How do scientists know what past climates were like?

How do scientists know what past climates were like?

Answer: 

Among the major sources of evidence are sediment cores from the ocean bottom. They preserve the fossils of tiny organisms called foraminifera. By measuring oxygen in the skeletons of these organisms, scientists can calculate fluctuations in temperature and moisture over millions of years. Some of the most dramatic climate fluctuations in all of Earth’s history occurred during the period of human evolution.