Abstract: Contrail cirrus – ice clouds forming in aircraft exhaust - are estimated to account for 2% of global anthropogenic climate impact from all sources. Understanding how contrail impacts are affected by changes in aircraft technology and aviation practices, such as the timing and location of flights, is essential for developing effective mitigation strategies but relevant information is limited. Furthermore, investigations of contrail impacts have almost exclusively focused on single years, with no single consistent study of how contrail impacts have changed over time. This has made it difficult to interpret differences in impacts between studies of different years.

In this study, we investigate how the radiative forcing (climate impact) of contrails has evolved from 1980 to 2019. We simulate contrails sampled from an inventory of this period using the Aircraft Plume Chemistry, Emissions, and Microphysics Model (APCEMM). This model captures the dynamics of contrail formation, coagulation of solid and liquid particles, particle settling, and particle growth and evaporation. We use ERA5 reanalysis data to estimate prevailing atmospheric conditions and analyze results with particular emphasis on annual changes in the location, timing, and composition of emissions.

We find that, from 1980 to 2019, total contrail impacts have increased but the impact per unit of contrail distance has decreased. This is due to a combination of technological and policy factors. We discuss how falling soot emissions appear to have reduced contrail optical depth but not lifetime while increasing flight coverage over Europe has a greater impact than the increases over Asia. However, we also find that trends in these impacts are highly sensitive to the criteria used to determine whether a contrail is likely to have evaporated.

This work implies that differences between prior studies of contrail properties may be driven in large part by differences in contrail inventory. Our work also suggests that a small number of additional factors should be considered alongside total flight distance to understand how future changes in aviation are likely to affect its climate impacts.

Abstract: With renewed interest in commercial supersonic transport (SST) aircraft due to increased demand for air travel, the environmental impacts on ozone and climate from proposed supersonic fleets need to be analyzed. In this study we have examined two such proposed fleets developed by MIT, flying at Mach 1.6 with a cruise ceiling at 17 km and representing either high or low economic growth. The high scenario burns 43.1 Tg of fuel with 0.39 Tg NOx and 0.14 Tg BC emission whereas the numbers for the low scenario are 9.6 Tg, 0.008 Tg and 0.03 Tg respectively. The UIUC analyses was done using the global climate chemistry model CESM2 – WACCM6 with model top ~140km with comprehensive troposphere-stratosphere-mesosphere-lower-thermosphere chemistry at a horizontal resolution of 0.9^o×1.25^o, and 70 vertical levels, compared to the GEOS-Chem simulation done by the MIT group. Our presentation will also include a comparison of the UIUC and MIT findings. The UIUC study indicates a global ozone column reduction by 0.33% and 0.06% respectively for the high and low scenarios, which can be attributed to the NOx emissions. For both the scenarios, the maximum ozone loss in Northern Hemisphere (NH) occurs during the summer and early fall (June-October), and during late fall (April-May) in the Southern Hemisphere (SH). Although fraction of SST emission is lower in SH compared to NH, atmospheric transport results in significant ozone loss in SH too. Although CO₂, SO₂, and soot emissions contribute to climate impacts, we find that the largest impacts are likely due to stratospheric ozone and water vapor perturbations. The UIUC net non-CO₂, non-contrail stratospheric-adjusted radiative forcing (climate impact) from the high and the low scenario was 14.7 mW/m² and 4.26 mW/m² respectively, indicating an overall warming effect of the order of 10% of that from modern subsonic aviation which grows non-linearly with fleet size.

Abstract: Halogenated greenhouse gases (such as HCFCs, HFCs, PFCs, and SF6) have global warming potentials thousands to tens of thousands of times greater than carbon dioxide on a per kilogram basis. Estimating the emissions of these gases on a global scale is challenging since direct measurements are unavailable. Instead, they are inferred using measured global atmospheric concentrations and knowledge of their lifetimes. The ocean uptake for halogenated species can impact their lifetimes, but this process has been assumed to be largely negligible in the past. Further, reaction with hydroxyl radicals (OH) is a major atmospheric loss pathway for HCFCs and HFCs. Emission estimations usually assume OH is constant over time, but recent chemistry-climate models suggest OH increased after 1980, implying underestimated emissions. Here, we use a coupled atmosphere-ocean model to explore how the inferred lifetimes and emissions of certain HCFCs, HFCs, PFCs, and SF6 can be affected by ocean processes and time-varying OH. We show that by including the ocean uptake, the lifetimes are shortened by 2 – 15% for HCFCs and HFCs, and 20 – 40% for PFC-14 and SF6. Certain HCFCs and HFCs can be further destroyed in the ocean due to microbial activity; this could lead to up to an another 8 – 25% decrease in their lifetimes. We also show that increases in modeled OH imply an additional underestimation in HCFC and HFC emissions by ~10% near their respective peak emissions. These species are considered under the Montreal Protocol and its amendments and the Paris Agreement. Evaluating the success of these global agreements requires accurate knowledge of contributions to global warming from these gases and consideration of these processes.

Abstract: Large amounts of terrestrial carbon and nutrients are routed to the ocean through the Land-Ocean Aquatic Continuum (LOAC). Once in coastal waters, these terrestrial inputs impact ocean carbon chemistry. Lateral carbon export from rivers has been estimated to be responsible for global-ocean outgassing of roughly 0.45 Pg C yr^-1. However, the biogeochemical pathway for this outgassing has not yet been quantified. In this study, we have carried out a set of model sensitivity experiments, in which we introduce terrestrial carbon and nutrients in the ECCO-Darwin global-ocean biogeochemistry state estimate. We compute daily riverine export by combining the GlobalNEWS2.0 watershed model with point-source freshwater discharge from the JRA55-do atmospheric reanalysis. We quantify the litter and soil carbon pool for mangrove forests worldwide and the tidally-driven flux from this intertidal carbon pool to the open ocean following the time-volume change of water estimated from the combination of the FES2014 barotropic tidal model and the Global LiDAR Lowland Digital Terrain Model. We evaluate the impact of terrestrial exports on the global ocean by comparing a suite of experiments against a baseline simulation that does not include terrestrial carbon and nutrient export for the 1995–2017 period. Our study explores the role of terrestrial carbon and nutrients in the ocean’s biological and carbon chemistry. By including processes that occur at the land-ocean interface, we aim for an improved understanding of how the LOAC impacts global carbon cycling.