Oceans and Life

The One Thing

Oceans are interconnected physical, ecological and economic systems: their relief, waves, tides and currents shape climate, habitats and human activity. Because oceans also contain valuable resources and present serious hazards, they require sustainable management, preparedness and effective maritime rules.

Who and What

• Ocean relief: The different physical features and depths found on the ocean floor. Its main divisions include the continental shelf, continental slope, continental rise, abyssal plains, mid-ocean ridges, seamounts, submarine plateaus and trenches.
• Continental shelf: The gently sloping, shallow submerged edge of a continent, usually extending to about 200 metres depth. It is important for fishing, petroleum, natural gas, ports and other economic activities because it receives sunlight and nutrients.
• Continental slope: The steep zone connecting the continental shelf with the deep ocean floor.
• Abyssal plain: A broad, nearly level area of the deep ocean floor covered by sediments.
• Ocean trench: A long, narrow and very deep depression in the ocean floor, often formed where tectonic plates meet.
• Mid-ocean ridge: An underwater mountain chain formed mainly where oceanic plates move apart and new crust is created.
• Wave: A rhythmic movement of seawater, usually caused by wind. The water mainly moves up and down rather than travelling forward with the wave. Wave height is the vertical distance between the crest and trough, while wavelength is the horizontal distance between two successive crests or troughs. Wave frequency is the number of waves passing a point in a given time.
• Tide: The regular rise and fall of sea level caused mainly by the gravitational pull of the Moon and the Sun and the rotation of Earth.
• High tide: The time when sea level reaches its highest position along a coast.
• Low tide: The time when sea level falls to its lowest position along a coast.
• Spring tide: An unusually high high tide and low low tide occurring near the new moon and full moon, when the Sun, Moon and Earth are nearly aligned.
• Neap tide: A tide with the smallest difference between high and low water, occurring near the first and third quarter phases of the Moon.
• Ocean current: A large-scale, continuous movement of seawater in a particular direction. Currents are influenced by prevailing winds, Earth’s rotation, differences in temperature and salinity, coastline shape and ocean-floor relief.
• Warm current: A current carrying warm water from lower latitudes toward higher latitudes, generally raising nearby coastal temperatures.
• Cold current: A current carrying cold water from higher latitudes or deep ocean areas toward warmer regions, generally lowering coastal temperatures.
• Upwelling: The movement of cold, nutrient-rich deep water toward the surface, supporting rich fishing grounds.
• Marine resources: Useful materials and living organisms obtained from oceans, including fish, seaweed, salt, petroleum, natural gas, sand, gravel, polymetallic nodules and renewable energy from tides, waves and offshore wind.
• Sustainable use: Using ocean resources carefully so that they remain available for future generations while marine ecosystems are protected.
• Tropical cyclone: A powerful rotating storm formed over warm tropical oceans, bringing strong winds, heavy rain and storm surges. Tropical cyclones generally require ocean water at about 26.5 degrees Celsius or above, sufficient moisture and favourable atmospheric conditions.
• Storm surge: An abnormal rise of seawater pushed toward the coast by strong cyclone winds and low air pressure.
• Tsunami: A series of long, powerful sea waves usually caused by an undersea earthquake, volcanic eruption, landslide or another sudden displacement of seawater.
• Maritime rules: International and national laws regulating navigation, safety, marine boundaries, resource use, pollution control and peaceful activities at sea.
• Exclusive Economic Zone: A sea area extending up to 200 nautical miles from a coastal state’s baseline, where that state has special rights to explore and use natural resources.
• Nautical mile: A unit used in marine and air navigation. One nautical mile equals 1.852 kilometres.
• Knot: A speed of one nautical mile per hour.
• Sonar or echo sounding: A method of measuring ocean depth by calculating the time taken for a sound signal to travel to the seabed and return.
• United Nations Convention on the Law of the Sea: A broad international framework governing maritime zones, navigation, resource rights and protection of the marine environment.

Causes and Consequences

• Ocean relief creates varied physical environments. Continental shelves, continental slopes, continental rises, abyssal plains, mid-ocean ridges, seamounts, submarine plateaus and trenches produce differences in depth, terrain and habitat. Ocean depth is commonly measured using sonar or echo sounding.

• Ocean-floor relief influences currents and marine life. The shape of the ocean floor and coastlines affects the movement of seawater, while currents influence the distribution of nutrients and habitats.

• Waves, tides and currents continually redistribute seawater. Waves are mainly generated by wind, tides result principally from the gravitational pull of the Moon and Sun together with Earth’s rotation, and ocean currents are shaped by winds, Earth’s rotation, temperature, salinity, coastlines and ocean-floor relief.

• Tides have practical and economic uses. They assist navigation, fishing and coastal transport, contribute to tidal energy generation, and help clear sediments from river mouths and harbours. Spring tides occur around the new moon and full moon, whereas neap tides occur around the first and third quarter phases.

• Differences between warm and cold currents affect climate and fisheries. Warm currents generally raise nearby coastal temperatures, while cold currents lower them and may produce fog where they meet warm air. Where warm and cold currents meet, nutrients can support plankton and marine food chains, creating productive fishing grounds.

• Upwelling supports marine productivity. The movement of cold, nutrient-rich deep water toward the surface supports rich fishing grounds and demonstrates the connection between ocean processes and food supplies.

• Oceans have global ecological and economic importance. They cover about 71 percent of Earth’s surface and contain most of its water. They provide fisheries, seaweed, salt, petroleum, natural gas, sand, gravel, polymetallic nodules and renewable energy from tides, waves and offshore wind. They also provide transport routes, livelihoods and climate regulation.

• Resource use creates environmental pressures. Plastic pollution, oil spills, sewage, chemical waste, overfishing, habitat destruction, ocean warming and acidification threaten marine ecosystems. Pollution prevention, responsible fishing, coastal conservation and international cooperation are therefore necessary.

• Tropical cyclones develop over warm oceans. Warm water, generally about 26.5 degrees Celsius or above, supplies energy, while moisture and favourable atmospheric conditions support storm formation. Cyclones can produce high winds, intense rainfall, flooding, landslides and storm surges.

• Preparedness reduces cyclone impacts. Coastal evacuation, following official warnings, early-warning systems, disaster-resistant infrastructure and community preparedness reduce loss of life and are most effective when combined with rapid evacuation.

• Tsunamis result from sudden displacement of seawater. Undersea earthquakes, volcanic eruptions and landslides can generate waves that travel rapidly across deep water and become higher and more destructive near shallow coasts. Following a strong coastal earthquake or an unusual rapid withdrawal of seawater, people should immediately move to higher ground and remain there until authorities issue an all-clear.

• Maritime rules are required because oceans cross national boundaries. The United Nations Convention on the Law of the Sea provides an international framework for maritime zones, navigation, resource rights and marine environmental protection. The Exclusive Economic Zone gives coastal states special rights over resources up to 200 nautical miles from their baselines.

• Navigation rules promote safety and peaceful use. Ships follow navigation and collision-prevention rules, use lights and signals, maintain safe speed, and respect designated shipping lanes and restricted areas. Accurate use of the nautical mile and knot is important in marine navigation.

• Sustainable management connects human welfare with conservation. Protecting marine biodiversity and reducing pollution help maintain food supplies, livelihoods, climate regulation and the health of coastal communities.

What Gets Asked

• Compare the continental shelf, continental slope, abyssal plain, ocean trench and mid-ocean ridge, including their physical characteristics and significance.
• Explain how waves, tides and ocean currents differ, and assess their effects on navigation, coastlines, climate and marine life.
• Distinguish between spring tides and neap tides, including the Moon phases associated with each and their practical uses.
• Explain how warm currents, cold currents and upwelling influence coastal temperatures, fog, nutrients and fishing grounds.
• Assess the value of marine resources alongside the threats created by plastic pollution, oil spills, sewage, chemical waste, overfishing, habitat destruction, ocean warming and acidification.
• Explain the causes and impacts of tropical cyclones and tsunamis, and evaluate the importance of warning systems, preparedness, evacuation and maritime rules.