- The aviation sector accounts for approximately 1.5 percent of global anthropogenic greenhouse gas emissions per year. U.S. aviation activities account for nearly 40 percent of these emissions.
- Gains in aircraft efficiency have been entirely offset by rapidly growing demand for air travel. From 2000 to 2006, global demand for passenger aviation grew at an average rate of 3.8 percent annually.
- A combination of operational practices, lower-carbon fuels, and higher aircraft fuel efficiency could reduce annual greenhouse gas emissions from global aviation by more than 50 percent below “business-as-usual” projections, causing emissions from aviation to only double, as opposed to quadruple by 2050. Aggressive implementation of lower carbon fuels could improve that outlook considerably by replacing a larger share of traditional jet fuel more quickly.
- The international component of air travel and international treaties affect the policies appropriate for reducing emissions from the aviation sector.
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:
- Operational Efficiency
Optimizing flight paths and reducing airport congestion could immediately reduce the aviation sector’s GHG emissions. Adopting advanced communication, navigation, and surveillance and air traffic management (CNS/ATM) systems can reduce the time aircraft spend idling on runways or circling airports waiting to land, thus reducing fuel use and associated emissions.
- Aircraft EfficiencyAircraft efficiency technologies reduce the amount of fuel aircraft use per unit of distance traveled. Several technological improvements exist to improve aircraft aerodynamics, such as applying laminar flow control to an aircraft to reduce drag and, as a result, fuel consumption. More radical innovations include blended wing body aircraft that not only reduce drag but allow the entire aircraft to generate lift, as opposed to just the wings. More fuel-efficient engines and incorporation of super-lightweight materials, such as fiber-metal laminate, into the airframe offer additional avenues to improving aircraft efficiency.,
- Alternative Fuels
Alternative fuels have lower net GHG emissions than traditional petroleum-based aircraft fuel. Biofuels, Fischer-Tropsch fuels, and liquid hydrogen could all present feasible alternatives in the future. While these fuels do not present an immediate alternative, their adoption presents a long-term path toward lower carbon flight. To be seriously considered as a mitigation strategy, alternative fuels must be both cost-competitive and offer significant reductions in GHG emissions.
- Alternative Modes of Transport
Switching from aviation to less carbon-intensive modes of transport can also help mitigate GHG emissions. High speed rail (HSR) is especially suited to replace short-distance passenger air travel in some circumstances, such as in high density corridors. The energy use per passenger-mile for HSR could be as much as 65 to 80 percent less than air travel, but the overall reduction in GHG emissions would depend on a number of factors, including the design of the system (operating speeds and distances between stops) and passenger load factors (i.e., capacity utilization). The European and Japanese experience has shown high speed rail to generally be competitive with air travel on routes of up to 300-500 miles, where there is existing high demand for intercity travel and where several high-population areas can be connected along a single corridor. The total infrastructure and operations environmental impact should be considered for valid comparison of modes.
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
- International Jurisdiction
The international dimension of aviation emissions complicates their mitigation. In fact, no paradigm exists for assigning transnational GHG emissions to individual countries. While the International Civil Aviation Organization is analyzing options for curtailing global aviation emissions, little agreement exists on specific policy actions.
- Lack of a Price on Carbon
With the exception of future flights to and from the European Union (see above), businesses and consumers do not face a financial cost associated with GHG emissions from aviation. This means firms and consumers do not fully take into account the social cost of GHG emissions when considering investments in new technology or travel decisions.
- High Level of Risk in Research and Development
The most advanced mitigation technologies come with high capital costs and considerable investment risks. Many firms are reluctant to invest in developing advanced technology such as blended wing body or laminar coating, as the effectiveness and payoff of these technologies are unknown.
- Government Regulations
Many operational strategies are beyond the control of airlines and strongly dependent on government regulation and support.
Policy Options to Help Promote Aviation Emissions Mitigation Strategies
- Carbon Price
Government policies that put a price on carbon, such as a GHG cap-and-trade program, would guide firms and consumers in making cost-effective decisions regarding limiting emissions from aviation—ranging from blending lower-carbon fuels to choosing alternative transport modes or reducing consumption of aviation services—all relative to the cost of comparable avoided emissions from other sectors of the economy.
- Aviation Emission Standards
Governments could mandate fuel efficiency standards for new aircraft or implement standards to limit the carbon intensity of the fuels used.
- Regulatory Changes
Policies that facilitate the transition to more advanced air traffic management systems would improve operational efficiency in the aviation sector. The revamping of government-mandated operational protocols could reduce congestion, thus saving fuel by reducing idling time and taxiing distance.
- Government Sponsored R&D
Government-sponsored R&D can be an effective driver of innovation, especially when it is targeted at basic research that is beneficial to many industries (e.g., low-carbon fuels and advanced lightweight materials) or is focused on risky projects (e.g., radical changes to airframe designs) that individual companies may not be willing to fund. Public R&D has been a particularly important driver of aviation innovation in the past, and an increase in government R&D funding could accelerate the rate of innovation and development of new technologies.
- Supportive Agricultural Policy
While the technical barriers to low carbon biofuel for aviation are rapidly being retired, the availability of feedstocks is a barrier to fuel capacity building. Policy that would reduce the risk of investing in new crops aimed at aviation biofuel could accelerate capacity and lessen reliance on fossil fuels.
- Department of Defense Leadership
As the single largest user of aviation fuels, the Department of Defense could help build capacity and mitigate risk of initial capital investment by implementing a program to “lead the fleet” in sourcing low carbon fuels. The assurance of market demand would help green entrepreneurs with initial start-up challenges.
- Increased Government Spending on Infrastructure
Strategic government spending could enhance efficiency within the aviation sector, such as funding airport expansion projects or improving air traffic control to reduce congestion. Governments could also invest in alternatives to air travel, such as high speed rail. The American Recovery and Reinvestment Act of 2009 (i.e., the economic stimulus package) includes more than $8 billion to help finance high speed rail corridors throughout the United States.
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.
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.
P.L.-111-5, American Recovery and Reinvestment Act of 2009 (H.R.1), 111th Congress.