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6.3. Radiative Forcing from Aircraft-Induced Changes in Greenhouse Gases
This section presents RF calculations for perturbations to greenhouse gases
attributable to aircraft. Greenhouse gases that have been identified as aircraft-perturbed
are CO2, O3, CH4,
and H2O. Each gas presents a special case in terms of
predicting its perturbation or deriving radiative forcing. The RF calculations
presented here are derived from radiative-balance and comprehensive climate
models by subtracting net radiative flux (incoming solar minus outgoing terrestrial
infrared) for a control run from that for a run that includes the specified
perturbation. In general, these calculations integrate over a full range of
latitudinal and seasonal variations that are typical of the Earth's climate,
consider the imbalance at the tropopause, and account for the adjustment of
stratospheric temperatures. Important factors in deriving representative radiative
forcing include realistic temperatures, water vapor, surface albedo, clouds,
and tropopause. RF calculations represent the instantaneous imbalance in the
troposphere-land-ocean system, thus do not include responses that are considered
part of the climate feedback system, such as changes to clouds and tropospheric
water vapor.
RF values depend on atmospheric composition as well as temperature, water vapor,
and clouds because all of these factors interact with the radiation field. For
these calculations, we have adopted the changing composition as specified in
IS92a from IPCC (1995) and summarized in Table 6-2.
This composition includes substantial increases in CO2
and CH4 that alter the Earth's radiation spectrum, thus
change the RF for a given unit increase of gas. Although we expect mean global
warming of about 1 K by 2050, with concurrent changes in water vapor and possibly
cloud cover, there is no consensus in IPCC (1996) regarding what this future
atmosphere would be. Thus, these RF values for 2050 are not based on a future
climate, and this discrepancy must add to the uncertainty of this assessment.
However, all such potential, systematic errors apply equally to the baseline
scenario IS92a, and the relative climatic impact of aircraft will have less
uncertainty.
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