Tasked by the European Commission to conduct an update on the non-CO2 effects of aviation on climate change, Europe’s regulatory agency EASA has issued a report that highlights the latest understanding of the science and suggests technological, operational, policy and financial tools to address the issue. In addition to CO2, aircraft emit a wide variety of gases and aerosols at cruising altitude that influence climate directly and indirectly. The analysis confirms their significance is at least as important as those of CO2 alone, although the complexity of measuring non-CO2 climate impacts, together with the uncertainty regarding trade-offs between the various impacts, makes targeted policy development in this area challenging, say the authors. However, potential policies suggested include a levy on aircraft NOx emissions and/or the inclusion of such emissions under the EU ETS, and mandatory use of cleaner burning sustainable aviation fuels.
Steve Arrowsmith, Chief Expert for Environmental Protection at EASA, who led the project, told a webinar organised by NGOs Carbon Market Watch and Transport & Environment there had been past studies of the topic in 2006 and 2008 that primarily focused on NOx emissions but it was considered the scientific understanding was not sufficiently mature to propose policies to address non-CO2 impacts. He said the understanding had evolved considerably over the past decade, including on some new effects, although there remained significant uncertainties with regard to the magnitude of these impacts.
The report was compiled by renowned climate science, technology, ATM and policy experts from the EU, Norway and the UK. The non-CO2 climate impacts assessed arise from aircraft engine emissions of oxides of nitrogen (NOx), soot particles, oxidised sulphur species and water vapour. The chemical and physical processes can lead to contrail and contrail cirrus impacts in particular local atmospheric conditions, and complex impacts arising from NOx and particulate matter (PM) emissions during cruise.
The net impact of aviation non-CO2 emissions is a positive radiative forcing (warming), although there are a number of individual positive and negative (cooling) forcings, for which large uncertainties remain. The largest aviation non-CO2 impacts that can be calculated with best estimates are those from net-NOx (NOx is not a climate warming agent per se but its emission results in changes in the chemical balance of the atmosphere to ozone and methane which have radiative impacts, quantified as a net-NOx effect) and contrail cirrus, both of which have significant uncertainties in their magnitude, particularly contrail cirrus. Contrails predominantly cool if the sun is close to the horizon and warm if the sun is high in the sky. However, they exclusively warm at night, thereby resulting in a net positive (warming) radiative forcing.
The scientific community has adopted Effective Radiative Forcing (ERF) as a better metric of an absolute impact when compared to Radiative Forcing (RF) as it shows better proportionality to changes in global mean surface temperature response, particularly for short-lived climate forcing agents such as clouds and aerosols. The usage of ERF rather than RF is potentially significant for aviation NOx and contrail cirrus impacts. Aviation ERFs are less well quantified than RFs for net-NOx impacts but better quantified for contrail cirrus forcing effects.
Research shows the ERF from the sum of non-CO2 impacts yields a net positive (warming) that accounts for more than half (66%) of the aviation net forcing in 2018. However, in the same year, the uncertainty distributions showed that non-CO2 forcing terms contributed about eight times more than CO2 to the overall uncertainty in aviation net forcing.
While the confidence level on the magnitude of the impact of NOx remains low, the current understanding is that NOx still has a net positive climate forcing effect. However, say the scientists, if surface emissions of tropospheric ozone precursors (NOx, CO, methane and non-methane hydrocarbons) decrease significantly in future and aviation emissions increase, it is possible that the net aviation NOx ERF will decrease, or even become negative (i.e. cooling), even with increasing total emissions of NOx.
“This highlights one of the problems of formulating NOx mitigation policy based on current emissions/conditions,” says the report.
A major scientific and policy challenge also remains comparing long-lived aviation CO2 emissions with short-lived non-CO2 emissions and their impacts on a common scale. CO2 has multiple lifetimes in the atmosphere because of different timescales but a significant fraction – around 20% – accumulates and remains in the atmosphere for millennia.
The CO2 equivalent emissions metric (CO2-e) that is currently widely used, including within the EU ETS for stationary installations, is the Global Warming Potential for a time-horizon of 100 years (GWP100). This metric estimates an overall CO2 multiplier of 1.7 to account for future impacts of aviation non-CO2 emissions. To address the challenge, the scientific community has proposed a number of alternatives to the GWP100, including the Global Temperature change Potential (GTP) metric, which estimates a 1.1 multiplier.
The report points out that there is no exclusively correct choice of an equivalent emissions metric as the choice depends on the policy (for example whether it is a temperature target or an emissions reduction target) and the subjective choice of the time horizon of interest.
The simple approach of applying a multiplier to account for the climate effects of non-CO2 emissions – for example a net GWP100-based multiplier – averaged across the aircraft fleet and all atmospheric conditions may not be appropriate, say the experts. Also, they argue, the use of the multiplier does not incentivise reductions of non-CO2 emissions independently of CO2 emissions, neither at the global/regional fleet level nor on an individual flight-by-flight basis.
Another option would be to calculate the total climate impact of individual flights and then determine the CO2 equivalent emissions on a flight-by-flight basis. Such equivalents could be used as the basis for a policy instrument but, says the report, once again the magnitude of the equivalency depends on the choice of metric and time horizon.
The report states that a relatively new application of the GWP, referred to as GWP*, produces a better temperature-based equivalence of short-lived non-CO2 climate forcers by equating an increase in the emission rate of a short-lived climate forcer (SLCF) with a one-off ‘pulse’ emission of CO2. The GWP* is an example of a flow-based method that represents both short-lived and long-lived climate forcers explicitly as ‘warming-equivalent’ emissions that have approximately the same impact on the global average surface temperature over multi-decade to century timescales. Based on this method, the indication is that aviation emissions are currently warming the climate at around three times the rate of that associated with aviation CO2 emissions alone.
“It could be argued that temperature-based metrics, and the GWP*, are potentially more useful for temperature-based policy objectives, such as the temperature targets of the Paris Agreement. They also provide a more physical basis of actual impacts than GWPs for SLCFs,” says the report.
However, it adds: “This report does not recommend one specific metric or choice of time horizon. These choices partly depend on the suitability of the metric to a particular mitigation strategy and partly upon the user’s choices, which may be influenced by socio-economic factors, such as equity valuation.”
Technological or operational measures to mitigate aviation’s non-CO2 impacts that involve a reduction of a SLCF, such as NOx or contrail cirrus, are covered by the report. Because they can result in increased CO2 emissions, however, measures need to be considered carefully to ensure the net impact is beneficial, it cautions. “The ratio between benefits and disbenefits will change with the time horizon being considered but a reduction in SLCFs might make it easier to achieve climate change targets in the next decades and up to a century.”
Avoiding contrail cirrus-forming in ice-supersaturated regions of the atmosphere is an example of an operational measure to reduce the climate impact of aviation. There is some evidence, say the experts, that most of the total forcing comes from a few events where formation is large and long-lasting – sometimes referred to as ‘big hits’. Flights impacting these events should be targeted for avoidance, rather than all flights, and research into reliably forecasting ‘big hits’ should be undertaken. Avoidance of ice-supersaturation regions requires accurate prediction at least 24 hours in advance and meteorological forecast modelling needs to be improved as the capability to forecast persistent contrails is limited, says the report. The potential impacts of trade-offs from increased CO2 emissions as result of flight re-routings also need to be more thoroughly understood to ensure ‘no regret’ policies, it adds.
Reducing contrail cirrus impact, as well as improving air quality, could also be met without modification of flight trajectories or incurring an additional fuel consumption/CO2 penalty by reducing soot particle emissions. This could be achieved by using low-carbon sustainable aviation fuels (SAF). The reduction in the use of aromatics in fuel is seen as an important mitigation measure to reduce non-CO2 aviation emissions. SAF has shown a reduction in non-volatile particulate matter (nvPM) emissions in landing and take-off (LTO) operations and cruise due to their lower aromatic and sulphur content.
The report says there is scope for improving emission characteristics through the hydrotreatment of conventional fossil fuels to reduce aromatics and sulphur although the extra costs and energy requirements would need to be examined in order to balance the differential environmental benefits.
The global aircraft fleet NOx performance, in terms of certified data, is likely to improve as older high-NOx engine designs are replaced with new engine combustion technologies, with NOx emissions on a per passenger kilometre basis expected to show a reduction over time, although significant reductions may be limited. Levels of nvPM emissions are likely to improve as engines with technology designed for NOx control enter the fleet, although technologies to mitigate nvPM are less well understood than NOx. Beyond 2040-2050, hybrid-electric aircraft and revised configurations could offer significant reductions in NOx emissions.
Non-CO2 emissions charging
In the meantime, potential policy options to reduce non-CO2 climate impacts could include a NOx charge or the inclusion of aircraft NOx emissions in the EU ETS.
A charge on NOx would cover total NOx emissions over an entire flight and calculated using certified LTO NOx emissions data, the distance flown and a factor accounting for the relation between LTO and cruise emissions. The report cites a 2009 legal analysis that suggested neither ICAO’s Chicago Convention nor its recommended policies on taxes and charges prevented the implementation of such a measure. The charge would incentivise engine manufacturers to reduce LTO NOx emissions during their design process and airlines to minimise NOx emissions in operation, while taking into account associated trade-offs. Further research and monitoring is still needed on the climate impact of aviation NOx, caution the experts, but if there is the political will to take the option forward then they suggest the measure could potentially be implemented in five to eight years.
Incorporating aviation NOx emissions into the EU ETS would also take five to eight years and the same caveats over research and the uncertainty about the issue would apply, says the report, along with the incentive by manufacturers and airlines to reduce NOx. As existing EU ETS legislation uses the GWP100 metric to convert other greenhouse gases to CO2 equivalents for stationary installations, so it is assumed this would be the metric applied to the aviation sector. The measure could be implemented by adjusting the existing legislation and building on existing administrative processes and precedents, for example baseline, cap, auctioned allowances and MRV and accreditation. The same EU ETS geographical scope for aviation could be applied to NOx as that for CO2 emissions.
However, Arrowsmith told the webinar: “There is clearly uncertainty with regard to the climate impact and that was identified [by the experts] as a political risk in terms of the integrity of the EU ETS, recognising that as the science evolves, the intended effect of something that is put in place may change.”
Other policy measures could entail reducing the maximum volume concentration of aromatics within fuel uplifted at European airports and an EU blending mandate to boost the use of sustainable aviation fuels. If the political will was there to take these options forward, the aromatics measure could potentially be implemented in the five-to-eight-year period or perhaps longer, while a SAF blending mandate, already under consideration by the European Commission, could be achieved in a shorter timeframe, advises the report.
Another option, although more complicated and taking longer to implement, would be to levy a charge on the full climate impact of each individual flight, so having the broadest coverage of all the policy measures. The introduction of such a charge requires a good estimate of the climate costs at a flight level and, says the report, there is no scientific consensus on the methodology to calculate these costs.
“It could be argued that a levy that aims to internalise the external costs would be considered a charge and not a tax,” it says. “In this case, the charge would be related to recover the external costs of the climate impact of aviation.” Significant research is needed to develop and define this measure, it adds.
Arrowsmith said key messages to be taken from the report included a need to continuously review the latest scientific understanding on non-CO2 impacts and conduct further research, potentially through the EU’s planned Horizon Europe scientific research initiative, to increase certainty, consider different metrics and time horizons, enhance existing analytical methods to estimate aircraft non-CO2 emissions, and enhance capability to accurately predict the formation of persistent contrails. In addition, he said, there was a need to maintain and regularly review existing ICAO environmental certification standards on CO2, NOx and nvPM, and to incentivise the uptake of sustainable aviation fuels.
Responding to the report, Transport & Environment said the EU could not afford to wait the five to eight years proposed to implement aviation non-CO2 mitigation policies. It said the measures should be included in the Commission’s upcoming Sustainable and Smart Mobility Strategy that is due out shortly. Contrail avoidance should also be prioritised in the revision of the Single European Sky, it recommends, and pricing for non-CO2 emissions used to incentivise airlines to use eco-friendly flight paths.
“The European Commission was first tasked with addressing the non-CO2 emissions of flying in 2008. It shouldn’t waste any more time in implementing the solutions that are available today,” said Jo Dardenne, T&E’s Aviation Manager. “Contrails and other non-CO2 effects need to be urgently tackled to avert climate crisis.”