The Ocean Planet

Oceanography Fundamentals 🏆

Course 1 · Chapter 1 · The Ocean Planet: Scope & Sources

Between December 1872 and May 1876 the British ship HMS Challenger sailed about 70,000 nautical miles around the world, lowering weighted lines and nets into water that nobody had ever properly sampled (check the figures). Its crew sounded the depths at hundreds of stations, took temperatures at different depths, dredged up animals nobody had named, and returned with material that took nearly twenty years to publish. The expedition is usually treated as the birth of oceanography as a science. This chapter sets the scene for the course: how much of the planet the ocean covers, how it is divided, how deep it goes, how little of it has been seen, and where you can look at the data yourself.

The Scale of It

Seen from space, Earth is mostly blue. A few numbers fix the scale.

Area

The ocean covers about 71 per cent of the surface, roughly 361 million square kilometres. The land is the minority.

Water

About 97 per cent of all the water on Earth is in the ocean (check the exact figure). Ice caps, groundwater, lakes, rivers and the atmosphere share the rest.

Depth

The average depth is a little under 3,700 metres, and the deepest point, the Challenger Deep in the Mariana Trench, is nearly 11,000 metres down (check the latest survey value).

A thin skin, not a deep one Earth's radius is about 6,371 kilometres, so even the deepest trench is under two-tenths of one per cent of the way to the centre. On a school globe the ocean would be a film of water thinner than the varnish. Yet that film holds some 1.3 billion cubic kilometres of water (check), and it shapes the weather, the climate and most of the living space on the planet.

Ocean or Oceans?

Geographers talk of one connected world ocean, divided by convention into named basins. Water moves between them (Chapters 6 and 7), so the boundaries are administrative rather than physical.

Basin	Roughly how much of the ocean	Note
Pacific	Nearly half	Largest and deepest; larger than all the land put together
Atlantic	About a fifth to a quarter	S-shaped; the one that touches Scotland's west coast
Indian	About a fifth	Mostly in the southern hemisphere; seasonal monsoon winds drive its currents
Southern	A few per cent	Ring of water around Antarctica; recognised as a fifth ocean by the US National Geographic Society in 2021 (check)
Arctic	About 4 per cent	Smallest and shallowest; partly covered by sea ice

The proportions are approximate because they depend on where the edges and the marginal seas are drawn (check against one source). Smaller regions, such as the North Sea, the Mediterranean and the Caribbean, are called seas; many are partly enclosed by land.

Land and Water Are Not Evenly Shared

The northern hemisphere is about 60 per cent water and the southern hemisphere about 80 per cent (check). Seen from a point near New Zealand, almost the whole hemisphere is ocean. This lopsidedness matters for climate: water heats and cools more slowly than land, so oceanic regions have milder seasons than continental interiors. Edinburgh, for example, has far milder winters than places at the same latitude in central Canada, and the Atlantic is the reason.

Going Down: The Depth Zones

Light fades with depth, and so does temperature and the amount of life. Oceanographers divide the water column into layers. The boundaries below are the commonly used ones, though textbooks differ slightly.

Zone	Depth	What it is like
Epipelagic (sunlight zone)	0 to 200 m	Enough light for photosynthesis; most marine life
Mesopelagic (twilight zone)	200 to 1,000 m	Faint light; many animals that rise at night to feed
Bathypelagic (midnight zone)	1,000 to 4,000 m	No sunlight; near-freezing and high pressure
Abyssopelagic	4,000 to 6,000 m	Covers the vast abyssal plains (Chapter 2)
Hadal	Below 6,000 m	Found only in trenches; named after Hades

Pressure, in plain terms Pressure rises by about one atmosphere for every ten metres of seawater (check). At the bottom of the Challenger Deep it is more than a thousand times the pressure at the surface. Chapter 3 returns to this when it looks at how seawater behaves.

How Much Do We Actually Know?

A common claim is that we know more about the surface of the Moon than the ocean floor. It is half true. Satellites measure the sea surface from space, and the slight bulges and dips caused by seafloor mountains give a coarse map of the whole seabed. But detailed maps need ships or underwater vehicles with sonar (Chapter 2), and the project Seabed 2030 reports that only about a quarter to a third of the seafloor had been mapped to modern standards by the mid-2020s (check the latest figure). The water column and its inhabitants are even less well sampled, and new species are described every year.

Claims to treat with caution Popular articles often say "we have explored only five per cent of the ocean" or "the ocean produces most of our oxygen". Both are loosely used. "Explored" is rarely defined, and estimates of the ocean's share of oxygen production are commonly given as about half, with real uncertainty (check). Whenever you see a striking ocean statistic, ask what was measured and by whom.

Why Study It?

Climate. The ocean has absorbed the large majority of the extra heat trapped by greenhouse gases in recent decades, with figures above 90 per cent often quoted (check; Chapter 8).
Weather. Hurricanes draw energy from warm water, and sea temperature steers storms and rainfall (see Weather & Meteorology Fundamentals 6).
Food and trade. Fisheries feed billions of people, and the great majority of world trade by volume goes by sea (check).
Hazards. Tsunamis, storm surges and rising sea level threaten the many people who live near the coast.
Life. The sea is the largest habitat on the planet by volume, and life probably began there.

What the Course Covers

Chapters	Theme
2–3	The seafloor and the properties of seawater
4–5	Waves and tides
6–7	Surface currents and deep circulation
8	The ocean and climate
9–10	Marine life, exploration and protection

It is an introduction for the curious, with plain language and a minimum of mathematics. It does not teach navigation or seamanship, and it does not replace a degree course.

Where to Get Data and Read More

National agencies. The UK Met Office and the National Oceanography Centre publish ocean and sea-state information; in the US, NOAA publishes a very large open collection.
Tide tables and gauges. The UK Hydrographic Office and port authorities publish predictions and records for most harbours.
Satellite sea-surface data. Agencies such as ESA and NASA release sea-surface temperature and sea-level maps free of charge.
Bathymetric charts. The GEBCO project publishes a global gridded map of the seafloor (check the current release).
A coast visit. A beach at low and high tide, with a tide table in your hand, makes Chapter 5 concrete.

Exercises

Exercise 1

The ocean covers about 71 per cent of the surface of Earth, which is about 510 million square kilometres in total. Work out the ocean's area in square kilometres, and say what the answer tells you about the land area.

→ Solution

Exercise 2

Name the depth zone for each of these and say whether sunlight reaches it: (a) 50 m; (b) 600 m; (c) 2,500 m; (d) 8,000 m.

→ Solution

Exercise 3

Find one striking statistic about the ocean in a news article or on a website. Write three sentences saying what it claims, what it would have to measure, and whether you can trace it to an agency or a paper.

→ Solution

Quick Reference

Coverage: about 71 per cent of the surface; about 97 per cent of Earth's water
Depth: average a little under 3,700 m; deepest about 11,000 m (Challenger Deep)
Basins: Pacific, Atlantic, Indian, Southern, Arctic, joined as one world ocean
Zones: epipelagic, mesopelagic, bathypelagic, abyssopelagic, hadal
History: HMS Challenger, 1872 to 1876, the first great scientific voyage
Knowledge gaps: only a quarter to a third of the seafloor mapped in detail
Check: basin proportions, trench depth, mapped percentage, oxygen share

Coming next Chapter 2 goes to the bottom: the continental shelf and slope, the flat abyssal plains, the mid-ocean ridges and the deep trenches, and how sonar and satellites reveal them.