Joan Smith: An unpredictable world, Mr Blair? It's you who was surprised by Iraq

What is it about Tony Blair and weapons of mass destruction? In a week when a cross-party committee of MPs warned that British troops in Iraq and Afghanistan are "desperately short" of equipment, the Prime Minister announced that he wants to spend up to ?20bn on building a new generation of submarines for Trident missiles. The warning from the Select Committee on Defence followed an outspoken attack on the Government by the former head of the armed forces, General Sir Mike Jackson, reinforcing complaints from soldiers about shortages of equipment on the front line.

It's inconceivable that we would use nuclear weapons in any of the operations our over-stretched armed forces are involved in, and British troops in Afghanistan are struggling against a deadly enemy with fewer helicopters than they need. Yet Blair has chosen this moment to begin a "debate" on a decision he's already taken, committing the country to the huge expense - more like ?65bn if you include running costs - of updating our nuclear weapons, on the grounds, and I'm paraphrasing only slightly here, that it's an unpredictable old world.

Not as unpredictable as all that, you might think, given that the Prime Minister was warned that invading Iraq would increase the risk of terrorism. He took no notice, embarking on a conflict which has recruited thousands of young men to the terrorist cause, in the name of countering a threat - Saddam's WMD - which didn't exist. When he has such a dreadful track record, I can't think of one reason why I should take Blair's assessment of future risks seriously, although I am tempted to speculate about the role subconscious guilt has played. Could it be that spending all this money on nukes is a weird act of atonement on the part of a politician whose poor judgement is responsible for one of the most catastrophic wars in living memory?

At a time when events in Iraq and elsewhere cast such a long shadow, I doubt whether there is broad popular support for unilateral nuclear disarmament. Whether we need the capacity to wipe out quite so many of the world's cities is another matter, and Blair's suggestion that we might be able to get by with 160 warheads and three submarines is hardly a significant contribution to the cause of non-proliferation. With support from the Conservatives, Blair is likely to get his way when it comes to a vote next year, and we'll be committed to updating a weapons system designed for something that might never happen.

What is undoubtedly going to happen - and this is where Blair's preference for addressing hypothetical threats is most infuriating - is climate change. The tornado which swept through north-west London on Thursday may not have been caused by global warning, but the increased incidence of hurricanes around the world is.

How much evidence does the Government need that the most pressing threat is the accelerating damage the human race is inflicting on the planet? Think what might be achieved if Blair diverted some of the cash he's earmarked for nukes into schemes to protect the environment.

The Cold War is over, but his fascination with nuclear weap- ons shows that his thinking has not progressed. His determination to rush a decision on Trident through Parliament before he steps down isn't just a refusal to face present facts in Iraq and Afghanistan, although that's bad enough. It's a positively Neronian gesture, playing with boys' toys while Rome burns.