Journalists were bizarrely banned from asking questions at the launch of the Stern Review yesterday, so the question several reporters wanted to ask Tony Blair, who was presiding, was never put: "What message do you think this sends to George Bush?"
For a whacking great message there certainly is, even if the Prime Minister was excused from being asked to spell it out himself. In 2001 President Bush took the United States out of the Kyoto protocol, the ground-breaking climate change treaty, on the premise that its requirement to cut US greenhouse gas emissions would damage the American economy. The Stern Review tells him bluntly: not cutting your emissions will eventually damage your economy a hell of a lot more.
This is not a Greenpeace campaigner talking, remember, not someone who might be suspected of harbouring a secret socialist agenda. This is a former chief economist of one of global capitalism's most revered institutions, the World Bank. And what Sir Nicholas Stern has done with his report on the economics of climate change is remarkable: he has ripped up the last excuse for inaction.
The scientific case to fight climate change has long been overwhelming, the moral case even more so. The last rational place where an opponent of acting to tackle global warming might take refuge has been the economic argument. No longer. If you oppose action now, you are simply burying your head in the sand.
Stern's loudly trumpeted conclusion that the cost of not acting will be five to 20 times the cost of dealing with the problem marks a genuine political turning point. It suddenly delivers a new orthodoxy. One wouldn't want to exaggerate, but it does feel like one of those moments that are truly historic, when the balance of affairs tips in a decisive way - like the day in November 1942 when the Russians surrounded the Germans at Stalingrad. Things are suddenly different.
That's not to say that the Americans, or indeed, the Chinese and the Indians, soon to take over from the US as the biggest CO2 emitters, will follow right away the course Sir Nicholas spells out. Stalingrad didn't end the Second World War.
But afterwards, things had changed.