ome time ago, in celebration of my 40th birthday, I decided to embark on a journey unlike any I'd taken before. I would travel around the world strictly by land and sea, avoiding all forms of air transportation. My notion was that, by sticking to the earth's surface, I'd regain a visceral sense -- lost in innumerable airline terminals -- of the size of the world. It seemed an appropriate birthday gift for a travel writer who had become somewhat jaded by travel.
I also imagined that, by shunning airplanes, I was giving the planet a gift. Air travel is one of the most notorious polluters in the world of transportation. Emissions from commercial aircraft contribute significantly to global warming, a fact that belies the "friendly skies" image invented by the airlines.
Aircraft pollution affects climate in several ways. Nitrogen oxides in engine exhaust contribute to the depletion of the ozone layer. Jet fumes also contain other greenhouse gases, including carbon dioxide. But another bit of information took me by surprise. The home page on my computer's Internet browser is the "Astronomy Picture of the Day," which usually features ethereal images of star clusters, planets, and moons. A couple of months ago I turned on my computer and saw an amazing satellite photograph titled "Contrail Clutter Over Georgia." There one could behold the entire southeastern United States, looking as if it had been attacked by a grizzly.
What appeared to be long gray claw marks were actually jet contrails, clouds produced when jet engines pump extra moisture into the air. These man-made cirrus clouds, so pretty at sunset, trap the earth's heat in regions where air traffic is heavy. Although scant resources are earmarked for studying the impact of airplanes on the environment -- NASA has had to recruit student volunteers for "contrail counting" exercises -- the space agency's scientists are finding that contrails are a growing global warming concern. A 2004 NASA study projects that contrails alone will increase temperatures in the lower atmosphere over the United States by at least one degree every 20 years. In the early 1990s, contrails were responsible for less than 1 percent of global warming, and aircraft emissions overall contributed 4 percent. By 2050, the effect of contrails will have increased sixfold, and air travel will be responsible for about 17 percent of global warming.
Carbon dioxide and water vapor make up the bulk of airplane emissions. According to the Edinburgh Centre for Carbon Management, a consulting company that advises businesses and governments on strategies to mitigate climate change, an average commercial flight in the United States releases nearly 1,800 pounds of greenhouse gas, per passenger, into the atmosphere. This seems like an impossibly large number, since a commercial airplane carries only some 10,000 pounds of fuel. But when those exhaust molecules mix with oxygen, the impact soars. To put it another way, a Boeing 747 traveling from New York to London and back exhales some 440 tons of carbon dioxide -- roughly equal to what 80 SUVs cough up during a year of rush-hour driving.