In An Inconvenient Truth, Al Gore produces a graphic of the cities to which he has flown (sometimes by private jet) to talk about climate change. There are dozens of them, all over the world. I am glad that he gave those talks–his contribution to the fight against climate change is equal to that of all other environmental campaigners put together. But I was shocked not so much by his mode of travel as by his total lack of embarrassment–even, perhaps, of awareness–about the contradiction between what he was saying and what he was doing.

In Europe, where the environmental impacts of transport have been subject to furious debate for years, a climate change campaigner would never have exposed himself in this way. Had he flown as much as Gore, he would have felt it necessary to explain that he could not otherwise have been so effective. He would never set foot in a private jet. He might have conducted his talks by video link–and made a point of that in the film.

Jets produce staggering amounts of carbon dioxide and other gases that accelerate global warming. But in North America the impacts of transport–especially flying–are only beginning to nudge the political surface.

Carbon dioxide emissions per passenger mile from a standard airliner are very similar to those from cars. But you can cover nearly 15,000 miles in one day by plane. The CO2 produced by planes is augmented by the other greenhouse gases they release, magnifying its effect by 270 percent. This means that flying is one of the most destructive things we can do.

It appears that the only ethical option–and I realize that this will be an even less popular message in North America than it is in Europe–is greatly to reduce the number of flights we take. Because our economy has been built around the rapid mobility of goods and people (the volume of US airfreight grew by 372 percent between 1980 and 2004), this could be our greatest political challenge.

Officially, transport is the world’s second-fastest-growing source of CO2 emissions; between now and 2050, they are expected to double. But these numbers tell only half the story. The fastest-growing source of carbon emissions is the power sector, whose output is expected to quadruple by mid-century. But much of this growth should in fact be attributed to transportation, as it is partly driven by the use of coal and gas to make synthetic liquid fuels used for vehicles. Cars and planes could soon become the primary cause of global warming.

The automobile problem is–in engineering terms–relatively easy to address: All the necessary technologies required to slash emissions from surface transport exist already. But dealing with airplanes is a far more complicated problem. It means confronting not just the political and cultural resistance we meet while trying to clean up emissions from cars but also massive technical barriers.

Lifting a large passenger plane into the air and keeping it there is a feat subject to strict physical constraints, and it requires a massive amount of fossil fuel. The standard jet engine has more or less reached the limits of efficiency, and there is no replacement anywhere near production. There has been a great deal of talk about “blended wing bodies”: planes in which the passengers sit in swollen wings. In principle they could cut fuel use 30 percent. But this is no more than a concept, whose stability and controllability have not been proven.

New fuels are the stuff of fantasy. Richard Branson, CEO of Virgin, is looking for ways to create biofuel for jets; to this end he has started a new company called Virgin Fuels. But biofuels cause more climate change than they prevent. Forests in South America and Southeast Asia are being cleared to plant oil palm, sugarcane and soya for transport fuel. A study by Wetlands International with the Dutch scientific consultants Delft Hydraulics found that the production of every ton of palm oil results in up to 33 tons of CO2 emissions, as trees are burned and peat is drained. This means that palm oil causes up to ten times as much global warming as petroleum. Even if you could put it in planes in large quantities, biofuel would not solve the carbon problem.

A more promising alternative fuel is hydrogen, if made from renewable electricity. Jet engines can run on hydrogen; however, because it is a far less dense fuel than kerosene, the planes would have to be much wider to carry it. This means that they must fly in the stratosphere–otherwise they’d encounter too much drag. Unfortunately, the water vapor produced by burning hydrogen in the stratosphere would cause a climate-changing effect thirteen times greater than that of an ordinary subsonic plane.

As the Intergovernmental Panel on Climate Change discovered, “there would not appear to be any practical alternatives to kerosene-based fuels for commercial jet aircraft for the next several decades.”

The obvious replacements for planes are scarcely better. Fast passenger ships appear to be even worse for the environment than jets. One set of calculations I have seen suggests that the Queen Elizabeth II, the luxury liner run by Cunard, produces 9.1 tons of emissions per passenger on a return trip from Britain to New York. This is 7.6 times as much carbon as you produce when traveling by plane.

or are ultra-high-speed trains the answer. Though trains traveling at normal speeds have much lower carbon emissions than airplanes, Professor Roger Kemp of Lancaster University shows that energy consumption rises dramatically at speeds above 125 miles per hour. Increasing the speed from 140 to 220 mph almost doubles the amount of fuel burned. If the trains are powered by electricity, and if that electricity is produced by plants burning fossil fuels, they cause more CO2 emissions than planes. Running trains on renewable electricity is certainly possible, but this faces problems: Trains must run on time, and that means there is little room for “demand management,” which means reducing the electricity load in response to fluctuations in supply. In all transport systems, high performance is incompatible with low consumption. The faster you go, the more energy you need.

There is one form of transoceanic transport that might help us reduce emissions, but this will not be a popular proposal. The total climate impact of a zeppelin, blimp or airship is 80 to 90 percent lower than the impact of a jet plane. Though forever associated with the Hindenburg disaster, airships are now quite safe. They have a range of up to 6,000 miles. Their top speed is around 80 mph. This is faster than ships but much slower than jetliners, which cruise at more than 500 mph. A flight from New York to London by airship would take forty-three hours. They also have trouble landing and taking off in high winds and making headway if the wind is against them. This makes travel times less reliable than those of jets.

If we really have to cross the ocean, and if we are to make the reduction of carbon emissions a top priority, airships might be the best form of transport. Already dirigibles are being developed for lifting superheavy cargo in and out of inaccessible areas.

It seems to me that the only way the number of flights by passenger jet can be significantly and permanently reduced is through a reduction in the capacity of airports. Unfortunately, all over the world, airports are expanding. In Britain, for example, Tony Blair’s government has instructed airports to double their capacity to accommodate a projected rise in the number of passengers from 228 million in 2005 to 500 million in 2030.

Reversing this trend is extremely difficult, but it is necessary if we are to have a high chance of preventing runaway climate change. Some sectors–tourism and hotels, for example–will undoubtedly suffer. We would need to get used to vacationing closer to home, or traveling less frequently and for longer. Corporations would have to start making better use of technology, conducting much of their business through video conferences and electronic gatherings. But it is surely not beyond the wit of humankind to maintain a healthy economy without having to load 200 pounds of human being onto an airplane every time something needs to be discussed.

Less jet travel will be extremely hard to sell. It flies in the face of everything we have been encouraged to regard as progress. But climate change appears to demand that progress be redefined. It suggests that the sum of human welfare will now be enhanced not by new economic freedoms but by new restraints.