Oceans and climate: HSC Earth and Environmental Science Module 7
“Investigate the role of the oceans in regulating climate, including but not limited to ocean currents, thermohaline circulation, carbon and heat uptake, and the El Nino-Southern Oscillation in the Australian context”
The oceans store and move most of Earth's heat and carbon. Wind-driven surface currents such as the warm East Australian Current redistribute heat and moderate coastal climate; deep thermohaline circulation, driven by temperature and salinity, stores heat and carbon for centuries. The oceans have absorbed over 90 percent of the extra trapped heat and about a quarter of human carbon dioxide, slowing surface warming but causing thermal expansion and acidification that bleaches the Great Barrier Reef. ENSO, a coupled ocean-atmosphere cycle, dominates Australian rainfall, drought and flood variability.
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What this dot point is asking
NESA wants you to explain how the oceans regulate climate by storing and transporting heat and carbon. You need surface currents and thermohaline circulation, the ocean's uptake of heat and carbon dioxide (and the resulting acidification), and the El Nino-Southern Oscillation as the ocean-atmosphere system that dominates Australian climate variability.
The answer
The oceans hold far more heat and carbon than the atmosphere and move them around the planet, so they are central to climate. They buffer temperature change, redistribute heat from equator to poles, and absorb much of the carbon dioxide and excess heat that humans add, slowing surface warming but with consequences of their own.
Surface currents and heat transport
Wind-driven surface currents carry warm water poleward and cold water equatorward, moderating climate. The warm East Australian Current flows south down the eastern coast, keeping the New South Wales coast milder and supporting marine ecosystems; it has been extending further south as the ocean warms, shifting species ranges. Because water has a very high heat capacity, coastal regions have milder, more even temperatures than inland areas at the same latitude, which is why coastal Australian cities have smaller temperature swings than the interior.
Thermohaline circulation
Beneath the surface, a slow global circulation is driven by differences in temperature and salinity (thermo for heat, haline for salt), together controlling density. Cold, salty water in polar regions is dense and sinks, driving a deep flow that connects all the ocean basins in a circulation sometimes called the global conveyor belt, taking around a thousand years to complete. This deep circulation stores heat and carbon for centuries and helps regulate long-term climate.
The oceans have absorbed more than 90 percent of the extra heat trapped by the enhanced greenhouse effect, and roughly a quarter of human carbon dioxide emissions. This uptake has slowed the warming of the air at the surface, but it has also driven ocean warming, sea-level rise from thermal expansion, and ocean acidification.
Heat and carbon uptake, and acidification
Carbon dioxide dissolves in seawater, where some forms carbonic acid. As the oceans absorb more human carbon dioxide, surface waters become more acidic, lowering the carbonate available to corals and shellfish that build calcium carbonate skeletons. Combined with ocean warming, this stresses reefs: the Great Barrier Reef has suffered repeated mass coral bleaching events as warm water expels the algae corals depend on. The oceans thus both slow atmospheric warming and pay a price for doing so.
The El Nino-Southern Oscillation
ENSO is the coupled ocean-atmosphere cycle that dominates Australian climate variability. In normal (and La Nina) conditions, trade winds pile warm water in the western Pacific near Australia, bringing rain. In El Nino, the trade winds weaken and warm water shifts east toward South America, so rainfall over eastern Australia falls, raising drought and bushfire risk. La Nina does the opposite, bringing heavy rain and flooding. ENSO is monitored using ocean temperatures and the Southern Oscillation Index, allowing the Bureau of Meteorology to give seasonal outlooks.
- Confusing surface currents with thermohaline circulation
- Surface currents are wind-driven and fast; thermohaline circulation is density-driven, deep and slow (about a thousand years).
- Saying ocean carbon uptake is harmless
- Absorbing carbon dioxide acidifies the ocean and, with warming, drives coral bleaching on the Great Barrier Reef.
- Treating ENSO as just an air phenomenon
- ENSO is a coupled ocean-atmosphere cycle; the shifting of warm surface water across the Pacific is central to it.
Try this
Q1. Explain how the high heat capacity of the ocean moderates the climate of coastal regions. [3 marks]
- Cue. Water absorbs and releases large amounts of heat with little temperature change, so coastal areas have smaller temperature swings than inland areas at the same latitude.
Q2. Explain how ocean uptake of carbon dioxide affects the Great Barrier Reef. [4 marks]
- Cue. Dissolved carbon dioxide acidifies surface water, reducing carbonate for coral skeletons; combined with ocean warming this drives mass coral bleaching.
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
2021 HSC-style1 markA graph shows changes in atmospheric CO2 and the pH of seawater over time. Which row correctly identifies the changes? A. Atmospheric CO2 increasing, seawater pH decreasing. B. Atmospheric CO2 increasing, seawater pH increasing. C. Atmospheric CO2 decreasing, seawater pH increasing. D. Atmospheric CO2 decreasing, seawater pH decreasing.Show worked answer →
The correct answer is A: atmospheric CO2 increasing and seawater pH decreasing.
The graph shows atmospheric carbon dioxide rising over time. The ocean takes up a large share of this extra carbon dioxide; when carbon dioxide dissolves in seawater it forms carbonic acid, which raises the hydrogen-ion concentration and therefore lowers the pH. This is ocean acidification.
So as atmospheric CO2 goes up, seawater pH goes down, an inverse relationship. The other options either pair the two in the same direction (B, D) or have CO2 falling (C, D), which the data do not show.