I just came across this cunning way of working out what the global temperature levels were in geologically distant times.
Some oxygen is heavy, some oxygen is light,
But both are fine for filling lungs and making things burn bright.
18O’s the weighty one, though in abundance less
Than 16O, the common form. Here’s what you’d never guess:
The ratio between them tells you really quite a lot
About past global temperatures – were summers cold or hot?
Now, oxygen’s in water, which evaporates from the sea;
The light form does so faster – that simple fact’s the key.
And, if it fell as snow on frozen lands in times of old,
It never could have melted if the climate was so cold,
And therefore it would not have run back seawards; so one finds
That sea-water’s enriched in 18O in ice-house times.
Now, creatures living in the seas incorporate some O
As carbonate material in the shells in which they grow.
So those which lived in chillier climes than we have ever seen
Built up their shells from water where more O’s numbered 18.
That’s why our oceanographers extract a lengthy core
Of shelly sea-bed sediments from many miles offshore:
They check for 18O and, if they find it, they conclude
That that bit of the core formed in an icy interlude.
We know by this bizarre but fairly accurate technique,
That twenty thousand years ago the climate was real bleak.
It was a glacial maximum – the last one that we know –
When northern lands were covered all the year with ice and snow.