What is an ocean?

The global ocean covers 71% of the planet. While there is one global ocean, it is split into five geographical regions. These are the Atlantic Ocean, the Indian Ocean, the Pacific Ocean, the Southern Ocean, and the Arctic Ocean. There are also dozens of seas, such as the Mediterranean Sea, the Greenland Sea, and the Weddell Sea. Seas are usually associated with a particular ocean. For example, the Mediterranean Sea is linked to the Atlantic Ocean – via the Straits of Gibraltar. The key difference between a sea and an ocean is that a sea is much smaller, and is commonly bordered, or partly enclosed, by land. When we use the term ocean, or ‘global ocean’, we are usually referring to all ocean and sea water.


The oceans contains around 96% of the world’s water. NOAA estimate that this amounts to 1,335,000,000 km3 (1.3 billion cubic kilometres) of water. So far, we have explored only 5% of it. Their colossal size means that oceans play a major role in moderating the Earth’s climate. This is because they are the main conveyor of heat around the globe.


The five components of the global ocean

The five components of the global ocean



How are oceans changing?

The marine environment is determined by: temperature; salinity; ocean circulation; and the exchange of heat, water and gases with the atmosphere. Marine ecosystems depend on these parameters, as well as acidification, to function effectively.


Over the last few decades there have been major changes to ocean conditions and marine processes. This has been largely due to do increasing levels of greenhouse gases in the atmosphere which, through processes associated with global warming, have contributed to a rise in sea surface temperature and to ocean acidification. Global warming has also led to a rise in sea level – due to thermal expansion and the melting of the cryosphere . In the UK, for example, average sea level has risen by about 14 cm since the start of the 20th century.
The impacts of climate change may also influence commercial industries. Sea level rise, flooding, and storms, for example, might affect ports, shipping, and coastal towns. Temperature and water chemistry changes may influence marine ecosystems and fisheries. That is not to overlook some potential positive impacts including: enhanced marine recreation, coastal tourism, and the opening of new seasonal shipping routes in the Arctic.


Researchers have highlighted six key ways in which marine environments will be affected by contemporary climate change. These cover a range of issues from physical processes, ocean chemistry, and marine ecosystem change. In collaboration with researchers at the Sir Alister Hardy Foundation for Ocean Science (www.sahfos.ac.uk), we explore these issues in the following series of articles…



  • Changes to temperature, circulation and ice cover
  • Sea level, coastal erosion and storms


  • Ocean acidification
  • Marine eutrophication


  • Primary productivity and microorganisms
  • Changes in species composition and biodiversity