The Medieval Warm Period.
That was always inconvenient.
Inconvenient enough, apparently, that getting rid of it became almost a necessity if the Hockey Stick was going to stick.
But there is more. So much more.
There was the Roman Warm Period — in some regions longer and warmer than the Medieval Warm Period — coinciding with the rise and extraordinary expansion of Rome.
And let us be precise about Rome, because even history gets simplified when it becomes useful to a narrative.
Rome did not begin when Augustus took power. By then, it was already centuries old.
Nor did Rome disappear when the Western Empire fell. The Roman Empire survived in the East for another thousand years.
But the period of Rome’s greatest territorial extent, enormous political influence and extraordinary cultural impact coincided with a climate that was, in many places, warmer and more favourable than what came before and after.
Warm enough, in fact, for vineyards to exist in parts of northern Britain.
Try growing red wine grapes there today.
The point is not that warmth created Rome. History is never that simple.
The point is that the climate history of the last several thousand years is considerably more complicated than the neat line we are often presented with: a relatively stable pre-industrial climate followed by an unprecedented modern warming.
The evidence suggests something considerably messier.
Since the end of the last Ice Age, we have experienced a succession of warmer and colder periods. More than two. Warm intervals followed by colder intervals. Regional changes layered on top of broader climatic shifts. Periods of relative warmth giving way to periods of cooling, and then warming again.
The climate has not been sitting quietly in one place waiting for industrialisation to disturb it.
It has been moving.
And there is another observation that deserves considerably more attention: over the longer Holocene record, some reconstructions suggest a pattern in which warm intervals become less pronounced while subsequent cold phases can become more severe and persistent.
If that pattern continues, the Modern Warm Period is not necessarily the beginning of an endless climb.
It may simply be another warm interval.
And warm intervals end.
Eventually, the temperature turns.
The uncomfortable possibility is that the climate system may be moving towards another prolonged period of cooling — one that will last far beyond the political horizons of anyone alive today.
If that happens, the great question will not be how to stop the planet from becoming warmer.
It will be how to keep civilisation warm enough to function.
That is a rather different problem.
And it is one for which ideological enthusiasm is a remarkably poor substitute for energy.
So by all means, buy the heat pump.
Just remember that a heat pump does not create heat. It moves it.
And if the world really does enter another long cold phase, there may come a moment when we discover that the problem was never that we had too much heat.
It was that we had forgotten how precious it was.
Good luck with your heat pump.
There may not be quite as much heat left to pump as you were promised.