Ocean Warming and Acidification – Coral Bleaching and Dissolving Shells
GeographyWater & OceansAges 15–16
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Sign in to playRaise atmospheric CO₂ and sea surface warming and watch the ocean respond: CO₂ dissolves to form H₂CO₃, which releases H⁺, so pH falls (about 8.2 before industrialisation, about 8.05 today), carbonate ions and the aragonite saturation state Ω drop, and CaCO₃ shells and skeletons become harder to build or even dissolve. Warmer water bleaches corals as degree heating weeks (DHW) build up, expands and raises sea level, and pushes species ranges towards the poles. Compare tropical, temperate and polar seas and emission scenarios to 2100, and record results in a table.
Lesson: Ocean warming and acidification: carbonate equilibria, pH, aragonite saturation, coral bleaching, thermal expansion of seawater and species range shifts
What it shows
About a quarter of the CO₂ we emit dissolves in the ocean. Dissolved CO₂ forms carbonic acid, which releases hydrogen ions; these combine with carbonate ions, so pH and the carbonate available for shells fall even though the total carbon in the water rises. When the aragonite saturation state Ω drops below 1, unprotected shells dissolve. At the same time the ocean absorbs most of the extra heat: warm water expands and raises sea level, marine heatwaves push corals past their bleaching threshold, and species move towards the poles to stay in water of their preferred temperature.
How to use
Drag Atmospheric CO₂, Sea surface warming and Summer heatwave, or press a scenario button such as Today or SSP5-8.5 (2100). Choose a Sea region to compare tropical, temperate and polar water, and use Graph to switch between pH and Ω, carbon species, reef degree heating weeks, species ranges and thermal expansion. Press Record to add the current results to the table and compare runs.
Parameters you can change
- Atmospheric CO₂ concentration 280–1200 ppm
- Sea surface warming since pre-industrial times 0–5 °C
- Sea region Tropical (coral reef), Temperate (kelp forest, mussel bed), Polar (pteropods, krill)
- Summer marine heatwave (extra temperature) 0–3 °C
- Graph pH and aragonite saturation against CO₂, Inorganic carbon species against pH, Reef temperature and degree heating weeks, Species ranges by latitude, Thermal expansion and sea level rise
Questions to explore
- Why do carbonate ions decrease when CO₂ dissolves, even though the total carbon in seawater increases?
- At what CO₂ level does polar surface water become undersaturated in aragonite, and what happens to pteropod shells?
- How much extra warming makes severe coral bleaching happen almost every year?
