Commercial aviation could be heading for an image problem.
The growing demand for air travel will require so many additional flights that, despite huge improvements in the fuel-efficiency of airliner engines, aircraft and flight routings, aviation could account for a significantly higher percentage of global carbon dioxide (CO2) emissions by 2050 than it does now.
Last year's Aviation & Environment Summit (A&ES) in Geneva found that aviation accounted for 2 percent of worldwide fossil-fuel CO2 emissions in 2005. But the Intergovernmental Panel on Climate Change forecast in 1999 that aviation's global CO2 impact would rise to 3 percent by 2050.
"It's ? fairly inconceivable that we can cap emissions at where they are at the moment," said Peter Morrell, an economist at Cranfield University's Department of Air Transport, in England. "The industry is expected to grow faster than any technology that can reduce emissions."
Yet in the last 30 years airliner manufacturers and airlines have made enormous progress in becoming more environmentally friendly. They continue to do so.
For instance, Lufthansa has reduced the amount of fuel its aircraft burn -- and the amount of CO2 they emit -- by 70 percent per passenger since the 1970s. In the same period, airliner noise levels have fallen by 75 percent. Levels of oxides of nitrogen (NOx) from aircraft engines have fallen 50 percent in the last 15 years.
But the aviation industry considers this just a start. The Advisory Council for Aerospace Research in Europe expects airliners in Europe to emit 50 percent less CO2, 80 percent less NOx and 50 percent less noise by 2020 than they did in 2000. NASA has set similar targets for flights in the United States.
An additional problem
However, the burgeoning growth of air travel isn?t aviation's only environmental problem. Commercial aircraft fly in the upper atmosphere, and the water-vapor contrails their engines produce may create climate change at a rate disproportionate to aviation's overall greenhouse-gas contribution.
"The effective and overall impact of aviation on climate change is greater than" the 1.6 percent contribution the industry made in 2005 to global greenhouse gas, the Society of British Aerospace Companies' Stern Report noted last October.
This phenomenon is called "radiative forcing," said Morrell. Studies in the United States suggest that contrails can create 30 percent cirrus-cloud cover on days that are otherwise cloudless.
Without a major change in aviation operations, radiative forcing could make "aviation emissions account for ? 5 percent of the total warming effect (of all global CO2 emissions) in 2050," warned the Stern Report. By then, aviation will produce three times the level of CO2 emissions it does now.
The European Union "has grasped the nettle" by including commercial aviation in a major CO2 emissions-trading scheme, said Morrell. The European Commission, the EU's policy-making body, plans to cap CO2 emissions for all flights within the EU by 2011.
If an airline can't operate within its cap, it will have to buy rights to the excess CO2 it creates. "A critical issue," however, is the extent to which the EC can impose on EU nations and other countries its will to make aviation environmentally friendly, said Morrell.
The A&ES reported that 80 percent of aviation's greenhouse gas emissions come from passenger flights of more than 900 miles, "for which there is no practical alternative." So the EC decided that all flights to and from EU nations should be CO2-capped by 2012.
"There was a furious reaction from the U.S.," said Morrell. The U.N. civil aviation governing body, ICAO, forbids carbon-pricing programs.
The U.S. might get its way. The EU nations' transport ministers won't complete their study of the aviation CO2-cap proposal until the end of this year. "It's by no means a fait accompli -- it's more likely that it's going to be diluted," said Morrell.
Misleading fuel taxes
Britain and the Netherlands have started imposing fuel taxes. But both countries are adding these revenues to their general treasuries and there is no evidence they are using the new levies to promote environmental responsibility.
Separately, some airlines are offering voluntary carbon-offsetting plans. These plans ask customers booking flights on the Web to click on special links that result in the customers making donations to environmental programs. Canada's WestJet has done this.
The A&ES concluded that, to become truly environmentally responsible, the aviation industry will need to make progress on a variety of regulatory, technological and operational fronts.
Operational and technological possibilities
Today in Europe, for instance, there are 22 separate air traffic management authorities. Creation of a unified system would simplify air routes and operating procedures, reducing European airlines' CO2 emissions by as much as 12 percent, said Lufthansa. The FAA's Next-Generation air traffic system should create similar savings.
Fuel cells could start replacing airliners' auxiliary power units -- small jet engines -- soon. More efficient jet engines are being developed and new ducted-fan or unducted-fan designs, 15 to 20 percent more efficient than today's engines, could enter service by 2020.
Airliner manufacturers are using advanced materials to make aircraft lighter and are experimenting with new, aerodynamically efficient body shapes. Aviation companies are exploring alternative jet fuel sources such as biomass waste and liquid natural gas.
The International Air Transport Association's goal is to make aviation "a green industry producing zero emissions" by 2050. By then, concluded the A&ES, hydrogen could be a viable, clean fuel source for aircraft.