Airplane pollution

Emissions from aviation are a significant contributor to climate change. Airplanes burn fossil fuel which not only releases CO2 emissions but also has strong warming non-CO2 effects due to nitrogen oxides (NOx), vapour trails and cloud formation triggered by the altitude at which aircraft operate. These non-CO2 effects contribute twice as much to global warming as aircraft CO2 and were responsible for two-thirds of aviation’s climate impact in 2018.

Emissions from aviation have been growing faster than any other mode of transport, and have more than doubled between 1990 and 2019. Aviation emissions went from 1.5% of all European emissions in 1990 to 4.7% in 2019. The sector is expected to fully recover from the Covid ‘shock’ as early as 2024.

If unmitigated, aviation emissions could more than double (compared to 2019) by 2050 and in doing so, the sector will consume more than 10% of the remaining carbon budget to stay below 1.5°C of warming.

Unlike the Kyoto protocol, which had specific emission targets only for developed countries, the Paris agreement calls on all states to adopt “economy-wide” emission reduction targets. This “economy-wide” requirement means that despite there being no explicit reference to aviation – nor indeed are any other specific industrial sectors mentioned – 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. The same argument holds for shipping. The EU’s 2030 target already includes outbound aviation emissions (although these are in practice not included in any legislation), and other jurisdictions should follow suit, building on the UK’s decision in 2021 to be the first major economy to account for its airline and shipping emissions under its carbon budget.

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). 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 and European level.

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. This results in an increase in greenhouse gases, and the potential formation of persistent contrail cirrus. These high, linear clouds trap the Earth’s heat.

The consequence is a net warming effect, which may be up to three times worse than the warming caused by aviation’s CO2 emissions.

Nevertheless, these effects are also short lived, meaning that acting against them would quickly reduce aviation’s contribution to global warming, scoring important wins in our fight against climate change.

Despite their considerable impact, no measures are in place at regional or global level to reduce aviation’s non-CO2 climate impacts. As of 2025, the EU will start monitoring non-CO2 emissions on all flights departing from the bloc.

Persistent contrail formation is mainly the result of soot and other emissions on cold, high humidity atmospheric areas known as Ice Super Saturated Regions (ISSRs). But solutions exist to tackle the problem.

To reduce non-CO2 effects, airplanes can use clean fuels to reduce the amount of pollutants released in the air. Jet fuels with high aromatics and naphthalene concentrations increase soot formation, which in turn leads to persistent contrail cirrus. One way to reduce aromatics and naphthalene in jet fuel is to perform hydrotreating on fossil jet fuels. Reducing aromatics content of fossil jet fuel down to 8-10% can be achieved without significant costs and could lower non-CO2 effects significantly.

Avoiding flying through areas with very cold and humid conditions, known as Ice Super Saturated Regions (ISSR), is a key axis to reduce non-CO2 effects. Changing flight paths to fly at a lower altitude, or performing small diversions, can avoid contrail formation. For instance, rerouting less than 2% of flights in Japan can reduce the warming effect of contrails by 60%.

In order for researchers to better predict which regions should be avoided, airlines must cooperate with the EU and make the data collected on their flight routes available.

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.

The European Commission was first tasked with addressing the non-CO2 emissions of flying in 2008, and commissioned a landmark report to the European Union Aviation Safety Agency (EASA). The report, published in 2020, analyzed the latest available science, quantified non-CO2 climate impact of aviation as twice that of CO2, and proposed some mitigation measures.

However, these measures – and non-CO2 effects in general – are hardly reflected in EU. One provision exists in the EU ETS text, whereby airlines will have to start monitoring non-CO2 emissions of all flights departing the EU as of 2025. Europe should step up its efforts, as it is losing valuable time to mitigate most of the climate impact of aviation.

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. T&E has developed a tracker with all the e-fuels plants developing in Europe that you can find here. Read more on sustainable aviation fuels for planes here.

Zero emissions aircraft, such as hydrogen or electric planes, can help decrease aviation emissions for shorter ranges, but they will need significant amounts of funding to be in operation in the mid 2030s.  Read more on our views on new plane designs here.

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. By reducing corporate travel to 50% of pre-COVID levels, we can cut CO2 emission by as much as 32.6 MtCO2 by 2030 in Europe, which is equivalent to taking 16 million polluting cars off the road.

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.