Analysis By Alex Kirby BBC News Online environment correspondent

The ice caps are under threat from global warming

He laid his credibility on the line in explaining his belief that this was the biggest environmental challenge of all.

"If there is one message I would leave with you and with the British people today, it is one of urgency," he told his audience of senior decision makers.

He announced no new policies to tackle the threat, but relied instead on an appeal to the country to trust him.

Doubters dismissed

Almost everything the prime minister said will encourage those who believe climate change is a serious and growing problem.

Only members of what he called "a diminishing handful of sceptics" will take exception to what was in the speech.

One of the most difficult things in politics is working out what the balance of risk is, especially where it involves drastic action Tony Blair

Mr Blair mentioned the "mismatch in timing between the environmental and electoral impact" of our actions - the fact that the consequences of our actions today may not show up for a generation.

He deftly summarised the scientific evidence that the climate is changing, and threw his weight behind it.

Mr Blair said: "If what the science tells us about climate change is correct, then unabated it will result in catastrophic consequences for our world. The science, almost certainly, is correct."

He rehearsed the things the government is doing to reduce emissions of carbon dioxide and the other greenhouse gases - good things, like making schools and houses more environmentally sound.

Setting a limit

He spelt out the need for international action on pollution from aircraft and other problems.

He put some detail on his stated wish to use the UK's presidency in 2005 of the G8 group of industrialised countries to argue both for Africa and for action on climate change.

Mr Blair will press the G8 leaders to agree the basic climate science, to speed it up, and to bring countries like China and India on board.

There is even to be "an international scientific meeting,... more than just another scientific conference" in the UK next February. It will try to decide how much atmospheric pollution is too much, and what to do about it.

Weather conditions could worsen as an effect of global warming

So far, so good. There probably are no glamorous, bold policy announcements the prime minister could have made anyway.

Dr Steve Howard of the Climate Group told BBC News Online: "There aren't any miracle policies: what we need are a lot of small steps."

Plea for support

Mr Blair acknowledged his inability so far to persuade President Bush of the strength of his case, but suggested no way he might change this.

There is no doubting the sincerity of his conviction that climate change is a terrible and urgent threat.

But he did not - perhaps because he could not - say how he would persuade the rest of us not only to agree, but to act.

In an extempore passage inserted towards the end of his speech, the prime minister said: "One of the most difficult things in politics is working out what the balance of risk is, especially where it involves drastic action.

"Often it's not that the politicians can't see the problem, or lack the courage to act. It's that they need to know the political support is out there for them among the electorate."

He ended: "Now is the time to sound the alarm firmly and put this on the agenda."

Mr Blair has asked the electors for their trust in the past, and has sometimes been disappointed. He must hope that this time they will respond to him.