CO2 and non-CO2 emissions produced by aviation

Brief overview

Aviation has an impact on the climate and contributes to global warming.
CO₂ emissions can impact the environment for several hundred years.
Moreover, non-CO₂ emissions such as water vapour, nitrogen oxides, sulphur dioxide and black carbon are produced. These emissions cause additional warming and cooling at high altitudes.
The Federal Council's long-term climate strategy addresses both CO₂ and non-CO₂ emissions. Accordingly, measures are being implemented to minimise non-CO₂ emissions that cause warming.
The International Civil Aviation Organization (ICAO) has set itself the long-term goal of achieving net zero carbon emissions by 2050.
Aviation can achieve the net zero carbon target using sustainable aviation fuels, optimised aircraft technology and air traffic planning, negative emissions technologies (FOEN) and market-based measures.

A closer look

Aircraft emit CO2 and other exhaust gases during the fuel combustion process. CO2 contributes to global warming. In line with its fuel consumption, aviation accounts for 2–3% of global greenhouse gas emissions. Non-CO2 emissions both warm and cool the atmosphere, and contribute to additional global warming overall.

Key facts about CO2 emissions

Every type of carbon-based fuel generates CO2 during combustion. The CO2 accumulates in the atmosphere and intensifies the greenhouse effect. Every tonne of CO2 warms the climate, and once it has been emitted, its impact can last up to several hundred years. CO2 emissions can be calculated directly from information on fuel composition and consumption. The main way to combat climate change is to stop the emission of CO2 from fossil fuels.

Key facts about non-CO2 emissions

Water vapour, nitrogen oxides, sulphur dioxide and black carbon generate additional short-term climate effects at high altitudes. They can have both a warming and a cooling effect – the effects of non-CO₂ gases are influenced by the weather, the flight route, the time of day and the engines. Persistent contrails and nitrogen oxide emissions are the non-CO₂ emissions that have the greatest impact.

When aircraft fly at high altitudes and discharge black carbon particles, they can create contrails. These contrails can in turn form cirrus clouds with warming and cooling effects. While global air traffic has an overall warming effect, the situation is different for individual flights. For example, contrails formed during the day in clear skies are usually cooling. In fact, only around 14% of flights globally generate contrails with an overall warming effect. According to current knowledge, just 2% of flights are responsible for more than 80% of warming effects. These flights are the focus of the reduction measures.

It is difficult to quantify the effects of non-CO₂ emissions precisely. However, it can be assumed that these climate effects are at least as strong globally as those of CO₂ emissions.

From a global perspective, current nitrogen oxide emissions from aircraft are warming.

Calculating emissions

The CO₂ emissions of a flight can be calculated directly from the fuel consumption; models make it possible to estimate the climate impact. One tonne of CO₂ from an aircraft engine has the same effect as one tonne of CO₂ from another source such as heating or car traffic. Switzerland's greenhouse gas inventory contains statistics on the greenhouse gas emissions of air transport originating in Switzerland.

It remains difficult to determine the quantity of non-CO₂ emissions that aircraft emit while cruising. Exhaust gas measurements on engines must be carried out using test stands on the ground, but emissions levels change when the engine is cruising at high altitudes. Non-CO₂ emissions are not proportional to fuel consumption. Today's aircraft engines vary greatly in the emissions that they produce. The FOCA is working internationally to ensure that emissions differences become clearer and that international standards ensure that all engine developments move towards lower non-CO₂ emissions.

Determining the climate impact of non-CO₂ emissions is complex. This is due to the fact these emissions have both warming and cooling effects, depending largely on the condition and composition of the surrounding atmosphere. Climate models have to work with many assumptions here.

The FOCA publishes both the calculation of annual fossil-fuel CO₂ emissions from air traffic originating in Switzerland and an estimate of annual non-CO₂ pollutants emitted at high altitudes (Swiss civil aviation statistics 2023: Fuel consumption and pollutant emissions (section 7)). This makes it possible to track the development of non-CO₂ emissions in addition to the reduction in fossil-fuel CO₂.

The political debate on the impact of CO2 emissions and non-CO2 effects

The long-term climate strategy of the Federal Council makes clear statements on CO₂ emissions and non-CO₂ effects. Net climate-impacting emissions from international flights from Switzerland are to be reduced to a minimum by 2050. In specific terms:

Fossil-fuel CO₂ emissions are to be balanced (i.e. net zero).
The other climate impacts (non-CO₂ emissions and warming effects) are to decrease or to be offset by other measures. Sustainable aviation fuels are central to these targets. The increased use of sustainable fuels will reduce fossil CO₂emissions and the climate impact of non-CO₂ emissions in the future. Additional measures are required, which are listed below.

Measures addressing the climate impact of non-CO2 emissions

Adjustments to aircraft technology: Aviation works with global standards established by the ICAO. The FOCA is a member of the ICAO Committee on Aviation Environmental Protection and participates in its working groups, and also working to reduce non-CO₂ emissions. The FOCA was involved in the development of a system for measuring and limiting black particle emissions. These methods and limit values are today implemented globally as ICAO standards.
Adjustments to fuels: The use of sustainable aviation fuels (SAFs) has the benefit of reducing both fossil CO₂ emissions and non-CO₂ emissions. SAFs produce less black carbon particles and sulphur dioxide. Contrails become more permeable to heat radiation when SAFs are used, which reduces the contrails' warming component.
Adaptation of flight operations: If the formation of persistent contrails at night could be avoided, clouds formed by aircraft could have an overall cooling effect. The zones that enable strongly warming contrails to form are often only a few hundred metres thick and limited in size. The most effective way to avoid them is to adjust flight altitude, with the caveat that fuel consumption increases when an aircraft deviates from its ideal altitude or its flight path – which increases CO₂ emissions. The crucial point is having exact, real-time temperature and humidity information for the relevant altitudes (usually over 8,000 metres). The FOCA is therefore committed to ensuring that measurements on aircraft and data transmission and processing in air traffic control centres are reliable and timely.

Overview: Climate-impacting emissions and the differences between them

	CO₂ emissions	non-CO₂emissions
Characteristics:	CO₂ is a greenhouse gas	Other main climate-impacting emissions from aviation are not greenhouse gases
	Amount depends on the type of fuel and is proportional to consumption	Amount not proportional to CO₂ and depends on the engine and environment
		Contrails: Almost entirely produced from water vapour in the atmosphere
Impact:	Always warming	Warming and cooling Persistent contrails strongly warming at night, cooling during the day
	Can be calculated accurately	Cannot be calculated accurately
	Once emitted, impact continues for hundreds of years	Impact lasts minutes, hours For nitrogen oxides a maximum of several decades
	Impact increases even when emissions remain the same	Impact does not increase although emissions remain the same
	Impact does not depend on the location, time of day or weather conditions	Impact depends on the location, time of day and weather conditions