Aviation Environmental Impacts and Airport-Level Mit- Mit-igations
6.4 Climate Change Background
6.4 Climate Change
infrared radiation and thus may change the energy balance of the atmosphere. The key spe-cies of interest for climate change are the following:
• Carbon dioxide (CO2): CO2makes up 70 percent of exhaust emissions by mass. It is a long-lived GHG having an atmospheric residence time on the order of centuries.
It is thus of particular concern with respect to climate impact potential. As a result of its long lifetime, aviation CO2emissions get mixed in the atmosphere around the globe and become indistinguishable from CO2emissions from other anthropogenic sources.
• Water vapor (H2O): H2O makes up 29 percent of the exhaust emissions by mass and has a warming impact. However, in the troposphere (the portion of the atmosphere up to approximately 50,000 ft) where all subsonic commercial aircraft fly, water va-por only has a lifetime on the order of days and therefore has a negligible climate impact. Water emissions in the stratosphere (higher altitudes where supersonic air-craft can fly) can remain for much longer, but there are very few emissions in this region currently.
• Nitrogen oxides (NOx): NOx gas species are not themselves GHGs, but through at-mospheric processes they lead to ozone (O3) production and methane (CH4) de-struction, both of which are strong GHGs, so these processes have warming and cooling effects. The interactions are complex with differing temporal and spatial characteristics: short-lived ozone production causes warming effects that last on the order of months and primarily influence the northern latitudes where most aircraft fly, whereas long-lived methane destruction has cooling impacts over decades and occurs on a global scale. Therefore, although the globally averaged warming and cooling impacts largely offset, there can be significant regional variations.
• Sulfate (SOx) and soot aerosols: these are solid or liquid aerosols suspended in the atmosphere which can reflect sunlight (a cooling effect) or trap infrared radiation (a warming effect) depending on their characteristics (e.g., size, composition, concen-tration) and time of day. These aerosols have an atmospheric residence time of days to weeks, so impacts are short-lived. However, they can also act as nuclei for cloud condensation and trigger changes to naturally occurring cloud properties that may have much larger climate impacts.
• Condensation trails (contrails): these are the line-shaped trails visible from the ground that sometimes form behind aircraft (typically at high altitude) under certain atmospheric and engine conditions. They often only last a few minutes and have negligible environmental impact. However, under certain conditions, the contrails can persist for an hour or more and may also produce induced cirrus clouds that can
last for days. The climate impacts of these effects is not well understood; current estimates of their impact range from negligible to being more important than the impact from carbon dioxide while they exist.
Measuring Climate Change and Its Impacts
A variety of metrics are used to measure climate change impacts. The two most common ones are radiative forcing (RF) and global warming potential (GWP). RF is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth–atmosphere system and GWP is the cumulative RF effects of an emission over a specified time horizon. RF is often used because the estimated surface temperature change due to a given pollutant is directly proportional to its RF value. However, RF does not readily account for the vastly different timescales of the impacts of the pollutants described above and GWP can be more meaningful in that regard.
Figure 6.15shows the currently estimated RF impacts for key climate pollutants. Posit-ive bars represent warming effects, negatPosit-ive bars represent cooling effects, and the black whiskers represent uncertainty bounds. This figure also lists the spatial and temporal im-pact scales and the level of scientific understanding as of 2005. It is apparent that CO2
accounts for about half of the total estimated RF impacts from aviation and the level of scientific confidence is high for this GHG. NOxand linear contrails are the next most im-portant contributors, but for them the science is very much less certain. As the science pro-gresses, climate impact priorities will continue to be refined.
FIGURE 6.15 Radiative forcing impact estimates of aviation emissions. [Source: adapted from (ICAO, 2010a).]
Figure 6.16 shows the pathway from fuel combustion to climate change impacts. The emissions species and RF impacts are evident in the middle steps of the pathway. The cli-mate change effects include changes in surface temperature, sea level, ice/snow cover, and precipitation patterns. These have agriculture, ecosystem, energy, human health, and social consequences that can be monetized through appropriate damage functions (Mahashabde et al., 2011). As this figure illustrates, the latter stages of the pathway are most relevant from a policy-making perspective but also coincide with the greatest levels of scientific un-certainty.
FIGURE 6.16 Aviation emissions impact and damage pathways [Source: adapted from (ICAO, 2010a).]
From an airport perspective, climate change potentially has several important impacts.
First, it could alter the gross domestic product (GDP) of nations and hence the wealth of the traveling public in different countries. This would impact the evolution of demand for air transportation. Second, long-term changes to climate could change tourism patterns around the world (e.g., snow cover at ski resorts, peak temperatures of beach destinations, etc.).
The network appropriate to service this new distribution of demand would thus evolve.
Third, sea-level and weather changes (e.g., wind patterns, frequency of adverse weather conditions) would impact airport needs for drainage, snow/ice clearing equipment, runway orientations, etc. (EUROCONTROL, 2008).
Airport-Level Climate Change Mitigations
There is a growing library of guidance to airports, airlines, and other stakeholders to assist in GHG emissions reductions. For example, ACRP (2011a) lists strategies for reductions in a wide range of areas, including airfield design and operations; business planning; con-struction; carbon sequestration; energy management; ground service equipment; ground
transportation; operations and maintenance; performance measurement; and renewable en-ergy. Airlines can mitigate climate-related emissions by reducing fuel burn and by taking other actions, such as contrail avoidance, that involve changes to aircraft flight paths (ACI, 2009; EC, 2005; IATA, 2009). Because fuel accounts for a high proportion of airline op-erating expenses, there is alignment between the economic and climate impact pressures in this case. Figure 6.17 shows how some of the key opportunities for reducing climate emissions (in this case CO2) may be implemented over the next several decades. The re-lative potential impacts on emissions, implementation timescales, and barriers vary signi-ficantly between the alternatives. For example, changes to operations have relatively small impact reduction potential, but they can be implemented relatively quickly because their implementation barriers are lower compared to other actions. By contrast, new certifica-tion standards take longer to establish (because of the lengthy internacertifica-tional negotiacertifica-tions re-quired to reach agreement), but then they may greatly reduce environmental impacts once implemented. Key mitigations are discussed next.
FIGURE 6.17 Carbon dioxide emissions reduction opportunities. [Source: adapted from (Kar et al., 2010).]
Operational Procedures
The operational mitigations identified with a “C” inFig. 6.8 are effective at reducing cli-mate impacts of aviation. The proportion of total fuel burn in each phase of flight depends on the type of aircraft and mission being flown (e.g., short-haul vs. long-haul), but a gen-eral approximation is that 5 to 10 percent is burnt on the ground, 10 to 30 percent in climb and descent phases (including the terminal areas around the origin and destination airports), and the balance during cruise. There are ongoing efforts to reduce fuel burn in all phases of flight (e.g., the AIRE and ASPIRE initiatives previously discussed), but the cruise portion of flight obviously offers the biggest potential for GHG emissions reduction because it ac-counts for most of the fuel burn. However, the cruise phase also presents the biggest chal-lenge to implementing changes given its large geographic scope. Therefore, although the ground and airspace regions around airports account for relatively little of the fuel burn and climate-impacting emissions, airport stakeholders can play a relatively large role in their mitigation by promoting improved airport operations, airspace design, and procedures.
Many of the operational techniques for reducing noise and air quality impacts on the ground result in less engine-on time and therefore lower fuel burn and GHG emissions.
These include single-engine taxi; extended towing of aircraft; limiting the use of APUs;
and preferential runway assignment and airfield design aimed at reducing taxiing distances and time. Airports can also promote other ground, departure, and approach/landing flight phase operational improvements. Surface congestion management approaches are espe-cially effective at reducing taxi fuel burn and associated air quality and GHG emissions.
Every airport has a limit to the number of aircraft it can efficiently handle as a function of characteristics such as runway configuration, weather conditions, and demand. During periods of high demand, surface congestion management aims to allow just enough aircraft to taxi out to keep the airport operating at this limit. Excess flights are held at gates or other appropriate location with engines off until they can be released to the departure run-way efficiently, as Fig. 6.18 shows. By restricting the number of aircraft moving on the surface, it reduces engines-on taxi-out time, fuel burn, and emissions. Such concepts have been widely studied in operational trials in the United States (including Boston/Logan, New York/Kennedy and Memphis) and significant benefits are observed: for example, see Simaiakis et al. (2011).