Flying is the most carbon-intensive way to travel - producing greenhouse gases that warm the atmosphere, contributing to global warming and climate change.

Climate advocates Thrust Carbon say the potential 3.4 tonnes of CO2e emissions one England fan could generate travelling to every game is equivalent to the production of 34,000 plastic bags, or heating an average UK home for 19 months.

Dr Stuart Parkinson - from Scientists for Global Responsibility (SGR) - described the BBC's estimates as "deeply disturbing".

"For an England fan… generating 3.4 tonnes of CO2… that's equivalent to two to three times the whole year's emissions of an average person in a poorer country such as Haiti," he said.

Parkinson pointed out such levels of aviation emissions come as the world accelerates towards going beyond the Paris agreement to limit global temperature rises to 1.5C.

"It makes no sense when we look at the impacts of climate change and we're just shooting past climate targets," he said.

"We can't really afford those sort of emissions… it's untenable for the planet."

Scotland supporters wanting to follow their team would travel fewer miles in the group stage than their English counterparts, but they would also rack up significantly bigger distances if the team progress.

Members of the Tartan Army would travel 1,258 miles between host cities during the group stage.

Should they win the group and reach the final - and factoring in a return flight to Edinburgh before and after the tournament - that would take them beyond 12,420 miles, generating about 2.8 tonnes CO2e.

Reaching the final having finished runners-up would increase that to 13,771 miles - about 3.3 tonnes CO2e

Tens of thousands of Scotland fans are expected to make the trip to the US as they return to the tournament for the first time in 28 years.

Paul Goodwin - co-founder of the Scottish Football Supporters' Association - said awareness among fans of environmental impact was growing.

"As a Scotland fan, you really want to be there. You can't just not travel," he said. "And I think lots of fans do feel conflicted."

Goodwin said the SFSA was working on measures to educate fans about sustainability.

"We recognise that football has a responsibility when it comes to the environment," he said.

"We're working on plans to make sure the Tartan Army does recognise the impact they're having and take that responsibility seriously.

"It's about making sure that you can minimise the impact you're making and from a negative produce a positive."

In a lengthy statement to the BBC, Fifa said it recognised climate impact must be taken seriously and welcomed "informed scrutiny".

"Fifa also recognises that air travel is a significant contributor to the overall footprint of any major event, and it is clear that reducing emissions linked to flights is one of the biggest sustainability challenges major event organisers face," it said.

A spokesperson went on to outline "numerous" environmental initiatives related to the tournament implemented by Fifa and its host cities, including:

Fifa also said work was ongoing to "promote recycling and reduce food waste at key tournament sites", while a tree-planting initiative was about to begin across North America.

There will be 48 teams at the 2026 World Cup, and travel distances will vary depending on group location and knockout pathways.

While Fifa has regionalised some group-stage fixtures to limit coast-to-coast journeys, deep runs into the knockout rounds are likely to generate travel distances well beyond 12,000 miles for some fanbases.

The biggest group-stage travel burden will fall on the Uefa play-off qualifier in Group B, which could be Wales or Northern Ireland. Fans travelling to all three group games - in Toronto, Inglewood and Seattle - face clocking up more than 3,140 miles.

When return flights from the competing country are included, South Africa fans face the largest travel burden - both for the group stage and an admittedly unlikely run to the final:

The associated emissions for those flights are 4.7 tonnes CO2e per fan for the group stage, rising to as much as 5.9 tonnes CO2e per fan in the group-runner-up-to-final scenario.

The average South African's expected CO2 emission for a whole year is 5.8 tonnes CO2e, according to the International Energy Agency.

The seeded team with the greatest projected travel burden for fans is Germany:

At the other end of the scale, France's group travel - excluding international flights - is estimated at just 370 miles, and train travel is possible between two of their host cities.

A 2025 report from the SGR estimated the overall footprint of the 2026 World Cup could reach nine million tonnes of CO2e.

SGR says that is almost double the average for the past four World Cups and will make it the most polluting tournament ever staged.

"That's larger than many of the world's countries... equivalent to more than six million British cars being driven for a year," said Parkinson.

Aviation accounts for the overwhelming majority - about 80-90%, according to Parkinson - of a World Cup's carbon footprint.

"Minimising air travel is one of the most effective ways of reducing your carbon footprint," he said.

Parkinson believes Fifa's decision to increase the size of the World Cup "is thoroughly undermining any declared commitment to tackle climate change".

"They need to be looking at shrinking the tournament, not expanding it," he said.

That feels unlikely.

The 2030 World Cup will be jointly hosted by Morocco, Portugal and Spain, and - because the tournament will be celebrating its centenary - three matches will be played in Uruguay, Argentina and Paraguay.

In 2034, the tournament will take place in Saudi Arabia and will feature 11 stadiums - none of which have been built.

Qatar 2022 was the most compact World Cup ever staged, with minimal internal travel and centrally located stadiums accessible via public transport. Teams were often based in one location and fans could attend multiple matches in a day.

Previous large-nation hosts such as Brazil (2014) and Russia (2018) naturally meant longer distances were travelled - in some cases several thousand miles just in the group stage. They were, though, single-country tournaments with fewer teams (32) and therefore fewer games.

BBC Sport calculated the distances between host cities using the most local airport and assumed fans of competing countries would fly directly to the first and from the last game of the tournament where a direct international flight was available. If a direct flight wasn't available, the shortest available indirect route was used.

Train journeys for shorter routes - for France fans travelling between East Rutherford and Philadelphia, for example - were also factored in.

We used an air miles calculator, external for flights. Emissions were calculated based on the average passenger and weighted according to UK government greenhouse gas conversions., external

There are different calculations for CO2 per mile depending whether the flight is domestic, short haul to or from the UK, long haul to or from the UK or an international flight between two non-UK destinations.

The results are an indicator of air miles, travel and emissions only. Real-world totals vary depending on aircraft type, passenger load, seating class and routing.

Additional reporting by Dale Johnson