Commercial aviation has become a cornerstone of our economy and society. It enables us to quickly transport goods and people around the world, facilitates more than a third of all global trade in value, and supports 87.7 million jobs worldwide. However, the 80-ton flying machines we see whizzing through our skies at near-supersonic speeds also carry serious environmental baggage.
My team’s recent review paper highlights some promising solutions the aviation industry could put in place now to reduce the damage flying does to our planet. Simply changing the routes we fly could be the key to drastically reducing the climate impact.
Modern airplanes burn kerosene to generate the forward propulsion needed to overcome drag and produce lift. Kerosene is a fossil fuel with excellent energy density, which provides a lot of energy per kilogram burned. But when it is burned, harmful chemicals are released: mainly carbon dioxide (CO₂), nitrogen oxides (NOₓ), water vapor and particulate matter (tiny particles of soot, dirt and liquids).
Aviation is widely known for its carbon footprint, with industry contributing 2.5% to the global CO₂ load. While some might argue that this pales in comparison to other sectors, carbon is responsible for only a third of aviation’s full climate impact. Non-CO₂ emissions (mainly NOₓ and ice trails from aircraft water vapor) account for the remaining two thirds.
Taking into account all aircraft emissions, flight is responsible for about 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 decarbonising much faster, this number is expected to skyrocket.
It’s not just carbon
Airplanes 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, the NOₓ plane reacts with chemicals in the atmosphere to produce ozone and destroy methane, two very potent greenhouse gases. This aviation-induced ozone is not to be confused with the natural ozone layer, which sits much higher up and protects the Earth from harmful UV rays. Unfortunately, NOₓ emissions from aircraft cause more warming due to ozone production than cooling due to methane depletion. This leads to a net warming effect that accounts for 16% of the total aviation climate impact.
Also, when temperatures drop below -40 ℃ and the air is humid, the water vapor from the plane condenses on the particles in the exhaust and freezes. This forms an ice cloud known as a contrail. Contrails can be made of ice, but they warm the climate by trapping the heat emitted from the earth’s surface. Despite only lasting a few hours, contrails are responsible for 51% of the global warming of the aviation industry. This means that they are warming the planet more than all the carbon emissions from aircraft that have accumulated since the dawn of powered flight.
Unlike carbon, non-CO₂ emissions cause warming through interactions with the surrounding air. Their impact on the climate changes depending on the weather conditions at the time and place of release.
Reduce the impact on the climate without CO₂
Two of the most promising short-term options are optimal weather routing and formation flight.
The optimal climate path involves redirecting aircraft to avoid regions of the atmosphere that are particularly climate sensitive, for example where particularly humid air causes harmful and long-lasting contrails. 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%.
Flight operators can also reduce the impact of their aircraft by flying in formation, with one aircraft flying 1-2km behind each other. The pursuer “navigates” the wake of the lead aircraft, leading to a 5% reduction in both CO₂ and other harmful emissions.
But flying in formation can also reduce CO₂-free warming. When aircraft exhaust plumes overlap, emissions within them accumulate. When NOₓ reaches a certain concentration, the ozone production rate decreases and the heating effect slows down.
And when contrails form, they grow by absorbing the surrounding water vapor. During formation flight, the plane’s contrails compete for water vapor, making them smaller. Adding all three reductions, formation flight could reduce climate impact by up to 24%.
The decarbonisation of aviation will take time
The aviation industry has become fixated on combating carbon emissions. However, current plans for the industry to reach net zero by 2050 are based on an ambitious 3,000-4,000-fold increase in sustainable aviation fuel (SAF) production, problematic carbon offsetting schemes and introduction of hydrogen and electric aircraft. All of this could take several decades to make a difference, so it’s imperative that the industry reduces its environmental footprint in the meantime.
The optimal climate route and training flight are two key examples of how we could make change happen faster, compared to a purely carbon-focused approach. But there is currently no political or financial incentive to change course. It is time for governments and the aviation industry to start listening to science and take non-CO₂ emissions from aircraft seriously.
This article was republished by The Conversation under a Creative Commons license. Read the original article.
Kieran Tait is a PhD researcher at the University of Bristol. He receives funding from the Engineering and Physical Sciences Research Council (EPSRC).