I.        Introduction

In the grand scheme of things, aviation may not represent a huge source of concern with respect to climate change. But neither should the aviation industry (airports included) ignore the fact that aviation does contribute to climate change not only through the emission of carbon dioxide (CO2) but also through the emission of nitrogen oxides (NOx), aerosols and their precursors (soot and sulfate), and increased cloudiness in the form of persistent linear contrails and induced-cirrus cloudiness. The intent of this series of articles is to examine the effect aviation has on climate change, outline the regulatory and legal framework that is developing, and to suggest avenues for the aviation industry to pursue in the future.  The first challenge is to clear up some misconceptions about aviation and climate change so that we can move forward with accurate and up-to-date information.

II.      Some Facts About Aviation and Climate Change

In Aviation and Climate Change: the Views of Aviation Industry Stakeholders, the aviation industry makes several claims regarding the impact aviation has on climate change. First, the industry claims that “over the past four decades, we have improved aircraft fuel efficiency by over 70 percent, resulting in tremendous savings.” As a result, the industry continues, “given the significance of fuel costs to the economic viability of our industry, our economic and environmental goals converge.” Second, the industry claims that “because of our aggressive pursuit of greater fuel efficiency, greenhouse gas (GHG) emissions from aviation constitute only a very small part of total U.S. GHGs, less than 3 percent.” However, in order to assist the industry in its obligation “to further limit aviation’s greenhouse gas footprint even as aviation grows to meet rising demand for transportation around the world,” those claims of progress need to come under a microscope.

        A.            Contribution of Aviation to Climate Change Remains Subject to Debate

First, how much aviation contributes to climate change is still up to debate. Several governmental and aviation industry organizations have been reporting a “less than 3%” number for quite some time while environmental groups, particularly in Europe, claim that the percentage is anywhere from 5 to 9%. In examining the claims and counterclaims concerning emissions of GHG, one has to be very careful about the language and the metrics used in determining the “impact” any given industry will have on “climate change.” Many reports and studies focus only on CO2, since the amount of CO2 produced both naturally and by humans is overwhelming. However, as just about everyone knows by now, there are other gases and anthropogenic actions that exacerbate climate change. For example, the U.S. EPA recently proposed regulations that would require major emitters of six “greenhouse gases” to report their emissions to the EPA on an annual basis. Those six greenhouse gases are: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs), perfluorochemicals (PFCs), and other fluorinated 20 gases (e.g., nitrogen trifluoride and hydrofluorinated ethers (HFEs)). It also should be kept in mind when discussing climate change, especially with respect to aviation, that water vapor is estimate contribute anywhere from 36% to 72% of the greenhouse effect. This is important because the radiative forcing effect of cirrus cloud formation from the aircraft is a significant contributor to the greenhouse effect. As pointed out above, it is generally accepted that for aviation the GHGs of concern are CO2, nitrogen oxides (NOx), aerosols and their precursors (soot and sulfate), and increased cloudiness in the form of persistent linear contrails and induced-cirrus cloudiness.



The predominance of CO2 as the GHG of concern leads to another issue: measurement of GHG. Many reports state their findings in terms of “CO2e,” or CO2 equivalent. Carbon dioxide equivalency is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO2 that would have the same global warming potential (GWP), when measured over a specified timescale (generally, 100 years). For example, the generally accepted GWP for methane over 100 years is 25 and for nitrous oxide 298. This means that emissions of 1 million metric tons of methane and nitrous oxide, respectively, is equivalent to emissions of 25 and 298 million metric tons of carbon dioxide. This article will keep the convention of designating GHG other than CO2 in terms of “CO2e.”

Most reports and studies begin with the groundbreaking work of the United Nation Intergovernmental Panel on Climate Change (IPCC), which, in 1999 estimated that, based on earlier data, fuel combustion for aviation contributes approximately 2% to the total anthropogenic CO2 emissions inventory, and, if left unmitigated, this could grow to as much as 4% by 2050. Despite the age of the data, the 2% number has been used consistently throughout the first decade of the 21st century. The International Air Transport Association (IATA) in a 2006 press release relied on IPCC report by stating that “[a]ir transport contributes a small part of global CO2 emissions – 2%.” IATA press release , 2nd Aviation Environment Summit. Even as recently as September, 2009, the Transportation Research Circular of the Transportation Research Board fudges the issue by stating in the section about climate change and greenhouse gases that “fuel combustion for aviation contributes approximately 2% to the total anthropogenic CO2 emissions inventory.” What these estimates leave aside is the fact that CO2 emissions are only one facet of the greenhouse gas equation. 

The aviation industry tried to correct this in its paper Aviation and Climate Change: Views of Aviation Industry Stakeholders, published in February, 2009, by stating that “greenhouse gas (GHG) emissions from aviation constitute only a very small part of total U.S. GHGs, less than 3 percent.” However, the report that the paper cites to, the U.S. EPA’s Inventory of Greenhouse Gas Emissions and Sinks: 1990-2006 (April 15, 2008) (2008 EPA Inventory), only mentions emissions of CO2 in its discussion of its inventory of greenhouse gases in the creation of energy. See, 2008 EPA Inventory, Chapter 3. Moreover, the EPA only examined the aviation sector’s combustion of fossil fuel, and did not, for example, take into account the radiative forcing effect of cirrus cloud formation has on climate change. When the EPA published its next inventory, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007, (March 2009) (2009 EPA Inventory), the contribution of aviation to carbon dioxide emissions increased. It estimated that when international fuels were included, domestic and international commercial, military, and general aviation flights represented about 3.4 percent of the total emissions of CO2 in United States. 

There is no question that the emission of CO2, and, for that matter, the combustion of fossil fuels, does not tell the whole story with respect to aviation. However, there are relatively few studies that focus solely on aviation and examine the effects of all GHGs and not just CO2. In 2005, Robert Sausen and a group of climate scientists published their article Aviation Radiative Forcing in 2000: An Update on IPCC (1999) (Sausen 2005). That article concluded that when NOx emissions, contrails and cirrus clouds are added into the mix, aviation’s impact on climate change is about 2 to 5 greater than that of CO2 alone worldwide. This would mean that aviation would have an impact on climate change in the range of 4% to 10% when all aspects of emissions of GHG and other radiative forcing factors are taken into account. These numbers were updated in a July, 2009, article Aviation and Global Climate Change in the 21st Century (Lee et al., 2009) which appeared in the periodical Atmospheric Environment. The authors, a group of atmospheric scientists, concluded that when aviation-induced cirrus radiative forcing is included, aviation represents 4.9% of total anthropogenic “radiative forcing of climate.” While these studies are not United States specific, as the EPA inventories are, since these studies consider all GHGs emitted by aviation (not just carbon dioxide), are focused entirely on the climate effect of aviation, and are based more recent data, the conclusion that aviation contributes close to 5% of climate change is more accurate than the “under 2%” used by many in the aviation industry.

B.            Claims of More Fuel Efficient Aircraft Are a Little Exaggerated

If one were to rely solely on the aviation industry’s press releases, one could come to the conclusion that the aviation industry is doing its part to fight climate change by virtue of the fact that all aircraft have become more fuel efficient. In the aviation industry paper Aviation and Climate Change: The Views of Aviation Industry Stakeholders they state that “[o]ver the past four decades, we have improved aircraft fuel efficiency by over 70 percent, resulting in tremendous GHG savings.” February 2009, citing International Civil Aviation Organization, Environmental Report 2007, p. 107.   This is also the position that the International Air Transport Association has taken in its publications.   For example, in a press release regarding the Second Aviation and Environment Summit in 2005 IATA claimed that “Aircraft entering today’s fleets are 70% more fuel efficient than they were 40 years ago.” Likewise, the Air Transport Action Group (ATAG) website, www.atag.org states that “[t]oday’s world fleet is about 70% more fuel efficient than they were 40 years ago.” Seventy percent is also the number used by IATA for the amount of reduction of CO2 emissions per passenger kilometer. Aviation Environment Summit conclusions, 2005 (“Over the past 40 years, the commercial aviation industry has made tremendous progress in . . . reducing CO2 emissions per passenger kilometer (by 70%) and in improving fuel efficiency”). As recently as May 6, 2008, Douglas Lavin, Regional Vice President of North America for IATA testified before the U.S. House Subcommittee on on Aviation that “[o]ver the last forty years, the commercial airline industry . . . improved its fuel efficiency by 75%, leading to a similar reduction in CO2.” The improvement in fuel efficiency is at the heart of the industry’s proposals for meeting climate change challenges.

All of these statements, however, are based, in part, on the IPCC’s 1999 report, Special Report on Aviation and the Global Atmosphere. What one will note in reviewing the 1999 report is that it compares current jets with jets of the early 1960s. It does not, however, compare jets to piston engine aircraft. If they did that, a 2005 study from the Dutch National Aerospace Laboratory (NLR), which uses the IPCC data, concludes that aircraft have not made any progress in terms of fuel efficiency. “If one takes new aircraft from the early fifties (i.e. the last piston-engine aircraft) as the baseline, it shows that these last long-haul piston-powered airliners were as fuel-efficient as today’s average turbojet aircraft.” Fuel Efficiency of Commercial Aircraft: An Overview of Historical and Future Trends, NLR 2005, p.18. The GAO picked up on this dichotomy in its June, 2009, report Aviation and Climate Change GAO-09-554, p.4, fn. 5  noting that “some aircraft available in the 1950s were about equally as fuel efficient as jets currently available today.”

The Dutch report goes further and claims that even the report of 70% increase in the efficiency of jet engines from the early 60’s until the present may be overstated. Instead, the report claims, the fuel efficiency savings is closer to 55%. “If one takes new aircraft from the early sixties (i.e. the first jets) as the baseline (as presented in the IPCC report), an improvement of 55% is found rather than the 70% presented in the IPCC report.” NLR 2005, p. 18.   The Dutch report explains that main reason for this difference “is the different choice of baseline aircraft (B707 instead of DH Comet 4). The IPCC reference aircraft – the DH Comet 4 – has a rather atypical (i.e. very low) energy efficiency and only a very limited number were in operation. Further, the difference between the old and new aircraft chosen for the micro analysis is somewhat less than given by the IPCC.” NLR 2005, p,18. Thus, reliance on increases in fuel efficiency may not be an effective method to compensate for the effect that aviation has on climate change.

The industry reliance on innovation in creating more fuel efficient engines and aircraft, however, may be misplaced. Although recent innovations in engine and airframe design may eventually result in a more fuel efficient fleet of aircraft, they may not be sufficient to carry the industry forward to meet increasing demands on aviation to cut GHG emissions, at least in the short run. The GAO in its recent report concluded:

While airlines currently rely on a range of improvements, such as fuel-efficient engines, to reduce emissions, some of which may have limited potential to generate future reductions, experts we surveyed expect a number of additional technological, operational, and alternative fuel improvements to help reduce aircraft emissions in the future. However, according to experts we interviewed, some technologies, such as advanced airframes, have potential, but may be years away from being available, and developing and adopting them is likely to be costly. In addition, according to some experts we interviewed, incentives for industry to research and adopt low-emissions technologies will be dependent to some extent on the level and stability of fuel prices. Finally, given expected growth of commercial aviation as forecasted by IPCC, even if many of these improvements are adopted, it appears unlikely they would greatly reduce emissions by 2050.

GAO 2009, p.1. Over the short run, then, (i.e., between now and 2050) increases in fuel efficiency cannot be relied upon for decreases in GHG emissions. This was also the conclusion of the authors of Aviation and Global Climate Change in the 21st Century (2009). They concluded that “[a]n examination of a range of future technological options shows that substantive reductions in aviation fuel usage are possible only with the introduction of radical technologies.” Despite the aviation industry’s claims of increased fuel efficiency and its belief that reducing GHG emissions makes economic sense, it may very well be that the reductions necessary to achieve the goals currently under discussion will not be possible.

Moreover, there are distinct trade-offs between fuel efficiency that may not necessarily reduce emission of elements that cause climate change. As Mahmood Manzoor, a Senior Specialist with Messier-Dowty, Inc., points out in his article Sustainable Development – A Major Challenge to the Aviation Industry (Manzoor 2009):

Over the past 40 years, the aviation industry has made tremendous progress in improving fuel consumption (by 70%) and reducing gaseous emissions of CO and hydrocarbons by 50% and 90% respectively. However, the high combustion temperatures and pressures of aircraft engines tend to increase the production of particulate matter and NOx [both of which contribute to climate change].

Manzoor, § 4.2. Resolving this dilemma has proven to be a nettlesome problem for the industry. Mahmood Manzoor continues:

Environmental trade-offs between NOx and other emissions, noise and CO2 that are inherent in aircraft and engine design, continue to be discussed in detail within CAEP [Committee on Aviation Environmental Protection]. The low emissions TALON II™ combustor reduced NOx by over 25%, but at the expense of an increase in smoke from 30% to 93% of the ICAO [International Civil Aviation Organization] standard. Similarly, a trade-off for a Dual Annular Combustor (DAC) where NOx and smoke were reduced by approximately 30% and 67% respectively while hydrocarbons and CO increased by 15% and 130%. . . . All trade-offs are important, but with the emphasis on minimizing fuel burn (therefore CO2) and reducing noise, manufacturers are being forced to optimize engine design within a narrow physical design space.

Manzoor, § 4.3. The result is that there is not a direct correlation between an increase in fuel efficiency and decrease in the impact of climate change. Fuel efficient engines, operating at higher temperatures at high altitudes could create more of an impact on climate change even if they are emitting less CO2.


C.            Aviation is More “Climate Intensive” Than Previously Thought


As a corollary to the previous section, the aviation industry has long claimed that it is the least “climate intensive” of all of the transportation sectors. That is, on “liter per passenger kilometer,” or “gallon per passenger mile,” modern aircraft are more “climate-friendly” than cars, trucks, buses and even high-speed trains. IATA trumpets this fact on its website: “modern aircraft achieve fuel efficiencies of 3.5 litres per 100 passenger km [approximately 78 passenger miles per US gallon] . . . The A380 and B787 are aiming for 3 litres per 100 passenger km – better than a compact car!” http://www.iata.org/whatwedo/environment/fuel_efficiency.htm; see also, Airbus A380 website  http://events.airbus.com/product/a380_backgrounder.asp (“The green giant, more fuel-efficient than your car”).

Environmental groups, on the other hand claim that Aviation is between two and ten times more climate-intensive than surface transportation. They claim that the aviation industry data ignore four crucial factors:

  1. The figure of 3 or 3.5 liters per 100 passenger kilometer assume a full aircraft, i.e., a load factor of 100%. Thus, the number is representative of “aircraft seat” rather than “passenger.”
  2. The occupancy rate of cars (and trucks) at distance competing with aircraft (i.e., long hauls) is higher than the average occupancy rate of 1.6 that is frequently used when assessing the climate impact of cars.
  3. The figure of 3 to 3.5 liters per 100 seat kilometers applies to long-haul flights with large aircraft. Aircraft that do indeed compete with surface transport are smaller and fly shorter distances and are hence less efficient than 3.5 liters per 100 seat kilometers.
  4. The climate impact of non-CO2 emissions is ignored. Because of the effects of NOx, contrails and cirrus clouds at high altitude, a liter of fuel burnt in an aircraft at such altitudes has a greater climate impact than a liter burnt by surface transportation.

With load factors between 70% to 80% currently, the actual amount “per passenger” will be higher than the 3 to 3.5 liters per 100 passenger kilometer. The GAO avoided this trap by showing how modern aircraft fuel efficiency has increased on a “available seat miles per gallon.” As a result, the GAO’s report shows the current efficiency to be at 58 gallons per available seat mile, which is significantly lower than the 78 gallons per available seat mile reported by the IATA.

Taking the above factors into consideration, a CE Delft report To Shift or Not to Shift, That’s the Question: The Environmental Performance of the Principal Modes of Freight and Passenger Transport in the Policy-Making Context concluded that aviation performs three to ten times worse in terms of climate impact than cars on competing distances, and some two to ten times worse than high-speed trains. Likewise, when one examines aviation as a freight hauling industry, it does not do any better when compared to surface modes of transportation. The study External Costs of Transport (INFRAS/IWW 2004) showed that when it comes to freight transport, aviation is even worsein terms of emissions than passenger transport. The external costs of aircraft-related climate change are approximately ten times greater than for trucks, the second worst mode. Although none of these reports can be said to be the definitive word on whether aviation is more or less climate intensive than surface transportation, it does highlight the fact that aviation is probably more climate intensive than what was thought.


I.        Conclusion: Policy and Legal Implications

So what are the policy and legal implications of these facts? First and foremost, it is evident that aviation plays a larger role in climate change than most in the aviation industry would like to admit. This means that now is not the time for complacency or resting on illusory laurels. If aviation is not to be left behind by the auto and truck industry as well as shipping, it needs to take action sooner rather than later to control its impact on climate change. Second, these facts indicate that, at least in the short run, technological innovations will not noticeably affect the impact that aviation has on climate change. As both the GAO and Lee et al. pointed out, although the aviation industry is making technological advances that will reduce emissions that create climate change, these advances will not be available for implementation in the near future. Third, airports cannot walk away from issues surrounding the climate change impact created by aircraft. Although according to a 2006 Seattle-Tacoma International Airport greenhouse gas inventory 90% of total CO2 emissions associated with that airport were form aircraft operating above 3,000 feet, the airport is still responsible for those emissions. Using simple “but for” logic, if it were not for Seattle-Tacoma Airport, those airplanes would not be landing there, therefore, the airport should take responsibility for all incoming flights.