BACK in Medieval times, around 1300 to 1500 or so, the surface temperature was as warm or perhaps even warmer than it is today.

But then sometime around the year 1600 the earth cooled and went into what is called the ' Little Ice Age'.

This was a hard time for plants, animals and us humans alike.

Shorter growing seasons, frozen rivers and harbours, crop failures.

Why were the Medieval times so warm? We don't know.

Why did the temperatures drop suddenly to the Little Ice Age? We don't know.

Why did temperatures drop around 1600 and not 1400 or 1800? We don't know.

After a very cold century or so temperatures started to rise again.

Since the year 1700 or 1750 or so, temperatures have been rising, in fits and starts, at about a half of one degree per century for the last two-plus centuries.

Why didn't the temperature continue to cool after the Little Ice Age and put us into glaciation? We don't know.

Why did it start to warm at the end of the Little Ice Age, rather than staying cold? We don't know. Why did it start to warm around 1700 or so, rather than in 1900? We don't know.

Why have we seen slow warming since the Little Ice Age? We don't know.

Although we think we know about the climate, if we can't know the reasons for the past, how can we predict the future?

Shuffle a pack of cards, let your 'model' and supercomputer predict the card on top of the deck.

For the next model prediction, replace the card and fully reshuffle the pack.

Let the model again name the top card.

If you have a process that is random or for which you do not understand the underlying phenomenon, then you can not predict the result by adding more computing power or fine tuning your model.

Dave Christian