Two degrees is the talisman. Limiting climate change to C above the pre-industrial level has been the target for the European Union for 13 years, for the G8 group of nations since July, and next week is likely to be officially enshrined in the Copenhagen agreement as the great symbolic climate target for the whole world. You'll see it in the headlines (if the conference doesn't fall apart in the meantime). Two degrees, everybody!
So what are we to make of the results from the British Government's Avoid research programme, which were released yesterday and show that hitting this target is now, in practical terms, pretty much impossible?
The scientists involved aren't using those words, of course. They are merely providing the best analysis possible, so that policymakers may be informed.
But read closely: their analysis leaves no doubt whatsoever that the pathways to the C limit are simply too steep for the world to climb with its present level of ambition for cutting greenhouse gas emissions, even if this ambition is translated into a global agreement next week – something that is still far from certain.
What it means is that the radical prescriptions for climate change, the ones that come from the green pressure groups, the ones of which politicians instinctively think, "Nah, the electorate will never wear that" are the only ones that are actually going to work.
Not being an eco-fundamentalist, I find that quite hard to take on board myself, but the implication is unavoidable. In the end it's a simple choice. One way will work, the other won't. It's going to take quite a time to filter through to public opinion, though. And as for the climate sceptic lobby, the phrase "living in a parallel universe" comes more and more to mind.