More fuel-efficient planes have halved commercial flights’ emissions over the past 30 years–and we are partnering with NASA to develop and test a full-scale demonstrator airplane that could inform the designs of future airplanes to be as much as 30% more fuel-efficient.

However, achieving net zero in the next 28 years will also require rethinking which non-fossil energy sources should power the aircraft itself.

The problem

When it comes to changing airplanes’ energy carriers, safety and the laws of physics rule. Finding renewable energy alternatives with the same energy density and volumetric efficiency as fossil fuels is difficult.

Today, flights over 1,000 miles account for 80% of the industry’s emissions. New batteries will enable electric aircraft to be developed for short-haul travel–but batteries weigh too much for longer flights.

Many challenges must be overcome before hydrogen-powered long-haul planes can safely complete flights, refuel, and leave for their next destinations within an hour as most do today.

Loading the smallest molecule in the universe into aircraft is tricky. Due to their small size and required cryogenic conditions, hydrogen molecules can leak through minute pores of welded seams and be absorbed into metal, since hydrogen and cooler temperatures can make metal brittle. Airports will need new infrastructure and training to safely load and handle the frigid fuel that must be chilled at -423 degrees Fahrenheit.

Storing hydrogen poses equally complicated challenges. Planes will have to be redesigned since hydrogen requires more space, as well as cryogenic conditions. Hydrogen takes up to four times the volume as jet fuel.

Hydrogen must be proven every bit as safe and practical as traditional jet fuel to be viable. That means governments will need to establish an alternative set of airworthiness requirements.

What the numbers say about reaching net zero by 2050

NASA’s inspiring Artemis 1 mission reminds us of valuable lessons for hydrogen aircraft and what it means to achieve net zero emissions by 2050.

The Space Launch System rocket we developed with NASA used supercooled hydrogen to launch the uncrewed Orion spacecraft. It is the latest in a long line of spacecraft and test aircraft to prove hydrogen-powered flight possible over the past half century. With NASA, we flew on hydrogen when Apollo 11 took humanity to the moon in 1969.

Using hydrogen directly in commercial aircraft requires substantial changes to both the aircraft and the infrastructure around it. Learning how to safely and efficiently load, store, fly and scale hydrogen-powered aircraft will be the challenge of a generation of engineers.

By contrast, short and long-range aircraft can fly on sustainable aviation fuels, or SAF, which are non-fossil certified variants of traditional petroleum jet fuel. The primary materials can be sustainable bio-based waste sources like plant oils and cooking oil, industrial capture, or even the electric grid itself. SAF can be dropped into today’s aircraft and infrastructure to mitigate emissions immediately by up to 80% over their life cycle.

While flying on hydrogen does not emit carbon, its production often does. Since most hydrogen today is produced from fossil fuels, truly reducing emissions by flying on hydrogen will also require a wholesale transformation of the energy industry to ensure sufficient so-called green hydrogen produced from renewable electricity exists for aviation. Aviation has to compete with other sectors for green hydrogen as it becomes available.

Even then, it is arithmetically impossible to replace the world’s fleets with hydrogen-powered airplanes in time to meet the industry’s 2050 target.

By the late 2030s, we estimate more than 40,000 non-hydrogen commercial jets will be in service. Each will last decades. Given that the most airplanes ever produced around the world in a year so far have been about 1,800, we can’t just switch overnight to hydrogen. Emissions from those aircraft will need to be mitigated with SAF.

Hydrogen-powered aircraft may make a small contribution to moderating emissions in 2050. And while research today may develop solutions for the latter half of the century, it must also consider the 2050 challenge at hand.

To ensure future generations can continue to fly and enjoy the other global benefits the aerospace industry contributes to a strong economy, aviation must focus on developing and scaling SAF. Advances in the science and engineering of complex hydrogen propulsion technologies should be pursued, but likely apply to the longer term.

Our founder Bill Boeing once said, “Let no new improvement in flying and flying equipment pass us by.” As the Artemis 1 mission shows, our industry achieves the seemingly unattainable. The world must scale sustainable aviation fuels that can be dropped into existing aircraft today, while exploring decarbonized propulsion technologies like hydrogen and electricity that can make an impact in the second half of the century.

Christopher Raymond is the Chief Sustainability Officer of Boeing

The opinions expressed in Fortune.com commentary pieces are solely the views of their authors and do not necessarily reflect the opinions and beliefs of Fortune.

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