Another innovation has been on the radar since at least the Cold War: hydrogen-powered aircraft. But the engineering challenges here are significant. Hydrogen as a gas is too voluminous to be stored in useful quantities on board an airplane, so it has to be cooled to minus 253 Celsius, or about minus 423 Fahrenheit, the temperature at which hydrogen condenses into a liquid. Cryogenic infrastructure for fueling and storage would also need to be built at airports around the world.
But the technology exists: NASA and the European Space Agency have long used it successfully, and researchers at companies like Airbus and Rolls-Royce are working to adapt the technology for commercial aviation.
“With hydrogen, you can actually get to the point where you have zero carbon emissions,” said Lahiru Ranasinghe, the senior sustainability manager at easyJet, a European low-cost airline that is investing in hydrogen-powered technology.
Electric-powered flight is another low-carbon option. Because of the current limitations of the batteries that power their engines, fully electric aircraft lack the energy to propel large planes over long distances, but they could offer a solution for smaller planes flying shorter routes. On this front, Norway appears to be leading the way: According to Avinor, the Norwegian airport operator, all of the country’s domestic flights should be fully electric by 2040. Wideroe, a Norwegian regional carrier, plans to have its first fully electric aircraft in service by 2026.
A wispy, icy climate culprit
The aviation industry has focused on cutting back on carbon, but several academics say that there is a low-hanging fruit in terms of reducing the climate impact of flying. It turns out that contrails have a profound impact on the planet’s temperature.
“At a very high level, we’ve known for more than 20 years that contrail warming has been very significant — and comparable to CO2,” said Dr. Barrett of M.I.T.
The science is complicated, Dr. Barrett said, because their effect depends on the time of day. At night, contrails trap heat radiating off the earth, leading to additional warming. But during the day, contrails also radiate the sun’s energy back into the atmosphere and can actually have a cooling effect. But studies have shown that the overall impact is one of significant warming — anywhere from half to three times the effect of the aviation industry’s carbon dioxide emissions.
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