- Climate change
image sourceMaggie Robertsonimage captionMaggie Robertson used to fly to Edinburgh, but she has switched to the train
For Maggie Robertson, it was a long-haul flight to Texas that changed her mind about flying.
It was 2017 and she was having a great holiday. But then Hurricane Harvey came along – and she and her family narrowly sidestepped floods that cost more than 100 lives.
"That brush with natural disaster helped put things in perspective," she says.
Previously a regular flyer, visiting friends in Scotland and holidaying abroad, she says the penny dropped during that trip. And in the end, the decision was easy.
"It was a relief to say I'm not doing it any more," she says. "I knew that what I was doing wasn't consistent with what I thought was right."
She is one of a small band of people who have found flying just too uncomfortable to contemplate any more.
Many more people are still boarding the planes, but wrestling with a growing sense of shame.
They now feel that their desire for a holiday in the sun or a far-flung adventure is playing a small but undeniable part in the growing crisis of extreme weather events, rising sea levels and melting polar ice.
Flying is only responsible for around 2% of global emissions. That may not sound much, but if you are a flyer, it's a much higher proportion of your own carbon footprint. That's because more than 80% of the world's population never fly at all.
One flight from London to New York emits around 1,360kg of carbon. Even if you eat vegan and cycle everywhere, you'd struggle to make up for the emissions from a return trip.
Those are the kind of calculations that persuaded Maggie Robertson to sign up to Flight Free, an online club, where members pledge to avoid flying for a year at a time.
Anna Hughes, its founder, says she doesn't want to make people feel guilty about flying, but she would like them to be more aware.
"We're not suggesting that flying should go back to being the preserve of the rich. But we should definitely start to see it as once-in-a-while, if it is necessary, not just to go to Prague for a stag do," she says.
Yet last month, the government told us it was fine to keep on flying.
Saving the planet wasn't about stopping doing things, it was about doing them differently, said Transport Secretary Grant Shapps, announcing the government's plan for decarbonisation. He raised the prospect of super-efficient aircraft, running on hydrogen, batteries and sustainable fuel.
That's the view of the broader aviation industry, that flying is becoming cleaner and greener, thanks to the march of technology.
"The industry has got a clear plan to decarbonise aviation, and there are a lot of opportunities and proven technologies that can do that," says Andy Jefferson, programme director at an organisation called Sustainable Aviation, which works on behalf of the government and the industry, including manufacturers and airports.
"Part of the solution will be the evolution of existing types of aeroplanes, as each time they come online, they're in the region of 15% to 20% more efficient and better than the aircraft they're replacing," he says.
In addition, things such as making sure a plane doesn't carry more fuel or water than it needs, not maintaining planes in holding patterns as they wait to land, and keeping the engines well-maintained to ensure they operate efficiently, can all make flying greener.
In the longer run, aircraft could switch away from using fossil fuels altogether, says Mr Jefferson.
"In terms of battery electric aircraft and hydrogen-powered aircraft, the industry has already got two-seater, five-seater type aircraft flying today," he points out.
media caption"It flies cleaner and is more efficient" – Hybrid-electric plane trialled over Scotland
By the mid-2030s, Mr Jefferson believes we could be taking holidays to the Med in that kind of aircraft or using sustainable fuels from waste materials or crops.
So does that mean people can fly with a clear conscience?
The trouble with the message, "Carry on with your lifestyles, we're solving it," says Anna Hughes, is that it is rather "promise-heavy".
In the past, efficiency gains from new planes and better flight management have been swallowed up by big increases in the volume of air traffic, so that aviation as a whole is still emitting more than ever.
And many experts question industry optimism about how quickly renewable fuels will become mainstream.
In the meantime, say environmentalists, if you fly, you are contributing to the build-up of carbon in the atmosphere that causes global warming. So short-term carbon savings are just as important as hitting longer term goals.
"I think it's misleading for the consumer to say, 'Keep on flying, It's OK.' It's not giving the whole picture," says Ms Hughes.
image sourceGetty Imagesimage captionMany young Swedes are already turning their back on flying after "Flygskam" or flight-shame campaigns
So what about offsetting? Is that the answer for travellers who want to compensate for the impact of their flight?
There are hundreds of offsetting schemes around the world, offering passengers the opportunity to pay towards climate-friendly projects such as protecting the rainforests, installing renewable energy and distributing carbon efficient cookstoves in poor African communities.
But offsetting is controversial. In fact, Mike Childs, Friends of the Earth's head of policy, describes it as "the greatest con on Earth".
- Should I offset my summer holiday flights?
"They're trying to pretend that your emissions don't count because you can offset, but the offsets aren't real, they don't last, they're not permanent."
He suggests that, instead you use the offset money to take a train next time or donate to your local nature reserve.
image sourceAtmosfairimage captionCarbon offsetting schemes include giving people in African countries more efficient stoves
On the other hand, he doesn't think individuals should necessarily feel racked with guilt every time they board a plane.
"We have got a downer on flying, but we're not saying nobody should ever fly at all," he says.
"We don't want to attack people who go on holiday once a year, who perhaps can't afford to go by train or don't have the time to go by train."
Rather, he thinks it is up to governments to make change happen, by taxing aviation fuel, making long-distance train travel more affordable and putting in place policies to discourage frequent flying.
"We can spend our whole life feeling guilty or we can do what we can to try and minimise [our impact]," Mr Childs says.
"Some people will be carbon angels and rule out absolutely everything, but we also have to live in the real world and recognise that other people can go so far."
Maggie Robertson doesn't see herself as a carbon angel. But she can't imagine being able to fly with an easy conscience again any time soon.
She volunteers for Flight Free and has already managed a pre-pandemic summer holiday in Switzerland by train.
"Low-carbon flying will be important for people who have to fly, but I'm not optimistic that it is going to transform things to the point where we can all fly as much as we want," she says.
Even if it it can be achieved in the medium-term, she adds, it won't remove the emissions of the flying we do now, "so it's not an excuse to carry on as normal."