Current best estimates indicate that aviation contributes ~5% to global warming, with a significant proportion caused by non-CO2 effects. The largest of these non-CO2 effects is due to contrail cirrus, which also have the largest associated uncertainty. Another important effect is likely to be caused by aerosol-cloud interactions, although to date, due to the substantial challenges to simulate it in models, there are no best estimates for this effect. With several ambitious targets having been set for aviation to reduce its climate impact, there is an urgent need to improve our understanding of this impact today, together with developing reliable models suitable to advise on the most appropriate mitigation options. This project will make substantial advances in reducing the current uncertainty in aviation non-CO2 climate impacts by addressing its two largest sources: contrail cirrus and aerosol-cloud interactions. A major limiting factor in reducing the large uncertainty in these two aviation climate impact terms is the fact that only two climate models are currently able to simulate them, the German ECHAM and the American CESM models. Moreover, they employ very different methodologies, making identifying their main sources of uncertainty very challenging. Building on our team's expertise, we will further develop the capabilities of the UK Met Office climate model to simulate both contrail cirrus and aerosol-cloud interactions with methodologies consistent with both ECHAM and CESM. This will allow us for the first time to consistently quantify and compare these two important aviation climate effects in different climate models. In addition, we will also quantify how they are likely to change for a range of future aviation scenarios consistent with Net Zero CO2 strategies, including the use of alternative fuels (e.g. Sustainable Aviation Fuel and hydrogen), kerosene with direct air capture and storage, and contrail avoidance strategies. To develop these realistic scenarios, we will use our team's unique global aviation systems model which is able to account for aircraft operations developments, together with examining how they may change in the future given the current pledges and other system trends. Finally, we will explore and assess the most efficient technological and air traffic management solutions using our FaIR climate model emulator, one of only two calibrated probabilistic climate models used across the latest IPCC assessment report to quantify the global temperature response to emission scenarios. By making significant advances in our ability to robustly quantify the two largest sources of uncertainty in aviation climate impacts, our project will directly guide future aviation technology solutions by informing both policymakers and industry on the best future policy and investment decisions. Throughout the project, we will engage regularly with our project partners and other key stakeholders, including aircraft and jet engine manufacturers, airlines, government departments/agencies, and NGOs.

The main objective of this project is to collaborate with aviation stakeholders (airspace regulators, airline operators, air traffic controllers and engine manufacturers) to enable the UK airspace infrastructure sector to use existing environmental science to minimize the risk of volcanic ash to aircraft. The specific project partners for this project are the Civil Aviation Authority (the UK's airspace infrastructure regulator), and British Airways (one of the UK's largest airline operators). The challenge facing the CAA is that new aircraft engine susceptibility guidelines from the engine manufacturers describe engine tolerance limits in terms of a dosage (i.e accumulated concentration over time) rather than a peak concentration. However, there are no fit-for-purpose tools for the aviation industry to estimate along-flight volcanic ash dosage. The CAA thus need new tools to support any decision to change volcanic ash regulations from current peak ash concentration limits to along-flight ash dosage limits. The challenge facing BA is, given such a change in regulation, how to plan safe flight-routes and evaluate post-flight exposure to volcanic ash. In this project we will address these challenges by combining volcanic ash concentration charts and optimal flight-routing software to create a proof-of-concept tool which will allow along-flight ash dosages and the associated uncertainty to be calculated for the first time. We will use this tool to determine the sensitivity of along-flight ash dosage estimations to the spatial and temporal resolution of the volcanic ash information. This will be achieved by combining volcanic ash data, generated during the NERC funded PURE programme, with time optimum routing software, developed as part of the NERC funded EXTRA project. The new knowledge developed in the project will be used by the CAA to support strategic decision making and will enable new regulations to be developed that are based on the latest understanding of volcanic ash hazard to aircraft engines. These new regulations will result in a more resilient UK airspace infrastructure. The proof-of-concept tools developed in the project will demonstrate how airline operators, such as British Airways, could implement these changes in operational flight planning procedures. This tool will also encourage the use of uncertainty information in operational decision making procedures. In summary, the project will take existing research and translate it into information that is relevant to the aviation industry leading to clear benefits for the whole aviation sector. The project will last 12 months and cost £111k at 80% FEC

Clouds formed by aircraft (contrails) are the most easily visible human forcing of the climate system. Trapping energy in the Earth system, they contribute more than half of the total climate impact of aviation. This makes reducing contrails an important goal to achieve the UK's climate commitments. Theoretical considerations indicate two pathways for reducing contrails. First, improving engine design to emit fewer particulates may reduce contrail lifetimes and so their climate impact. Second, rerouting aircraft to avoid contrail forming regions. Assessing these pathways requires accurate models of contrail formation, evaluated at the level of an individual aircraft. This evaluation requires observations of contrails across their lifetime, coupled to details of the generating aircraft. Even where they are matched to specific aircraft, existing observations typically view a contrail once, (limiting their use for measuring contrail lifecycles) or cannot provide the detail on the contrail microphysical properties (such as ice crystal number or shape) necessary to assess the efficacy of different pathways to contrail reduction. Improving confidence in our contrail models urgently requires novel observations of contrail properties and lifecycles from individual aircraft. The impact of aircraft on clouds is not limited to contrails forming in clear air. Over half of contrails form embedded in existing clouds and the particulates emitted by aircraft can affect cloud formation several days after they were released. These effects produce a cooling, potentially large enough to offset all other warming effects of aviation, but are not represented in aircraft-level models used for planning contrail avoidance strategies. There are few observational constraints of these effects, targeted observations of the impact of individual aircraft on cloud microphysics are required to assess them and to improve future model simulations. To address these uncertainties and around contrail formation, persistance and climate impact as well as aerosol-cloud interactions, COBALT has three core components: 1. A measurement campaign in the southern UK, combining an array of ground-based cameras with a steerable cloud radar, to make high resolution observations of contrail formation from individual aircraft. Guided by aircraft transponder information, these observations will be focused on contrails and clouds modified by aircraft, characterising contrail formation and perturbed cloud properties within the first few hours of their lifecycle. 2. Counterpart satellite observations, using novel techniques to characterise contrail and cloud development from an hour to several days behind the aircraft. Building on techniques for studying natural cirrus, this will produce a complete characterisation of the contrail lifecycle, along with the first estimate of the aviation aerosol impact on existing cirrus clouds at a global scale. 3. The complete lifecycle characterisation will be combined with flight data from aircraft operators to produce a unique dataset designed specifically for the evaluation of aircraft-level models of contrail formation. An initial focus will be placed on evaluating aircraft-scale models, as these are currently being used to plan aircraft diversions. A comparison of climate model parametrisations of contrail formation will assess the ability of the parametrisations to reproduce the wide-area (>1000km2) contrail observations taken by the camera array. Led by an inter-disciplinary team of scientists and engineers, with partners in key international research centres and industry groups, COBALT will provide the tools necessary to evaluate our current models and ability to avoid contrails, guiding future modelling and operational trials of sustainable fuels and contrail avoidance.