Earth's oceans are all connected. While we have separate names for the world's oceans, in reality they are all connected and they form one global ocean. The Earth's global sea level, the surface level of the ocean, has varied over time. It has risen and fallen by hundreds of meters throughout our planet's history. Global sea level rises in response to melting ice and glaciers during interglacial times (periods of climate warming), and falls during ice ages.

Global sea level also changes in response to tectonic forces that raise or lower the seafloor: a rising seafloor causes rising ocean levels, whereas a sinking seafloor causes a drop in global sea level. The tides also affect global ocean levels because they cause sea level to rise or fall in a regular pattern. Additionally, changes in atmospheric pressure and weather, including precipitation amounts, affect global sea level. Changes in water temperature affect water density and water volume, which also raises or lowers global ocean levels. Increased or decreased flow of water into the oceans from rivers also changes global sea level.

In recent years, scientists have observed a global rise in sea level, hypothesized to be caused by the melting of ice sheets and glaciers due to global climate change and warming atmospheric temperatures.