Aviation and marine transportation combined are responsible for approximately 5% of total GHG emissions in the United States and 3% globally and are among the fastest growing modes in the transportation sector.

Under business-as-usual forecasts, CO₂ emissions from global aviation are estimated to grow 3.1% per year over the next 40 years, resulting in a 300% increase in emissions by 2050. International marine transportation emissions are estimated to grow by 1 to 2% per year, increasing by at least 50% over 2007 levels by 2050. Controlling the growth in aviation and marine transportation GHG emissions will be an important part of reducing emissions from the transportation sector.

A range of near-, medium- and long-term mitigation options are available to slow the growth of energy consumption and GHG emissions from aviation and marine shipping. For the near- to medium-term (to 2025), improvements in operational efficiency (e.g., advanced navigation and air traffic management systems for aviation and slower marine vessel speeds) have the potential to reduce GHG emissions below BAU projections by about 5% for aviation and up to 27% for marine shipping in the near to medium term (to 2025).

For the long-term (by 2050) advanced propulsion systems and new airframe designs could further reduce aviation CO₂ emissions by up to 35% below BAU projections. For marine transport, larger ships, new combined cycle or diesel-electric engines, and optimized hull and propeller designs could provide an additional 17% reduction in emissions below BAU projections by 2050.

Reducing the carbon intensity of the energy sources used in aviation and marine transportation, by transitioning to alternative fuels and power sources, also could reduce GHG emissions over the medium to long term, although the level of potential reductions is uncertain. Aircraft and marine vessels could be powered by low-carbon biofuels or perhaps even hydrogen. While numerous technical challenges still exist, the main challenge to the use of alternative fuels will be the ability of aviation and shipping to compete with other transportation subsectors for a potentially limited supply of low-carbon biofuels. This could particularly be an issue with marine shipping, where the industry currently consumes the lowest-cost fuels available, namely residual fuel oil. Marine vessels could also benefit from switching to lower-carbon, conventional fossil fuels (e.g., liquefied natural gas and marine diesel oil) or to other renewable energy sources, such as wind or solar power.

—McCollum et al.

Looking beyond technical measures, the authors note that while reducing the demand for aviation and shipping could achieve GHG reductions, the potential impacts are probably limited.

The challenge for these subsectors is that there are few suitable alternatives for the services provided by aviation and marine shipping. High speed rail could replace some passenger air travel, but currently there are few alternatives to marine shipping. Marine shipping is already the most efficient, lowest-cost form of transportation, aside from pipelines, which compete with shipping in just a few markets. With only modest cost increases likely to be achievable through policy intervention, and a limited number of alternatives, a large reduction in demand compared to BAU seems unlikely from these subsectors.

—McCollum et al.

For these reductions to be realized, however, international and domestic policy intervention is required, the authors write. Developing an effective path forward that facilitates the adoption of meaningful policies remains both a challenge and an opportunity.

Resources

• David McCollum, Gregory Gould and David Greene (2009) Aviation and Marine Transportation: GHG Mitigation Potential and Challenges