Climate processes

Abstract:

Aviation’s climate impact continues to grow, with little progress toward emission reductions aligned with global targets. While technological advances attract attention, operational efficiency across aircraft, airlines, airports, city pairs, and regions remains underexplored. Here we assess carbon dioxide efficiency for 27.5 million flights between 26,156 city pairs in 2023, using data from Airline Data, International Civil Aviation Organization, International Air Transport Association. Results show wide variation: 32–890 gram carbon dioxide per revenue passenger kilometres across routes and 60–360 gram carbon dioxide per revenue passenger kilometre across aircraft models. Efficiency differs by region and is lowest in Africa, Australia, and Norway, and highest in Brazil, India, and Southeast Asia. Operating all routes at their demonstrated optimum could cut emissions by 10.7%. A theoretical 50% reduction is possible with the most efficient aircraft, all-economy layouts, and 95% load factors. Efficiency-focused policy could swiftly reduce fuel use without limiting air transport capacity.

Abstract:

<jats:p>Abstract. Cloud microphysics remains a major source of uncertainty in km-scale atmospheric models. While cloud-resolving models have advanced our understanding of cloud-climate interactions, their predictability remains limited. Most studies have examined either microphysics schemes or domain-size sensitivities, but their interactions are poorly understood. This study examines cloud structure and precipitation sensitivity to microphysics schemes and how they vary between regional and global configurations within a single, consistent modelling framework. We analyse three convection-permitting simulations over the Amazon: two regional runs employing single- and double-moment microphysics schemes and a global single-moment run, with all other configurations consistent. We find that cloud hydrometeor characteristics are sensitive to the microphysics scheme. Specifically, the double-moment scheme produces up to five times more graupel and twice as much rain, but half as much cloud water and one-fifth as much fog as the single-moment scheme. Despite these differences, precipitation, water vapour, and outgoing longwave radiation remain consistent across schemes, suggesting large-scale constraints primarily govern integrated quantities. Furthermore, domain configuration further amplifies sensitivities. The global simulation exhibits up to 150 % more fog and nearly double the cloud ice compared to the regional single-moment run, highlighting the role of large-scale circulation and lateral boundary conditions. These findings demonstrate that microphysics schemes primarily influence cloud processes, while the domain setup determines how these sensitivities manifest. Improved observational constraints and perturbed-parameter ensembles are therefore needed to evaluate model performance, assess the broader generalisability of these findings and separate tuning effects and structural uncertainty.</jats:p>