The two most common isotopes of oxygen in nature are oxygen-16 (8 neutrons) and oxygen-18 (10 neutrons). When the Earth cools down, the lighter, oxygen-16 found in seawater is locked away in the ice of high latitude glaciers due to evaporative processes, leaving behind relatively more oxygen-18 in the oceans. During warm global climates, melted ice returns oxygen-16-rich waters to the oceans. So the proportion of oxygen-18 to oxygen-16 in the ocean reflects the Earth's climate even if we can't see the ice. Earth Scientists recognize this oxygen isotope pattern between glaciated and ice-free climates, referring to it as the “ice volume effect”, and have since used it to reconstruct ancient Earth climates.
Oxygen isotope records are also preserved in the shells of marine organisms and the proportion of oxygen-16 and oxygen-18 can be revealed by analyzing the chemistry of pristine fossils. The fossils of larger organisms like corals or clams can be especially informative for revealing annual and seasonal temperature variations because these marine animals live for multiple years secreting season growth bands in a similar fashion to tree rings.
Are we heading to normal temperatures?