As the number of extremely wealthy people worldwide has grown, so too has the market for private flights. Despite global warming being one of the major issues discussed at Davos every year, some attendees will be arriving at the World Economic Forum in Davos, Switzerland by private jet.

Some 1,500 private jets are expected this week, according to an estimate from Air Charter Service, up from 1,300 last year, according to Air Charter Service (ACS), a private-plane operator. “We have had bookings from as far as our operations in Hong Kong, India and the U.S.,” Andy Christie, private jets director at ACS, said in a statement. “No other event has the same global appeal.”

PrivateFly.com, a private-jet service, said it expects between 1,200 and 1,500 private jets in and out of local airports around Davos, double the average daily volume of flight traffic to the area. “The primary airport used by private jet customers flying to Davos is Zurich Airport. This is 148 kilometers away, or 1 hour 45 minutes by road,” the company said.

World Economic Forum disputes those estimates

Oliver Cann, head of strategic communications at the World Economic Forum, disputes those estimates. “How many private jets are going to Davos in 2019? It’s a question that always gets asked. And so it should,” he wrote in a blog post on Wednesday. “From an environmental perspective, taking a private jet is the worst way to travel to Davos.”

“ ‘We have been offering incentives to participants to use public transport for some years.’ ” — —Oliver Cann, head of strategic communications at the WEF

“It’s also a complicated question, as the data is not always easy to determine,” he wrote. “For example, air traffic authorities use a metric called ATM, or Air Traffic Movements. A normal flight would be two ATMs, however during busy times, when parking is not available at the airport, planes are required to take off again and park at another airstrip nearby. This adds a further two legs to the journey.”

In 2019, there were 350 movements of fixed-wing aircraft between Jan. 20 and Jan. 22, a 32% decrease from 462 for the first three days of the meeting last year. There were 136 ATMs at St. Gallen-Alltenrhein this year, up 46% from 93 last year for the same period, he wrote. “We have been encouraging participants to use this airport to reduce congestion on the roads,” Cann added.

Still, use of private jets for the first three days of the Davos show an “encouraging” annual decrease of 14%, he added. “We have been offering incentives to participants to use public transport for some years,” Cann wrote. “We also ask that they share planes if they have to use them; something that has been gaining popularity in recent years.”

Air travel comes with a hefty carbon footprint

Air travel comes with a hefty carbon footprint, generating greenhouse gas emissions that contribute to climate change. Davos attendees will have an opportunity to discuss that issue in depth, with “Safeguarding our Planet” the title of one of the panel sessions on Tuesday.

“ ‘What we do in the next few years will profoundly affect the next few thousand years.’ ” — —Naturalist Sir David Attenborough, speaking at the WEF this week.

“The World Economic Forum Climate Initiative provides a global platform to help raise ambition and accelerate climate action,” according to the WEF. “Public- and private-sector collaboration is essential to create a marketplace that will enable dramatic reductions in emissions and build resilience.”

In an interview with Prince William, 36, who is second in line to the British throne, the 92-year-old naturalist Sir David Attenborough told attendees at the forum on Tuesday, “There is more power in this room than any gathering anywhere. The people here need to do something about the natural world.” In a separate speech earlier this week, Attenborough said, “What we do in the next few years will profoundly affect the next few thousand years.”

Many rich people take shares in private jets

The world’s most elite aren’t necessarily fueling demand for private jets. The lion’s share of growth in private aviation is not due to people buying their own planes, according to a 2018 report from VistaJet, a private jet charter company, and Wealth-X, a firm that specializes in researching ultra-high net worth individuals.

“ Rich people are turning to more economical means of private air travel such as fractional ownership of planes and jet-card membership programs. ”

Instead, rich people are turning to more economical means of private air travel such as fractional ownership of planes (where you split the costs and title to the aircraft with a small group of other users almost like a timeshare), jet-card membership programs (where travelers typically pre-purchase a package of flight hours on a specific model of plane to be used by a certain date), and on-demand chartering (think an Uber-style service for private planes where you can book one on short notice).

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In the U.S., the number of flights by fractionally-owned aircraft increased 4.7% between 2016 and 2017, but the number of on-demand charter flights and flights through jet card memberships rose 9.2% over that same period, according to ARGUS International, an aviation technology firm.

Luxury travel expert Doug Gollan, who is also editor of travel website Private Jet Card Comparisons, estimates that 25,000 to 30,000 people hold jet card memberships, versus 8,000 to 10,000 people who are fractional owners of private jets.

“Jet card memberships are the sweet spot of the private aviation market with the number of providers and programs having more than doubled in the past decade against a backdrop of lower deliveries of new aircraft and a static market for fractional ownership,” he said.

(This story was updated with figures from the World Economic Forum.)