Today, glaciologists are able to reconstruct paleotemperatures by studying the isotopic composition of ice.
The water molecule is composed of one oxygen atom associated with two hydrogen atoms. Oxygen is a mixture of three natural isotopes: 16O (99.76%), 17O (0.04%) and 18O (0.20%). In the water cycle, the heavy isotope 18O travels more difficultly than the light isotope 16O. It evaporates less easily at the equator and falls more frequently with the precipitation that punctuates water’s journey to the poles. The polar ice caps are therefore poorer in the heavy isotope than the oceans, especially in a cold climate.
Thus, for glaciologists, ice poor in 18O comes from a period of cold climate while ice less poor in 18O comes from a period of warm climate.
At the same time, the study of ice made it possible to analyze the carbon dioxide content of air bubbles contained in the ice (not shown in the animation). Scientists were thus able to detect a link between this variable and the evolution of temperatures over time.
The study of marine sediments allows a similar analysis: Marine organisms develop their shells from the chemical elements contained in seawater, including oxygen. The isotopic ratio of oxygen constituting these shells found in marine sediments allows us to trace the climatic history of our planet. The evolution of the 18O / 16O isotope ratio in water evolves at the opposite rate from that of the ice caps. An ocean, and therefore sediments, rich in 18O implies ice caps poor in 18O and a cold climate.
Thanks to these different fields of study, whose results are in perfect agreement, scientists have demonstrated that climatic variations present alternating glacial and interglacial periods in a cycle of approximately 100,000 years. They also showed that atmospheric CO2 concentrations have evolved in parallel with temperatures, proving the link between these two factors.