As the doors of the 2019 Paris Air Show open to the public this weekend, cleaner transport in our skies is the challenge of our times according to the protagonists of the aviation industry.
Yet this comes against a background of growing pressure from regulators and from a blossoming environmental movement which wants us to shun air travel altogether.
The problem, according to aviation powerhouses gathered this week at Le Bourget in the Paris suburbs, is that growing world populations and economies mean that the number of planes in the sky could double over the next 20 years. And today's clean-aviation technologies aren't ready to keep up.
Here are some options already out there, and what's on the horizon.
Electric flying time
If carmakers figured out how to go electric, why can't we do it in the air, too?
Well it's primarily down to weight, altitude and storage. A mid-sized passenger plane weighs 100 times as much as a mid-sized car, and there is no battery system currently strong enough to lift a machine that size off the ground and keep it there.
So the dozens of companies working on electric planes including Boeing and Airbus are starting small.
Israeli company Eviation unveiled its 9-seat, all-electric plane named Alice at the air show, and won its first customer, US-based Cape Air. It hopes to get the plane certified and in service by 2022.
Urban air taxis may be the first widespread use of electric aviation. Uber wants to start flying them by 2023 around Houston and Los Angeles, but needs to overcome technology and certification challenges first.
Airbus Vahana Flying Taxi
Until someone solves the battery weight problem, most of the industry is betting for now on a middle-ground solution instead, combining electric power and traditional jet fuel. Analysts say some 200 hybrid projects are under development.
Some use electric technology for takeoff and standard fuel for cruising. United Technologies announced this week that it's aiming to have a "city hopper" regional passenger hybrid jet within three years that would reduce fuel consumption by 30 percent.
That's also the idea behind Voltaero, started by a former Airbus technology chief, Jean Botti.
Botti wants to see it enter in service by 2021 or 2022, and says they've had interest from operators in Scotland, Norway and Switzerland for regional jets.
Manufacturers are also looking at recycling fuels or mixing them adding synthetic fuel or renewable biofuel to kerosene to reduce its carbon footprint or using hydrogen, a far-off but increasingly talked-about option.
Paying to pollute
The industry is also looking for financial ways to ward off regulators and retain environmentally conscious passengers.
A UN-driven system called CORSIA will allow airlines to buy credits to compensate for, or "offset," their emissions. But critics say CORSIA or Carbon Offsetting and Reduction Scheme for International Aviation isn't ambitious or punitive enough.
Germany's Lufthansa is among the airlines that let customers offset their flights by calculating how much damage they are doing to the environment and paying a voluntary tax.
While the aviation industry is doing more than ever to lower emissions, at times it seems overwhelmed by the task, or unwilling to dramatically rethink the business.
Even if Europeans shun planes for cleaner high-speed trains, most US travelers don't have that option, Asia's growing middle class will increasingly take to the skies, and low-cost airlines are making air travel ever more accessible.
The airline industry has committed to halving greenhouse gas emissions by 2050 compared with 2005 levels. But that's far short of the target increasingly set by governments and backed by scientists to almost eliminate emissions by 2050.
The doors to the Paris Air Show at Le Bourget are open to the public from Friday 21 to Sunday 23 July.