The amount water found in the oceans has varied over time, and will continue to do so. The main reason for this variation is the amount of water that is stored in icebergs and continental glaciers.

Examining ice cores from the inland ice in Greenland, allows us to calculate how the global volume of ice and the sea level has fluctuated over the last 130 000 years. This is done by measuring the oxygen trapped in air bubbles found in ice cores.  During the evaporation of water (H2O) from the sea, water molecules containing lightweight oxygen isotope (O16) evaporate more readily than the heavy oxygen isotope (O18), thus concentrating light oxygen in the snow that precipitates down to form ice on glaciers. This accumulation of light oxygen in the ice coincides with the increasing concentration of heavy oxygen isotopes in the sea. The ratio between these two types of oxygen in water will therefore vary, depending on how much of the global volume of water is tied up in glacial ice. In the same way, one can calculate the ice volume and sea level far back in time, by measuring the oxygen isotopes from the Microfossils found in sediments on the greatest depths of the ocean where temperatures have been nearly constant.

Gjennom studie av luftbobler i iskjerner fra Grønland kan vi finne ut at det har vært store variasjoner i det globale isvolumet de siste 130 000 år. Etter Johnsen m.fl. (2001).
Through the examination of air bubbles in ice cores from Greenland, we discover that there have been great variations in the global ice volumes over the last 130 000 years. Figure based on Johnsen, etc. (2001).

From the ice core samples from Greenland we have determined that the last 130 000 years have been characterized by large variations in climate and global ice volume. In the northern hemisphere, temperatures have fluctuated 5-10° C. The last time the earth's climate was similar to today was during the former inter-glacial ca. 125 000 years ago. During that time there was less ice on land, so the sea level was 4-6 m higher than today. The last ice age started when the previous interglacial period ceased, which is about 117 000 years ago, and it continued until the current interglacial period that began about 11 700 years ago. During the last glacial maximum, about 20,000 years ago, the sea level was about 120 m lower than today and a large area of land in northern hemisphere was covered by thick ice.

The last ice age is also characterized by large climate variations with great variations in the global ice volumes. Ice cores from Greenland reveal that were frequent fluctuations between the warm and cold phases. The warm phases began with pronounced temperature increases that were proceeded by drawn out phases of cooling.   The long, cooling stages culminated in a large discharge of icebergs into the North Atlantic.

Under siste istids maksimum for ca. 20 000 år siden var jorden dekket av store isdekker og isbreer. Denne figuren viser hvordan et stort isdekke spredte seg utover og var sammenhengende fra Storbritannia i vest, langt sørover i Europa, østover mot Moskva og nordover til Svalbard og inn i Karahavet. Isbreer eksisterte også i Alpene og på Island, men dette framkommer ikke av figuren. Kotene på isdekket angir mektighet av isen i meter. Figuren er etter Svendsen m. fl. 2004
During the last glacial maximum about 20,000 years ago, the Earth was covered by large  ice sheets and glaciers. This figure shows the extent of ice cover, which spread continuously from the West in United Kingdom, far to the Southern Europe, East towards Moscow, North to Svalbard and into the Kara Sea. Glaciers also existed in the Alps and on Iceland, but these are not shown. The black contour lines indicate the thickness in metres of the ice sheets. Figure based on Smith m. fl. 2004