Climate forcing of aviation emissions in high altitudes and comparison of metrics

An update according to the Fourth Assessment Report, IPCC 2007
Hartmut Grassl, Max Planck Institute for Meteorology, Hamburg
Dietrich Brockhagen, atmosfair gGmbH, Bonn

December 2007

Abstract

Based on the 4th Assessment Report of the IPCC released in 2007, this paper assesses the actual
range of the Radiative Forcing Index (RFI), discusses its relevance in comparison to the Emission
Weighting Factor (EWF) that is similar to the Global Warming Potential (GWP) and tests the
hypothesis, according to which CO₂ emissions from aviation enhance global warming to a larger
extent than the non-CO₂ impacts in the long term. The RFI is calculated with a range of 1.9 - 4.7. A
“best guess” estimate is not given, in contrast to the 1999 IPCC Report. The EWF, calculated with a
time horizon of 100 years, has the same order of magnitude, but is altogether smaller than the RFI
(1.2 – 2.7). Thus, the warming effect of non-CO₂ emissions of aviation (e.g. ozone formation, aviation
induced cloudiness) can be either larger or smaller than the warming effect of CO₂ emissions over this
time horizon. The comparison of the two metrics shows the ongoing validity of the original rationale of
the IPCC for the RFI. During the chosen time horizon, the EWF has a delay in reflecting the actual
radiative forcing and thus underestimates the climate impact in this phase. The EWF only starts to
catch up on the RFI when aviation emissions decrease. With respect to climate policies, the RFI may
be more meaningful, because it better reflects the climate forcing of aviation for the relevant coming
decades than the EWF. Based on radiative forcing, the aviation contribution to global warming
amounts to about 2% from CO₂ emissions, and about 3 – 7% for the combined aviation effects. Put
into larger context of defining policy issues, both metrics can be used for environmental policy making.

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