Microorganisms use elements, including oxygen from the ocean around them, to build their shell.
The oxygen comes in a heavy form and a light form.
During warm periods the light oxygen evaporates and falls to the earth as rain, and then it runs back into the sea.
During cold periods, though it’s evaporating, it falls as snow and gets trapped on the land.
This means there’s less light oxygen in the ocean, and therefore less in the microorganism’s shell.
By studying fossilised shells we can infer how the climate has changed in the past,
and this is helping us to understand how our climate is changing today.