Anyone concerned with our climate should have this chart* committed to memory. (Click on the chart above to see a larger version)
It is a reconstruction of global temperatures for the past 800,000 years, based on data gathered from Antarctic ice cores.
It is a tribute to the scientists who have laboured to retrieve the ice cores in a hostile environment.
It is also a very sobering picture.
For most of the last 800,000 years, the earth has been much colder than now - as much as 10C cooler during the last ice age. However, there have been 4 short periods when it has been warmer than today - up to 4C warmer during the last major interglacial around 125,000 years ago. It is striking how short the warm periods have been - the last one was around 10,000 years long.
It is also notable just how rapidly the world warmed up - and cooled down. Typically no more than a few thousand years from the deep freeze to balmy conditions - and back again.
* Data prepared for BBC Online by Dr Robert Mulvaney, British Antarctic Survey, derived from Deuterium anomaly data
