Oceans and heat at Higher Level: The same statement as SL, with different examples: the ocean's share of global warming's heat, the Benguela Current and Walvis Bay, Vancouver and Winnipeg. HL adds the ocean conveyor belt and how currents change storms and the gases in the air.
Practise this as you read
- Explain how a current changes a coast, using air density.
- Trace the conveyor belt: where water sinks, and why.
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The Earth's hot-water tank: The oceans absorb most of the sunlight that reaches the Earth and store it as heat. Because of their low albedo and high specific heat capacity, they hold far more heat than the land and change temperature slowly.
The points to remember
- The oceans cover about 71% of the Earth's surface, so they receive most of the Sun's energy.
- Water has a low albedo: it reflects little sunlight and absorbs most of it.
- Water has a high specific heat capacity: it stores a lot of heat and warms and cools slowly.
- Most energy is absorbed in the tropics, where the Sun is high all year.
- So the oceans are a huge heat store, and the sea's temperature is more stable than the land's or fresh water's.
Real example: since 1971, about 91% of the extra heat trapped by greenhouse gases has gone into the oceans, and only about 1% has warmed the air (IPCC, 2021). Without the oceans, the air would be far hotter.
Say why water absorbs so much: Name the property and its effect: 'low albedo, so it absorbs most sunlight'; 'high specific heat capacity, so it stores heat and changes temperature slowly'.
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The heat absorbed in the tropics does not stay there. An ocean current carries it across the world, so the oceans act like the Earth's central heating.
The points to remember
- The oceans are joined into one connected circulation.
- Warm currents carry heat (and moist air) from the tropics towards the poles.
- Cold currents carry cold water from the poles back towards the equator.
- Surface currents are driven mainly by wind, turned by the Earth's rotation; tides help too.
- Deep currents are driven by density differences: cold, salty water sinks.
- Without currents, the tropics would be hotter and high latitudes far colder.
Remember it as: Warm water goes out to the poles; cold water comes back.
Real example: the warm Brazil Current carries tropical water south past Rio de Janeiro, while the cold Benguela Current carries Southern Ocean water north along Africa.
A current changes the climate of the coast it passes. The heat moves in steps: water, then air, then land.
The points to remember
- A warm current warms the air above it; winds carry that air over the coast, so winters are milder.
- A cold current carries cold water; it cools the air above; winds carry the cool air inland.
- So the average annual temperature of the coast is lowered.
- Warm currents raise evaporation, so the air is moist and brings rain.
- Cold currents limit evaporation, so the air is dry: coasts can be deserts (the Atacama).
- Coasts have a smaller yearly temperature range than places far inland.
Air density on a cold coast: Air over a cold current is cooled from below, so it becomes denser and stays low. This cool, dense air spreads over the coast and pushes the warmer, less dense air up and inland, so the coast stays cool in summer and has a small yearly range.
Real example: beside the cold Benguela Current, Walvis Bay in Namibia averages about 17 °C all year and gets under 20 mm of rain; cool, dense air brings fog most mornings, while the Namib Desert inland passes 35 °C.
Name density AND temperature: For a density question, say both: 'cooler, denser air'. And the current carries cold water; the wind carries cool air.
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Water helps keep the Earth's average temperature steady in several ways: as liquid, as vapour and as ice.
The points to remember
- Water absorbs more solar energy than land: high specific heat capacity, low albedo.
- Evaporation takes in heat, cooling the sea surface.
- Water vapour carries that heat to higher latitudes, in rising air and in tropical storms.
- Condensation into clouds releases the heat in colder places.
- Currents move heat from the equator to the poles; cold currents flow back.
- Oceans absorb CO₂; water vapour warms (a greenhouse gas); ice, snow and white clouds reflect sunlight and cool.
Absorb
- In August 2005 the Gulf of Mexico had absorbed enough heat to reach about 30 °C.
Evaporate
- Katrina drew up vast amounts of water vapour, cooling the sea behind it.
Carry
- The storm carried the moist air north over Louisiana and Mississippi.
Condense
- Condensing rain released the heat over land, far from where it was absorbed.
Water that warms
- Water vapour: a greenhouse gas
- Oceans store absorbed heat
- Warm currents heat cold coasts
Water that cools
- Evaporation takes in heat
- Ice, snow, white clouds reflect sunlight
- Oceans absorb CO₂
Four different roles: For 'the role of water', give four different processes, each with its effect: absorbing, evaporating, carrying and condensing, currents, reflecting. Four points about currents alone repeat one idea.
Surface and deep currents join into one slow loop round all the oceans, the ocean conveyor belt.
The points to remember
- The ocean conveyor belt is one loop joining the surface and deep oceans.
- In the North Atlantic, water cools and gets saltier (evaporation, sea ice forming), so it is dense.
- The dense water sinks and flows south as a cold deep current, round Antarctica, into the Indian and Pacific.
- There it slowly rises (upwelling) and returns as warm surface water.
- It is driven by temperature and salt: thermohaline circulation. One loop takes about 1000 years.
- It carries heat to north-west Europe; meltwater from Greenland could slow it.
Real example: measurements suggest the Atlantic part of this loop has slowed since the mid-1900s, as Greenland's ice melts. Its heat is one reason Bergen in Norway has ice-free harbours at 60°N.
Dense water sinks: Cold AND salty water sinks. Fresh meltwater is light, so it floats and slows the sinking. Do not say warm water sinks.
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Currents do more than move heat. They change winds, rain and storms, and even the gases in the air.
The points to remember
- Currents change air pressure and winds: warm water heats the air, which rises.
- Warm currents raise evaporation, humidity and rain; cold currents make coasts dry.
- Warm water feeds tropical cyclones: sea above about 26 °C gives them energy.
- Cold water absorbs more CO₂, so cold seas take carbon out of the air.
- Upwelling brings nutrients up: phytoplankton photosynthesise, taking in CO₂ and giving out O₂.
- El Niño and La Niña change currents and bring droughts and floods (2.4.10).
Warm current
- Warms the coast
- More evaporation, more rain
- Feeds tropical cyclones
- Holds less CO₂
Cold current
- Cools the coast
- Less evaporation, dry coast
- Often brings upwelling
- Absorbs more CO₂
Real example: off Peru, winds push surface water away from the coast and cold, nutrient-rich water upwells. Phytoplankton bloom, taking in CO₂, and support one of the world's biggest fisheries, the Peruvian anchovy.
Origin: one mark only: When a question asks about climate and the air, how currents form earns one mark at most. Spend your points on what the currents do.
How this comes up: Paper 2, Section B (b): explain the role of ocean currents in climate and the air [7].
Ocean currents link the tropics and the poles, and the sea surface and the deep ocean.
Explain the role of ocean currents in regulating climate and the composition of the atmosphere.
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