Oceans, Currents and Tides

Oceans cover about 71 per cent of the Earth's surface. The Pacific is the largest and deepest ocean, covering more than 30 per cent of the Earth's surface and containing the Mariana Trench, whose deepest point, Challenger Deep, is about 10,984 metres below sea level. The Atlantic is the second-largest, the Indian Ocean the third, and the Southern Ocean surrounds Antarctica, while the Arctic Ocean, the smallest and shallowest, surrounds the North Pole. Oceans regulate climate by storing and moving heat, supply moisture for rain, support fisheries and trade, and absorb a large share of the carbon dioxide humans emit.

Ocean currents are large-scale movements of seawater, driven mainly by winds, the Earth's rotation, temperature and salinity differences, and the shape of ocean basins. Warm currents, such as the Gulf Stream in the Atlantic and the Kuroshio in the Pacific, carry warm water from the tropics towards the poles; cold currents, such as the Canary Current and the Peru (Humboldt) Current, carry cold water towards the equator. Currents affect coastal climates: western Europe is warmer than places at similar latitudes in North America partly because of the Gulf Stream system, while cold currents off Peru and Namibia help create coastal deserts by cooling the air and reducing rainfall.

Tides are the periodic rise and fall of sea level, caused mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun, together with the Earth's rotation. When the Sun, Moon and Earth align, at new moon and full moon, their pulls combine to produce spring tides, with higher high tides and lower low tides. When the Sun and Moon are at right angles to each other, neap tides occur, with a smaller tidal range. Tides matter for ports, fishing, tidal energy and coastal ecosystems, and extreme tides combined with storms can cause coastal flooding.

For civil studies, oceans are a shared global commons. No single country owns the high seas, so fishing, shipping, pollution, deep-sea mining and climate change all require international rules, notably the United Nations Convention on the Law of the Sea, adopted in 1982. Ocean warming, acidification and sea-level rise are already affecting coasts, fisheries and island nations, and they connect ocean geography directly to climate policy, food security and the rights of small island states.