Darling ducked the difficult decisions

The Chancellor can no longer afford to ignore the contribution of international aviation and shippin

Like the Lord Almighty, the Chancellor giveth, and the Chancellor taketh away. On the one hand a 10 per cent increase in plane duty will force aviation to pay more of its environmental costs and help reduce emissions. On the other, Alistair Darling's explicit support for the expansion of both Heathrow and Stansted airports will force emissions ever upwards. A higher rate of first-year tax on polluting 4x4s will reduce emissions. But postponing the increase in fuel duty will increase them.

If his Budget speech to the Commons is to be believed, Darling has made up his mind: climate change is the greatest challenge facing us all, and "there will be catastrophic economic and social consequences if we fail to act". In response to this, with great determination and steely efficiency, the Chancellor . . . fails to act. There was no more money for the cash-strapped low carbon buildings programme, so the UK domestic renewables sector will continue to decline. Aviation can expand virtually unchecked. By caving in to the roads lobby and postponing the increase in fuel duty, he is making fossil fuel slightly cheaper in real terms, helping to increase consumption.

Darling also wants to "encourage sustainable biofuels", apparently not realising that in today's world the phrase is an oxymoron. He is happy to jump on the Daily Mail's plastic bags bandwagon - a campaign of marginal importance environmentally - but unwilling to do anything to encourage manufacturers to produce goods more sustainably. And so it goes on.

Big decisions have been postponed. Instead of agreeing that the UK's reductions targets should be bumped up to 80 per cent by 2050, in line with the latest science, this decision has been handed to the Committee on Climate Change and put off until December. There were no headline announcements on road pricing; it will be subject to further study. There was no announcement on feed-in tariffs to support micro-renewables, despite this being heavily trailed.

New houses will be zero-carbon from 2016, and commercial properties zero-carbon from 2019. But there is nothing substantial to reduce pollution from the existing housing stock, which at 27 per cent of UK emissions is one of our big gest sources of CO2. The government will give £26m to something called the Green Homes Service, but that has yet to be launched - and £26m really isn't very much money. At this rate of progress, our existing homes will be carbon-neutral by about the year 5000, when most of Britain will be under water.

The inescapable conclusion is that if the government does pass the Climate Change Bill as intended and set itself legally binding cuts in carbon, it will be hard-pressed to achieve them - particularly if the 2050 target is indeed raised to 80 per cent, as the green coalition group Stop Climate Chaos and many others are demanding. A little-noticed win for the climate-change movement was achieved recently when the government agreed to annual indicators of progress on carbon cuts, rather than just the five-yearly budgets. But this will make it even more difficult for ministers to duck difficult decisions, as Darling is doing by pledging commitment to acting on global warming while doing nothing substantial to reduce emissions.

The beauty of the Climate Change Bill approach is that it will forcibly iron out these inconsistencies in government policy. Future chan cellors will not be able to stand up before the country and simply pledge action; they will be judged by what happens with carbon emissions from year to year. If a future Alistair Darling wants to make petrol cheaper for motorists, thereby increasing emissions, he must force even deeper cuts in another sector of the economy to make up for it. There is no middle way.

But the bill still has a rather large hole in it - one large enough to fly a jet or sail a tanker through. International aviation and shipping are still excluded from our domestic targets, on the grounds that this aspect of our carbon footprint is shared with other countries. Ministers pretend that the issue is terribly complicated, but it really isn't. We could simply count all the emissions from each departing plane or ship, but ignore those that arrive. It's all the same to the planet.

Mark Lynas has is an environmental activist and a climate change specialist. His books on the subject include High Tide: News from a warming world and Six Degree: Our future on a hotter planet.

This article first appeared in the 24 March 2008 issue of the New Statesman, The truth about Tibet