Campaigning by climate activist Greta Thunberg and filmmaker-naturalist David Attenborough is persuading pollution-conscious fliers to try and mitigate the environmental damage caused by their flights.

Sales of so-called carbon offsets are soaring: Myclimate, a Swiss nonprofit whose clients include Deutsche Lufthansa AG, reported a five-fold uptake in its credits in a year. At Ryanair Holdings Plc, Europe’s largest discount carrier, the number of customers making voluntary offset payments has almost doubled in 18 months.

This summer’s heatwaves have boosted sales. Europe has seen rivers dry, temperature records fall and sporting events canceled in heat scientists blame on man-made global warming. The offsets may offer a salve to the emerging “flight shame” anti-flying movement spreading from Sweden, home of 16-year-old Thunberg, who recently said she’s crossing the Atlantic by sailboat to attend a United Nations climate summit in New York.

“There’s always some extreme weather story in the news,” said John Buckley, managing director of Carbon Footprint Ltd., an offset provider that has seen demand quadruple this year. “We’re now getting a higher level of interest than we’ve ever had before. It’ll put money back into the system, which will create more projects.”

What is a carbon offset? How much do they cost?

Carbon offsets are certificates that mitigate a passenger’s flight emissions by reducing greenhouse gases elsewhere in the world. The money passengers pay on top of their ticket goes to low-carbon or clean energy projects such as planting trees, installing solar panels or handing out cleaner cooking stoves.

Travelers face a dizzying array of options. Some airlines offer offsets directly when you pay for your ticket, while dozens of online companies advertise personal certificates tailored to your flight.

Prices vary widely, too, ranging from 10 cents per ton of carbon dioxide to more than $70, depending on the offset provider and the project being funded, according to Forest Trends Association, which provides data on voluntary carbon markets.

For example, on the website of Atmosfair, a German nonprofit, offsetting a one-way flight from London to New York can cost from $40 to $93 depending on the airline. Such a trip emits about half a ton of carbon emissions per passenger.

Verra, the biggest program for voluntary credits globally, has seen the monthly retirement, or usage, rate for offsets jump about 23% this year to 3.8 million tons a month. When a credit is used, it’s retired or cancelled. They aren’t used again.

Are carbon offsets here to stay?

Putting a value on the offsets is difficult because different projects cut greenhouse gases in a wide variety of ways. Global voluntary offsetting transactions, which include at least some flight credits, reached a value of about $200 million a year, according to a first-quarter 2018 survey by Forest Trends.

While public demand may be growing, it’s still from a very low base. Airline uptake for carbon-offsetting remains below 2% on average, though that figure doesn’t include third-party providers, the International Air Transport Association said by email. At Ryanair, fewer than 3% of customers are buying credits with the biggest portion from Germany.

Airlines Clash Over CO2 With Industry Vilified in Climate Debate

Airline emissions are beginning to be tackled by policymakers. The International Civil Aviation Organization is developing a global carbon market, while the U.K. is considering requiring airlines and other forms of transport to offer prices that include the cost of emissions. The British plan would allow consumers to opt out in order to pay a lower price.

The option to offset air-travel emissions has become public discussion, said Anne Thiel, communications manager for Washington-based Verra. While media attention and European heatwaves may have boosted offsetting, the yearly increase may be due to a more general growing appreciation of the problem, she said.

–With assistance from Christopher Jasper, Jesper Starn and Niklas Magnusson.

©2019 Bloomberg L.P.

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Photo Credit: A (carbon-emitting) flight prepares to land in Hong Kong. Anthony Kwan / Bloomberg