The project ‘REACT4C’ explored the feasibility of operational measures such as flight altitude and route changes to reduce the climate impact from aviation. Simplified mitigation studies were conducted as part of the project to quantify the environmental benefit of simplified Air Traffic Management (ATM) measures and these results were used to formulate general principles of environmentally optimized flight on particular timescales and for specific climate effects. The emissions model, FAST, was used to estimate the global emissions from civil aviation for the year 2006 and two mitigation scenarios, where the cruise altitudes were shifted higher and lower by one flight level or 2,000 ft. It was found that the fuel increased by 1.3% when flying lower and decreased by 0.8% when flying higher. The changes in the atmospheric chemical composition caused by aviation NOx emissions such as ozone (short- and long-lived), methane and stratospheric water vapour, were identified and quantified by five Chemical Transport Models (CTMs). These resulted in a mean net global radiative forcing (RF) increase of 1.6 mW/m2 for the ‘flying higher’ case and a decrease of 2 mW/m2 for the ‘flying lower’ case. Similarly, the direct global RF impacts from aerosols and the indirect impacts on cloudiness (soot-cirrus and contrails-cirrus) were also found to be beneficial to climate for the flying lower case, decreasing the RF by 0.2 mW/m2 for soot-cirrus and 5 mW/m2 for contrail-cirrus. The biggest contributor to aviation RF on a short-term horizon was the impacts on cloudiness (contrails-cirrus and soot-cirrus). The individual short-term impacts were also compared with the longer-term CO2 impacts using other metrics such as Global Warming Potential (GWP) and Global Temperature Potential (GTP). It was found that the case for cruising at ‘lower’ altitudes produced less non-CO2 effects. However, these mitigation scenarios were idealized and were ultimately used to formulate general principles of climate-optimized flight. The implementation of such simplified measures may not be feasible due to ATM operational restrictions.
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