Michael McCarthy: Will history see this as a turning point for climate change?

History is only patterns, patterns we impose with the benefit of hindsight on the chaotic flow of human events, and these patterns are eminently fallible, even reversible. Was the French Revolution a Good Thing? Absolutely, for generations of French Marxist intellectuals; then along comes someone like Simon Schama to argue persuasively in Citizens that it was not such a good thing at all.

Was Richard III a vile hunchbacked villain who murdered the Princes in the Tower? Well, he was according to Shakespeare, repeating a century later the propaganda of the dynasty which overthrew him, the Tudors; but not according to the contemporary citizens of York, who on 23 August 1485, publicly recorded their reaction to his death at the Battle of Bosworth in their municipal diary as "King Richard late mercifully reigning upon us was through great treason ... piteously slain and murdered, to the great heaviness of this city".

So trying to impose the right pattern on events is an enterprise fraught with pitfalls, and an even more chancy affair is the spotting of Turning Points, even as they happen. It might look like a turning point now, but it might well not in a few years' time; which is why I am somewhat hesitant in saying that what happened last week, on Wednesday and Thursday, was perhaps the key turning point in the efforts of human society to deal with climate change.

That was when first the US, and then the following day China, announced they were setting themselves targets to reduce their share of the carbon dioxide emissions which are causing the atmosphere to warm. It was widely reported, but maybe at a few days' distance we can see its significance even more clearly. These targets on their own are not remotely adequate to deal with global warming, but the setting of them publicly was momentous, the moment when the two biggest carbon emitters, who between them account for 40 per cent of all greenhouse gases, finally accepted that there was a grave problem which they would have to act to tackle.

If there is a successful all-round agreement at the Copenhagen climate conference which begins next week, that of course will receive the headlines and be prominent in the immediate memory; but even if I hesitate to say so, I still think, having followed this process for 20 years, that 25 and 26 November 2009, the dates of the US and Chinese targets, will mark the real shift in the way we responded to the gravest threat to our planet. The task ahead is colossal and some people, including some good judges, think it cannot be accomplished; but something has changed.