Ocean acidification
Big idea: Ocean acidification is the evil twin of climate change. As oceans absorb more CO2, they become more acidic, threatening shellfish, corals, and entire marine food webs.
The chemistry (simplified)
- CO2 from atmosphere dissolves in ocean water
- CO2 + H2O forms carbonic acid (H2CO3)
- Carbonic acid releases H+ ions causing pH to decrease (more acidic)
- Ocean pH has dropped from 8.2 to 8.1 since pre-industrial times
- This is a 30 percent increase in acidity (pH is logarithmic!)
Impacts on marine life
- Shell formation - harder for molluscs, corals, and plankton to build calcium carbonate shells
- Coral bleaching - combined with warming, accelerates reef decline
- Phytoplankton - base of marine food web affected
- Fish behaviour - acidification affects sensory systems
- Food webs - impacts cascade up from plankton to fish to humans
Impacts on human societies
- Fisheries decline - shellfish and fish stocks threatened
- Food security - 3 billion people depend on seafood
- Employment - fishing communities lose livelihoods
- Tourism - coral reef degradation reduces tourism income
- Coastal protection - healthy reefs protect coastlines from storms
7-mark question (MAR 2024, NOV 2017): Explain the impacts of ocean acidification on environmental systems AND societies. You need BOTH! Systems = shells, corals, plankton, food webs. Societies = fisheries, food security, jobs, tourism. Max 5/7 if you only cover one side!
IB-style question — ocean acidification from fossil fuels [7]
Burning coal, oil and gas has raised the amount of carbon dioxide in the atmosphere.
Describe how this can change the abiotic conditions of the ocean, including its pH. [7]
How to answer it, step by step
- CO₂ dissolves and lowers pH
• Extra atmospheric CO₂ dissolves into seawater and forms a weak acid.
• This lowers the ocean's pH — this is ocean acidification, which harms coral and shell-building animals. - Warming changes other conditions
• CO₂ is a greenhouse gas, so the ocean warms; warmer water holds less dissolved oxygen.
• Melting ice adds fresh water (lowering salinity) and warming can disrupt ocean currents.
Final answer
Acidification here is from dissolved CO₂ — do NOT write 'acid rain / NOx / SO₂', those are not credited for ocean pH.