Ask the pilot

I'm greener than most people, I'd guess. I don't own a car, for instance, and much of the furniture in my apartment was scavenged from curbsides and refurbished by hand. I'm low impact, abiding as best I can by the three R's of good stewardship: I reduce; I reuse; I recycle.

Then I go to work and expel hundreds of tons of carbon into the atmosphere. Yes, flying planes for a living -- big planes -- would seem to negate my efforts on the home front.

Commercial aviation has come under increasingly virulent attack for its perceived eco-unfriendliness. In Europe especially, powerful voices have been lobbying for the curtailment of air travel, proposing heavy taxes and other disincentives to restrict airline growth and discourage people from flying. I was introduced to a new term recently, "binge flier," a name for Europeans who take advantage of cheap airfares and indulge in short-stay leisure junkets. Thanks to ultra-low-cost airlines like Ryanair and EasyJet, it's not only possible but affordable to leave Britain in the morning, fly to a beach in Spain and be home again before dark. Many eco-activists find this appalling.

How much of their criticism is fair and how much is gratuitous airline-bashing is open to some debate. I'm the first to agree that airlines ought to be held accountable for their fair share of ecological impact. But that's the thing. Globally, commercial aviation accounts for only about 2 percent of all fossil fuel emissions. (In the United States, that 2 percent also drives approximately $3.4 billion in daily economic activity. Worldwide, it accounts for an estimated 8 percent of global GDP.) Airlines are easy targets these days, but in the hierarchy of environmental threats they appear to be disproportionately villainized. Commercial buildings emit a far higher percentage of climate-changing pollutants than commercial planes, yet there is little outcry and few organized movements to green them up. With cars it's similar. Americans have staggeringly gluttonous driving habits, yet only rarely are we made to feel guilty about them.

So it seems that 90 percent of the flak is aimed at 2 percent of the problem. But, actually, it's not so simple.

Gauging aviation's full influence on the environment is more complicated than simple percentages. Aircraft exhaust is injected directly into the upper troposphere, where its effects on climate change aren't fully understood. The production of water vapor (evidenced by contrails) and the release of compounds such as nitrogen oxides are factors as well. (In the days following Sept. 11, 2001, when commercial aircraft were temporarily grounded, climatologists noticed marked changes in cloud patterns over the United States, possibly attributable to the absence of cloud-forming contrails.) As a rule of thumb, experts recommend multiplying that previously cited 2 percent fossil fuel figure by an additional 2.5 to get a more accurate total of the industry's greenhouse gas contributions. Using this formula, airlines now account for about 5 percent of the problem.

Still that's not much, but commercial aviation is growing at an unprecedented rate. In countries like China, India and Brazil, emerging middle classes have spawned the birth of scores of new airlines. China alone intends to construct more than 40 large airports over the next several years. In the U.S., the number of annual airline passengers, already approaching a billion, is anticipated to double by 2025. Greenhouse gases from planes could rise to as much as five times current levels. If indeed we begin reducing the carbon output from other sources, as we keep promising to, such as those from automobiles, power plants and buildings, the output from aviation will rise drastically as a percentage of the whole. According to a study in Britain, if air travel in that country grows as predicted, carbon emissions would need to be reduced to zero in other sectors of transportation and manufacturing in order for the British government to meet stated reduction goals by 2050. A 1999 report by the United Nations concluded that regardless of improved air traffic flow and the greater efficiency of modern aircraft engines, such things "will not fully offset the effects of the increased emissions resulting from the projected growth of aviation."

Be that as it may, aircraft engines are remarkably efficient. For airlines operating on razor-thin margins and beholden to the whims of petroleum prices, engines need to be. Fuel costs make up a third of an airline's costs. According to the Air Transport Association, the industry's largest trade group, U.S. air carriers have increased fuel efficiency 70 percent over the past 30 years, and 35 percent since 2001 alone, mostly through the retirement of older, fuel-thirsty aircraft. Average fuel efficiency of the American automobile, on the other hand, hasn't changed in 18 years, and has stayed more or less stagnant for three decades, with car manufacturers vociferously opposing new rules. ATA has lobbied Congress to increase funding of air traffic control upgrades that could further reduce fuel consumption by 10-15 percent per flight. Imagine the automakers' lobby asking the government to mandate tighter mileage standards. The International Air Transport Association (IATA) says that 12 percent of global carbon dioxide from aircraft would be saved if air traffic control systems were modernized.

As for fuel consumption, let's look first at a short trip, from New York to Boston and back again. This flight is slightly under an hour in each direction. A typical aircraft on such a route, an Airbus A320, will consume somewhere around 10,000 pounds or 1,500 gallons of jet fuel over the course of the round trip. Assuming 140 passengers, that's 71 pounds of fuel, or just over 10 gallons per person. A lone occupant making the same trip by car would consume twice those amounts.

However, traveling by train the numbers would be considerably better. And as the distances drop, so does the presumed time advantage of going by plane. By air it's an hour or so, gate to gate (assuming no ATC delays), but considerably longer when you account for the transit times between airports and the traveler's actual origin/destination points, which might be many miles away. The carbon emissions of the connections (bus, taxi or private car) to and from the airport must also be considered. Amtrak's Acela train covers the Boston-New York run in three-and-a-half hours, but it's downtown to downtown. Overall, a train is the least harmful way to go over short distances, and greens have a good argument that railways ought to be the traveler's mode of choice, where available and practical.

But now let's calculate the emissions per passenger on a longer journey, where neither train nor car is a viable option. New York-London is one of the most popular and profitable pairings in the world, and one that I have flown several times of late, allowing me to use actual performance data from a round trip I made about two weeks ago. (Calculating a round trip gives us a better average. A one-way analysis would be skewed by the prevailing west-to-east winds.) The aircraft was a Boeing 767-300, one of the most common models used on transatlantic crossings.

The first leg, New York to London, covered 3,180 nautical miles and burned 72,400 pounds, or 10,800 gallons of fuel. The return trip was slightly shorter in distance but longer in time. It covered 3,160 miles and used 85,700 pounds, or 12,800 gallons of fuel. With a full complement of passengers in both directions, that worked out to 752 pounds, or 112 gallons per person. Maybe it sounds like a lot, but each passenger driving that same distance would have consumed double the fuel. Granted you cannot drive to England, but you see my point.

Of course, the reason most people make such trips isn't because they have to. It's because hopping on a plane is relatively cheap and easy. That may change. Air travel will always be an economic necessity, but the kinds of flying we've become used to might not always be possible. Should petroleum prices continue to climb, or skyrocket to some ghastly value with the arrival of so-called peak oil, the airline business will likely cease to exist as we know it. We'll still have airplanes, but the binge fliers and quickie vacationers will be long gone in the face of exponentially higher fares. Those anti-expansionists might get their wish after all.

Next week: Coffee, tea or plastic? The scourge of on-board trash. Plus, what some airlines are doing about the emissions problem.