(CNN) — Gleaming airplanes have been returning to the empty blue skies above 30,000 feet in recent months, opening up travel possibilities as they skim the clouds on their way towards unfiltered sunshine.
On board, safely behind face masks, many passengers have been shrugging off the old discomforts of plane travel in their excitement at being in the air again. Airline food has never tasted so good.
And yet, something is leaving a bitter taste. The world is a changed place since we were last exploring it, and one of the main changes is the heightened concern we have for the damage we've been causing to it through activities such as flying.
Aviation generates 2.8% of global CO2 emissions and, even before the recent COP26 climate talks, has become a lightning rod for the "flight shame" movement.
On the heels of the climate summit agreement by 200 countries to slash greenhouse emissions by 2030, the industry has been steeling itself for a crisis as more and more people raise concerns over the impact of air travel.
Numerous airlines have already pledged to offer carbon-neutral travel and explore alternative fuels to reduce pollution. Developments in electric airplanes, though still a distant possibility for long-haul flight, have also raised hopes for green air travel.
Net-zero carbon emissions by 2050
So how close are we to guilt-free and genuinely low-impact flying?
It's definitely a priority for the industry. At its annual meeting in October, IATA, the International Air Transport Association, rubber stamped a resolution in support of net-zero carbon emissions by 2050.
That's a tall order. It's anticipated that 10 billion people will be flying annually by 2050, which by today's emission standards means that the aviation industry will need to deal with a cumulative total of 21.2 gigatons of carbon over the next three decades.
IATA reckons that figure won't be quite so high as some emissions will be mitigated through the adoption of cleaner energy sources and better airplane design.
"A potential scenario is that 65% of [carbon] will be abated through sustainable aviation fuels," Willie Walsh, the former British Airways boss who is now IATA's director general, said in a press release.
Sustainable fuels, made from materials such as wood residue deposits, tobacco or sugarcane, can already be used in many of today's newer jet engines without any modification.
Walsh says new propulsion tech, including hydrogen, will take care of another 13% while efficiency improvements will account for a further 3%. The remainder, he says, could be dealt with "through carbon capture and storage and offsets."
Sustainable aviation fuel
Airlines seem to be on board. British Airways' parent company IAG plans to power 10% of its flights with sustainable aviation fuel (SAF) by 2030, and says it is investing $400 million over the next 20 years into fuel development.
In the Gulf, Abu Dhabi-based Etihad Airways has collaborated with Khalifa University, Boeing, and jet engine maker SAFRAN on producing sustainable biofuel from salt-tolerant halophyte plants that can be grown in seawater.
In June 2021, London Heathrow became the first UK major airport to successfully integrate SAF into its fuel distribution system, with a trial supply of SAF to power between 5-10 short-haul flights.
Over its life cycle, sustainable aviation fuel reduces up to 80% of greenhouse gas emissions compared to fossil jet fuel use. And that's why its deployment is key to reaching the 2050 targets.
So what's the downside? A key one is price. Sustainable jet fuel costs around three times as much as its fossil-based counterpart. In the pre-Covid era of 2019, less than 200,000 metric tonnes of SAF were produced globally -- a tiny fraction of the 300 million tonnes of jet fuel needed by commercial airlines in a normal year.
Post-Covid, air travel is already expected to get more expensive due to uncertain demand. Green air travel is likely to be even more costly, which could lead to slower uptake.
The value of offsets
Until less harmful fuel becomes commonplace, fliers can still try to mitigate the impact of their journeys through offsets -- schemes that compensate for carbon footprint by funding emission-reduction projects such as tree planting, wind farms or methane capture.
Several airlines operate their own. Cathay Pacific says its Fly Greener program has offset over 160,000 tonnes of CO2 since its launch in 2007 -- the equivalent of 30 million taxi journeys between Hong Kong International Airport and downtown.
No doubt, offsets bring societal benefits and help reduce the problem, but ultimately they're a form of accountancy -- they don't actually cut the amount of carbon spewing out of the back of jet engines.
The Compensaid platform developed by the Lufthansa Innovation Hub is trying to help passengers play a more direct role in making airplanes operate with less CO2. It's set to become SWISS and Lufthansa's prime carbon offsetting option.
As well as offsetting their emissions via investment in climate protection projects, SWISS travelers can also reduce their CO2 emissions by purchasing sustainable aviation fuel (SAF).
While SAF is expected to do the heavy lifting in aviation's green revolution, alternative technologies are developing at pace, especially for the sub-1,000-mile range flight market.
Europe's biggest airplane maker, Airbus, is betting on hydrogen to change aviation's trajectory for short- and medium-haul flight.
In a high-profile pivot last year, Airbus switched from a strong focus on the development of small electric planes to a new initiative exploring the potential of hydrogen.
It revealed a trio of hydrogen-powered zero-emission airliner concepts, under the banner ZEROe, which could enter service by 2035.
There are still plenty of obstacles to the widespread adoption of hydrogen. For one thing, airports don't have the infrastructure to store and deploy it.
LA-based startup Universal Hydrogen has a solution to that. It's developing a fuel distribution network that means hydrogen can be delivered in modular capsules straight to the airplane by freight.
It's also designing conversion kits that can be retrofitted to existing regional airplanes.
"We see the near-term decarbonization of regional aviation as a first step and catalyst, setting the whole industry on a path to meeting Paris Agreement emissions targets," Paul Eremenko, Universal Hydrogen co-founder and CEO, tells CNN Travel.
Then there's the hybrid model.
Ampaire Inc, another LA-based player, is developing hybrid electric systems for existing commuter airplanes in the nine to 19-seat category, such as the Cessna Grand Caravan and Twin Otter.
Tens of thousands of such aircraft would be suitable recipients for the upgrades -- in fact, the Union Bank of Switzerland estimates the global value for hybrid-electric aircraft could be worth $178 billion by 2040.
Ampaire recently flew its Electric EEL technology demonstrator (a modified six-seat Cessna 337 Skymaster) on a potential airline route from the Orkney Isles to northern mainland Scotland.
The Electric EEL runs on a combination of battery power and a conventional combustion engine, thereby reducing emissions and operating costs by as much as 25%.
It has the potential to change the economics as well as the ecology of regional aviation.
"Hybrid-electric aircraft achieve two objectives," explains Susan Ying, Ampaire's senior vice president for global operations Susan Ying. As well as hitting environmental targets, "They can also make current routes more profitable while lowering fares and strengthening connectivity."
A rendering shows what the Alice aircraft would look like in flight.
Another approach to cleansing the skies comes from Washington State-based Eviation Aircraft which recently unveiled the production version of its nine-passenger all-electric Alice aircraft, which produces no carbon emissions.
The aircraft, which has a range of 440 miles, is intended for feeder routes and also comes in a cargo version -- DHL Express has ordered 12 slated for service in 2024.
While energy sources are still evolving, UK-based Faradair Aerospace is currently working on a design that will squeeze the maximum efficiency out of whichever fuel prevails.
Its 18-passenger BEHA aircraft, made from lightweight composite, can carry a five-tonne payload, operate from short runways, and has a 1,150-mile range.
It's quiet too, driven by contra-rotating pusher propfans, and has a solar panel for "always on" cabin ground power.
Company boss and founder Neil Cloughley tells CNN Travel that a first flight potential for 2024/25 is possible, followed by certification for commercial operations by 2027.
The plan, he says, is to use a Honeywell turbo-generator which is based on the Airbus A350 Auxiliary Power Unit (the A350's onboard electricity generator) as the core power source.
This provides the "lower emission and noise benefits of replacing twin turboprop operations with all of the operational costs savings of electric motor propulsion, powered by a single efficient generator."
That means, it won't require significant reconfiguration to adapt to different fuel types -- smoothing the path for regulatory approval on adoption of improved systems.
This, Cloughely says, will "future-proof the asset's lifespan, providing longevity of operations without obsolescence threat for the near future."
In an uncertain world with fuel prices having never been so erratic, keeping energy options open could indeed be a smart strategy for the aviation sector as it strives for carbon neutrality.
For fliers, anything that brings true zero carbon flying closer is likely to be welcome, but as they continue to head up into those blue skies, it's clear that the promise of impact-free air travel is still a long way off.
Paul Sillers is an aviation journalist specializing in passenger experience and future air travel tech. Follow him at @paulsillers