If environmentalists ever ascribe an unusually hot spell to global warming, sceptics rightly come down on them like a ton of bricks. As they point out, weather is different from climate: it will always vary, but it is the trend that counts.
But now that we are having a record-breaking cold snap, sceptics gleefully suggest that it shows global warming is not happening. It's even said that the Met Office's bodging – again – of the seasonal weather forecast invalidates all climate science.
Surely they don't need to resort to such inconsistency to make their case? They were right first time. Nothing can be inferred either way from one, or even a few, episodes of blazing heat or freezing cold; it takes a trend stretching over many years. And while harsh winters can be predicted to get commoner if the world cools down, this big freeze does not show that this is happening.
While we are at it, it's not true either, as some sceptics say, that the whole of the northern hemisphere is now gripped by unusual cold. Alaska, Canada, Greenland, northern Africa, East Asia and most of Europe are warmer than usual, while northern Europe, Siberia, China and most of the United States are cooler.
We are being chilled by two anti-cyclones. One over Greenland is sweeping Arctic air south; the other, over Russia, is spreading Siberian cold east. Mild warmer air from the Atlantic, which we normally enjoy, has been diverted southwards. It's the weather, sceptics.
