Planes

Why are international aviation emissions not counted in national climate targets?

International aviation emissions are rarely covered in climate targets. Yet, the Paris agreement calls on all states to adopt “economy-wide” emission reduction targets. This “economy-wide” requirement means that the aviation sector remains covered by the agreement just like all other sectors.

To ensure compliance with the agreement, T&E is calling for the inclusion of aviation emissions in countries’ climate pledges under the Paris agreement – known as nationally determined contributions (NDCs). Such an inclusion would encourage states to take action, both at national and international level as appropriate, to address aviation’s climate impact.

Currently, parties are working with the UN’s aviation agency, the International Civil Aviation Organisation (ICAO), to adopt global measures, such as a long-term emissions reduction goal and a global offsetting scheme (CORSIA). This scheme has proven ineffective to date. However there is a limit to the level of ambition that a UN agency with 193 members can agree to. There is therefore a need for more ambitious and effective action at national level.

Green aviation technologies

Cleaner fuels and changes to aircraft design can play an essential role in reducing emissions from flying. Ensuring planes are as efficient as possible and are capable of being powered by alternatives to fossil kerosene, is essential.

However, the right type of fuels need to be pursued. Using crop-based biofuels would cause negative effects such as driving deforestation or increasing food prices. Therefore the focus needs to be on truly Sustainable Aviation Fuels (SAFs). These can be advanced biofuels, which are produced using feedstocks which don’t compete with food, or feedstocks derived from renewable electricity, known as e-kerosene.

E-fuels such as power-to-liquid are a potential source of zero or lower carbon alternative fuels. E-kerosene is the only fuel that can be sustainably scaled up to reduce aviation’s climate impact. But they require enormous amounts of renewable energy and their environmental effectiveness depends on the source of the CO₂ required to produce the fuels. However, they do provide the potential to significantly reduce emissions from the sector. Read more on sustainable aviation fuels for planes here.

To deliver savings in the short term, we will need to maximise the efficiency of traditional aircraft design. Unlike most other transport modes, there are no credible CO₂ efficiency standards for aircraft design. It is therefore important that regulators step in with more effective standards, and provide further incentives through higher taxation of the aviation sector.

But tweaks to traditional aircraft design are not enough. We need zero-emission airplanes powered by clean electricity or hydrogen from renewable sources to decarbonise the sector. Several concepts for such aircrafts have emerged, with potential entry into service in the 2030s. With the right measures in place, electric, hybrid-electric and hydrogen planes could start flying in the 2030s.

An "all of the above" approach is necessary, focusing on efficiency improvements, new fuels, and government support for breakthrough designs, considering that zero-emission aircraft may initially be limited to short and medium-range flights.

What are aviation’s non-CO₂ climate impacts?

On top of CO₂, aircraft engines emit other gases – nitrous oxides (NOx), sulfur dioxide (SO2) and water (H2O) – and particulate matter (soot). When emitted at high altitudes, these emissions affect atmospheric physical and chemical properties. The consequence is a net warming effect, which may be at least as bad as the warming caused by aviation’s CO₂ emissions.

The first piece of evidence highlighting the importance of aviation’s non-CO₂ effects came from the UN’s Intergovernmental Panel on Climate Change (IPCC) back in 1999. Since then, a lot of excellent research has been carried out to better understand the effects of these emissions and how to tackle them.

How do these contrails form? When a plane flies, the engine releases tiny particles and water vapour. But, sometimes, in cold and humid atmospheric conditions, the water condenses around the particles - creating an ice cloud. Most contrails disappear within a few minutes but if the plane flies through a particularly humid and cold area, the ice crystals persist and linger in the atmosphere for hours.

In the daytime, contrails will reflect some of the sun’s heat back into space - helping to cool the Earth. But at the same time, contrails block some of the heat radiating from Earth. During the day, the net effect may be cooling or warming. But during the night, there is no heat from the sun to reflect into space. The only climate effect the contrails have is acting like a blanket, trapping heat on Earth and warming the planet (watch our video explaining contrails here).

Not all flights cause warming contrails. In fact, less than 3% of flights cause 80% of the contrail warming. So, on those very targeted flights, a plane could adjust its flight path to avoid the very humid and cold regions - known as ISSRs (ice supersaturated regions). The climate benefits would be significant.

Although this may cause a slight deviation and the flight would burn a bit more jet fuel, if planned carefully, simulations have shown that the warming from the extra CO₂ emitted from these detours would only be 1/100th of the warming saved by avoiding the contrails. For instance, rerouting less than 2% of flights in Japan can reduce the warming effect of contrails by 60%.

And the good news doesn’t stop here. As a climate solution, contrail avoidance is particularly cost-effective. On a transatlantic flight, it would cost less than €4 per ticket to avoid contrails (read more in our study and on our explainer page).

Should we fly less?

As it is currently difficult to reduce emissions from this sector, flying less is the only way to reduce emissions in the critical decade before 2030. Flying less could mean taking a more sustainable mode of transport, such as train, or changing your destination to one which can be reached by less climate-intensive means. It could also mean teleconferencing instead of attending business meetings, or going on longer individual holidays, rather than several short, carbon intensive, trips.

During the COVID-19 pandemic, the ease with which many employees adjusted to being home and flying less revealed that those long-held ideas of the need to fly for work no longer stand. Reducing corporate travel is an easy way to cut aviation emissions.

Whatever the means of achieving it, demand reduction should not be a taboo subject. The sector’s emissions are soaring at a rate too high to ignore and policy-makers should at the very least not recklessly champion endless aviation growth while at the same time knowing how difficult it is to decarbonise the sector.