Pre-pandemic, aviation was responsible for nearly one gigaton of CO2 emissions annually, around 2.4% of the global total, although through its additional non-CO2 impacts it contributes around 4% to human-induced global warming, a share that is increasing. A paper published in Environmental Research Letters suggests emissions produced by the aviation industry must be reduced each year if they are not to increase warming further, otherwise the sector could consume up to one-sixth of the remaining budget to limit warming to 1.5°C by 2050. The UK researchers behind the study developed a simple technique for quantifying the temperature contribution of historical aviation emissions, including both CO2 and non-CO2 impacts. They then modelled a number of potential post-Covid industry recovery scenarios and their climate impacts through to 2050, together with the potential effects of the industry scaling up the use of low-carbon fuels. The authors show that the only way to ‘freeze’ the temperature increase from the sector is to decline emissions by about 2.5% per year. However, they found a 90% mix of carbon neutral fuels by 2050 could achieve a similar outcome, with no further temperature increase from the sector.
Aviation is projected to cause a total of about 0.1°C of warming by 2050, half of it to date and the other half over the next three decades, should aviation’s pre-Covid growth resume. The industry would then contribute a 6%-17% – so up to one-sixth – share of the remaining 0.3-0.8°C to not exceed 1.5-2°C of global warming. Under this scenario, the reduction due to Covid-19 to date is small and is projected to only delay aviation’s warming contribution by about five years as it is the cumulative emissions that matter, found the researchers of the study, ‘Quantifying aviation’s contribution to global warming’, from the University of Oxford, Manchester Metropolitan University and the NERC National Centre for Earth Observation.
“Covid has reduced the amount people fly, but there is little chance for the aviation industry to meet any climate target if it aims for a return to normal,” warned Milan Klöwer, a climate physics researcher at the University of Oxford who led the study.
To estimate aviation’s contribution to current and future anthropogenic global warming, the researchers analysed the total climate forcing, taking both CO2 and non-CO2 (those aircraft emissions of water vapour, NOx, sulphur and soot at altitude that have accounted for more than 50% of aviation-induced warming) effects into account. A large fraction of the increase in atmospheric CO2 naturally stays for centuries, hence why recent emissions of CO2 alone do not drive global warming, but the accumulative emissions. The accumulated carbon emissions of aviation for the period 1940-2019 are 33 GtCO2, equivalent to the historic emissions of Canada and about 2% of the world’s cumulative CO2 emissions. By contrast, most non-CO2 effects vanish within a year, the exception being the negative forcing from methane. Taking both CO2 and non-CO2 effects into account, the total aviation-induced warming up to 2019 is about 4% of the almost 1.2°C that the planet has warmed so far, found the researchers.
They have designed four scenarios to capture a possible future of global aviation to 2050:
Scenario 1: No pandemic – Assumes there had been no Covid-19 pandemic and a continuous growth in air traffic CO2 emissions of about 3% per year.
Scenario 2: Back to normal – Assumes a post-Covid recovery for 2021-2024 at 16% annual growth and 3% thereafter, so the pre-Covid level is reached in 2024.
Scenario 3: Zero long-term growth – Assumes a 13% annual growth for the 2021-2024 recovery period, zero growth thereafter and about 90% of the pre-Covid level is reached in 2024.
Scenario 4: Long-term decline – Assumes a 10% annual growth for the recovery period but a 2.5% decline thereafter, with air traffic levels about 50% lower in 2050 compared with 2019, similar to the pandemic year 2020.
The second scenario, ‘back to normal’, leads to the 6% to 17% aviation share of the remaining budget to stay within the 1.5-2°C limit. Without policy intervention, this contribution will continue to increase beyond 2050, say the authors. The recovery period will delay aviation-induced warming, reducing it by about 10% in 2050, but future annual growth will have a much greater impact than Covid, which is projected to only delay the warming contribution of aviation by about five years should the pre-Covid growth resume.
In the third ‘zero long-term growth’ scenario, aviation-induced warming will keep rising over the next decades, as the CO2 emissions continue to accumulate and start to dominate over the non-CO2 effects. Interestingly, points out the paper, if global aviation were to decline by about 2.5% a year (scenario 4), even with no change in the current fuel mix or flight practices, the impacts of the continued rise in accumulated CO2 emissions and the fall of non-CO2 climate forces would balance each other, leading to no further increase in aviation-induced warming with immediate effect.
Said co-author Professor David Lee of Manchester Metropolitan University: “One of the important nuances is that the non-CO2 impacts, like the formation of contrails and cloudiness, have been thought to dominate the total impact: this is true at present, but it’s not widely understood in the stakeholder community that if you take care of CO2, the non-CO2 fraction actually decreases in importance, even more so with sustainable aviation fuels that generate fewer contrails. This emphasises the importance of tackling aviation’s CO2 emissions.”
The study designed two additional scenarios to look at the potential impact on aviation-induced warming of the introduction of low/zero-carbon fuels (bio or synthetic):
Scenario 5: 55% zero-carbon fuels by 2050 – Assumes an air traffic growth following the ‘back to normal’ scenario but with a 3% increase of zero-carbon fuels per year from 2024.
Scenario 6: 90% zero-carbon fuels by 2050 – Makes a similar assumption to scenario 5 but with an annual 5.8% increase in zero-carbon fuels. The increased CO2 emissions from increased air traffic are therefore overcompensated, lowering annual CO2 emissions over time.
While for scenarios 5 and 6 with zero-carbon fuels the CO2 emission indices are respectively lowered, the non-CO2 climate forcings continue to scale with annual fuel consumption (regardless of the carbon neutrality of the fuel mix). However, due to fewer soot particles in bio or synthetic fuels, contrail formation is predicted to be reduced by low-carbon fuels.
The authors conclude that scenario 5 would reduce aviation’s contribution to global warming insufficiently to be sustainable, nor will it stop the non-CO2 effects from increasing, and only scenario 6 would limit aviation-induced warming.
“Any growth in aviation emissions has a disproportionate impact, causing lots of warming,” explained co-author Professor Myles Allen, University of Oxford. “But any decline also has a disproportionate impact in the other direction. So the good news is that we don’t actually need to all stop flying immediately to stop aviation from causing further global warming – but we do clearly need a fundamental change in direction now and radical innovation in the future.”
While solutions such as moving to alternative fuels present a clear pathway to minimising warming, said Dr Simon Proud of the National Centre for Earth Observation and RAL Space, there are actions that could be taken right now, such as more efficient air traffic control. “A ban on fuel tankering – where aircraft carry more fuel than they need, and hence burn extra fuel, to save the cost of refuelling at the destination – would reduce CO2 emissions in Europe alone by almost one million tonnes,” he also suggests.
In the paper, the authors recommend that those using carbon footprint calculators and add a multiplication factor to include the non-CO2 effects of aviation in a simplified way, should use a factor of 2.6, on the assumption of a 3% continuous growth in aviation. In general, though, multiplication factors are scenario and time-dependent, and therefore should be used with caution in carbon footprint calculations, they say.
“Nevertheless, for all scenarios the ‘warming footprint’ of aviation is at least twice as large as its carbon footprint in the coming decade, clearly highlighting that non-CO2 effects are non-negligible to assess the contribution of aviation to global warming,” they conclude.