Meghan Hodges
Aviation is one of the most challenging sectors to decarbonize. Aircraft emit gases and particles directly into the upper troposphere and lower stratosphere, altering the concentration of atmospheric greenhouse gases, including carbon dioxide (CO2), ozone (O3), and methane (CH4). These emissions have a significant effect on atmospheric chemistry and cloud cover, with possible implications for the ozone layer. While the total amount of aviation fuel burned and the resulting carbon dioxide, NOx, and water vapour emissions are well-known, the climate impacts of these emissions are more complex and challenging to quantify. This complexity arises from the interaction between the emissions and the atmosphere, leading to regional and global climate forcing. The development of a fleet of supersonic aircraft flying at high altitudes is a growing concern, as it could significantly perturb the ozone layer in the stratosphere, causing a reduction in global column ozone. Jumbo jets, in particular, spend a considerable amount of time in the stratosphere, and their emissions are damaging the ozone layer.
| Characteristics | Values |
|---|---|
| Aircraft emissions as a percentage of global CO2 emissions | 2.5% in 2019, projected to grow to 3.5% by 2030 |
| Global aviation emissions in 1990 | 0.5 billion tonnes |
| CO2 emissions per passenger-kilometer in 1990 | 357 grams |
| CO2 emissions per passenger-kilometer in 2019 | 157 grams |
| CO2 emissions per revenue passenger per km in 2018 | 88 grams |
| Projected aviation emissions in 2050 | 43 billion tonnes |
| Projected aviation emissions as a percentage of global emissions in 2050 | 5% |
| Projected aviation emissions in 2050 without regulation | 3 billion tonnes |
| Projected growth of aviation emissions by 2050 | 300% |
| Percentage of global warming effect caused by aviation | 3.5% |
| Projected fuel burn of a new proposed fleet of supersonic aircraft | 122.32 trillion tonnes annually |
| Projected NOx emissions of a new proposed fleet of supersonic aircraft | 1.78 trillion tonnes annually |
| Projected global column ozone depletion due to a new proposed fleet of supersonic aircraft | 0.74% |
| Projected maximum global column ozone depletion due to a new proposed fleet of supersonic aircraft | 1.4% |
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What You'll Learn
- Jets emit gases and particles into the upper troposphere and lower stratosphere
- The emissions from jets alter the concentration of atmospheric greenhouse gases
- Aircraft emissions have a significant effect on atmospheric chemistry and cloud cover
- Jumbo jets spend a lot of time in the stratosphere, where their emissions damage the ozone layer
- Aviation is one of the hardest sectors to decarbonize
Jets emit gases and particles into the upper troposphere and lower stratosphere
Aircraft emit gases and particles directly into the upper troposphere and lower stratosphere, where they have a significant impact on atmospheric composition. These emissions are predominantly anthropogenic and have been shown to alter the concentration of atmospheric greenhouse gases, including carbon dioxide (CO2), ozone (O3), and methane (CH4). The total amount of aviation fuel burned, as well as the emissions of carbon dioxide, nitrogen oxides (NOx), and water vapour, are well-known parameters.
The gases and particles emitted by aircraft contribute to climate change by triggering the formation of condensation trails (contrails) and increasing cirrus cloudiness. The warming effect of ozone increases from NOx emissions, comparable to the warming effect of CO2 emitted by aircraft. Additionally, the warming effect of contrail formation through changes in cloudiness is of the same magnitude as the warming effect of CO2.
The impact of aircraft emissions on the ozone layer is a complex issue. While aircraft fly in the lower stratosphere to avoid turbulence and increased atmospheric drag, their emissions can still rise into the stratosphere under certain conditions, such as low pressure below. On the other hand, under high-pressure conditions, pollution from aircraft emissions will descend, and planes can fly higher without damaging the ozone layer.
The concentration of pollution in specific areas can trigger larger effects than more dispersed pollution. Polar regions, for example, may need to be avoided due to their greater sensitivity to pollution. Additionally, the development of a fleet of supersonic aircraft flying at higher altitudes could potentially perturb the ozone layer in the stratosphere.
Overall, while aircraft emissions are small compared to other man-made emissions, they have a significant impact on atmospheric chemistry and cloud cover, with possible implications for the ozone layer.
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The emissions from jets alter the concentration of atmospheric greenhouse gases
Carbon dioxide is the largest component of aircraft emissions, making up approximately 70% of the exhaust. It has a long atmospheric residence time of over 100 years, during which it mixes with the atmosphere and traps heat, contributing to the greenhouse effect. The warming effect of carbon dioxide emitted by aircraft is comparable to the warming effect associated with the ozone increase from NOx emissions. The other gases and particles have shorter atmospheric residence times and remain concentrated near flight routes, mainly in the northern mid-latitudes.
The non-CO2 effects of aviation, such as warming induced by aircraft contrails, also contribute to the total climate influence of aviation. The formation of condensation trails (contrails) and increased cirrus cloudiness due to soot and hydrocarbon particles are significant climate impacts of aviation. The climate impacts of aviation emissions are challenging to quantify, but they can be compared to other sectors using radiative forcing.
While aviation emissions are small compared to other man-made emissions, they could have a significant effect on atmospheric chemistry and cloud cover, with possible implications for the ozone layer. The number of civil aircraft flights is projected to grow faster than almost any other part of the world economy, which will further increase the concentration of atmospheric greenhouse gases.
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Aircraft emissions have a significant effect on atmospheric chemistry and cloud cover
The principal emissions from aircraft include carbon dioxide, water vapour (H2O), nitric oxide (NO), nitrogen dioxide (NO2), sulfur oxides (SOxO), and soot. While aircraft emissions are small compared to other human-made emissions, they can have a significant impact on the atmosphere. The gases and particles emitted by aircraft can alter the chemical composition of the atmosphere, leading to potential implications for the ozone layer.
Water vapour, a product of jet fuel consumption, makes up about 30% of aircraft exhaust. While it has a minimal direct warming impact due to its short lifespan in the atmosphere, water vapour contributes to the formation of contrails. When the ambient temperature is low enough, water vapour in the exhaust instantly freezes, forming ice crystals. These ice crystals can expand and spread horizontally and vertically, forming contrail-induced cirrus clouds.
Contrails and contrail-induced cirrus clouds have a serious warming effect, trapping infrared rays. The warming impact of these clouds can be up to three times that of CO2. Additionally, the NOx emissions from aircraft lead to an increase in ozone, producing a warming effect comparable to the warming effect of CO2 emitted by aircraft.
The development of a fleet of supersonic aircraft flying at high altitudes could further impact the ozone layer in the stratosphere. While there are considerations to restrict flight altitudes to reduce emissions in the stratosphere, implementing such restrictions may be counterproductive, potentially leading to more fuel burned without a net environmental benefit. Instead, a more sophisticated approach that dynamically adjusts height restrictions based on real-time atmospheric conditions has been suggested.
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Jumbo jets spend a lot of time in the stratosphere, where their emissions damage the ozone layer
Jumbo jets spend a significant amount of time in the stratosphere, a fragile layer of the atmosphere that is crucial for protecting life on Earth. This is due to several factors, including the desire to keep fuel costs down, the concentration of aircraft into relatively narrow flight corridors, and the jet stream, which provides a strong tailwind for eastbound flights.
The emissions from jumbo jets contain nitrogen oxides (NOx) and water vapour, which have been shown to play a key role in destroying the ozone layer. NASA estimated that about 25% of the NOx in the stratosphere originates from aircraft, and this figure may rise to 60% in the lower stratosphere, where ozone destruction is concentrated. Aircraft engines also discharge an estimated 80 million tonnes of water vapour into the stratosphere annually. These emissions contribute to the formation of polar stratospheric clouds, which provide surfaces for the chemical reactions that destroy ozone.
The impact of aircraft emissions on the ozone layer was first recognised in the 1980s with the development of the Concorde jet, which cruises at around 16.5 kilometres (55,000 feet). However, the issue was largely forgotten when the focus shifted to halogen compounds such as CFCs. Recently, concerns have been renewed with plans to build a new generation of supersonic aircraft, which could result in even greater destruction of the ozone layer.
There have been debates among atmospheric chemists about whether aircraft should be banned from the stratosphere to protect the ozone layer. Some suggest that aircraft could fly at lower altitudes or take more southerly routes to avoid the most vulnerable sectors of the ozone layer. However, it is important to consider the potential environmental impact of lower flight paths, as emissions at those altitudes can have different effects on the atmosphere. Additionally, restricting flights to lower altitudes may result in more fuel being burned, potentially negating any positive impact on the environment.
Overall, the complex interaction between aircraft emissions, atmospheric conditions, and the ozone layer highlights the need for careful consideration and further research to develop effective strategies that balance environmental protection with the demands of the aviation industry.
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Aviation is one of the hardest sectors to decarbonize
While aviation contributes only around 2% of global greenhouse gas emissions and 2.5% of CO2 emissions, it is considered one of the most challenging sectors to decarbonize. This is due to a combination of factors, including the unique requirements of the industry, the high costs of alternative fuels, and the slow development and adoption of new technologies.
Firstly, aviation has specific constraints related to weight and size, which limit the potential for innovation and the implementation of new technologies. For instance, the development of electric aircraft is hindered by the weight of batteries, which would need to be substantial to power a plane. As a result, the focus has been on improving fuel efficiency, with a 39% improvement from 2005 to 2019, but this has been outpaced by the growth in emissions due to increased air travel.
Secondly, alternative fuels such as Sustainable Aviation Fuel (SAF) and hydrogen are still in the early stages of development and adoption. SAF, which is essentially a biofuel, can offer up to an 80% reduction in emissions compared to traditional jet fuel, but it is more expensive. Hydrogen, particularly green hydrogen produced through electrolysis using renewable energy, is a zero-emissions fuel but is currently very costly to produce. Airlines are piloting hydrogen planes, and the technology is expected to become more affordable and widely used in the coming decades.
Thirdly, the aviation industry has a strong focus on safety, which can slow down the implementation of new technologies and fuel sources. Thorough testing and long innovation cycles are required to ensure any changes do not compromise the safety of passengers and crew. This can result in a lag in the adoption of new, more sustainable practices and fuels, even when they have the potential to significantly reduce emissions.
Finally, the popularity of air travel continues to grow, and with it, the sector's emissions footprint. This growth, combined with the slow progress in decarbonization, could lead to a substantial increase in aviation's greenhouse gas emissions in the coming decades. Strategies to address this include cleaner fuels, new aircraft designs, improved aircraft technology, and smarter operational practices.
In summary, aviation is a challenging sector to decarbonize due to its unique requirements, the high costs and slow adoption of new technologies, safety considerations, and the increasing demand for air travel. A combination of strategies will be necessary to achieve meaningful reductions in the sector's greenhouse gas emissions and meet net-zero goals.
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Frequently asked questions
It is difficult to quantify the exact amount of ozone pollution jets emit annually, but it is known that aircraft emit gases and particles that alter the concentration of atmospheric ozone.
The principal emissions from jets that contribute to ozone pollution are carbon dioxide (CO2), water vapour (H2O), nitrogen oxides (NOx), sulfur oxides (SOxO), and soot.
The emissions from jets can trigger the formation of condensation trails (contrails) and increase cirrus cloudiness, which contribute to climate change and can have a significant effect on atmospheric chemistry.
Yes, there have been discussions about implementing altitude restrictions on aircraft to minimise their impact on the ozone layer. Researchers have suggested that dynamic height restrictions based on real-time atmospheric conditions could be more effective than a blanket altitude restriction.
A study by Zhang et al. (2023) found that a fleet of supersonic aircraft targeting service entry around 2030 to 2035 would burn 122.32 Tg of fuel annually and emit 1.78 Tg of NOx. This fleet is projected to cause a reduction of -0.74% in global column ozone, with regional depletions reaching up to -1.4%.
