Stratospheric flight environmental impact: Analysis of trends and tradeoffs
When jetliners fly in the stratosphere, their emissions tend to be longer-lived and therefore have greater environmental impact. Since the bottom of the stratosphere can be lower than the cruise altitude of most commercial flights, the amount of stratospheric emissions must not be overlooked. The impacts of stratospheric emissions are different from tropospheric emissions, and the amount of stratospheric emissions today need to be evaluated. On the other hand, some studies suggest placing more flights into the stratosphere, as flying in the stratosphere can significantly reduce the presence of contrails. The tradeoff between the effect of contrails and that of stratospheric emissions is still unclear, but contrails can often be avoided without entering the stratosphere. In this study, we develop simple quantitative ways of assessing current stratospheric fuel burn using publicly available data, and then we develop a way of assessing stratospheric flight strategies. Our analysis covers 78% of the total travelled distance reported by the United States Bureau of Transportation Statistics, and shows that these flights burned ~ 9 million tons of fuel annually, or ~ 25% of cruise fuel, in the stratosphere between 2008 and 2012. Our results also show that the chance of flying into stratosphere varies by area because of variations in the tropopause height, but flights within the contiguous United States tend to stay below the stratosphere. The stratosphere fuel burn of Asia-US flights may be significantly reduced by taking jet stream routes since the stratosphere is lower near the poles. For the feasibility of contrail avoidance, our result showed that the chance of finding an Ice-Supersaturated-free region within 1000 ft. of the current flight level below the tropopause is significant for mid-latitude regions. We also found that if the region right below the tropopause is occupied by an Ice-Supersaturated Region (ISSR), this ISSR tends to be thicker. Thus, if a flight in the uppermost tropopause observes ISSR along the route, it may be easier to avoid the ISSR by raising the flight altitude into the stratosphere. However, thicker ISSRs along the routes require larger movements, while thinner ISSRs are harder to be forecasted. Flying into the stratosphere may be the best solution to avoiding contrails at this moment, but studies on related tradeoffs are necessary in the future.
Marais, Purdue University.
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