TL;DR

Aviation is on track to account for 25% of all GHG emissions by 2050. To prevent this, the aviation sector must reach net-zero CO2 emissions AND reduce non-CO2 radiative forcings like water vapor and soot.
Most innovation focuses on new fuel sources for existing aircraft (such as hydrogen and biofuels), but significant technological, sourcing, and cost barriers exist.
With the right design, electrification can replace 50-80% of all air traffic, eliminating direct emissions and at lower maintenance and operational costs than a traditional aircraft.

The impact of air travel

Burning one kg of aviation fuel produces 3.16 kg of CO2. In 2018, the total CO2 emissions from air travel (passenger + freight) were 918 million tonnes—or 2.4% of global emissions. Air cargo is growing, but passenger transportation was 81% of all 2018 aviation emissions, or 747 million tonnes of CO2 from 38 million flights.

CO2 emissions, however, are only part of the problem. If you look at everything else that comes out of a jet engine, the warming effects are 3x higher than those of just the CO2.

Source: Lee et al., 2021

Water vapor

Aeroplane exhaust is made up of 30% water vapor, and water vapor is a potent GHG that amplifies global warming. As the earth’s temperature increases, more water evaporates into the atmosphere. Because water itself absorbs heat, the warming effect gets more potent, and the cycle continues.

Soot

Soot is black carbon particles produced by burning fuels, and jet engines leave a trail of these through the sky. In the atmosphere, soot particles absorb energy from the sun and release that as heat, but they also become condensation nuclei (aka cloud seeds), meaning a surface where water can condense. These particles create contrails along the path of the aircraft (the long white trails you see following planes in the sky).

Source: Kärcher, 2018

<aside> ⚠️ The direct warming effect of contrails is larger than that of CO2 and NOx emissions.

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