About Ocean Acidification
This simulation models seawater carbonate chemistry as atmospheric CO₂ dissolves into the surface ocean. It applies Henry's law to find dissolved CO₂, then solves the carbonate equilibria using temperature- and salinity-dependent dissociation constants (K0, K1, K2, Kw). Holding total alkalinity fixed near 2300 µmol/kg, it uses Newton's method to find the hydrogen-ion concentration, giving pH, carbonate-ion content and the aragonite saturation state Ω.
The CO₂ slider sweeps atmospheric concentration from 180 to 1200 ppm, while sea surface temperature and salinity sliders adjust the equilibrium constants. Preset buttons jump between pre-industrial (280 ppm) and extreme (1000 ppm) scenarios, and the animation steadily raises CO₂. As pH falls and Ω drops below 1, the on-screen coral shifts from healthy to bleached to dissolving, illustrating why acidification threatens reefs and shell-forming marine life.
Frequently Asked Questions
What is ocean acidification?
Ocean acidification is the ongoing fall in seawater pH caused by the ocean absorbing roughly a third of human carbon dioxide emissions. When CO₂ dissolves it forms carbonic acid, releasing hydrogen ions and lowering pH. Average surface pH has fallen from about 8.18 before industrialisation to around 8.08 today.
How does the simulator calculate pH?
It first uses Henry's law to find how much CO₂ dissolves at the chosen temperature and salinity, then sets up the carbonate alkalinity balance. With total alkalinity held near 2300 µmol/kg, it solves for the hydrogen-ion concentration using Newton's method over up to 30 iterations, and pH is the negative base-ten logarithm of that concentration.
What do the three sliders control?
The CO₂ slider sets atmospheric concentration from 180 to 1200 ppm, which drives how much carbon dioxide dissolves. The sea surface temperature slider (0 to 35 °C) and the salinity slider (25 to 40 PSU) change the equilibrium constants K0, K1, K2 and Kw, so they shift pH and saturation indirectly rather than adding CO₂ directly.
What is the aragonite saturation state, Ω?
Aragonite saturation Ω compares the actual product of calcium and carbonate ion concentrations against aragonite's solubility product. When Ω is above 1 the water is supersaturated and corals can build skeletons easily; below 1 the water is corrosive and aragonite skeletons begin to dissolve. The simulator flags reefs as healthy at Ω above 3 and dissolving below 1.
Why does adding CO₂ lower the pH?
Dissolved CO₂ reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. More CO₂ means more hydrogen ions, so pH drops. Those extra hydrogen ions also combine with carbonate ions to form bicarbonate, reducing the carbonate available for shell and skeleton building even as total dissolved carbon rises.
Is the chemistry in this model physically accurate?
It uses recognised empirical fits for the carbonate equilibrium constants and a genuine numerical solver, so the trends in pH and saturation are realistic. It is simplified, however: total alkalinity is held constant, borate and other minor species are approximated, and it assumes equilibrium surface water. It is a teaching tool, not a substitute for a full model such as CO2SYS.
Why does a small pH change matter so much?
Because pH is logarithmic, a drop of 0.1 units represents about a 26 percent increase in hydrogen-ion concentration. So even modest-looking pH declines mean substantial changes in acidity. The simulator shows the running pH change since the 280 ppm pre-industrial baseline so you can see the cumulative effect of rising CO₂.
How do temperature and salinity affect the result?
Warmer water holds less dissolved gas, so higher temperatures reduce CO₂ uptake, but they also shift the dissociation constants and lower aragonite solubility. Salinity changes the activity of ions and the equilibrium constants too. Adjusting these sliders shows how regional conditions, from cold polar seas to warm tropical reefs, modify the chemistry.
Why are coral reefs especially vulnerable?
Corals build skeletons from aragonite, a form of calcium carbonate. As acidification lowers carbonate ion concentration and the aragonite saturation state, building and maintaining skeletons becomes harder and eventually impossible. Combined with warming-driven bleaching, falling saturation pushes reefs from growth toward net dissolution, which the simulator depicts as the coral whitens and crumbles.
What real-world scenarios do the presets represent?
The presets mark key carbon milestones: 280 ppm is the pre-industrial baseline, 420 ppm is roughly the present day, 560 ppm is double pre-industrial, and 800 to 1000 ppm reflect high-emissions futures. These mirror concentration pathways used in climate projections, letting you compare ocean pH and reef health across plausible end-of-century outcomes.