Warmer water stresses marine life and raises sea level. Dissolved carbon dioxide also changes ocean chemistry, making shell-building harder.
Oceans absorb much of the climate system's excess heat. Warming, freshwater input and changing winds affect water density, mixing and currents. Stronger layering can keep nutrient-rich deeper water from reaching the surface. Changing currents also move heat and organisms differently. Effects vary by region; they do not mean that every current has stopped.
Unusually warm water can make corals lose the algae that supply much of their energy. This is coral bleaching. Bleached coral may recover, but prolonged or repeated stress increases the chance of death. As reef habitat is lost, other organisms lose food and shelter, changing the food web.
Ocean acidification is a separate process. More atmospheric carbon dioxide dissolves in seawater, increasing hydrogen ions and lowering pH. Less carbonate is then available for organisms that build calcium-carbonate shells or skeletons. Acidification means becoming more acidic: seawater can become less alkaline while its pH remains above 7.
Warming water expands and raises sea level. Melting land ice adds more water; melting floating sea ice does not make an equivalent direct contribution. Rising seas and damaged ecosystems then affect coastal habitats, fisheries and settlements. Follow that chain from physical change to ecological and human effects.
Step by step
Heat stress
Unusually warm water can cause corals to lose their algae and bleach.
Chemical change
Dissolved carbon dioxide lowers pH and carbonate availability for shell-building.
Wider losses
Damaged organisms and habitats affect food webs and the people who depend on them.
Worked example: A cascade from reef to fishing community
Persistent heat stress can cause coral mortality. Reef habitat then deteriorates, affecting organisms that depend on it. Changes in fish populations can reduce the catch and income available to a reef-dependent fishing community. The size of the impact also depends on fishing pressure and alternative livelihoods.
Watch out for this
Acidification means the whole ocean must have become an acid below pH 7.
Acidification describes a decrease in pH. Seawater can become less alkaline and still remain above pH 7.
Check your understanding
A reef study reports falling pH and reduced carbonate availability. Which process directly explains these observations?
- Additional carbon dioxide dissolving into seawater.
- A daily sea breeze reversing at night.
- Corals losing algae is the only possible cause.