David Cameron has spent most of his time as Conservative leader trying to steal the Lib Dems' thunder by concentrating on the environment. As well as changing the party logo to a green tree, he has travelled to the Arctic to be snapped next to a melting iceberg. He has cycled to work, has a windmill on his house and even threatens to have one installed at Number 10. The problem is he has to get elected as prime minister first, and he is learning that winning awards is a lot easier than winning elections.
Standing in his way is the Chancellor of the Exchequer, who is also going green. The thing about Brown is that he thinks longer and harder than most about decisions before he makes them. Sometimes this works against him, as it did when he dithered about standing for party leader over a decade ago. Usually, however, it works to his advantage.
On climate change, Brown has been waiting months for Sir Nicholas Stern's report. Deliberately leaked last weekend for maximum media impact, it has at a stroke killed off Cameron's point-of-difference. It is the first report to give an economic case for climate change. For years, business, especially in the US, argued green measures would slow the economy. Stern suggests that the opposite is true.
Having brought in Al Gore as an adviser, a slightly more hard-hitting green than Zac Goldsmith, Brown has established himself as a serious thinker on climate change.
What to do about it is the tough question. The Treasury was not exactly over the moon with the leaking of David Milliband's green tax proposals, but since Brown will never make the mistake of hitting motorists too hard again, it matters little. The Chancellor will want to wait to see what the Tories come up with before moving too far down the tax road.
What he will be doing is calling for joint European action. Britain is responsible for two per cent of CO2 emissions, so acting alone isn't an option.
Brown has plenty of experience of making his case in the corridors of world power, having fought long and hard for Third World debt relief, and we can expect plenty more posturing on the international stage. It won't spoil his chances of becoming PM, either.
It seems increasingly likely that Brown will be elected unopposed as Labour leader, but how ironic if it is Cameron's number-one issue of climate change that propels him to power.