When Is Flying Greener than Driving?: Nitty-gritty

Q: In terms of greenhouse gas emissions, is it better for a family of four to drive a 20-mpg car from San Francisco to Los Angeles, or to fly? At what mpg rating (for the car driven) are the emissions equal?

Asked by Steve Schmidt, ’83, MS ’92, Los Altos, Calif.

Just as cars differ in fuel efficiency, so do planes. Here is a chart showing differences in CO2 output by plane manufacturer and model.

Courtesy Christian Jardine, Oxford University Centre for the Environment

Not surprisingly, airlines' level of environmental friendliness varies too, depending on the type of fuel used, cruising speed (going a bit slower saves gas), as well as the type of airplane used. Here is another carbon calculator listing the amounts of CO2 expelled for the same trip by different airlines.

Another factor to consider when comparing airplanes to cars is that aircrafts produce contrails—ice clouds created by jet engines—that may accelerate global warming. During the three-day flight moratorium after September 11, 2001, scientists had the opportunity to analyze climate patterns without thousands of planes zipping above us. They noticed a one-degree difference between night and day highs and lows, especially in heavy air traffic areas like the Northeast. NASA researcher Patrick Minnis found that contrails tend to warm the earth because they trap the sun's heat and prevent rays from reflecting back into space.

Major airlines and others are working to decrease GHG output with more efficient designs and alternate fuels, especially biofuels. Virgin and Continental are heavily pursuing these technologies, which they say could reduce carbon emissions by up to 40 percent. Boeing, in partnership with GE and Virgin, tested a biofueled 747 on February 24, 2008. Many airlines are now testing alternative fuels since the European Union will subject airports to a carbon trading program by 2012, meaning airlines will have to limit how much CO2 they emit and will be able to buy or sell carbon credits if they're over or under emission targets.

However, there are some drawbacks to alternative fuels. First, fossil fuels produce more energy per pound than biofuels do, and airplanes need that extra kick for take-off and landing. And biofuels tend to solidify at higher temperatures than fossil fuels do. At cruising altitude, the air temperature is usually about -50 degrees Celsius—cold enough to turn your low-carbon fuel into slush. And then there's the problem with any biofuel use—the vast amounts of land needed to grow fuel crops. In a Seattle Times interview, Dave Daggett, who heads energy and emissions research at Boeing Commercial Airplanes' product-development unit, said that creating a 15 percent blend of bio-plus-jet fuel for the entire U.S. fleet would require an area the size of Florida planted with soybeans.

Biofuels research continues, with several promising leads for aviation. They include algae-based biofuels and fuels comparable to petroleum-based jet fuel from the oilseed crop camelina. Compared to conventional fuel, camelina could reduce GHGs by 84 percent, according to the Sustainable Futures Institute at Michigan Technological University. Boeing is also testing this fuel, which performs as well as petroleum, but so far has failed to convince farmers to switch to the crop.

Airlines are making strides in decreasing fuel consumption by boosting efficiency, however. Boeing's new 787 Dreamliner produces about 20 percent lower emissions per passenger than comparable planes, for example, owing mostly to using carbon fiber instead of heavier aluminum for the fuselage.

Courtesy Boeing

Boeing 787 Dreamliner testing in January, 2010.

Still, even the most fuel-efficient flight will have significant negative impacts. That makes current proposals to develop high-speed rail links in California, Florida and elsewhere all the more important—and might give you a very appealing third option for that San Francisco-Los Angeles trip. The federal government will provide $8 billion toward high-speed rail construction, 2.25 billion designated to California.

America's love of the automobile has so far kept our train technology in the Stone Age. Japan introduced its first bullet train in 1964, and France's TGV reached 187 mph in 1989. Meanwhile, an Amtrak train from San Francisco to LA through the less-than-breathtaking Central Valley takes nine long hours—if you opt for the admittedly spectacular coastal route, you're looking at 12 hours of "Are we there yet?" on a train instead of six in your car!

Construction of the California high-speed rail line is slated to begin in September 2012 with the first passenger trip estimated to start around 2020. That's a long wait, but the trip averaging 220 mph is expected to take about two and a half hours and cost just $55 each way. There are still a lot of roadblocks along the way, however—including a major one in the form of protests from Peninsula residents who worry about the high-speed rail's construction effects on existing Caltrain tracks, and noise pollution. Some residents would like the train to go underground. Peninsula residents created a high-speed rail blog to voice concerns over the bullet train's development, and are working with Caltrain and California High-Speed Rail Authority officials to compromise over issues.

Of course, driving could be more efficient, too. Last year, the Obama administration announced that by 2016, the efficiency standard for passenger vehicles would be 35.5 mpg. Here again, we've lagged behind the rest of the world. Europe already averages 36 mpg and Japan 31, while the United States is chugging along at 21.

Even with our poor fuel standards, driving is still the best combination of practical and green. But new options are on the way—we just have to hope they're in place before your kids are ready to make the trip on their own. In the mean time, have a good drive!

DAVID CARINI plans to receive his master's degree in communication in 2010.