Boris Johnson is famously a politician who sees the glass half full, and whose instincts are to hope that a crisis solves itself before he has to impose painful sacrifices on himself and us. So it is all the more curious that he is strikingly gloomy that far too little is being done across the world to halt global warming. What’s more, his plans to ‘green’ the British economy are more ambitious than many other governments’ and relatively expensive.

Why then is Johnson being less Micawberish and more pre-emptive on climate change than he was – for example – when Covid-19 arrived here in February 2020? Why does he ‘get’ the catastrophic logic of global warming, when he didn’t – till too late, many would say – appreciate the merciless logic of a viral pandemic?

The other day he said it was because the scientists showed him a chart demonstrating that above a certain level of warming, the desertification, flooding and species extinction becomes catastrophic and irreversible. And, by contrast, it is true that he didn’t get timely and unambiguous advice from his scientific advisers that locking down would minimise damage from Covid-19 both to health and wealth.

But this disjunction between the climate and virus scientists doesn’t feel like the whole story. And although part of what makes him an unlikely eco warrior is probably his social circle, notably the two Goldsmiths, his upbringing and his partner, there’s a political calculation too.

The point is that, in recent days, his officials – both the political ones and the Whitehall lifers – have been much more positive and optimistic than him, both about what the G20’s most powerful world leaders agreed on climate change at the weekend and the prospects for this fortnight’s talks.

Some of these officials have been scarred over many years by how short-term nationalist economic interests trumped a collective duty to save the planet.

So when the G20 members agreed to achieve net zero greenhouse gas emissions ‘by or around mid century’, they were thrilled – and weren’t even conspicuously shocked or upset when super emitter India, or rather its PM Narendra Modi, interpreted ‘by or around’ as 2070 (on that logic, I am currently ‘by or around’ 40 years old). Per contra, all Johnson’s rhetoric in recent days is that we’re doomed, on the basis of promises and even half promises made so far to cut emissions.

It is true that he has signed himself up to the compelling notion that the only route to salvation is for the rich nations both to do more to cut their own emissions and also to provide free or cheap finance to the poor ones, to subsidise the green reinvention of their respective economies. But simultaneously he shrugs and implicitly concedes that the instinctive selfishness of the nation state means his own conversion to global altruism is highly unlikely to win the day.

So have you solved the puzzle? The answer is not that Johnson is Saint Attenborough installed in 10 Downing Street. Instead he has made the reasonable calculation that it is better to be on the side of the climate angels, to shame his peers into doing more. And you never know, he might just succeed, by securing new promises on ‘coal, cars, cash and trees’, that would allow him to claim credibly in 12 days that ‘1.5 has been kept alive’, and that there remains a viable path to avoid the worst of climate change.

That said, he has also taken out a personal insurance policy – such that if COP26 ends with a whimper rather than a bang, which is the outcome in the centre of the fan chart of expected outcomes, who on earth would think to blame him?