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4 Future Directions The panel's specific charge for this phase of its work was to provide guidance for the supersonic aircraft component of NASA's Atmospheric Effects of Aviation program, and the previous chapter of this report listed some specific recommendations for future research on this topic. However, because this will be the last report produced by this panel, and because (as of this writing) continued funding for AEAP is highly uncertain, the panel believes it is important to also express some general concerns about the future of research on the atmospheric effects of aviation, both subsonic and supersonic. These concerns are raised to help ensure that those responsible for making decisions about future research on this topic are fully aware of the benefits to be gained from maintaining a focused research program. Multiagency / Multidisciplinary Coordination. NASA's AEAP has provided an important focal point for coordinating the work of researchers from a variety of federal agencies and other organizations in studying the atmospheric effects of aviation. It has also provided an opportunity for modelers to work directly with the scientists who carry out field measurements and laboratory studies. Unless there is a program clearly designated to fulfill this role in the future, it seems likely that much of this effective coordination will be lost, which could result in important research gaps and the less efficient utilization of scarce research funds. Participation in International Assessments. Maintaining a focused U.S. research program on the atmospheric effects of aviation has consequences far
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beyond just the scientific community. International scientific assessments are playing an increasingly important role in guiding the formation of national environmental policies and regulations. U.S. scientists played a significant role in the production of the new IPCC assessment Aviation and the Global Atmosphere; but without a continuing research program on this issue, it is likely that they will play a much smaller role in any future assessments. The European Community, in particular, is maintaining a strong research program on this topic, and they have also begun to take a leadership role in developing more stringent regulations on aircraft emissions, including the first regulations for emissions at cruise altitude. Without an organizational center for research on this topic, the U.S. regulatory community, as well as aircraft manufacturers and airline operators, may ultimately be placed at a relative disadvantage with respect to these larger international efforts. Current Atmospheric Climatology. As technology evolves and demand for air travel increases, it seems highly likely that a substantial number of aircraft will be flying in the stratosphere within the next few decades (either supersonics or higher flying subsonics). Until that time, there is a unique window of opportunity to study the chemical climatology and dynamical structure of a (relatively) unperturbed stratosphere. Having an understanding of ''baseline'' conditions and near-term trends will make it much easier to forecast future conditions and to detect any chemical and dynamical changes that may occur in the future due to aircraft emissions or other anthropogenic perturbations. Future Stratospheric Aircraft. It must be stressed that the AESA assessment was restricted to studying the effects of only one type of aircraft (known as the Technology Concept Aircraft [TCA] [Baughcum et al., 1998]) that cruises at Mach 2.4 and has a NOxEI between 5 and 15. This restriction seems reasonable given the fact that the TCA was the only type of stratospheric aircraft that has been seriously considered for commercial production recently. It is quite possible, however, that other types of stratospheric aircraft may be considered in the future (for instance "hypersonic" aircraft that cruise at higher speed and altitude). In such a case, it is imperative that the assessment calculations be redone to specifically test the effects of the appropriate mach numbers and emission indices. This is important because some very preliminary studies have shown that hypersonic aircraft could have quite devastating effects on ozone (Oliver, 1994). Continuing Research Needs. It is important that certain issues continue to be studied even if the AEAP is terminated. There are several areas of research highlighted in this report that do not have an obvious "home" in other existing research programs. These include: further quantification of stratospheric dynamics and stratosphere-troposphere exchange, major intercomparison and validation exercises for chemical-transport models, and studies of fundamental aircraft
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engine combustion and particle formation processes. Also, as noted earlier, AEAP has helped sponsor several large field programs featuring coordinated in situ observations of multiple chemical species, which have led to numerous advances in understanding upper tropospheric and lower stratospheric chemistry. It is important that such programs continue, as they will help improve understanding of both aviation impacts and many other issues. Although this report focuses on issues associated with a fleet of HSCTs, it is also implicitly concerned with the stratospheric impacts of all types of aircraft. The current fleet of subsonic aircraft spends about 20 to 30 percent of cruising time in the lowermost stratosphere (10–12 km). It is likely that aircraft passenger use (passenger-km) and fuel use will continue to increase at 5 percent and 3 percent per year, respectively.5 For the foreseeable future, this will largely be a subsonic fleet. The effects of subsonic aviation have been studied under the aegis of the AEAP/SASS, which was recently reviewed by this panel (see NRC, 1999). Subsonic aviation impacts were also the main focus of the IPCC (1999) report Aviation and the Global Atmosphere . Some of the main points of future concern that these reports have identified are (1) the chemical and radiative impacts of contrail induced cirrus clouds, (2) impacts of chemically induced changes in the radiatively active gases methane and ozone, and (3) uncertainty in transport processes, in particular, stratosphere-troposphere exchange. For example, the increase in global cirrus cloud cover induced by subsonic aircraft contrails is estimated to be up to 0.2 percent for the late 1990s, and this may expand to up to 0.8 percent by the year 2050. These cirrus cloud increases could be much larger on a regional level in areas of heavy air traffic. Because cirrus clouds generally lead to greater trapping of infrared radiation, this would likely result in an increase in radiative forcing of climate; however, the uncertainty attached to this estimate is very large and clearly requires further study. As another example, NOx increases due to subsonic aircraft emissions are estimated to have increased the northern mid-latitude ozone column by about 6 percent in the early 1990s, and this figure could rise to 13 percent by 2050 (although the global mean column ozone changes are much smaller). The increases in ozone occur in a region of the atmosphere that is particularly sensitive to radiative forcing. This ozone increase is also calculated to result in an increase of OH, which in turn would decrease methane and reduce its radiative impact. Finally, transport of gases in the lower stratosphere and upper troposphere remains an important issue, regardless of the type of the aircraft, supersonic or subsonic. These will remain important issues to be addressed in the future by means of a comprehensive program of measurements and modeling. In particular we note 5 Using the IPCC (1992) IS92a scenario for growth, which is a mid-range growth estimate (IPCC, 1999)
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the emphasis placed on climate and chemistry modeling and underscore the requirements that this will impose on computing resources. A recent NRC report (NRC, 1998b) concerning U.S. climate modeling efforts recommends that better coordination of goals and objectives is necessary and that improvement of supercomputer facilities is required. These recommendations also apply to future work in assessing aviation's climate impacts, and thus the panel encourages more coordination between the various agencies charged with studying atmospheric chemistry and climate, as well as more attention to addressing concerns about improving U.S. supercomputing capabilities.
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