Aviation has reshaped the world we live in—allowing for affordable and rapid travel to almost any point on the globe. In recent years, economic growth and rapid globalization have made air travel affordable to an even larger part of the global population. In this context, demand for aviation, in terms of passenger-miles flown, has grown at a rapid pace. In China, for example, domestic air transport grew at 15.5 percent annually from 2000 to 2006. Globally, the rate of air travel increased at 3.8 percent per year over the same time period. This growing demand for air travel has resulted in increasing levels of greenhouse gas (GHG) emissions from the aviation sector, despite efficiency improvements. Currently, the aviation sector—including both domestic and international travel—accounts for approximately 1.5 percent of global anthropogenic GHG emissions per year. The U.S. accounts for nearly 40 percent of the global GHG emissions from aviation.Barring policy intervention, GHG emissions from aviation are projected to quadruple by 2050.
A number of strategies can mitigate the rising level of GHG emissions from the aviation sector. In the near term, adopting navigation systems and air traffic control techniques that minimize fuel use and idling can reduce emissions by as much as 5 percent. Over the long term, advanced propulsion systems, utilization of lightweight materials, and improved aerodynamics and airframe designs hold the promise of further reducing aviation emissions. The development of cost-effective, lower-carbon alternative fuels could result in even deeper reductions. Strategies also exist to reduce the demand for aviation by switching to other modes of travel – such as high speed rail connections where high speed rail can compete with aviation on time, price, and convenience.
To be effective, policies to reduce GHG emissions from aviation will require a high degree of international coordination. The trans-national nature of airline ownership and travel complicates how responsibility for aviation emissions is assigned to individual countries. The International Civil Aviation Organization has taken some steps toward understanding these complexities, and policies to reduce emissions will need to adequately take account of international sovereignty issues. The industry has proposed a global sectoral approach where GHGs are managed at the sectoral level instead of on a country-by-country basis.
While the discussion herein focuses on CO2 emissions from aviation, there are other emissions and effects of aviation that also contribute to climate change. In addition to CO2 from fuel combustion, airplanes can also emit methane (CH4), nitrous oxide (N2O), hydrocarbons (HC), particulate matter (PM), sulfur oxides (SOx), and nitrogen oxides (NOx). Moreover, certain high-altitude aviation emissions can spur heat-trapping cloud formation. Compared to these non-CO2 emissions and cloud formation, there is more scientific certainty regarding the impacts of CO2 emissions from aviation and greater consensus on the optimal policies for reducing aviation’s CO2 emissions.
Strategies available to mitigate GHG emissions from aviation include the following:
It is important to also focus on the timeframe in which these strategies are adopted. The operational lifetime of an aircraft ranges from 20-30 years. As such, it takes a number of years for any new technologies to penetrate through the entire fleet. It also takes many years to implement a new HSR route.
Environmental Benefit / Emission Reduction Potential
Combining all available mitigation strategies could reduce global GHG emissions from aviation by as much as 53 percent below “business-as-usual” (BAU) projections in 2050. As Figure 1 shows, owing to large projected increases in aviation demand, absolute emissions from aviation are projected to increase from current levels by mid-century even in the case of significant policies to limit emissions. However, policy interventions can limit global aviation emissions to about double current levels by 2050, as compared to a nearly four-fold increase under “business as usual” over the same timeframe.
While more easily implemented, adoption of a broad array of more efficient operational practices—from improved landing techniques to reduced taxiing times—would only produce emissions savings of 5 percent below the BAU projection in 2050.
Technological advances offer the potential for more significant reductions. Current trends in aviation efficiency improvements are expected to continue; the efficiency of the U.S. and global aircraft fleets will continue to improve as older, less efficient aircraft are retired and then replaced with new, more efficient aircraft. Under “business as usual,” a projected 30 percent decrease in aviation energy intensity will be achieved by utilizing currently known technologies: more efficient propulsion systems (engines), advanced lightweight materials, and improved aerodynamics (e.g., winglets, increased wingspans). Added support through government sponsored research and development (R&D) and other policy interventions could yield an additional 35 percent reduction below BAU emissions in 2050. Much of this 35 percent would come from application of the more ambitious and therefore riskier technological alternatives. Blended wing body or other innovative airframes, for example, could reduce fuel consumption by as much as 32 percent when compared to an Airbus A380 (a currently operating state-of-the-art aircraft model). Advanced laminar coatings that reduce drag could increase fuel efficiency by a further 16.5 percent.
The emission reduction potential from alternative fuels is slightly less certain. While a number of technologies exist to produce alternative fuels, it is unclear at this time which technologies will prove viable in the long term. Conservatively, these alternative fuels could provide an additional 24 percent emission reduction against a BAU scenario.
The potential emissions reduction from shifting to alternative modes of transport is more difficult to quantify and likely the option with the smallest emission reduction potential.
Table 1. Global GHG Emissions Abatement for Aviation Sector
Reductions from “Business as Usual” in 2050 (%)
Advanced CNS/ATM systems (e.g., NextGen, SESAR)
Aircraft Design and Propulsion
Unducted fan (open rotor) engines where feasible, greater application of advanced lightweight materials, improved aerodynamics (e.g., laminar flow control), new airframe designs (e.g., blended wing body)
Medium term: Biofuels;
Long term: Biofuels, hydrogen
Total Reduction from BAU Emissions in 2050
Figure 1: Global GHG Mitigation Potential from the Aviation Sector
Source: McCollum, David, Gregory Gould, and David Greene, Greenhouse Gas Emissions from Aviation and Marine Transportation: Mitigation Potential and Policies, 2009. http://www.c2es.org/technology/report/aviation-and-marine 
The cost of mitigating emissions from aviation depends on the strategy adopted. Not only do cost estimates vary among studies, but some costs are inherently uncertain, given the experimental nature of many of the technologies analyzed. A study that focused on European aviation fleets estimated that GHG abatement costs for the aviation sector range widely, depending on abatement option, spanning -$222 to $308 per metric ton of CO2, with most of the abatement options at costs below $110 per ton.
At the lower end of the cost spectrum, operational innovations, such as CNS/ATM technologies, hold the potential to yield cost savings via reduced expenditures on fuel. More advanced technologies, such as laminar coating, come with higher costs, including significant R&D costs. It is especially difficult to estimate the costs of these longer-term solutions, as the estimated capital costs for development and implementation are often highly speculative.
Similar uncertainty surrounds the costs of alternative fuels for the aviation sector. Existing biofuels cannot currently compete against jet fuel and are not expected to achieve significant penetration in the aviation sector through 2050 under “business-as-usual” conditions. Even in the event of significant improvements in biofuel technology, a number of cost barriers would remain. New fuel types might require re-engineering airplane propulsion systems, increasing the costs of fuel switching.
Building new HSR infrastructure for shifting transportation from air to rail is very capital-intensive. For example, constructing a proposed HSR line between Los Angeles and San Francisco has an estimated cost of $45 billion. Reliable estimates of the dollars-per-ton cost of CO2 emissions reductions from transportation mode shifting are lacking, and some estimates of the marginal cost of emission reductions from HSR are very high.
Current Status of Aviation Emissions Mitigation Efforts
Efforts are already underway to mitigate GHG emissions in the sector. For example, airlines regularly retire older aircraft and make adjustments to airframe design through the addition or repositioning of winglets. Future state-of-the-art aircraft, like the Boeing 787 and Airbus A350 (first deliveries of the former are expected in 2010 with the Airbus A350 about three years later), will combine a number of technologies—from lightweight materials to advanced propulsion systems—to achieve even greater fuel efficiency. Nonetheless, in terms of absolute emissions, these gains in efficiency are entirely offset by burgeoning demand.
Efforts to improve operational efficiency include the U.S. NextGen initiative, which uses satellites to track aircraft routes and uses the satellite data to shorten travel distances and reduce congestion. A similar initiative known as the Single European Sky ATM Research (SESAR) project is underway in Europe.
Some airlines are also testing the possibility of blending jet fuel with alternative fuels. A number of commercial airlines, such as Continental Airlines, have conducted or plan to conduct test flights that make use of biofuels. The International Air Transport Association (IATA) has set a goal for its member airlines to use 10 percent “alternative” fuels by 2017. Initial formal technical approval of these new low carbon biofuels is expected in 2010.
The European Union is on track to integrate the aviation sector into its GHG cap-and-trade system beginning in 2012. The EU regulations cover GHG emissions from all flights either landing at or departing from airports within the European Union.
Obstacles to Further Development or Deployment of Aviation Emissions Mitigation Strategies
Policy Options to Help Promote Aviation Emissions Mitigation Strategies
Related Business Environmental Leadership Council (BELC) Company Activities
Related C2ES Resources
Climate TechBook Biofuels Overview 
McCollum, David, Gregory Gould, and David Greene, Greenhouse Gas Emissions from Aviation and Marine Transportation: Mitigation Potential and Policies , 2009.
Further Reading / Additional Resources
Congressional Research Service (CRS) – Aviation and Climate Change , 2010
International Civil Aviation Organization (ICAO) - Aircraft Engine Emissions 
National Aeronautics and Space Administration (NASA), Environmentally Responsible Aviation (ERA) Project 
Partnership for AiR Transportation Noise and Emissions Reduction (PARTNER )
Transportation Research Board (TRB) of the National Academies - Aviation 
Boeing, Current Market Outlook 2008-2027 .
Karagozian, A., W. Dahm, et al., Technology Options for Improved Air Vehicle Fuel Efficiency: Executive Summary and Annotated Brief, United States Air Force Scientific Advisory Board, 2006.
Greener by Design, Mitigating the Environmental Impact of Aviation: Opportunities and Priorities. Report of the Greener by Design Science and Technology Sub-Group, 2005.
McCollum, David, Gregory Gould, and David Greene, Greenhouse Gas Emissions from Aviation and Marine Transportation: Mitigation Potential and Policies, 2009. Unless otherwise noted, all facts and figures in this document are drawn from this report.
McCollum et al. 2009.
For more detail on this topic, see the box on p.11-12 of McCollum et al. 2009.
CNS/ATM refers to technologies that enhance the ability of air traffic control to monitor and direct multiple aircraft within an airspace. For example, satellites can more accurately pinpoint an aircraft’s location, allowing more planes to safely operate within close proximity. This allows for more fuel efficient routing and landing procedures, thus reducing GHG emissions.
Laminar flow control (LFC) refers to technologies that modify the aircraft’s boundary layer (the layer of air that clings to the surface of the airframe). LFC increases fuel efficiency by reducing turbulence in this layer. There are two types of LFC technology: passive and hybrid. Passive LFC reduces aerodynamic drag by modifying the air-wing interaction through the shape of the front of the wing. Hybrid LFC removes a portion of the boundary layer (e.g. through slotted or porous wing designs) to reduce drag.
Liebeck, R. H., “Design of the Blended Wing Body Subsonic Transport,” Journal of Aircraft 41(1):10-25, 2004.
Karagozian, A., W. Dahm, et al., , Technology Options for Improved Air Vehicle Fuel Efficiency: Executive
Summary and Annotated Brief, United States Air Force Scientific Advisory Board, 2006.
Greener by Design, Mitigating the Environmental Impact of Aviation: Opportunities and Priorities, Report of the Greener by Design Science and Technology Sub-Group, 2005.
While the direct GHG emissions from combustion of biofuels or Fischer-Tropsch fuels will be similar to or the same as the direct GHG emissions from combustion of traditional petroleum-based aviation fuel, such alternative fuels can have significantly lower net lifecycle GHG emissions since they can be manufactured from biomass feedstocks so that, in effect, combustion of such alternative fuels emits CO2 that was earlier absorbed from the atmosphere by the biomass feedstocks. For more information on biofuels, see C2ES’s Climate TechBook “Biofuels Overview .”
Fischer-Tropsch synthesis of transportation fuels involves gasification of a carbon-containing feedstock (e.g., biomass, coal) and production of a synthetic crude oil, which can then be processed into refined liquid fuel products.
Winglets are vertical extensions of wingtips that reduce drag and increase fuel efficiency.
Greener by Design, The Technology Challenge. Report of the Technology Sub-Group, 2001.
Morris, J., A. Rowbotham, et al., A Framework for Estimating the Marginal Costs of Environmental
Abatement for the Aviation Sector, Omega and Cranfield University, 2009.
Greener by Design, Annual Report 2007-2008.
Government Accountability Office (GAO), High Speed Passenger Rail: Future Development Will Depend on Addressing Financial and Other Challenges and Establishing a Clear Federal Role, 2009.
See, for example, Morris, Eric, “High-Speed Rail and CO2,” blog post, New York Times, 24 July 2009.
GAO, Aviation and the Environment: NextGen and Research and Development Are Keys to Reducing
Emissions and Their Impact on Health and Climate, Statement of Gerald L. Dillingham, 2008.
International Air Transport Association (IATA), “Fact Sheet: Alternative Fuels,” 2009.
For more details, see C2ES’s brief, Climate Change Mitigation Measures in the European Union , 2009.
P.L.-111-5, American Recovery and Reinvestment Act of 2009 (H.R.1), 111th Congress.
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