Studies towards assessing the effects of aviation on climate

Emissions from aviation are an important component in the overall concerns about the effects of human activities on climate. Aviation emissions modify the chemical and physical properties of the upper troposphere and lower stratosphere (UTLS) in various ways. Aircraft emit gases and particles that can either directly or indirectly affect climate and air quality, including: carbon dioxide (CO2); nitrogen oxides (NOx) that can increase ozone (O3) production and increase the destruction of methane (CH4); water vapor that under certain atmospheric conditions can lead to contrail formation; and soot and other particles that along with contrails can affect the amount and characteristics of cirrus clouds. Soot and sulfate particles can also change the cloudiness by acting as cloud condensation nuclei. Due to the high growth in air traffic that is projected to continue, it is important to understand the effects of aviation on air quality and climate. Based on then existing analyses of the emissions and their effects, the aviation contribution in changing the radiative forcing on the climate system was about 5% of the total human-related emissions (relative to 1750) in 2005 (Lee et al., 2009). This contribution is a result of various effects, especially the direct effects of CO2, NOx-induced effects, aerosol direct and indirect effects, and increased cloudiness from contrail formation and aerosols acting as cloud condensation nuclei. One of the main challenges of the aviation scientific community has been to increase the level of scientific understanding of these effects, especially with respect to those most uncertain (i.e. NOx effects, contrail-cirrus and aerosol effects). Another challenge has been to develop a simple climate model (SCM) that has the level of sophistication necessary to accurately assess aviation induced climate effects while being easy to use by policy makers for use in policy considerations. The main objectives in this study were: (1) to evaluate the capabilities and limitations of simple climate models for evaluating aviation policy options and tradeoffs, and (2) to increase the scientific understanding of aviation NOx-induced effects on climate. With regard to the first goal, enhancing the evaluation of SCMs, the carbon cycle and energy balance treatments in several widely used simplified climate models were evaluated. The findings from this study resulted in modifications to the carbon cycle and energy balance model components of the APMT model that is used extensively by FAA in aviation policy analyses. With regard to the second goal, 3 lines of research were pursued to increase the scientific understanding of aviation NOx-induced effects on climate. First, aviation NOx-induced effects were quantified using three-dimensional (3-D) climate-chemistry models and further, an intercomparison of NOx-induced effects in 3-D climate-chemistry models was performed. The NOx-induced forcings obtained in 3-D simulations were further used to update the parameterization of these effects in SCMs. Second, two additional NOx-induced effects (i.e., reduction in long-term O3 concentrations and lower stratospheric water vapor (SWV)) that have not been fully accounted for in previous studies were quantified based on parameterizing the results obtained in the 3-D simulations. Results indicate that the inclusion of long-term O3 and SWV RFs decreases the net aviation-induced RFs by about 21 to 31% for different range of scenarios studied. Finally, the representation of aviation NOx-induced effects in SCMs were evaluated and improved. The parameterization was improved based on the results of the 3-D simulations and by including the lifetime of the perturbed species and their emissions history into RF calculations. This resulted in 10 to 36% higher aviation NOx-induced net forcing than the net forcings that were reported in the literature, previously. Third, a set of experiments were performed to directly calculate the aviation NOx-induced changes in CH4 that were otherwise calculated through a simple parameterization, and also to determine the CH4 feedback factor on its own lifetime. Moreover, the accuracy of the parameterization that is used for the calculation of NOx-induced changes in CH4 was evaluated. Results indicate that parameterizing the change in CH4 concentration based on the change in its lifetime overestimates the change in CH4 by 8.2% while decreases the computational requirements by nearly a factor of 8