AGREEMENT on a third runway at Heathrow may become, literally, one of the most costly environmental decisions ever made. It sets a trajectory for aviation to emit more than a third of the UK's greenhouse gas emission quota by the middle of this century.
This will limit the options for growth of our domestic industries, energy infrastructure and agriculture. It will also seriously damage our international standing on climate policy.
In the UK, and Scotland, we have set carbon emission reduction targets of 80 per cent by 2050. These set a legal framework for a series of ever tightening carbon limits.
However, this is not "job done". There are no legal teeth to deliver reductions to meet this policy. These can only come with measures to tackle emission sources.
Putting a price on carbon, and trading it as a commodity, will help find the lowest cost across technologies and countries. All emission sources will eventually be included, with aviation and shipping no exception. This may be the best way to encourage emission reductions for international transport.
Flying and shipping transcend countries' borders. As many emissions occur over water, or in transit through other countries, they defy traditional climate policies.
The most likely option for allocation is given by the government's own advisers, the Committee on Climate Change. Emissions will be accounted for on a "bunker fuel basis" – the UK will be responsible for the emissions of flights and ships, leaving its airports and ports.
So, what are these emissions likely to be? Forecasts of unconstrained growth (precipitated, by, for example, expansion of Heathrow) have been provided by the Department for Transport. Its "central bunker fuel predictions" suggest UK-related emissions for domestic and international flights could reach about 60 million tonnes by 2030, then remain relatively stable for the next two decades.
By 2050, if we follow this anticipated projection, the report concludes that 35 per cent of the country's total emissions quota will be attributable to aviation.
We will be able to deal with this in a couple of ways: either we reduce emissions elsewhere to compensate, or buy credits to make up the shortfall.
Let's take these in turn. The Committee on Climate Change anticipates we will rely on electricity more and more as a power source for heat and transport. This is because, if generated via renewables, electricity provides a viable alternative to burning fossil fuels. However, with 70 per cent of our electricity generation capacity due to be obsolete in the next 20 years, and the land constraints on wind power, it will be almost miraculous if we plug this gap, let alone switch from gas for our homes, and petrol for our cars or compensate for rising aviation emissions.
That leaves us with the carbon markets. A widely anticipated cost of carbon, as the markets kick-in post 2012, is 150 per tonne of . Emissions, per passenger on a return flight to New York, are 1.3 tonnes of . That means the cost of a ticket will need to rise by 200 to compensate for its climate impact. But there is another sting in the tail. The effect of aviation on the climate is higher than impact alone, due to the white contrails we see and invisible nitrous oxides. Current estimates are that these factors at least double, and perhaps quintuple effect. Could this mean a 1,000 carbon supplement on the price of a New York ticket?
There are also huge political repercussions on the Heathrow decision. December's Copenhagen conference, to hammer out a post-Kyoto agreement, will influence the geopolitical landscape of the next century.
In embracing a UK expansion of air travel, the government has undermined our international climate credibility. Our hand, for negotiating targets and settlements, will be weakened.
This decision will be unimaginable in a few years time. The Copenhagen settlement will have laid the foundations for a global carbon market, making aviation an expensive luxury. Fuel prices that put airlines out of business not 12 months ago will be at new highs.
In the world's eyes, we have demonstrated our true environmental colours. We have bet on brown, when the world's money is moving to green.
• Charles Henderson is director of Climate Futures.