Interview with Nicolas Bellouin by Sorbonne University

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.

This article is republished by Sorbonne University. Read the original article..

The solutions to reduce the climate impact of aviation

Move the mouse over the table to get more information.

Technological and operational solutions CO2 Contrails/
Induced cirrus
NOx Deployment complexity Timeframe for large-scale deployment
Carbon offsetting
Reduces CO2 effects but effectiveness and quality of compensation variable and difficult to verify
Avoiding areas where contrails are formed
Slight increase due to change of course
Slight increase but possible decrease with lower altitude


Introduction of metrics to identify trade-offs between CO2 and non-CO2 effects to ensure a beneficial effect on the climate
10-15 years
Formation flights
Slight decrease due to fuel savings
Little or no effect?
Slight decrease due to fuel savings


More constraints on flight planning and air traffic management
De-aromatized fuel
Small decrease during flight but potential increase during production
Reduced radiative effects in the absence of aromatics?


Introduction of a new fuel category
CO2 reduction compared to kerosene (life cycle)
Reduced radiative effects in the absence of aromatics?


Availability of sustainable biomass for production, investment and scale-up of the industry, cost.
15-25 years
Potentially neutral if made from atmospheric CO2 and decarbonised electricity
Reduced radiative effects in the absence of aromatics?


Technological maturity, energy efficiency and the need for decarbonised electricity, cost.
Potentially CO2 neutral if made from low-carbon energy sources
More frequent? But potentially lower optical thickness and shorter life?

Very high

Complete redesign of aircraft and refuelling infrastructure. Associated investment. Production development. Cost.
>30 years

Source: Updated analysis of the non-CO2 climate impacts of aviation and potential policy measures pursuant to the EU Emissions, European Union Aviation Safety Agency (EASA), 2020

What fraction of the CO2 radiative forcing can be attributed to aviation?

The article written by Olivier Boucher, Audran Borella, Thomas Gasser and Didier Hauglustaine can be found at:

Estimating how much of the CO2 radiative forcing can be attributed to the aviation sector may sound easy. Indeed CO2 emissions from aviation are well known; the rise in CO2 atmospheric concentration is well observed and the radiative impacts of CO2 are well understood and quantified. However, there are also a number of complicating factors: the CO2 radiative forcing depends logarithmically with the change in atmospheric concentration and the efficacy of natural sinks of CO2 is changing over time. .All these effects need to be accounted for if a proper attribution is to be made.

A popular method, used by Lee et al. (2021) and others, is the residual attribution method, whereby the radiative forcing for a particular sector (the aviation sector in this case) is calculated as the difference between the total CO2 radiative forcing and the CO2 radiative forcing should that particular sector had not existed. However this method suffers from a major drawback that was overlooked by previous authors. Since the CO2 radiative forcing is not linear in concentration, the total radiative forcing from all sectors considered together is not the same as the sum of the radiative forcings from each sector considered individually. Furthermore aviation is different from many other sectors in that it has occurred relatively late in the industrial period. It is thus essential to differentiate the impact of early and late emissions because they do not contribute equally to the current atmospheric concentration and radiative forcing. Aviation started only a few decades ago, its emissions can therefore contribute relatively more to the change in CO2 concentrations, but relatively less to the CO2 radiative forcing because of the logarithmic dependence.

Different methods exist to address those issues. In this study we used the proportional, differential, and time-sliced attribution methods. The last two methods require to compute the CO2 concentration at time t due to emissions from aviation and all anthropogenic activities up to a time t’ before time t. We have used the OSCAR compact Earth System model and historical CO2 emissions data to estimate the different values. This allows us to account for how the CO2 concentration decreases as natural sinks sequestrate the emitted CO2 over time.

We found that the more rigorous methods (the proportional, differential, time-sliced methods) lead to aviation CO2 radiative forcing 20%, 13%, and 12% larger than the marginal method which underestimates the true CO2 radiative forcing by aviation. However, this is compensated by the lower contribution to the increase in CO2 atmospheric concentration that we estimated using our well calibrated model. We estimate that aviation contributed 2.18 ppm to the rise in CO2 atmospheric concentration in 2018, which is less than the values of 2.9, 2.4 and 2.4 ppm found in a previous study relying on less sophisticated models. Our study thus provides a clear basis and methodology for future assessments of the aviation impact on the carbon cycle and CO2 radiative forcing.