Oceans as a system

Covering most of the planet, the oceans are a vast, interconnected system that is central to how Earth works:

• Climate regulation — oceans absorb, store and redistribute huge amounts of heat, moderating global temperatures.
• Carbon store — the oceans are the largest active store of carbon, absorbing much of the CO2 emitted by human activity.
• Biodiversity — they support ecosystems from coral reefs to the deep sea.
• Resources — food, energy, minerals and transport routes for billions of people.

Systems thinking: oceans have inputs, stores, flows and outputs of energy and matter. Because everything is linked, a change in one part (e.g. temperature) ripples through circulation, chemistry and ecosystems.

Ocean morphology

Bathymetry is the study of the shape of the ocean floor, which is closely linked to plate tectonics:

• Continental shelf — the shallow, gently sloping submerged edge of a continent.
• Continental slope and rise — the steeper drop to the deep ocean floor and the wedge of sediment at its base.
• Abyssal plain — the vast, flat deep-ocean floor.
• Mid-ocean ridges — undersea mountain chains at constructive plate margins where new crust forms.
• Ocean trenches — the deepest zones, at destructive margins where crust is subducted; seamounts are isolated undersea volcanoes.

Name the feature

Ocean water properties

The physical properties of seawater control its movement and its ability to support life:

• Temperature — warm at the sunlit surface, cold at depth. The thermocline is the zone where temperature falls rapidly with depth, separating warm surface water from cold deep water.
• Salinity — the dissolved salt content; higher where evaporation exceeds freshwater input.
• Density — increases with lower temperature and higher salinity; denser water sinks, driving deep circulation.
• Dissolved gases — oxygen and CO2 dissolve more readily in cold water, important for both life and the carbon cycle.

Link it up: temperature and salinity together set density - and density differences are what drive the global deep-ocean circulation.

The thermocline

Ocean circulation

Oceans move heat and nutrients around the planet through linked surface and deep circulation:

• Surface gyres — large wind-driven circular current systems in each ocean basin.
• Thermohaline circulation — the global deep conveyor driven by differences in temperature and salinity: cold, salty, dense water sinks at high latitudes and returns slowly elsewhere.
• Upwelling — where winds push surface water away and cold, nutrient-rich water rises to replace it, fuelling highly productive fisheries.

Why it matters: the conveyor redistributes heat that shapes regional climates, while upwelling supports some of the world's richest fishing grounds - so circulation links physics, ecology and human livelihoods.

Your turn — pressure at depth

Oceans and the carbon cycle

Oceans are a huge carbon store and take up much of our CO2 through two main pumps:

• Solubility — CO2 dissolves directly into cold surface water.
• Biological pump — phytoplankton fix dissolved CO2 by photosynthesis; when organisms die, some carbon sinks to the deep ocean, locking it away.
• Carbonate pump — marine organisms build shells of calcium carbonate; these sink and are stored in seafloor sediments.

The catch: absorbing more CO2 makes seawater more acidic (ocean acidification), which harms shell- and reef-building organisms - so the ocean's carbon service comes at an ecological cost.

The biological pump

Ocean geopolitics

UNCLOS (the UN Convention on the Law of the Sea) sets the legal framework dividing the oceans into zones measured from a country's coast:

• Territorial waters — extend 12 nautical miles; the state has full sovereignty here.
• Exclusive Economic Zone (EEZ) — extends 200 nautical miles; the state controls the resources (fish, oil, gas, minerals) but not general navigation.
• Continental-shelf claims — states may claim seabed resources beyond 200 nm where the shelf extends further.
• The high seas — waters beyond national jurisdiction, a shared global commons.

Why it matters: valuable resources and overlapping claims make maritime boundaries a source of geopolitical tension - from fishing disputes to contested seabed oil and strategic sea lanes.

How far do territorial waters reach?

Your turn — percentage protected

Ocean resources and conflict

Oceans provide a wide range of resources, and competition over them drives conflict:

• Fisheries — a vital protein source, but many stocks are fully or over-exploited.
• Oil and gas — large reserves under continental shelves.
• Minerals — including polymetallic nodules on the deep-sea floor.
• Renewable energy — offshore wind, tidal and wave power.
• Tourism — coasts, reefs and cruises support major economies.

Evaluate: exploiting ocean resources brings jobs and food, but conflicts arise over fishing rights, seabed claims and the environmental cost - especially in the largely ungoverned deep sea.

Feature, threat or response?

Tap a card, then tap the category it belongs to.

Threats to the oceans

Human activity is placing the ocean system under mounting pressure:

• Overfishing — removing fish faster than stocks can recover, collapsing food webs.
• Plastic pollution — persistent waste that harms marine life and enters food chains.
• Ocean acidification — falling pH as seawater absorbs more CO2, weakening shells and coral.
• Warming — rising sea temperatures drive coral bleaching and shift species ranges.
• Deoxygenation — warmer, more stratified water holds less oxygen, expanding "dead zones".

Interlinked: these threats reinforce one another - warming, acidification and deoxygenation together stress ecosystems already weakened by overfishing and pollution.

What causes ocean acidification?

Your turn — percentage change in a stock

Managing the oceans

Because oceans cross borders, management ranges from national to global:

• Marine Protected Areas (MPAs) — zones where activities such as fishing are limited or banned, letting ecosystems recover.
• Fishing quotas — limits on catch to keep stocks sustainable.
• International agreements — including UNCLOS and efforts to govern the high seas and reduce pollution.

Evaluate: MPAs and quotas can work well where enforced, but much of the ocean is a poorly governed global commons, so international cooperation and monitoring are the key challenges.

Match the term

Tap a description on the left, then its match on the right.

The big ideas to know

You have worked through the whole Exploring Oceans debate. Press Finish to see your score.