Imagine if scientists found lots of polar bear fossils buried under what is now a tropical beach: that would tell them that at a specific time in the past that beach was a cold and icy place. It’s the same with the plankton but their relative abundance tells the story of much more subtle changes in temperature.
On land, plants and animals are sensitive to the humidity and temperature of the climate. Certain species like cold and dry, warm and dry, warm and wet, cold and wet etc.
Pollen, seeds, shells, and insect exoskeletons get washed from the land into sea sediments.
Marine sediments are therefore a treasure trove of information, telling us what the climate was like a very long time ago. Over decades, thousands of scientists have analysed both ice and marine cores to piece together, like detectives, the story of the Earth’s climate changes.