How do we find out about the climates of the past?
To discover what the earth’s climate was like in the past we use what are known as proxies. Proxies reveal how a certain thing behaved in the past. By knowing how it behaves in the present and how this behaviour changes with changing climatic conditions we can infer what the climatic conditions were like in the past.
For example, let’s look at foraminifera (or forams as they are affectionately known). These cute little single celled organisms record the chemistry of the sea-water in which they grow. The ratio of Magnesium to Calcium they incorporate into their shells is dependant on the temperature of the water in which they grow. The amount and type of oxygen (yes there’s more than one type, and they’re known as isotopes) they put into their shells is dependant on temperature, salinity and global ice volume. The type of Boron is related to the pH of the water they are in. The pH is typically dependant on the atmospheric concentration of the atmosphere. So here we have these tiny creatures, floating along in the surface waters of the oceans recording the chemistry of the seas, which is dependant on the climate.
After about a month, these forams die and slowly settle to the bottom of the ocean. Many are eaten, many dissolve in the deep waters, but some reach the bottom of the sea and are buried in the sediment.
Later, geologists/earth scientists/paleoclimatologists come along and drill deep-sea cores into the pile of sediment, bringing up a tube of sediment containing lots of dead shells of forams. Oldest at the base, youngest at the top. Forams from each layer are removed, cleaned and chemically analysed, typically in a mass spectrometer (a fancy instrument which can record tiny changes in the chemical composition of the shells).
By knowing, say the ratio of Magnesium to Calcium in the shell, we can match this ratio to modern forams with the same ratio, growing somewhere in the world or that have been grown in the lab in artificial conditions. And therefore, the conditions that these forams were growing in some time in the distant past is known. So we know what the world was like in the past. At least at this one particular location, at this one particular time, for this one particular variable (temperature, pH).
Its a lot of hard work to then repeat, for multiple locations, at multiple times, for multiple variables. But no-one said it was easy!
There are loads more proxies. Corals provide super high resolution of a snapshot in time. Stalagmites provide long term records on land. Pollen can show us what past vegetation was like. Ice cores trap atmospheric gases in tiny bubbles. Fossilised leaves, peat bogs, lakes, even tree rings all provide a little window into the climatic conditions of the past. The list continues to grow. Each new proxy from each new source material revealing another insight into the past