The oxygen isotopic composition of ocean water is measured by determining the ratio of 18O to 16O, expressed as 18O/16O. The isotope 16O preferentially evaporates into the atmosphere. During periods of global cooling, the 18O/16O ratio will:

During periods of global cooling, the 18O/16O ratio in ocean water will increase. This is because 16O, being the lighter isotope, preferentially evaporates compared to 18O. During cooler periods, there is less overall evaporation, but the effect still favors 16O removal from the ocean. As a result, water that condenses and falls as precipitation (snow or rain) tends to be enriched in 16O, leaving the ocean relatively enriched in 18O. Additionally, when it is cooler, more ice forms at the poles. This ice preferentially locks in 16O, due to its lower evaporation temperature, further depleting the ocean of 16O, and raising the 18O/16O ratio in the remaining ocean water.

Extra: The concept here is related to stable isotope geochemistry, a field used to study climate patterns of the past (paleoclimatology). Isotopes are atoms of the same element with different numbers of neutrons. Oxygen has three stable isotopes: 16O, 17O, and 18O. The isotopic composition of ocean water doesn't just reflect temperature changes but also global ice volume. When there are larger ice sheets on land, the ocean water is more enriched in 18O because more 16O is locked in the ice.

Paleoclimatologists use the ratio of oxygen isotopes in marine sediments, ice cores, and other geological materials to infer past temperatures and ice volumes. Since the isotopic composition of these materials remains relatively unchanged over geologic timescales, it serves as a "fossil" record of past environmental conditions.

In science, the differences in isotope ratios are typically described using the delta notation (δ). For instance, δ18O is a measure of the ratio of 18O/16O in a sample compared to a standard ratio. The standard is usually a known isotopic composition, such as Vienna Standard Mean Ocean Water (VSMOW) for ocean water. Positive δ18O values indicate enrichment of the heavier isotope (18O), which can signify colder periods with more ice volume, while negative δ18O values would indicate lower ratios of 18O/16O due to warmer conditions with less global ice.