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Frequently Asked Questions
1. Question: Why should an airport prepare a GHG an entity can have over the emissions), sources reflected
inventory and what are the benefits of having one? in the inventory should be categorized by ownership and
control. Thus, the sources that are owned and controlled
Answer: Currently, GHG inventories are voluntary on a
by the airport operator should be noted first, followed by
national level, but there may be state or local requirements
the sources owned and controlled by the airport tenants
to prepare an inventory. Such local requirements could
(airlines, fixed-based operators, concessions, etc), fol-
be to (1.) include the inventory in a climate action plan,
lowed by other ground access vehicular access to the
(2.) meet a local requirement to register a governmental
airport, referred to as the public category.
entity's emissions in a climate registry, (3.) meet a state
NEPA-like disclosure requirement. All other inventories For inventories prepared for purposes of a sustainability
would likely be voluntary. Section 1.4 of the Guidebook plan or NEPA-like state requirements, the inventories
discusses the categories of inventories. will typically focus on a subset of sources that are affected
by the plan or the airport project. These inventories can
The benefits of preparing an inventory include: being
present emissions using the same approach that is used
prepared for future legislation and related requirements,
understanding emissions and fuel consumption-related for climate action plans, but limited to the sources reflected
efficiencies, and demonstrating environmental leader- in the plan or project.
ship. By preparing GHG inventories over time, an air- Alternatively, sources can be reported as Scope 1, 2, or 3,
port can consider trends and compare these inventories which are shown in Table 2-2 of the Guidebook, relative
against other airport inventories as well as other emission to the specific sources, as follows:
sources.
Scope 1/Direct emissions are from sources that are owned
2. Question: What sources are to be captured in the or controlled by the reporting entity. For an airport, the
airport inventory? Scope 1 emissions would be those associated with ground
vehicles owned and operated by the airport, as well as
Answer: The sources to be reflected in an inventory stationary sources.
depend on the purpose of the inventory. As discussed in
Sections 2.1 to 2.4 of the Guidebook, a climate action Scope 2/Indirect emissions are those from the generation
inventory generally reflects all sources that operate at of purchased electricity consumed by the entity.
an airport, ranging from aircraft to GSE to GAVs, etc. Scope 3/Indirect and Optional emissions are a con-
The geographic boundary of the inventory ranges from sequence of the activities of the entity, but occur at
departing aircraft (the entire flight segment from the sources owned or controlled by another party. Scope 3
airport to its destination) to ground access vehicles for would be the largest quantity of emissions at an airport,
the entire ground movement associated with the air travel
as they would include aircraft-related emissions, emis-
(such as the ground travel to arrive at the airport and
sions from all tenant-related activities (including aircraft
then return upon completion of the air travel).
operations and the associated ground support activities)
To enable the inventory to be policy relevant (a term as well as the public's ground travel to and from the
used by various organizations to reflect the influence that airport.
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3. Question: For what emissions is the airport operator 6. Question: Do I need to account for emissions above the
responsible? mixing height (e.g., 3,000 ft or 914.4 m)?
Answer: When evaluating ways to reduce emissions, the Answer: Unlike criteria pollutants, GHGs emitted below
airport operator would be responsible for emissions the mixing height have an affect on global climate change;
that it owns and controls or over which it has some criteria pollutants emitted under the mixing height gen-
influence. The emissions that the airport owns and controls erally exert their primary effects locally. Therefore, the
are the primary responsibility. This is further explained approach recommended to compute aircraft emissions
in Section 2.4. reflects the emissions that occur below the mixing height
as well as above. Aircraft Method 1 (using fuel-dispensed
4. Question: How do I ensure that double counting does data) captures the fuel necessary to power aircraft from
not occur with flight segments at other airports? one airport to another. Method 2 uses the Method 1
quantity, but separates the emissions from below the mix-
Answer: The approach recommended by this Guidebook ing height versus above (with the LTO reflecting emissions
strives to avoid double counting for all sources. When below the mixing height). Method 3, as is expected to be
computing aircraft emissions, unless the airport operator provided annually by the FAA, will present emissions in
owns aircraft, such emissions are associated with the the following three areas: ground, ground to 3,000 ft, and
tenant category and are considered Scope 3. To avoid above 3,000 ft. Section 3.1 of the Guidebook discusses the
double counting among airports, the calculation of air- procedures associated with all three methods.
craft emissions is recommended to represent one of the
following two approaches: 7. Question: Should emissions reductions from recycling
1. Fuel dispensed at an airport (Method 1 or 2) or be included?
Answer: Most airports have employed some form of
2. If calculating emissions from each flight, only reflect-
recycling including passenger-related internal terminal
ing emissions from departures from the airport in
waste, concessions food waste, construction materials
Method 3.
and debris, etc. The Guidebook's Section 3.6 provides
In this later approach, aircraft emissions should be assessed methods that would enable the airport to capture the
by individual legs of a flight, rather than the departure and emissions reduction benefits associated with recycling
final destination of multi-leg flights. For instance, a flight (waste management) activities.
that leaves JFK, flying to ORD, then to DEN, and then to
SFO, should attribute the flight from JFK to ORD to JFK, 8. Question: Should indirect emissions (e.g., electricity
the flight from ORD to DEN to ORD, and the flight from generation off airport property) be accounted for in the
DEN to SFO to DEN. Section 3.1 of the Guidebook discusses inventories?
the methodology associated with all of these methods. Answer: In general, the answer to this depends on the
purpose of the inventory. For climate action plan-type
5. Question: Won't including aircraft emissions for an inventories, yes, indirect emissions should be captured. For
entire flight (i.e., attributing all of a flight's emissions project-related and sustainability plan inventories, this will
to the departure airport) give the wrong impression depend on the project. Section 3.5.3 of the Guidebook pro-
that the particular airport and jurisdiction is responsible vides an approach to account for electricity, which is con-
for GHGs other than occurring within the jurisdiction? sidered a Scope 2 emission (see Section 2.2.2), if acquired
Answer: Since the attribution of a flight's emissions is to the from local power companies. Most airports purchase
departure airport, the emissions are no longer "centered" electricity from the grid, but those that generate their
around the airport as they are for criteria pollutants under own electrical power through the consumption of various
the LTO cycle. As a result, this geographic distortion must fossil fuels would report such fuel consumed to generate
be understood when policy decisions are made concerning electricity as Scope 1. WRI and the various registries reflect
the attribution of emissions to airports. This Guidebook electrical power acquired as Scope 2 (indirect) emissions.
currently follows the IPCC methodology in attributing all
9. Question: Why is there a difference in approaches for
of a flight's emissions to the departure airport, which--
developing inventories for criteria pollutants and GHG
when used uniformly--results in the prevention of double
gases?
counting and consistency with the national EPA inven-
tories. Section 2.5 provides further information on this Answer: The international regime for computing criteria
issue. pollutants (carbon monoxide and ozone precursors)
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evolved to its current approach, which reflects local a CO2 equivalent (CO2e) is to be generated, reflecting the
emissions (emissions within the mixing height, gener- potential effect that non-CO2 pollutants have on climate
ally under 3,000-ft elevation) that occur by the main change, through the use of Global Warming Potentials
source categories of aircraft, GSE, GAV, and stationary (GWPs). As indicated in Section 3.8, the IPCC Fourth
sources. Assessment Report GWPs are suggested, but GWPs from
older IPCC reports (e.g., Second Assessment Report) may
Learning from this approach, the international approach
be used for consistency with previous inventories and/or
to quantifying GHGs has evolved to reflect similar inven-
other protocols. Appendix D contains a discussion of
torying methods, but based on ownership and control.
RFI, GWPs, and other metrics.
As is reflected in several of the documents supporting this
approach, the inventory should relate to the ability of
12. Question: The Guidebook recommends calculations for
various parties to effect change and reduce emissions
all sources. What should I do if data are not available
associated with the source. This is further discussed in for a specific source?
Sections 1.7 and 2.4. Also, unlike criteria pollutants, the
effects of GHGs cannot be attributed to a specific location Answer: The Guidebook identifies a preferred method for
(e.g., an "airport boundary"). As explained in Section 2.5, the quantification of emissions for each source category.
GHG emissions from aircraft and vehicles influenced by It was recognized that not all airports may have data at this
the airport need to be accounted for irrespective of where level of detail, and thus, alternative methods are identi-
those emissions occur (e.g., beyond the airport boundary fied, some of which require lesser quantities of data. The
and above the mixing height). Guidebook recommends using the highest level of data
available for each source so as to aid in identifying key
10. Question: How do the results of this Guidebook compare (major) sources.
with the online carbon calculators for air travel?
13. Question: Is it acceptable to use the preferred methods
Answer: A number of evaluations have been conducted for certain sources and alternative methods for other
by various parties; some of these are publicly available on sources?
Internet sites. As many of these reviews have shown, a
user may get one result with one online calculator and a Answer: Yes. The Guidebook recommends using the high-
different result with another. These calculators are pri- est level of data available for each source so as to aid in
marily designed to provide the user the emissions from identifying key (major) sources. Each data source and the
one person's air travel, principally associated with aircraft methods that are used should be clearly documented by
usage. In contrast, the inventory produced using this the developers of the inventory.
Guidebook is designed to capture all of the emissions
from sources at an airport (not just single flights), with 14. Question: Should multiple formats be used to report
aircraft likely to be the dominant source. The results the airport inventory?
of an airport inventory are not easily compared with Answer: This depends on the purpose of the inventory.
those of an online calculator unless the airport inven- The Guidebook recommends that the documentation
tory can identify the average flight distance of flights at specifying the inventory note the methods and sources
an airport. that are commensurate with the reporting format. Some
airports may wish to report their emissions in a simple
11. Question: Why do online carbon calculators use a format whereas others may wish to use multiple formats.
Radiative Forcing Index (RFI) and how do these relate For instance, if an airport chooses to report their inven-
to the methods in the Guidebook? tory to TCR, the format for this registry must be used.
However, to aid the airport in understanding and moni-
Answer: Several online calculators employ an RFI to
toring its emissions, the airport may choose to have an
account for many of the unknowns about the effects of
alternative planning format.
aircraft emissions on climate change and some incorrectly
use the RFI. The Guidebook does not recommend the use
15. Question: For aircraft emissions, the FAA AEDT/SAGE
of an RFI for airport-related GHG inventories. Rather,
data source presents current (historical) emissions. How
similar to protocols for other industries, this Guidebook
should I estimate future aircraft emissions?
recommends that at a minimum, Level 1 inventories be
prepared to show CO2 emissions. For the standard Level 2 Answer: Airports can estimate future emissions in a num-
inventories, reflecting emissions of the six Kyoto GHGs, ber of ways. First, Aircraft Methods 1 and 2 are available to
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airports for use if forecasts of fuel dispensed are available. the Guidebook as the preferred Aircraft Method 3. The
Many airports increase existing fuel-dispensed quantities fact that the mixing height is different from the 3,000-ft
in proportion to the change in aircraft operations over height does not affect the GHG inventories since the total
time. Some airports have aviation activity forecasts that AEDT/SAGE GHG data (above and below 3,000 ft) for
would enable use of Method 2 for future years. Although an airport still represents the total GHG emissions from
Methods 1 and 2 are not preferred, they are acceptable aircraft. In evaluating criteria pollutants, some airports use
alternative methods. the local mixing height in their modeling as recommended
by USEPA and state air agencies. Therefore, should an
16. Question: For aircraft emissions, the FAA AEDT/SAGE
airport choose to generate a Level 2 or Level 3 pollutant
data source presents emissions assuming the mixing
evaluation reflecting criteria pollutants using a different
height is 3,000 ft. At my airport, the mixing height is
mixing height, these airports should note the different as-
not 3,000 ft. Does this matter? And if so, where?
sumptions in the documentation. This would not affect
Answer: No. The FAA source is provided to enable all the total emissions for aircraft, but would affect the subset
airports to have access to a common source of aircraft of total emissions reported for "ground to 3,000 ft" and
emissions using the IPCC Tier 3 method, referred to in "above 3,000 ft."
