Britain was trying - and that it had contracted with experts and is seeking legitimate certification. He agreed that the details could be devilish and that estimating a carbon budget depends on how broadly the parameters of an event - its catering, beds, transport, security, prep work, waste and supply chains - are accounted for.

"We don't have uniform criteria for measuring yet," cautioned Fiona Pelham, chief executive of Positive Impact, a carbon emissions and event consultancy. "And transparency can be an issue, too."

She and others hoped the British government would issue a report on how well it did at the G-7 summit.