This project will use state-of-the-art models and observations to quantify the climatic effect of aviation non-CO2 emissions from current and future generation aircraft.

Global aviation emissions have grown strongly during the past 50 years, accelerating from an annually averaged growth rate of 2.2% over 1970-2012 to 5% during 2013-2018. While existing forecasts predict a continuation of this trend, the COVID-19 pandemic has raised new questions in terms of the evolution of air traffic during the next few decades (Forster et al., 2020).

Aviation already contributes about 2.5% of the total global anthropogenic CO2 emissions and its overall effect on climate is substantially larger. Current aviation radiative forcing estimates indicate that the contribution of air traffic to global warming is roughly 5% (2-14% uncertainty range), with a significant proportion (~60%) caused by non-CO2 effects. The largest of these non-CO2 effects are caused by aviation-induced cloudiness (AIC) (Kärcher, 2018) and emissions of NOx (leading to changes in ozone and methane), water vapour and aerosols.

Since 2010, the aviation industry has pledged to halve its global CO2 emissions by 2050 compared to 2005 levels. However, if we are to achieve the Paris Agreement long-term temperature goal of keeping global average temperature to well below 2C above pre-industrial levels, then even stronger aviation emission cuts are needed. A new aim of reaching a net zero-carbon aviation system has recently been proposed and in September 2020 Airbus have unveiled plans for the first commercial zero-emission aircraft fuelled by hydrogen that could be in service by 2035.

Despite significant progress in our understanding of the overall impact of air traffic on climate, large uncertainties remain especially in terms of AIC and aerosol-cloud interactions resulting from soot and sulphur emissions. With net zero-carbon aviation becoming a near-term target, providing reliable estimates of the aviation non-CO2 emissions from current and future generation aircraft is now more important than ever.

Objectives
The aim of this project is to investigate the climatic effect of aviation non-CO2 emissions from standard and future generation aircraft. The approach will involve a combination of radiation, chemistry-transport and emission-based climate models, together with simulations using the state-of-the-art UK Earth System Model. While relatively flexible to allow for your interests, the project is likely to involve:
- A comprehensive assessment of changes in contrail coverage due to COVID-19 restrictions during 2020 using satellite observations.
- Producing a range of scenarios for future air traffic, including the potential for a long-term shift driven by changing attitudes to flying.
- Extending the existing contrail parameterisation for the UK Met Office climate model (Rap et al., 2010) to hydrogen-fuelled aircraft.
- Quantifying the radiative forcing of non-CO2 aviation emissions (in particular AIC, and soot & sulphur aerosol emissions) for standard and hydrogen-fuelled aircraft.
- A detailed evaluation of the role of flight route optimisation (latitude/altitude effects) for standard and hydrogen-fuelled aircraft.
- Developing improved analytic response functions for aviation emissions within the Leeds-FaiR emission-based climate model (Smith et al., 2018).

Potential for high impact outcome
There are still large uncertainties in our understanding of the current aviation impact on climate and how it might evolve in the future. With access to state-of-the-art models and support from our world leading research groups, this project will address these uncertainties and will therefore have important implications for future climate projections. This will be of interest to both the general public and to policy makers working in climate mitigation and the transport sector.