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10 complex indirect effects), and the different moral and value- scenarios are required to estimate the relative effects of the based assumptions that may be implicit in these methods, there emissions. A simple multiplier is generally not appropriate. is a healthy scientific debate regarding the "best" equivalency As already noted, depending on the policy question, dif- methods. Scientists, economists, and others who study climate ferent equivalency methods may be more or less appropriate. recognize that the usefulness of different equivalency methods Further, the scientific understanding of some effects is still often depends on the question one asks and the way in which relatively immature, and both the equivalency methods and the equivalent emissions are judged. the specific equivalency values are likely to change over time. Nonetheless, equivalency methods are useful for taking action based upon the compiled inventories or, in some cases, for An important point in developing an emissions summing up the overall magnitude of several different species inventory is to collect sufficient data to enable (e.g., in a CO2e unit). different equivalency methods and more ad- For purposes of creating a CO2e, it is recommended that the vanced analyses to be conducted because there latest IPCC Fourth Assessment Report (IPCC 2007) be used and is currently no perfect equivalency metric. noted in the documentation of the emissions inventories. Sep- arately accounting for each pollutant and reporting their emis- sions alongside the GWP values (while noting the source of the Within Appendix D, several equivalency methods (radia- GWPs) would allow adjustments in the future as refined GWP tive forcing index [RFI], GWP, global temperature potential values become available (e.g., from future IPCC assessment re- [GTP], and others) are reviewed. The appendix describes the ports). Section 3.8 presents the specific process for calculating underlying assumptions, strengths and weaknesses, and es- CO2e. Although the GWPs from the Fourth Assessment Report timation methods. Although there are differing opinions are recommended, GWPs from prior reports can be used for about the relative usefulness of these different equivalency consistency with previous inventories and other existing pro- methods, it is recommended that the latest data from the tocols. In general, it is recommended that a GHG inventory IPCC Fourth Assessment Report (IPCC 2007) be used. How- should always include both the original mass quantities of each ever, as was noted in prior sections, many of the publicly pollutant as well as the CO2e masses. This allows comparisons available protocols advocate the use of the IPCC Second As- of the different pollutants as well as for potential changes to the sessment Report or Third Assessment Report. Although the CO2e values if improved GWPs become available. Fourth Assessment Report represents the state-of-the-art un- derstanding, the international approach to local-level in- 1.7 Allocating Emissions Reductions ventories has evolved using older data through the protocols developed by TCR, CCAR, and ICLEI. Therefore, for con- A dilemma that airport operators are likely to have in the sistency with previous inventories, GWPs from the second future quantification of emissions is how to allocate emis- and third reports could be used if the airport inventory is to sion reductions that the airport operator has implemented be integrated into an inventory that uses these earlier as- or funded through various funding sources (rates and sessments. The specific GWPs used should be specified as charges, grants, bonds, etc.) that result in emission reduc- part of the overall inventory documentation. tions associated with a source that is owned and controlled Consistent with the IPCC Fourth Assessment Report, this by another party (i.e., tenants or public ground travel to/ Guideline recommends the GWP as the primary means of es- from the airport). For purposes of this section, those actions tablishing equivalency for long-lived GHGs; however, in Ap- are referred to as actions where the airport operator has "in- pendix D, the Guideline notes the many shortcomings associ- fluence," but does not own the source or control the emis- ated with the use of GWPs. Further, consistent with the IPCC, sions (see Section 2.2 for ownership and control inventory the GWP is not recommended for all climate change pollu- boundaries). tants and impacts. For some short-lived climate change pol- There are numerous instances where one party may have lutants and impacts, the science is either too premature or too influenced the emissions of another party's sources. These physically and chemically complex to support their inclusion influences represent actions that have the potential to shift in these simplified equivalency methods. This is especially true emissions from one party to another. For instance, many for some cruise-level effects of airplane emissions (indirect ef- airports are implementing preconditioned air (PCA) and fects of NOx emissions, contrails, and aviation-induced cirrus 400-hz power at the gates. This type of project is designed cloudiness) for which the scientific understanding is not suf- to reduce the use of APUs on aircraft, by enabling the air- ficiently mature to enable equivalency metrics to be used with craft power needs when parked at the gate to be met with confidence. For these situations, more complex computer ground power. Although some 400-hz/PCA projects are simulations, exercised over a range of scientific and economic funded and implemented by the tenants, others are under-

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11 taken by the airport operator. Further, in many airport set- Nuys FlyAway) are designed to increase the use of higher- tings, the electrical demands of the terminal (including the occupancy vehicles accessing airports versus reliance on gates) are procured by the airport operator. For those loca- single occupancy vehicles. Such programs decrease emis- tions where the ground power system is funded by the air- sions from publicly owned vehicles. port operator, or where the airport operator pays for the electrical power, the accounting for emissions associated There may be merit to developing a separate methodology with this need has been transferred from the tenant (due to to allocate recognition or "credit" for emission reductions. the reduced reliance on the APU) to the airport operator Such a methodology is currently outside the scope of this (where the energy need is then electrical based). Other ex- Guidebook. amples where similar accounting needs may arise include the following: A separate methodology may need to allocate Airport infrastructure projects--These projects, such as new recognition for emissions reductions that cross runways or runway extensions are owned by the airport over between a party that owns and controls the operator, may be funded through federal grant monies or emissions and parties that influence the emissions. locally backed revenue bonds, but result in emission reduc- tion changes associated with airport tenants. National or regional airspace improvement--Delays in cer- We note that in the reporting protocol established by the tain regions of the country often result in ground holds at Global Reporting Initiative (GRI), there is the recognition of cities with air service to those regions. Control of the national emissions owned and controlled by one party where a sub- airspace and the aircraft that use it rests with the federal stantial influence is exerted by another party on the emissions government. of those sources. Although GRI has not developed an account- Surface traffic improvements--Improvements such as ex- ing protocol for such influences, their sustainability plans rec- tending regional light rail systems to the airport or airport- ognize the influence that various parties may have over another sponsored busing programs (such as the Los Angeles' Van party's activities (GRI 2008).