For "whether", read "when". That is how far scientific thinking has shifted since the last report from the UN's Intergovernmental Panel on Climate Change. The IPCC has a reputation for conservatism. When 2500 climatologists, oceanographers, glaciologists, meteorologists and the like try to pool their wisdom on a subject as complex and uncertain as this, it is hard not to come up with a bland document that reads like something written by committee. The first part of the fourth IPCC report, out today, verges on the apocalyptic. The reason is simple: since its third report, facts on the ground (and in the sea and up in the air) have started to speak for themselves.

Those who predicted that it would make Sir Nicholas Stern's report on the subject look like scaremongering are wrong. True, it dismisses the notion that the Gulf Stream could collapse, plunging Europe into a new ice age, and its predictions about rising sea levels are far more modest than the catastrophic five metres worst-case scenario suggested by Stern. But on the central issue of rising temperatures, there is broad agreement. Most sobering of all, the IPCC will warn that various feedback effects, such as forests and oceans becoming progressively less able to absorb carbon dioxide, could accelerate the speed of change.

The Blair government enjoys flaunting its green credentials, even if British cuts in greenhouse gas emissions owe more to the collapse of heavy industry than any imposed restraint. It is where we go from here that matters. In a muscular speech in Edinburgh today, Environment Secretary David Miliband will argue the case for urgency. Our emissions are a "dangerous boomerang" that is already heading back our way and climate change cannot be tackled by leaving it in a box labelled "the environment", he will say.

How far has he convinced Gordon Brown? Though Mr Brown commissioned the Stern Report, his rhetoric about global warming has stressed the need for international action rather than British legislation radically to alter personal behaviour. The government's ambitious climate-change bill, with its goal of a 60% reduction in carbon dioxide emissions by 2050, remains unconvincing without binding interim targets. And though a leaked memo from Mr Miliband last year revealed his support for green taxes, Mr Brown has so far avoided restoring the fuel-duty escalator, settling instead for a modest increase in air passenger duty. This is unlikely to curb the growth of emissions from travel because most travellers will simply absorb the increase rather than change their habits. The Chancellor is, rightly, wary of two factors. First, indirect taxes on necessities such as fuel are cruelly regressive, hitting the poorest hardest. Secondly, there is a gap between the public's green aspirations and their support for green taxes. Fundamental reform will be required if we are to shift part of the tax burden from incomes to polluting behaviour.

Mr Miliband is right to suggest that Britain cannot hope to influence India and China to limit their emissions unless we practise what we preach. With talks on a new global agreement on climate change due to begin in Indonesia in December, there has never been a better time to set a good example.