David Cameron has got it right on climate change. The Conservative leader's advocacy of binding annual limits on UK emissions of carbon dioxide is brave. It has forced the Government to respond: we now expect the Queen to announce a Climate Change Bill in her Speech next month. Meanwhile, David Miliband, the Secretary of State for the Environment, writes on page 43 to explain and defend the policies of Her Majesty's Government.

When Mr Cameron endorsed the proposal by Friends of the Earth for binding annual limits, we were entitled to be sceptical. It looked like a device to lend policy substance to Mr Cameron's green style, but without any visible means to achieve the desirable end of cutting Britain's carbon output. It is true that he enjoys the luxury of opposition while Mr Miliband has to act. Any action that is likely to reduce the amount of carbon dioxide that this country puts into the atmosphere will produce winners and losers. And it is a fact of democratic life that the gratitude of the winners is outweighed by the resentment of the losers at the ballot box.

However, Mr Cameron cannot be accused of merely indulging in the irresponsibility of opposition. He has set out in some detail how his plan would work. He proposes an Independent Climate Change Commission, charged with enforcing carbon targets in the way that the Bank of England is charged with hitting the inflation target. This system "will create a price for carbon in our economy," he said, "so things that produce more carbon will become more expensive." He has not spelt out the full implications of this for the cost of domestic gas and electricity or aviation fuel, which are the cheapest forms of carbon at the moment. But the Leader of the Opposition has been explicit enough to be described as brave.

This is what constructive opposition ought to be about: challenging the Government to be bolder in pursuit of goals that we all know to be right, but which may provoke short-term unpopularity. Mr Cameron has also succeeded in flushing out some of the Government's weaker excuses for inaction. In our pages today, Mr Miliband says that annual limits would be unworkable because carbon emissions will fluctuate with the weather and the economic cycle. Yes, but an independent commission would be able to take those fluctuations into account - five-year targets, which the Government prefers, would not have the same bite.

That said, Mr Miliband should be congratulated for the speed and energy with which he has risen to the climate-change challenge. In a sparkling speech last week, he quoted Mark Twain: "Everybody talks about the weather, but nobody does anything about it." But he also said that we must create "a sufficiently high and stable long-term carbon price". He and Mr Cameron understand the challenge. They know that if low-carbon economics is not hurting it is not working.

They know this means difficult choices for politicians - and the rest of us. For the rich residents of Richmond-upon-Thames, forced to pay extra to park their SUV (a plan opposed, incidentally, by local Conservatives); for the family planning a skiing holiday by low-cost airline this Christmas; and for the genuinely poor pensioners struggling to heat their homes.

If Mr Cameron and Mr Miliband understand the issue, Tony Blair does not. Last week, the Prime Minister said binding annual limits was a bad idea because they "are very, very difficult to deliver". Precisely. But the challenge is no longer for him. Tomorrow, Sir Nicholas Stern reports to Gordon Brown on the economics of climate change. The big question is how the Chancellor and PM-in-Waiting will respond.