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Global aviation operations combust fossil fuel and emit gases and aerosol to the atmosphere, altering its composition. In addition, aviation produces linear and spreading contrails that increase global cloudiness, and modify natural background clouds. Atmospheric composition and cloudiness largely control the balance in Earth's atmosphere between incoming radiation from the Sun and outgoing radiation from the atmosphere and surface. Any imbalance caused by human activities can lead to long-term changes in climate. At present, aviation emissions and cloudiness do contribute to an imbalance (i.e., net positive radiative forcing) in Earth's climate system that contributes to surface warming and other changes. The magnitude of the imbalance is a few percent of that caused by all human activities since pre-industrial times. Principal emissions that arise from aviation fuel combustion are carbon dioxide (CO2), nitrogen oxides (NO^sub x^), hydrocarbons (HC), sulfur species (SO^sub x^), black carbon particles (BC), and water vapor (H20). This paper addresses the scientific understanding of the processes that connect aviation emissions and aviation impacts on cloudiness to climate change, and highlights important remaining uncertainties. Scientific understanding helps guide choices concerning how climate change from aviation operations can be reduced in coming decades.
I.Introduction
International aviation operations contribute substantially to our world economy and societal wellbeing by carrying cargo and people to many points on the globe. Successful operations over many decades and the world's expanding population and improving quality of life imply continued growth in the coming decades. Aviation operations have long been known to contribute to Earth's changing climate, as first summarized in a Special Report by the Intergovernmental Panel on Climate Change (IPCC) in 1999.1 The Report comprehensively laid out the scientific and technical aspects of how aviation operations influence climate and relied on computer models of the atmosphere to evaluate the quantitative aspects of climate change from aviation and all other human activities. Since ^99, a number of follow-on reports and studies have been conducted in response to improved understanding of aviation effects and increased skill in atmospheric modeling.
Aviation operations have grown substantially over the past decades. Growth in aviation fuel use has been linear since r970, with use more than doubling by 2005, and has persisted despite global financial and other crises over that period.2 The growth...