These “non-CO2” effects are related to nitrogen oxides (NOx), water vapour, and particulate matter – also known as aerosols when these particles are airborne – emitted during the combustion of paraffin by engines.
Under current conditions, it is commonly accepted that the warming effect of increased ozone due to aviation outweighs the cooling effect of decreased methane. The resulting effect depends not only on the amount of NOx emitted, which depends on the aircraft models, but also on the altitude, latitude and season of flight.
Water vapour emitted by aircraft is a greenhouse gas that tends to warm the climate when emitted at altitude. But, above all, it is the source of contrails when atmospheric conditions allow their formation – and the occasional transformation of these into cirrus clouds.
In the latter case, we speak of “contrail-induced cirrus clouds”; the ice that makes up the cirrus clouds would not have condensed without the passage of the aircraft or it would have done so only later.
Finally, the particles emitted by aircraft can modify the optical properties of the different types of clouds they encounter before being carried away by rain or deposited on the surface. These effects are, however, very complex and still very uncertain, so that even their warming or cooling character remains unknown.
Different time scales
There are several different time scales associated with these different effects.
Conversely, NOx, particles and water vapour disappear from the atmosphere after a few weeks if they are emitted at the cruising altitude of aircraft. The same applies to their induced effects on ozone or clouds.
On the other hand, the effects of NOx emitted by aviation on methane materialise on an intermediate time scale since the lifetime of methane in the atmosphere following a disturbance is 12 years.
Beyond the time scales associated with the lifetimes of chemical species, we must also consider those of the climate system itself. A disturbance of energy introduced into the climate system, even over a short period of time, has a lasting impact on the climate, as the ocean absorbs this additional energy before gradually releasing it back into the atmosphere.
The radiative forcing index is a bad idea for accounting for emissions
Climate change is usually measured by the concept of “radiative forcing”: this quantity measures the radiative imbalance of the planet due to past emissions and is expressed in relation to a reference period generally set at 1850, a time when industrial activities were still low.
For CO2, which has a long lifetime in the atmosphere, and to a lesser extent methane, this incorporates past emissions that have a lasting impact on atmospheric concentrations. For short-lived pollutants, only the most recent emissions matter, as the oldest emissions no longer exert a radiative forcing.
For the aviation sector, according to current knowledge, non-CO2 effects are responsible for a positive radiative forcing that tends to warm the climate. The ratio of total radiative forcing to the radiative forcing due to CO2 is called the Radiative Forcing Index (or RFI).
Some carbon footprint calculators use the RFI as a multiplicative factor of CO2 emissions to take into account non-CO2 effects and thus “convert” CO2 emissions into “CO2-equivalent”. However, we believe that this does not make much sense.
To convince ourselves of this, we can perform the following thought experiment: let’s assume that before the Covid-19 crisis, in 2019, non-CO2 effects are responsible for twice the radiative forcing of CO2, which corresponds to an RFI of 3 (i.e., (2 + 1)/1). One ton of CO2 emitted by aviation would therefore correspond to 3 tons of “CO2-equivalent”.
The radiative forcing from non-CO2 effects, on the contrary, decreased in line with the decrease in traffic (even by more than a factor of 4, as the methane effects of past emissions persist over time). During Covid-19, taking into account the same radiative forcing for CO2 – which increases only slightly from one year to the next – but a forcing divided by 4 for the “non-CO2” effects to take into account the reduction in traffic in 2020, we arrive at an RFI of 1.5 (i.e. (2/4 + 1)/1). One ton of CO2 emitted by aviation would therefore correspond to only 1.5 tons of CO2-equivalent instead of the 3 tons of CO2-equivalent before Covid.
This is nonsense because flights in spring 2020 will of course have the same climate impact as the same flights in spring 2019! So there is no reason why flights in 2020 should “count” half as much as those in 2019.
The fundamental reason why RFI is not appropriate as a multiplier is that radiative forcing accumulates the effects of past emissions, whereas we wish to compare the climate effects of current emissions, either to allocate an aviation user’s fair share of emissions, or to assess different technological or operational options that could be implemented in the future.
Which climate metric to use?
Fortunately, there are climate change metrics that can be used to estimate the future climate impact of a flight today, despite the short time scale of radiative forcing from non-CO2 effects.
In particular, the Global Warming Potential (GWP), which measures the radiative impact over a future period, typically 100 years, of emissions that occur at a point in time. The GWP of one kg of pollutant (such as CH4 or NOx) can then be compared with that of one kg of CO2, and the concept can easily be extended to aircraft trails.
Another metric, the Global Temperature change Potential (GTP), is defined in a similar way, but based on the change in average surface temperature at a certain time (50 or 100 years) after an emission pulse.
These metrics lead to much lower CO2 multiplication factors than the RFI unless time frames much shorter than 50 years are chosen. The choice of time frame is a political choice that can have important implications. A short time frame neglects the substantial part of the CO2 warming that occurs beyond the time frame. Choosing a long time frame may minimise the short-term effectiveness of warming reduction solutions based on non-CO2 effects.
To illustrate the importance of the metric used to estimate the total climate impact of aviation or when calculating the carbon footprint, let us compare the RFI of aviation with the multiplicative factors associated with these other metrics.
According to a recent estimate, the various radiative forcings from aviation result in an RFI of 2.9. If the GWP at a time horizon of 100 years is used to calculate the equivalent emissions of the different aviation disturbances, a multiplication factor of 1.7 is derived. For the TMP with a time horizon of 100 years, the multiplication factor is only 1.1.
This shows that the choice of one metric rather than another is crucial for calculating the multiplication factor.
A decision aid
Some of the solutions envisaged to reduce the climate impacts of aviation have the dual advantage of reducing both CO2 and non-CO2 effects.
In this case, the metrics simply serve to quantify the net gain for the climate. In contrast, other solutions require a trade-off between the CO2 and non-CO2 effects of aviation. For example, engine manufacturers know how to reduce NOx emissions but often at the expense of CO2 emissions. A partially or fully decarbonised fuel, such as hydrogen, could lead to greater non-CO2 effects.
Strategies for modifying aircraft trajectories to reduce the effects of contrails or NOx can also be envisaged, but at the cost of increased fuel consumption and therefore CO2 emissions. In these cases, it is relevant to compare the different effects (CO2 and non-CO2) with several adapted metrics to understand which effect prevails over the other and at what timescale (20, 50 and 100 years for example) and to be able to take the best possible decisions.
As far as we know, the non-CO2 effects of aviation have, on the whole, a warming effect on the climate. It is therefore relevant to try to reduce them in order to reduce the overall warming impact of aviation.
It is also important to ensure that the technologies being developed to decarbonise aviation do not induce excessive non-CO2 effects. Each solution must be examined and its impacts assessed using the most appropriate climate metrics, without forgetting to take into account other possible impacts (air quality, noise, biodiversity, etc.).
Climaviation, an innovative project to reduce the impact of aviation on the climate
Interview with Nicolas Bellouin, climatologist at the Institut Pierre-Simon Laplace, conducted by Sorbonne University.
The Climaviation project aims to better understand and quantify the climate impacts of aviation. It is led by Nicolas Bellouin, a climatologist in the United Kingdom and holder of the Aviation and Climate Chair at the Institut Pierre-Simon Laplace (IPSL). This contributor to the sixth IPCC report explains the objectives of this ambitious project, which brings together scientists from the IPSL and the French Aerospace Lab (ONERA).
In what context was the Climaviation project born?
Nicolas Bellouin: Faced with global warming and the need to reduce carbon dioxide (CO2) emissions, the aviation industry has embarked on a worldwide decarbonisation strategy. The task is particularly difficult for this economic sector where CO2 remains very present and where each innovation envisaged on aircraft must be tested and approved before being implemented.
In this context, the French Civil Aviation Authority (DGAC) has funded the Climaviation project over five years to explore different solutions to reduce the climate impact of aviation.
What is the objective of this project?
N. B.: Everyone knows that aviation emits CO2 and that these emissions must be reduced. But CO2 is not the only culprit. Aircraft engines emit other compounds: nitrogen oxides, water vapour and particles. Under the right conditions, water vapour and particles form contrails behind the aircraft. Some of these contrails persist and continue to expand, forming large fields of ice clouds that disrupt the Earth’s radiative balance. This is one of the so-called “non-CO2” effects of aviation.
According to recent climate modelling, the impact of these effects could be greater than that of CO2. But it remains uncertain because of the complexity of the mechanisms to be modelled and the scales to be taken into account in the simulations. Many questions arise concerning the size and properties of the cloud cover induced by contrails, their lifetime in the atmosphere, the formation and composition of ice crystals, the impact of a fuel change on the chemistry of the atmosphere, etc.
The aim of the Climaviation project is therefore to understand and quantify these effects in order to take them into account in climate impact reduction strategies.
What solutions are you exploring to reduce the climate impact of aviation?
N. B.: The aviation industry is intensifying its efforts to improve the efficiency of existing engines or to use alternative fuels with a low carbon footprint, or even new decarbonised energy carriers such as hydrogen.
We are also looking at alternative strategies that rely heavily on the existing fleet: changing flight altitudes, using updrafts, adjusting flight times, and so on. While these strategies do not necessarily require technological change, it is necessary to verify their effectiveness and measure their impact in the short and long term.
The DGAC therefore needs our scientific advice to determine which of these solutions not only reduce CO2 emissions but also limit non-CO2 effects.
This is a multidisciplinary research project combining the strengths of Sorbonne University and ONERA. How is this collaboration organised?
N. B.: This project brings together around thirty scientists. They include atmospheric physicists, cloud physicists, chemists, observers, specialists in automatic pattern detection, etc.
ONERA scientists know how to model the impact on the atmosphere of an engine or fuel change on a space-time scale of a few seconds and a few metres behind the aircraft. At IPSL, we model what happens on much larger scales: on a global level and over several hours, years or even centuries. Through our collaboration, we are trying to bridge the gap between these two orders of magnitude.
Our ambition is to connect ONERA’s models to the climate models developed by IPSL in order to build perennial scientific tools that can be used to estimate the climate impact of any new solution proposed in aviation.