Measuring the "true" fuel (and thus carbon) efficiency of aviation depends on many assumptions, including load factors aboard planes and the distance of trips. (As with car trips, longer trips are more efficient.) Measuring on a per-mile basis ignores the fact that few people drive 10,000 miles a day; measuring per trip ignores the fact that most people fly infrequently. However, the best-case scenarios show that flying is no more carbon efficient than driving; the worst-case scenarios show emissions three times higher than driving.

The obvious solution to these externalized costs would be a carbon tax of some sort, and naturally the European Union would be the first the propose one: the European Parliament will vote in July on whether to add flights at European airports to its "emissions trading scheme" (ETS), a cap-and-trade system for carbon emissions. Yet this would constitute a first-ever tax on jet fuel: currently, tolls (landing charges and passenger fees) pay for airports, quite unlike how the gas tax pays for roads.

Furthermore, the financially teetering airline industry can't agree on anything. American carriers say they'd be exempt, as the U.S. is no longer party to Kyoto; European "legacy" and low-fare carriers disagree about how to allocate the initial carbon cap.

Worse yet, few alternatives exist to Jet A kerosene. New aircraft offer incremental efficiency improvements, but few breakthroughs are on the horizon. Alternative fuels and fuel cells are too heavy for use aboard planes, and alternative travel modes are far slower. (One exception is high-speed rail over short and medium hops, but even the best such services worldwide face stiff competition from low-fare bus and air routes, and besides, trips under 1,500 km account for only 20% of aviation carbon emissions.) Operational efficiencies, like reducing congestion at busy airports, could reduce emissions, but not by a whole lot.

I'll admit I feel vastly more guilty about the carbon emissions of the taxicab ride to the airport than the plane ride from the airport. I've often chosen cheap short-hop flights instead of overnight Amtrak trips, and the notion of offsetting that choice by buying carbon credits doesn't really cross my mind when the flight attendant hands over those pretzels. But perhaps higher prices might be the ticket: "fuel surcharges" of $100 atop already pricey transoceanic trips did partially dissuade me from a recent long-distance getaway.