LONDON — British Prime Minister Boris Johnson is promising to host the first "carbon-neutral" summit for the Group of Seven this week — seeking to slash the emissions of greenhouse gases by sourcing the crab and lamb locally, deploying ­generators powered by hydro-treated vegetable oil and offsetting all that international jet travel by building a composting facility in Vietnam.

Or at least that’s the idea. The summit is going to emit tens of thousands of tons of carbon dioxide. The final bill is still being tabulated.

Johnson’s green messaging might have been undercut a bit when he arrived in Cornwall, England, via jet plane, rather than taking one of the government-chartered trains 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 all-in 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 that the Cornwall meeting itself “will be completely carbon-neutral” and that, “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 in the details, and the British government has released scant information on how it 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.”

Reached on Friday, Natasha Connolly, associate director of sustainability for Arup, the consultancy that is doing the emission tabulations, said the government is trying to account for all the carbon that the summit will expend.

The Royal Navy has brought not one but three warships — including its new state-of-the-art aircraft carrier, HMS Prince of Wales — to idle 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). Connolly could not immediately say how much carbon the carrier burns, but she promised that its emissions would be included.

The British government has also committed to covering the carbon tab for the 6,500 police officers standing guard, many of them housed on a cruise ship anchored off Falmouth.

Also included will be the offset of the carbon emissions for the hundreds of journalists covering the event, and dashing about in taxis, rental cars, buses and trains.

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.

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” mantra.

“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,” Alok Sharma, government minister and president-designate of COP26, said in a statement.

The extra emissions can be offset, 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, said Connolly of the consultancy Arup. She said charter flights carrying world leaders clearly are more polluting per person than an economy row on a discount carrier.

“But the impact of these leaders will be offset by the huge decisions they will make for the climate,” she said. Connolly could not say Friday how much carbon the delegations will burn through, but she said she believes that the British government will release a full report in the future.

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.

Although a carbon calculation for a seat on Air Force One, as an example, is not available, the German nonprofit Atmosfair ­estimates that a business-class, round-trip flight from Washington Dulles Airport to London Heathrow would emit about 4,127 kilograms (4.5 tons) of carbon dioxide equivalents.

In comparison, Atmosfair calculates that driving a car for a year emits 2,000 kilograms (2.2 tons), while the annual emissions per capita in Ethiopia is 560 kilograms (0.6 tons). Another consultancy roundly estimates that one night in a five-star hotel equals 35 kilograms (77 pounds) 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 composting or small hydropower projects that 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. Robust science-based emission standards are still evolving for the event sector, he said.

But Hewlett said it was good that Britain was trying — and that it has contracted with experts and is seeking legitimate certification. He agreed that the details could be devilish and said that estimating a carbon budget depends on how broadly an event’s parameters — 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 said they hope the British government issues a public report on how well it did at the G-7 summit.