Scientists can learn about ancient climates from geological strata and fossils. First, regarding geological strata, if thick glacial sediments are found in the strata, it indicates the existence of an ice age; if a large-scale global sea-level drop and significant land expansion are discovered, it also confirms the existence of an ice age. During cold climates, more snow falls on land and does not melt into water before flowing into the ocean, while the evaporation of seawater decreases, causing sea levels to continuously drop. According to echo sounding, there are currently about 27×10^6 cubic kilometers of ice in Antarctica. Such a large amount of seawater turning into ice would cause sea levels to drop by 65–75 meters. Additionally, the oxygen-carbon isotope composition of geological strata can be used to determine changes in ancient climates. Since climate is related to greenhouse gases like carbon dioxide, measuring the oxygen-carbon isotope composition of geological sediments can reveal the carbon cycle at that time. Systematically measuring the oxygen-carbon isotope changes in thick loess layers, long stalactites, thick sediments on the ocean floor, and thick ice cores in Antarctica can provide data on the gradual changes in ancient climates over millions of years.
Regarding fossils, they can also reveal changes in ancient climates. Climate has a significant impact on organisms. Generally, animals living in tropical regions have faster metabolic rates and smaller bodies, while animals living in temperate regions have slower metabolic rates and larger bodies to withstand cold. Some animals can only live in tropical regions, while others can only live in temperate regions. For example, in China during the Middle Pleistocene, animals could be clearly divided into two groups: those living in the cold north, represented by the Peking Man and the Megaloceros, and those living in the tropical and subtropical south, represented by the Smeilodon and the Giant Panda. Similarly, before the third major ice age in the Late Ordovician, animals grew smaller. When the ice age arrived, temperatures plummeted and sea levels dropped by 50–100 meters, causing the extinction of many animals. During the subsequent long ice age, many animals gradually adapted to the cold climate, growing larger and forming unique ice-age faunas that differed significantly from those before the ice age, making their differences easily identifiable.