The classic proxies for palaeoclimatology are microfossils. Everyone knows the idea of a large fossil bone of a dinosaur. That type of fossil is useful but very rare. So what most palaeoclimatologists use for working with ocean sediments are microfossils – fossils of tiny algae or very small plankton.
These can't be seen with the naked eye. They can only be seen under a microscope. So the modern ocean is full of plankton, which prefer different temperatures.
So if you go to the modern tropics you'll find a type of algae called forams - foraminifera - and the species of foram you get living in the tropics will be different to the species living off the coast of Scotland, which again will be different from the species of forams living in the Arctic.
So you get different populations of these species of plankton.
Different in the morphology. In the case of foraminifera they produce a tiny calcium carbonate shell. So very small shells made of chalk. These creatures' shells are about the size of a grain of sand. If you look at them under a microscope there are very clear differences between the types of shell.
There are scientists who are experts in identifying these foraminifera in ocean sediments that might go back thousands or millions of years. They count the different types of species, and identify them under microscopes.
Then they look for changes in those species types over time, and relate those to changes in water temperature over time.
So they learn the relationship between body types and temperature from the modern ocean. You hold that assumption and apply it back through time.
They use a type of statistical analysis called regression. That shows that part of what controls their modern distribution is temperature, but there are other factors. It's temperature and salinity.
So that's the classic example – foraminifera – of something that's been used for the last 50 years.
In lakes – because we also look at lake sediments – we find the microfossils of algae called diatoms. They are also found in ocean sediments. So diatoms and foraminifera and pollen are the classic microfossil proxies.