Steve Richards: My green test... when will it be cheaper and easier to use a bus or train instead of a car?

While we are on the subject, what about more cycle lanes to compare with Berlin or Stockholm?

Beware the noisy clamour that surrounds the publication of a single report. The Stern Report is a brilliantly forensic analysis of the overwhelming challenges posed by climate change. Following yesterday's launch political leaders were competing to endorse its conclusions. Yet the political consensus in relation to this newly fashionable issue is not as great as it seems.

The leaderships of the two bigger parties walk tentatively on green terrain. The tentativeness is at odds with the sweeping declarations made after the report's publication. Only a few weeks ago senior government figures were ruling out authoritatively the inclusion of a climate change Bill in the Queen's Speech.

Now the Bill will take centre stage. For Gordon Brown the politics of the environment are especially complicated. Brown knows some unavoidably tough decisions will be required that will not be as popular in the short term as a broad aspiration to cut carbon emissions. He has become green because he has had no choice in the matter. He does not reach this position out of a lifetime's conviction.

Similarly political expediency partly shapes David Cameron's espousal of the green cause. He wants the Conservatives to be liked again. He needs to attract those that have voted for the Liberal Democrats in recent elections. The policies for a party that want to be hailed as pro-business and long-term tax cutters are incomparably more complicated. We await the detailed policies.

In the meantime the parties experiment gingerly in relation to the biggest challenge of our times, inevitably seeking political advantage as they do so. Echoing the increasingly influential Friends of the Earth, the Conservatives call for annual targets in relation to carbon emissions without outlining the measures to bring them about.

Such rigid targets are parochial and unworkable, the equivalent of advocating unilateral nuclear disarmament in the 1980s. The key to reducing carbon emissions is multilateral trading agreements with other countries. Yet in making such a case the Government faces a battle between sensible policy-making and the populism that can swamp green issues too easily. In refusing to agree annual targets it risks appearing less "green" even if it is being more practical. The battle over targets is the first big divide between the parties.

The other one relates to the issue of green taxes. The two bigger parties preach vaguely. To their credit the Liberal Democrats are more specific but have yet to be candid about whether they are introducing green taxes to change behaviour or raise revenue. There is a danger that vaguely popular green taxes will be used to justify unaffordable tax cuts.

Yesterday the shadow Chancellor, George Osborne, told the BBC that he would put up green taxes to pay for tax cuts for families. In which case Osborne is accepting that green taxes will not change behaviour, but become an alternative source of revenue. Similarly the Lib Dems argue that their more detailed tax proposals would pay for cuts in income tax. They will do so only if people continue to fly in the same numbers and use polluting cars with the same intensity.

Note Brown's cautious reference yesterday in an otherwise crusading speech at the launch of the report. He said that taxes have a "role to play". I am told that the Treasury regards green taxes as relatively marginal. Note also Cameron's reluctance to be specific about tax increases. These are early days in the politics of the environment.

For all the loud cross-party cheers greeting yesterday's report, I have a simple test that no government or potential government comes close to passing. When will it be cheaper and more reliable to use public transport rather than the car? I have lost count of the number of times I have made inquiries about travelling somewhere by train and concluded: Sod it, I will take the car. It will be quicker and cheaper.

Sometimes I have dared to inquire about taking the bike on a train journey only to discover that if a change of train is necessary it is not possible to take the bike for the whole route. Recently I made inquiries about taking a bike to Scarborough and was told I could take it as far as York. Again: Sod it, I will take the car and put the bike in the car.

And while we are on the subject what abut more cycle lanes, to compare with Stockholm, Berlin or Amsterdam? The sad fact is that many European countries are greener than Britain even if the Government manages to set the global agenda with the Stern Report. Without an integrated transport system that includes affordable and reliable public transport, it will be in no position to preach.

The congestion charge in London offers a fruitful model. The charge deters some drivers, while the rest pay a charge that funds much-needed improvements in public transport. Ken Livingstone was re-elected by a large majority at the mayoral election having made such a move. Voters would start to feel positive about the new green lifestyle if their additional payments made an immediate improvement in public transport.

For now all we can deem is that the political context is more benevolent than it was. Cameron's motives are irrelevant. He is where he is. Even some of those that work with Brown accept that Cameron "gets it" in relation to the environment's importance. No party that appoints Zac Goldsmith and John Gummer to review green policies can avoid proposing some constructive ideas. Brown commissioned the Stern Report before Cameron had won the leadership of the Conservative Party. He knew he would have to act and sought cover from a respectable economist, as he did when he commissioned the senior banker, Derek Wanless to make an assessment on the costs of funding the NHS. Now he has the room to move irrespective of the Stern Report. The Stem Report provides added cover, but in spite of the disagreements Cameron is the main shield as the Government prepares to act.

Compare the scope for some cross-party agreement with the crisis in the autumn of 2000 when a few loutish lorry drivers caused mayhem over petrol taxes. Within days the Tory leader, William Hague, was popping up to demand cuts in fuel duty and fleetingly becoming popular in the polls. The Government was relatively isolated and was forced to back down stealthily. Now it is freer to roam more widely. In that respect Cameron's shift is more important than Stern's report.

The most significant impact of the Stern Report is to give Blair and Brown the political language to accompany changes that were bound to be implemented in some form or other. Blair proclaimed a "pro-growth" strategy. Brown spoke of being "pro-growth and pro-green". Stern makes a powerful self-interested case for action, reinforces what was widely known, and follows a political consensus in Britain that is more passionate in generalities than specifics. Its publication is the easy bit. Now let us see what follows.

s.richards@independent.co.uk