The close connection that exists between the atmosphere and the ocean is not surprising considering that 70% of the planet is covered by water—about 360 million km²—and the few dozen km of air is extremely thin compared to the 12,750 km diameter of Earth. This means that about 90% of the atmospheric heating caused by rising concentrations of CO₂ is transferred to the oceans.

In approximate terms, about one-third of the carbon dioxide we add to the atmosphere by burning fossil fuels stays there for centuries, about one-third is captured and sequestered in various land forms such as forests, soils and vegetation, although one recent United Nations study suggests the terrestrial sequestration may only be about 25%, possibly because forest and plant cover is being diminished by agriculture and fires, and because a higher global temperature is reducing the photosynthesis process by which plants process carbon dioxide into carbon, sugars and oxygen. Much of the remaining one-third of our CO₂ emissions that is not absorbed by marine algae dissolves in the oceans to form carbonic acid.