Analyzing the Total Warming Footprint of Aircraft

By Laura ThomsonNov 24 2021

The aviation industry contributes largely to global warming because of the climate pollutants it generates. Looking at Klöwer, M. et al's journal paper published in Environmental Research Letters, this article analyzes aviation’s total warming footprint by modeling CO₂ and non-CO₂ effects.

Image Credit: Aerial-motion/Shutterstock.com

Aircraft alters the radiative balance of the planet through emissions and contrails, contributing to global warming. In recent years, global aviation has increased dramatically and the carbon footprint of top emitters is usually dominated by air travel.

Aviation is a huge industry and is one of the most carbon-intensive ways to travel. The pandemic resulted in the cancellation of the majority of flights from March 2020, which resulted in a large reduction in carbon emissions and other climate pollutants.

To limit global warming to below 2 °C, all emission sectors need to be decarbonized. The aviation industry was responsible for around 2.4% of global annual carbon emissions before the pandemic.

Nitrogen oxide (NO_x) emissions at altitudes of 8-12 km lead to complex chemical reactions in the atmosphere and also cirrus cloud formation by condensation trails. This article analyzes the total climate effects caused by aviation, taking both CO₂ and non-CO₂ effects into account. It also presents various scenarios that depict possible futures of aviation until 2050.

How Aviation Affects the Climate

Until 2016–2019, aircraft engines used more than 1 billion liters of fuel per day, emitting 1.23 kg of water vapor (H₂O), 3.16 kg of CO₂, 1.2 g of sulfur (SO₂), up to 15.14 g of NO_x, and 0.03 g of black carbon (soot) per kg of fuel (see Table 1). As nitrogen oxides react in the atmosphere, it alters the radiative balance of other gases, including ozone (O₃), methane (CH₄), and stratospheric water vapor (H₂O), thus indirectly affecting the climate.

Table 1. Best estimate emission indices and effective radiative forcing for aviation emissions and contrail formation from Lee et al. (2021). Effective radiative forcings from NO_x arise via reaction with CH₄, O₃ (short and long-term), and stratospheric water vapor and are noted therein. Consequently, the radiative forcings of these scale with the emission of NO_x. Source: Klöwer, et al., 2021.

Aircraft also form condensation trails on their paths, and these form cirrus clouds when persistent and increase cloudiness. These persistent contrail formations and emissions are transformed into effective radiative forcings.

The study employs data from the International Energy Agency (IEA) on jet fuel consumption and up-to-date evaluations of non-CO₂ radiative forcing.

Scenarios for 2050

The advent of the pandemic led to travel restrictions and there were fewer than 5000 flights—an 80% decrease from pre-pandemic air traffic (see Figure 1).

Figure 1. Daily flights over Europe between 2011 and the present. The seasonal cycle shows more flights in summer and less in winter with a strong decrease associated with holidays at the end of the year. The number of flights increased by about 2% yr⁻¹ pre-COVID in Europe. The pandemic forced many airplanes to the ground since March 2020 with only a partial recovery in summer 2020 and 2021. Data from EUROCONTROL. Image Credit: Klöwer, et al., 2021.

Even though the aviation industry partially recovered after the lockdown, the number of flights decreased by about 45% on average in 2020 (see Figure 2b).

Figure 2. Aviation’s contribution to global warming to 2050. (a) Annual historic and future annual carbon dioxide (CO₂) emissions of aviation following four scenarios: No Pandemic, Back to Normal, Zero Long-Term Growth, and Long-Term Decline as explained in the text. (b) Daily flights of selected airports globally between 2019 and November 2020 and annual averages for all scenarios. (c) Cumulative warming-equivalent emissions of CO₂ and non-CO₂ effects of aviation since 1940 and the corresponding aviation-induced global warming. Scenarios are color-coded as in (a). Image Credit: Klöwer, et al., 2021.

With the introduction of COVID-19 vaccines, the air traffic is anticipated to increase. It is still unclear if the air traffic would reach the same level as the pre-Covid period owing to the emergence of variants and also the boom in virtual technology, which has made a curb to business meetings. The research provides four scenarios to elucidate the possible futures of global aviation:

Scenario 1: No Pandemic—This scenario presumes that there is no COVID-19 pandemic and growth in air traffic CO₂ emissions of about 3% per year.

Scenario 2: Back to Normal—This scenario presumes a post-COVID recovery for 2021–2024 at 16% yearly increase and the pre-COVID level is reached in 2024.

Scenario 3: Zero Long-Term Growth—Assuming a 13% annual growth for the recovery period 2021–2024 and zero growth thereafter, around 90% of the pre-COVID level would be attained in 2024.

Scenario 4: Long-Term Decline—Assuming a 10% yearly growth for the recovery period 2021–2024 and a decline thereafter.

In all the scenarios, the emissions indices were unchanged. The researchers converted the annual air traffic from these scenarios to jet fuel consumption and CO₂ emissions.

Aviation’s Impact on Global Warming

Emissions of global aviation in 2019 were about 1 billion tons of CO₂ (GtCO₂) and this emission is expected to more than double by 2050.

As a huge fraction of the increase in atmospheric CO₂ stays for many 1000s of years, the recent emissions of CO₂ alone do not create global warming. The pandemic made a negligible impact on the CO₂-induced warming from aviation.

Aircraft also impact the climate through other non-CO₂ climate pollutants. Hence, the long-lasting effects of aviation are from different time scales. The aviation industry contributes 4% of global warming.

Considering the 3% annual growth scenario, Back to Normal, aviation would have bestowed 0.09 ± 0.04 °C to global warming by 2050. The pandemic only delayed the contribution of aviation by about five years.

However, if global aviation declines with no change in the present flight practices or fuel mix, this would result in an increase in aviation-induced warming with immediate effect.

Potential of Zero-Carbon Fuels

The obvious option to decrease aviation’s CO₂ emissions is introducing low-carbon fuel (bio or synthetic) as an alternative. Additional scenarios were created to identify the effects of zero-carbon fuels on aviation-induced warming.

Scenario 5—This scenario assumes a 3% increase of zero-carbon fuels per year from 2024 and a growth in air traffic. The increased CO₂ emissions from increased air traffic are thus compensated.

Scenario 6—Here a 90% zero-carbon fuel by 2050 is assumed with a yearly 5.8% increase of zero-carbon fuels. This lowers the annual CO₂ emissions over time.

Also, the fewer soot particles in bio or synthetic fuels contrail formation are assumed to be decreased by low-carbon fuels. Non-CO₂ effects can also be altered by changing flight routes such as adjusting aircraft cruise altitude.

The Back to Normal scenario with increased use of low-carbon fuels and attaining 55% carbon-neutrality by 2050 is analyzed (see Figure 3). This scenario decreases aviation’s contribution to global warming insufficiently.

Figure 3. Zero-carbon fuels (bio or synthetic) can limit aviation-induced warming only when they replace fossil fuels by 2050. (a) CO₂ emissions of aviation following the 3% annual air traffic growth of the Back to Normal scenario (as in Figure 1) but with increasing use of zero-carbon fuels. (b) Aviation-induced warming in two scenarios: 55% zero-carbon fuels by 2050 will not limit the warming, only the highly ambitious scenario of 90% carbon neutrality reaches maximum warming of about 0.04 °C. Scenarios are color-coded as in (a). Image Credit: Klöwer, et al., 2021.

Warming Footprints Instead of Carbon Footprints

The majority of the carbon footprint calculators employ a constant—multiplication factor—to include the non-CO₂ effects of aviation in a simplified way (see Figure 4). These multiplication factors are scenario and time-dependent.

Moreover, for all scenarios, the warming footprint of aviation is twice its carbon footprint in the coming years, emphasizing that non-CO₂ effects are non-negligible to analyze the contribution of aviation to global warming.

Figure 4. The warming footprint of aviation is a scenario and time-dependent multiplicative of its cumulative carbon footprint, about 2–2.6× larger in recent decades. Diagonal lines represent a constant multiplication factor often used in carbon footprint analyses to simplify the non-CO₂ effects of aviation. Dots represent decades for all scenarios and historic emissions. Warming footprints are the cumulative CO₂ warming-equivalent emissions, including both CO₂ and non-CO₂ effects. Image Credit: Klöwer, et al., 2021.

Discussion

A substantial ongoing reduction of 2.5% yr⁻¹ in aviation CO₂ emissions restricts the aviation sector’s contribution to further global warming. Low carbon fuels do not substantially decrease aviation’s contribution to warming as it gets compensated by air traffic growth.

Journal Reference:

Klöwer, M., Allen, M. R., Lee, D. S., Proud, S. R., Gallagher, L., Skowron, A. (2021) Quantifying aviation’s contribution to global warming. Environmental Research Letters, 16(10). Available at: https://iopscience.iop.org/article/10.1088/1748-9326/ac286e.

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