EXECUTIVE SUMMARY

• Aircraft emissions in conjunction with other anthropogenic sources are expected to modify atmospheric composition (gases and aerosols), hence radiative forcing and climate. Atmospheric changes from aircraft result from three types of processes: direct emission of radiatively active substances (e.g., CO2 or water vapor); emission of chemical species that produce or destroy radiatively active substances (e.g., NOx, which modifies O3 concentration); and emission of substances that trigger the generation of aerosol particles or lead to changes in natural clouds (e.g., contrails).

• Radiative forcing (RF) is the metric used here (and in IPCC) to compare climate perturbations among different aviation scenarios and with total anthropogenic climate change. RF is the global, annual mean radiative imbalance to the Earth's climate system caused by human activities. It predicts changes to the global mean surface temperature: Positive RF leads to global warming. Yet climate does not change uniformly; some regions warm or cool more than others; and mean temperature does not describe vital aspects of climate change such as droughts and severe storms. Aviation's impacts via O3 and contrails occur predominantly in northern mid-latitudes and the upper troposphere, leading potentially to climate change of a different nature than that from CO2. Nevertheless, we follow the scientific basis for RF from IPCC's Second Assessment Report and take summed RF as a first-order measure of global mean climate change.

• For the 1992 aviation scenario (NASA-1992*), radiative forcing of climate change from aircraft emissions (gases and aerosols) is estimated to be +0.05 W m^-2, which is about 3.5% of total anthropogenic radiative forcing as measured against the pre-industrial atmosphere of +1.4 W m-2 for combined greenhouse gases and aerosols (and +2.7 W m-2 for greenhouse gases alone). The components of aircraft-induced radiative forcing are as follows: CO2, +0.018 W m-2; NOx, +0.023 W m-2 (via ozone changes) and -0.014 W m-2 (via methane changes); contrails, +0.02 W m-2; stratospheric H2O, +0.002 W m-2; sulfate aerosol (direct effect), -0.003 W m-2; and black carbon aerosol (soot), +0.003 W m-2. Changes in "natural" cirrus clouds caused by aircraft may result in negligible or potentially large radiative forcing; an estimate could fall between 0 and 0.04 W m-2. Uncertainty estimates, typically a factor of 2 or 3, have been made for individual components and are intended to represent consistent confidence intervals that the radiative forcing value is likely (2/3 of the time) to fall within the range shown. The uncertainty estimate for the total radiative forcing (without additional cirrus clouds) is calculated as the square root of the sums of the squares of the upper and lower ranges of the individual components.

• Projection of subsonic fleet growth to 2015 (NASA-2015* scenario) results in a best estimate for total aircraft-induced radiative forcing of +0.11 W m-2 in 2015-about 5% of IS92a projected radiative forcing from all anthropogenic emissions that year.

• Various options for the future development of subsonic air traffic under the International Civil Aviation Organization (ICAO)-developed Forecasting and Economic Subgroup (FESG, or F-type) scenarios for aviation in the year 2050 assume an increase in fuel use by 2050 relative to 1992 by a factor of 1.7 to 4.8. These options result in a range for aircraft-induced total radiative forcing (without additional cirrus clouds) from +0.13 to +0.28 W m-2 in 2050, or 3-7% of IS92a total anthropogenic radiative forcing for that year. However, the upper and lower bounds represent aircraft scenarios that diverge significantly from economic growth assumed for IS92a. Alternative Environmental Defense Fund (EDF, or E-type) scenarios considered here adopt growth in 2050 fuel use by factors of 7 to more than 10 and result in a range of total radiative forcing (without additional cirrus clouds) from +0.4 to +0.6 W m-2.

• For the year 2050, a scenario that matches IS92a economic growth (scenario Fa1) gives total radiative forcing of +0.19 W m-2. Individual contributions to aircraft-induced radiative forcing are as follows: CO2, +0.074 W m-2; NOx, +0.060 W m-2 (via ozone changes) and -0.045 W m-2 (via methane changes); contrails, +0.10 W m-2; stratospheric H2O, +0.004 W m-2; sulfate aerosols (direct effect), -0.009 W m-2; and black carbon aerosols (soot), +0.009 W m-2. The contrail estimate includes an increase in fuel consumption, higher overall efficiency of propulsion (i.e., cooler exhaust), and shifting of routes. An estimate for the radiative forcing from additional cirrus could fall between 0 and 0.16 W m-2.

• As one option for future aviation, we consider the addition of a fleet of high-speed civil transport (HSCT, supersonic) aircraft replacing part of the subsonic air traffic under scenario Fa1. In this example, HSCT aircraft are assumed to begin operation in the year 2015, to grow linearly to a maximum of 1,000 aircraft by the year 2040, and to use new technologies to maintain very low emissions of 5 g NO2 per kg fuel. By the year 2050, this combined fleet (scenario Fa1H) would add 0.08 W m-2 on top of the 0.19 W m-2 radiative forcing from scenario F1a. This additional radiative forcing combines direct HSCT effects with the reduction in equivalent subsonic air traffic: +0.006 W m-2 from additional CO2, +0.10 W m-2 from increased stratospheric H2O, -0.012 W m-2 from ozone and methane changes resulting from NOx emissions, and -0.011 W m-2 from reduced contrails. In total, the best value for HSCT RF is about 5 times larger than that of displaced subsonic aircraft, although the recognized uncertainty includes a factor as small as zero. The RFs from changes in stratospheric H2O and O3 are difficult to simulate in models and remain highly uncertain.

• Although the task of detecting climate change from all human activities is already difficult, detecting the aircraft-specific contribution to global climate change is not possible now and presents a serious challenge for the next century. Aircraft radiative forcing, like forcing from other individual sectors, is a small fraction of the whole anthropogenic climate forcing: about 4% today and by the year 2050 reaching 3-7% for F-type scenarios and 10-15% for E-type scenarios.

• The Radiative Forcing Index (RFI)-the ratio of total radiative forcing to that from CO2 emissions alone-is a measure of the importance of aircraft-induced climate change other than that from the release of fossil carbon alone. In 1992, the RFI for aircraft is 2.7; it evolves to 2.6 in 2050 for the Fa1 scenario. This index ranges from 2.2 to 3.4 for the year 2050 for various E- and F-type scenarios for subsonic aviation and technical options considered here. The RFI increases from 2.6 to 3.4 with the addition of HSCTs (scenario Fa1H), primarily as a result of the effects of stratospheric water vapor. Thus, aircraft-induced climate change with RFI > 1 points to the need for a more thorough climate assessment for this sector. By comparison, in the IS92a scenario the RFI for all human activities is about 1, although for greenhouse gases alone it is about 1.5, and it is even higher for sectors emitting CH4 and N2O without significant fossil fuel use.

• From 1990 to 2050, the global mean surface temperature is expected to increase by 0.9 K following scenario IS92a for all human activity (assuming a climate sensitivity of +2.5 K for doubling of CO2). Aircraft emissions from subsonic fleet scenario Fa1 are estimated to be responsible for about 0.05 K of this temperature rise.

• At present, the largest aircraft forcings of climate are through CO2, NOx, and contrail formation. These components have similar magnitude for subsonic aircraft; for an HSCT fleet, H2O perturbations in the lower stratosphere, which are the most uncertain, are the most important. The largest areas of scientific uncertainty in predicting aircraft-induced climate effects lie with persistent contrails, with tropospheric ozone increases and consequent changes in methane, with potential particle impacts on "natural" clouds, and with water vapor and ozone perturbations in the lower stratosphere (especially for supersonic transport).

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