Decarbonizing aviation is challenging; you need to carry a lot of energy in the lightest possible form, and it's very hard to compete with jet fuel on that metric. Batteries are currently abysmal, so the last few years have seen a furious pace of development in the hydrogen space, generally taking hydrogen on board as a compressed gas, then running it through a fuel cell to power electric propellers.

Gaseous hydrogen might get your aircraft roughly double the range that a battery can, and that unlocks short-range regional flights with zero emissions. Universal is at the absolute bleeding edge here – last year, the company flew the largest hydrogen fuel cell-powered aircraft ever built, by a country mile. You can see Lightning McClean, a converted Dash-8 airliner, take its first flight in the video below.

Airline operators will need to give up some paying seats to fly clean like this; the Dash-8 can normally take up to 56 passengers, and this gaseous-hydrogen retrofit gobbles up 16 seats. But running on gaseous hydrogen lets operators run flights up to around 460 miles (740 km) in range, and that, says Universal, will cover around 75% of the routes normally serviced by this class of plane.

To go further than that, you'll need to upgrade to cryogenic liquid hydrogen. You need to store it at incredibly low temperatures, below 20 K (-253 °C/-423.4 °F) at ambient pressure levels, to keep it in its liquid phase and stop it boiling off. If you can manage that, it becomes a matter of how much passenger space you give up, but the potential is there to cover nearly all the flights this class of plane handles on jet fuel.