Environmental impacts from the aviation sector are in continuous growth. The total sector contribution to anthropogenic climate forcing is approximately 3.5%, representing up to 9% of US greenhouse gas emissions from transportation in 2018. Despite the COVID-19 crisis, it is also expected to grow at a global rate of approximately 4% per year in the next 20 years, and a full sector recovery is expected by 2024. The total impacts of aviation emissions on the climate, however, are still uncertain. This is due to factors including (i) the uncertainty regarding the radiative effects of short- and long-term climate forcers; (ii) the difficulty of validating modeling tools e.g. contrail formation and persistence or stratospheric chemical response to emissions; and (iii) the growing interest in new air transportation technologies such as unmanned aerial vehicles, supersonic aviation, or hydrogen and alternative fuels. These factors together require a persistent effort to improve the available tools assessing aviation environmental footprint. The objective of this thesis is to provide additional insights into aviation climate impacts, by improving current modeling capabilities. Specially, I aim to resolve elements that will be of increasing interest as the sector evolves. The work is divided into two parts. The first part focuses on improving climate impact estimates from contrails, ice clouds which form behind aircraft. Those are estimated to cause approximately half of the total climate forcing from aviation. The second part focuses on developing modeling tools for assessing climate impacts from future commercially viable supersonic fleets, as multiple companies are currently designing projects of that type (Aerion, Boom, Spike Aerospace, NASA, Lockheed Martin, etc.). In the first part, I develop a new contrail radiative forcing model with a new parameterization to model exchanges of radiation when multiple cloud layers overlap occur. My parameterization also reduces current uncertainties related to uncertainties in contrail microphysical structure. I find that, assuming maximum possible overlap, cloud-contrail overlap in 2015 increased the net radiative forcing from contrails. This effect was greatest in the North-Atlantic corridor. For 2015, contrail-contrail overlap results in a 3% net reduction in the estimated radiative forcing. Finally, using "in situ" measurements to constrain contrail microphysical evolution pathways, I find that the global net radiative forcing due to contrails in 2015 is between 8.6 and 10.7 mW/m2. Relative to the mid-point, this uncertainty range is less than one quarter of that previously reported in the literature. In the second part, I estimate the sensitivity of the global supersonic market and its climate impacts to factors such as design choice, regulations and economic assumptions. For this, I develop a detailed supersonic aircraft design model providing robust information on cruise altitude, fuel burn and emissions variation with aircraft design choice. I also, in order to address overland restrictions, develop a high-resolution routing algorithm, capable of assessing optimal routing for multiple regulatory options. I obtain that, in the absence of flight path restrictions, a fleet of 130-870 supersonic aircraft can be feasible, operating up to 2.5% of the seat-kilometers in the global aviation market. This will result in a net increase of fuel burn from commercial passenger aviation of up to 7%. However, between 78% and 100% of the global unrestricted market potentials cannot be addressed when supersonic flight is restricted over land or over areas with a population density of more than 50 inhabitants per square kilometer. When evaluating environmental impacts, aircraft design choice can change the sign of supersonic aviation impact on non-CO2 aviation climate forcing. In general, implementing supersonic aviation results in a global warming effect. However, if reducing fleet average NOx emission index by 58%, through an increase in fuel burn of 7%, climate forcing can change from positive (increase) to negative (reduction). Designs aiming to address high-value demand, at the upper bound of supersonic speeds (cruise Mach number = 2.2), are the most environmentally harmful because of their higher cruise altitude and fuel burn. While based on my results, we shouldn't expect any significant viable market from them, a 10% fleet substitution would be responsible of a doubling in global non-CO2 radiative forcing impact.

Aircraft affect global climate through emissions of greenhouse gases and their precursors and by altering cirrus cloudiness. Changes in operations and design of future aircraft may be necessary to meet goals for limiting climate change. One method for reducing climate impacts involves designing aircraft to fly at altitudes where the impacts of NOx emissions are less severe and persistent contrail formation is less likely. By considering these altitude effects and additionally applying climate mitigation technologies, impacts can be reduced by 45-70% with simultaneous savings in total operating costs. Uncertainty is assessed, demonstrating that relative climate impact savings can be expected despite large scientific uncertainties. Strategies for improving climate performance of existing aircraft are also explored, revealing potential climate impact savings of 20-40%, traded for a 2% increase in total operating costs and reduced maximum range.

Under the Federal Aviation Administration's (FAA) Aviation Climate Change Research Initiative (ACCRI), non-CO2 climatic impacts of commercial aviation are assessed for current (2006) and for future (2050) baseline and mitigation scenarios. The effects of the non-CO2 aircraft emissions are examined using a number of advanced climate and atmospheric chemistry transport models. Radiative forcing (RF) estimates for individual forcing effects are provided as a range for comparison against those published in the literature. Preliminary results for selected RF components for 2050 scenarios indicate that a 2% increase in fuel efficiency and a decrease in NOx emissions due to advanced aircraft technologies and operational procedures, as well as the introduction of renewable alternative fuels, will significantly decrease future aviation climate impacts. In particular, the use of renewable fuels will further decrease RF associated with sulfate aerosol and black carbon. While this focused ACCRI program effort has yielded significant new knowledge, fundamental uncertainties remain in our understanding of aviation climate impacts. These include several chemical and physical processes associated with NOx-O3-CH4 interactions and the formation of aviation-produced contrails and the effects of aviation soot aerosols on cirrus clouds as well as on deriving a measure of change in temperature from RF for aviation non-CO2 climate impacts -- an important metric that informs decision-making.

Aircraft affect global climate through emissions of greenhouse gases and their precursors and by altering cirrus cloudiness. Changes in operations and design of future aircraft may be necessary to meet goals for limiting climate change. One method for reducing climate impacts involves designing aircraft to fly at altitudes where the impacts of NOx emissions are less severe and persistent contrail formation is less likely. By considering these altitude effects and additionally applying climate mitigation technologies, impacts can be reduced by 45-70% with simultaneous savings in total operating costs. Uncertainty is assessed, demonstrating that relative climate impact savings can be expected despite large scientific uncertainties. Strategies for improving climate performance of existing aircraft are also explored, revealing potential climate impact savings of 20-40%, traded for a 2% increase in total operating costs and reduced maximum range.

A discussion of the opportunities and challenges involved in mitigating greenhouse gas emissions from passenger travel.

This Intergovernmental Panel on Climate Change Special Report is the most comprehensive assessment available on the effects of aviation on the global atmosphere. The report considers all the gases and particles emitted by aircraft that modify the chemical properties of the atmosphere, leading to changes in radiative properties and climate change, and modification of the ozone layer, leading to changes in ultraviolet radiation reaching the Earth. This volume provides accurate, unbiased, policy-relevant information and is designed to serve the aviation industry and the expert and policymaking communities.