Monbiot:

On a return flight from London to New York, every passenger produces roughly 1.2 tonnes of carbon dioxide: the very quantity we will each be entitled to emit in a year once the necessary cut in emissions has been made.

Wikipedia:

Carbon offsets can be purchased by individuals, businesses and governments from a variety of commercial and non-commercial organizations, for as little as $0.10 per ton of carbon dioxide sequestered. For example, the UK government purchased offsets for the air travel required for the 31st G8 summit.

All this fuss over carbon emissions that can be offset for 12 cents? There must be something wrong somewhere. Monbiot again:

The second reason is that the climate impact of aeroplanes is not confined to the carbon they produce. They release several different kinds of gases and particles. Some of them cool the planet, others warm it. In the upper tropo-sphere, where most large planes fly, hot, wet air from the jet engine exhaust mixes with cold air. As the moisture condenses, it can form "contrails", which in turn appear to give rise to cirrus clouds - those high wispy formations of ice crystals sometimes known as "horsetails". While they reflect some of the sun's heat back into the space, they also trap heat in the atmosphere, especially at night; the heat trapping seems to be the stronger effect. The overall impact, according to the Intergovernmental Panel on Climate Change, is a warming effect 2.7 times that of the carbon dioxide alone.

Ok, so multiply the necessary offset by 2.7 times. It still won't break the bank.

Unlike most environmentalists, who have also called for this measure, the government knows perfectly well that fuel tax cannot be imposed on international flights. It is prohibited under international law by article 24 of the 1944 Chicago Convention, which has been set in stone by 4,000 bilateral treaties - making it almost impossible to unpick. Now the government proposes that aviation be incorporated into the European Emissions Trading Scheme. If flights continue to grow, it will break the system.

The damn Chicago Convention of CIA secret flight fame.

As far as aircraft engines are concerned, major new efficiencies in the next 20 years or so are a pipedream. The Royal Commission reports that "the basic gas turbine design emerged in 1947. It has been the dominant form of aircraft engine for some 50 years and there is no serious suggestion that this will change in the foreseeable future." It is hard to see how it could be made much more efficient than it is already.

It's especially hard if you're not an engineer. Are there any in the room? How about tradeoffs? Trading fuel efficiency for power, for instance?

Those whom the Gods wish to destroy They first make mad. -- Euripides