Summary of Metrics

Numerous modeling approaches have been used to estimate aviation’s contribution to climate change. Radiative Forcing (RF), Radiative Forcing Index (RFI), Integrated Radiative Forcing, Global Warming Potential (GWP) and Integrated Change in Temperature over Time all express in some way the change in energy balance that GHG emissions are causing:

Radiative Forcing

Radiative Forcing is an instantaneous measure: it expresses the climate forcing of a greenhouse gas at a particular point in time. Yet it also has a temporal component: it is a backward-looking metric because it measures the RF of a GHG that has accumulated in the atmosphere over a certain period of time. Long-lived GHGs will continue to warm the atmosphere for the duration of their residence. Consequently, RF values include effects of past air travel and exclude future effects of current air travel. RF values reported in the IPCC aviation report (1999) and by Sausen et al. (2005) are therefore not the correct metric for determining total climatic response of current air travel (and were never intended to be used that way).
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Radiative Forcing Index

Radiative Forcing Index (RFI), as used in IPCC 1999, compares the non-CO2 warming effects of aviation to those of CO2.  RFI is the ratio of total radiative forcing (RF) of all GHGs to RF from CO2 emissions alone.  The RFI calculations in the IPCC aviation report from 1999 are based on RF values for aviation emissions from the last approximately 50 years. RFI has the same shortcomings as RF: RFI includes effects of past air travel and excludes future effects of current air travel. RFI is therefore not the correct metric for determining total climatic response of current air travel (and was never intended to be used that way).
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Integrated Radiative Forcing

Integrated Radiative Forcing enables us to calculate the future effects of GHGs. It sums the RFs of GHGs over a chosen time frame (in mathematical terms: integrating them; see Figure 3 for a schematic illustration of integrated RF). Integrated RF expresses the energy that is added to the system during a chosen time horizon due to GHG emissions. Integrated RF of current emissions excludes effects from past air travel and includes future effects of current air travel. Integrated RF of current emissions could therefore be an appropriate metric for our purposes. Yet results of integrated RF vary greatly depending on the chosen time frame and whether a pulse emission or sustained emissions are used (see further discussion below).
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Global Warming Potential

Global Warming Potential is used as a tool to compare the potency of different greenhouse gases with that of CO2.  In that way, GWP is similar to RFI. But whereas RFI is a backward-looking metric, GWP is a forward-looking metric that includes future effects of current emissions. GWP is based on integrated RF over a chosen time frame of each GHG relative to that of CO2e. GWP values depend on the time span over which the forecast warming potential is calculated. GWP with a time horizon of 100 years is used in the Kyoto Protocol, yet such a long time horizon might underestimate the importance of short-lived emissions. While GWP is widely accepted as a reliable proxy for the warming impacts of long-lived, well-mixed gases such as CO2, GWP with a 100-year time frame may not be suitable for measuring the kind of short-lived non- CO2 emissions associated with aviation.
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Global Temperature Change Potential

Global Temperature Change Potential (GTP) goes further than the metrics described above. It estimates the change in global mean temperature for a selected year in the future.  This metric is more complex because it calculates climate response and not just radiative forcing. Although uncertainty is increased, relevance is also increased since it is more useful to know what the actual temperature change will be than just to know the amount of energy that was added to the system. GTP excludes effects from past air travel and calculates future warming from current air travel.
GTP could therefore be an appropriate metric for our purposes. Yet, as with integrated RFI, the results of GTP vary greatly depending on the time frame and if a pulse emission or sustained emissions are used.
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Integrated Change in Temperature over Time

Integrated Change in Temperature over Time expresses the climate impacts as a theoretical number that is the sum of all the temperature changes that occur over a given period of time. The FAA/APMT model (see Economic Cost Calculations) can also be used to calculate such a metric, expressed in integrated-delta T-years. Integrated Change in Temperature over Time could therefore be an appropriate metric for our purposes. Yet, as with integrated RF, the results if this metric will vary greatly depending on the time frame and if a pulse emission or sustained emissions are used.

Economic Cost Calculations

Economic Cost Calculations go further than all the metrics discussed above: they calculate climate costs and not just radiative forcing. As mentioned above, assumptions that need to be determined by moral or political positions lie hidden in the economic assessment of the damages (e.g. the discount rate). This means that climate cost calculations are more complicated and their results more dependent on value-based decisions than simple radiative forcing calculations. Despite their shortcomings, economic models are important because they translate climate change into the currency that is most pertinent to policy makers and businesses: the monetary costs associated with expected climate effects. Yet economic cost calculations go beyond our purpose. A metric that compares the different forcings or climate responses is more appropriate for our task because it is more compatible with metrics that are used in other climate policy measures (such as the Global Warming Potential with a 100-year time frame used under the Kyoto Protocol).
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