Just a few days ago, Arnie redefined himself as an environmentalist. Then he went and broke a leg.

It wasn't cause-andeffect, but the skiing accident had the unexpected bonus of drawing attention to a major change of mind. Which, in itself, gave cynics pause for thought, partly because they have always doubted that he had a mind, but also because Schwarzenegger is (a) a Republican, a party whose environmental track record is somewhere between woeful and vile, and (b) an environmental troglodyte whose favourite vehicle is the gas-guzzling, slitty-eyed Hummer.

The Road to Damascus has nothing on this conversion . . . Saul of Tarsus may have come a cropper on the Road to Damascus, but he didn't shatter a femur in the process.

Arnie has a new religion: environmental protection.

Prior to shattering his appendage, the governor shattered his loyalty to George W Bush by contradicting the president's oftstated claim that the science around climate change is not definitive.

"I say the debate is over. We know the science, we see the threat and we know the time for action is now, " the governator said.

Schwarzenegger is committed to reducing California's greenhouse gas emissions to 2000's production levels in three years, with a view to hitting 80% below 1990's by 2050. He hasn't said precisely how this is going to be achieved . . . the lower orders, presumably, develop the policy details . . . but he's indicated that, starting in 2009, California, under his leadership, will set greenhouse gas emission standards for cars. This pits him squarely against the Bush administration which has linked arms with car manufacturers to challenge those standards in court.

Where he has got specific is in promising increased grants to universities for alternative energy research, funding for a 'hydrogen highways' programme to put enough hydrogen fueling stations across the state to make sense of buying zero-polluting vehicles, and subsidising the installation of solar panels on a million roofs.

He announced all this at a UN World Environment Day Conference, the first to be held in the US, at which Bush administration officials were nowhere to be seen. This raised the possibility that our Arnie knows a lame duck president when he sees one, and wants to put as much clear air as possible between himself and the duck.

It's a risk, of course. Conversion is always suspect. Indeed, one of the 'lessons' derived from the Reagan years is that voters love consistency and certainty in their presidents. Just how far this goes may be tested by the issue of climate change. When it comes to a choice between political conversion and having the ground floor of your home inundated by rising sea levels, the former looks quite attractive.

Schwarzeneggar's conversion extends even to his Hummers. He's had them converted so they can run on bio-fuels.

It's difficult to get the mind around the man as meriting a place in the new trio singing off the same green hymn sheet: Stern, Schwarzenegger and Gore. But if it was ever going to happen, 2006 was the year for it. Because 2006 was the year of The Environmental Tipping Point. The point at which the environment moved from being the worthy concern of a treehugging minority into mainstream.

Mainstream with a deadline. Sir Lawrence Stern's recent report effectively says we have 15 years before we fall over the environmental cliff. Fifteen years before climate change problems become irreversible, with rising oceans wiping out highly populated living areas and generating a tsunami of displaced people with all the violence and social disruption inevitably involved.

Although Al Gore's movie had set the scene for this new sense of urgency, it still might not have led to the real attitude change we're now seeing. Professor Jared Diamond's seminal book on the collapse of societies establishes the grim truth that even when cultures have a pretty good idea that extinction lies around the corner if they keep doing what they're doing, they tend to keep at it until there's none of them left.

What's new and different is the 'what's good for the environment is good for the profit line' rationale.

Arnie's pushing "tremendous rewards for our economy" from investments in alternative energy.

The infuriating thing is that Arnie's doing what Ireland should have been doing years ago: capturing the research, bolstering universities, changing behaviour by hammering in the infrastructure to support it and creating jobs.

We did it in industrial development, back in the '60s, courtesy of Michael Killeen and the IDA.

Now, instead of taking the same world leadership role on environmental industrial development, we're making a virtue out of buying carbon credits.

Diarmuid Doyle is on leave