This arsenal of facts brings Brown's big green chance

At last we are at a turning point on climate change. David Miliband likens it to the wartime discussions which produced the Beveridge report and the welfare state. Certainly the shifts in tax, spending and behaviour will be at least as big, and probably bigger. There is one huge difference, though. That was about giving people something new. Today's aptly named Stern report on climate change is defensive. It is about taking things away.

The challenge of climate change has long been the favoured cause of Liberal Democrats and, more recently, of David Cameron too. But the man who commissioned today's report from Sir Nicholas Stern, a former chief economist at the World Bank, is Gordon Brown, the prime-minister-in-waiting. It is already clear that the 700-page report hands Brown what he likes best, an economic underpinning for politics. The world must invest £184bn now or face a bill up to 20 times as much if it delays.

As a tactic, this is pure Brownism. You cannot imagine him posing happily on a bicycle, still less getting into serious husky-bothering in the Arctic circle. He likes an awesome arsenal of facts to have been assembled first, which he can then pick from to pound opponents until they emerge from the smoking ruins with their hands up. It is a well-established pattern. Way back there was Labour's poverty commission. Then there were the 18 volumes of Treasury reports into the consequences of Britain joining the euro. There was Derek Wanless on the future of the NHS, and Adair Turner on pensions.

Then what happens? Sometimes, as with the euro, the fact-arsenal has been used to stop something happening. Sometimes, as with the health service, it has been used to explain a huge shift in spending. But always, as their authors would confirm, Brown deploys the results as he wants to. He uses experts as his supply-train, not as his generals.

So he should. The awesome threat of climate change is not really in doubt. Nor is the inevitability of a new age of green taxes. Everybody is in this game, one way or another. When New Labour came to power, parties competed to talk about cutting taxes. Now they are competing about how they'll raise them. But what is up for political argument is the essential detail - who pays, how much, and who referees it all? For Brown, the opportunities and dangers are all too obvious. There have been acres of excellent high-minded commentary on the general need for green taxes. Let's look for once at the politics.

Labour comes late to the party. The Lib-Dems have been early and consistent green-taxers and have well-thought-through ideas about changing behaviour. When Cameron became Tory leader he identified the need to win over Lib Dem voters and young people, and saw that green issues were the best way to do that. Both the other parties have a strong vested interest in painting Labour as bad for the environment.

The dangers for Labour do not stop there, however. If there is a general belief that taxes have risen too much already, then Labour's high taxes, plus more green taxes, might be one jump too far. Could voters not salve their environmental consciences by choosing Cameron, and hoping he will hand back some of the money in other ways? It is worth remembering that any effective system of green taxes will bite hard on cherished middle-class privileges - those casually taken overseas holidays, the villas in France, the second homes, the huge plasma televisions, big cars and all-year-round summer fruit. People might not be so keen to give them all up in the privacy of the ballot box as they may pretend to an opinion pollster.

Yet the opportunities for Brown and Labour are bigger than the pitfalls. First of all, here is a policy agenda that is vital and unites the party. Blairites such as Stephen Byers are as keen on a new fiscal policy for the global-warming age as any Brownite. Charles Clarke has championed the environment, along with leftwingers such as Alan Simpson. This issue truly straddles the party divide.

Then there's the sheer scale of the issue. It dwarfs all those domestic policy reviews being commissioned by Tony Blair in a bid to tie Brown's hands. This is a radical change that Blair cannot claim credit for. (Indeed, Brown's commission on climate change coinciding with the police moving in on Downing Street over cash-for-peerages made about the best Sunday newspaper contrast the chancellor has enjoyed for years.)

Next, ministers can reflect that "Gordon come lately" jibes don't really matter. Cameron went early with high-profile gimmicks, Brown waited for the evidence and then moved. So what? By the time of the next election, nobody will be able to remember.

No, the crucial thing is what Brown does with the case for carbon emissions and green taxes now. The row over a climate-change bill is interesting. There are dangers in tying policy on carbon emissions so tightly that the country cannot properly respond to freakish weather or economic shocks, but some external system of measurement or regulation is surely not only right but chimes with Brown's proudest moment, the Bank of England's independence.

More important still is taxation. I get the impression that the Treasury is grumpy with David Miliband for suggesting specific "stealth taxes" on fuel, less for the idea than for the impression of policy-making via newspapers. Labour needs to achieve two things. It needs to show that its version of green taxes will reflect its overall values, so will not harshly penalise the worse off. And it needs to reassure those hard-pressed middle-ground families that green taxes don't simply mean extra costs for motoring, new dustbin charges, higher holiday bills, without any compensation.

The answer, surely, is to promise to use extra revenue from green taxes - which will always be a fluctuating resource because people's behaviour will change as they bite - to cut income tax or property taxes, particularly for the worse off. When people read about road-camera revenues being siphoned off for new bureaucracies, or councils taking extra "green" charges and using them for their own purposes, naturally they become cynical. If receipts from eco-taxation are guaranteed to be used to help those at the bottom of the heap, their moral value would be enhanced rather than diminished.

If Brown can make this shift happen, we could be about to witness a radical change in the way people pay taxes: instead of taxation solely by income, it will be taxation by behaviour. In a world where the sustainability of the planet is at risk, who could argue with that?

jackie.ashley@guardian.co.uk