Commercial aviation has become a cornerstone of our economy and our society. It allows us to move goods and people quickly around the world, facilitates more than a third of all global trade by value, and supports 87.7 million jobs worldwide. However, the 80-ton flying machines we see hurtling through our skies at near-supersonic speeds also carry serious environmental baggage.
My team’s recent review article highlights some promising solutions the aviation industry could implement now to reduce the damage theft does to our planet. Simply changing the routes we take could hold the key to drastic reductions in climate impact.
Modern aircraft burn kerosene to generate the forward propulsion needed to overcome drag and produce lift. Kerosene is a fossil fuel with excellent energy density, providing a lot of energy per kilogram burned. But when burned, harmful chemicals are released: mainly carbon dioxide (CO₂), nitrogen oxides (NOₓ), water vapor and particulates (tiny particles of soot, dirt and liquids).
Aviation is widely known for its carbon footprint, with the industry contributing 2.5% of the global CO₂ load. While some may argue that this pales in comparison to other sectors, carbon is only responsible for a third of aviation’s total climate impact. Non-CO₂ emissions (mainly NOₓ and ice trails produced by aircraft water vapor) account for the remaining two-thirds.
Taking all aircraft emissions into account, flying is responsible for around 5% of human-induced climate change. Given that 89% of the population has never flown, passenger demand is doubling every 20 years, and other sectors are decarbonizing much faster, this number is expected to skyrocket.
It’s not just carbon
Aircraft spend most of their time flying at cruising altitude (33,000 to 42,000 feet) where the air is thin, to minimize drag.
At these altitudes, NOₓ from aircraft reacts with chemicals in the atmosphere to produce ozone and destroy methane, two very potent greenhouse gases. This aviation-induced ozone should not be confused with the natural ozone layer, which sits much higher and protects the Earth from harmful UV rays. Unfortunately, NOₓ emissions from aircraft cause more warming due to ozone production than they cool due to methane reduction. This leads to a net warming effect that accounts for 16% of aviation’s total climate impact.
Additionally, when temperatures drop below -40℃ and the air is humid, water vapor from aircraft condenses on particles in the exhaust and freezes. This forms an ice cloud known as a contrail. Contrails may be made of ice, but they warm the climate by trapping heat emitted from the Earth’s surface. Although they only last a few hours, contrails are responsible for 51% of global warming in the aviation industry. That means they’re warming the planet more than all the carbon emissions from airplanes that have accumulated since the dawn of powered flight.
Unlike carbon, emissions other than CO₂ cause warming through interaction with surrounding air. Their impact on the climate changes depending on the atmospheric conditions at the time and place of the release.
Reduced climate impact without CO₂
Two of the most promising near-term options are weather-optimal routing and formation flying.
Climate-optimal routing involves re-routing aircraft to avoid regions of the atmosphere that are particularly sensitive to climate – for example, where particularly humid air causes long-lasting and damaging contrails to form. Research shows that for a small increase in flight distance (usually no more than 1-2% of the trip), the net climate impact of a flight can be reduced by around 20%.
Air operators can also reduce the impact of their aircraft by flying in formation, with one aircraft flying 1-2 km behind the other. The trailing aircraft “surfs” on the lead aircraft’s wake, resulting in a 5% reduction in CO₂ and other harmful emissions.
But flying in formation can also reduce non-CO₂ warming. When aircraft exhaust plumes overlap, the emissions they contain accumulate. When NOₓ reaches a certain concentration, the rate of ozone production decreases and the warming effect slows down.
And when contrails form, they grow by absorbing surrounding water vapor. In formation flight, the contrails of the aircraft compete for water vapor, making them smaller. Adding up the three reductions, formation flying could reduce the climate impact by up to 24%.
Decarbonizing aviation will take time
The aviation industry has focused on tackling carbon emissions. However, current plans for the industry to reach net zero by 2050 rely on an ambitious 3,000-4,000 fold increase in Sustainable Aviation Fuel (SAF) production, problematic emissions offset programs carbon emissions and the introduction of hydrogen and electric aircraft. All of this could take several decades to make a difference, so it’s crucial the industry reduces its environmental footprint in the meantime.
Climate-optimal routing and formation flying are two key examples of how we could accelerate change, compared to a purely carbon-focused approach. But there is currently no political or financial incentive to change course. It’s time for governments and the aviation industry to start listening to science and take CO₂-free emissions from aircraft seriously.