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Don Wuebbles, Piers Forster, Helen Rogers, and Redina Herman

Metrics such as radiative forcing and global warming potential have proven to be useful tools in climate policy–related studies, including evaluation of the effects of aviation on climate, to relate different emissions to one another in order to maximize the application of mitigation policies and their benefits. In order to be an effective tool for policymakers and their communication with scientists and industry, a metric should be easy to use and as scientifically well grounded as possible. Thus, the best metrics will be simple and will include uncertainties that reflect the state of knowledge in order to give users confidence in their scientific quality. A concern with developing new metrics is the need to weigh their applicability against the ease of understanding the results. Radiative forcing is commonly used in analyses of aviation effects on climate and is integral to other metrics, but it has known deficiencies. Well-recognized metrics like global warming potential and global temperature potential are dependent on radiative forcing but also have their own advantages and recognized limitations. Simplified integrated assessment modeling may eventually represent a useful alternative to such metrics. The objective of this study is to examine the capabilities and limitations of current climate metrics in the context of the aviation impact on climate change, to analyze key uncertainties associated with these metrics and, to the extent possible, to make recommendations on future research and development of metrics to gauge aviation-induced climate change that could potentially affect decision making, including aircraft design and operations.

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Andrew Heymsfield, Darrel Baumgardner, Paul DeMott, Piers Forster, Klaus Gierens, and Bernd Kärcher

This article reviews the current state of understanding of the science of contrails: 1) how they are formed, 2) their microphysical properties as they evolve into contrail cirrus and whether their microphysical properties can be distinguished from natural cirrus, and 3) the ice-nucleating properties of soot aerosols and whether these aerosols can nucleate cirrus crystals. Key gaps and underlying uncertainties in our understanding of contrails and their effect on local, regional, and global climate are identified. These include 1) better quantification of the fraction of ice number and mass that survives the vortex phase and the aircraft-specific influences on the vortex dynamics, 2) more accurate measurements of the ice crystal size distributions of contrail cirrus and cirrus in general, which are uncertain because of instrument limitations, and 3) more measurements of the ice-nucleating properties of aircraft exhaust and other ambient ice nuclei in situ under cirrus-forming conditions. Future field campaigns aimed at satisfying measurement needs are proposed.

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Steven C. Sherwood, Sandrine Bony, Olivier Boucher, Chris Bretherton, Piers M. Forster, Jonathan M. Gregory, and Bjorn Stevens


The traditional forcing–feedback framework has provided an indispensable basis for discussing global climate changes. However, as analysis of model behavior has become more detailed, shortcomings and ambiguities in the framework have become more evident, and physical effects unaccounted for by the traditional framework have become interesting. In particular, the new concept of adjustments, which are responses to forcings that are not mediated by the global-mean temperature, has emerged. This concept, related to the older ones of climate efficacy and stratospheric adjustment, is a more physical way of capturing unique responses to specific forcings. We present a pedagogical review of the adjustment concept, why it is important, and how it can be used. The concept is particularly useful for aerosols, where it helps to organize what has become a complex array of forcing mechanisms. It also helps clarify issues around cloud and hydrological response, transient versus equilibrium climate change, and geoengineering.

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Guy P. Brasseur, Mohan Gupta, Bruce E. Anderson, Sathya Balasubramanian, Steven Barrett, David Duda, Gregg Fleming, Piers M. Forster, Jan Fuglestvedt, Andrew Gettelman, Rangasayi N. Halthore, S. Daniel Jacob, Mark Z. Jacobson, Arezoo Khodayari, Kuo-Nan Liou, Marianne T. Lund, Richard C. Miake-Lye, Patrick Minnis, Seth Olsen, Joyce E. Penner, Ronald Prinn, Ulrich Schumann, Henry B. Selkirk, Andrei Sokolov, Nadine Unger, Philip Wolfe, Hsi-Wu Wong, Donald W. Wuebbles, Bingqi Yi, Ping Yang, and Cheng Zhou


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.

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