Chemistry begins in the air
The ocean absorbs carbon dioxide. Reactions in seawater increase hydrogen ions and lower pH: this is ocean acidification.
A building material becomes less available
Carbonate ions become less available to organisms building calcium-carbonate shells and skeletons. Changes in this chemistry can affect wider food webs.
Connect the steps
Draw a chain from air to water to a living organism. Where would you measure change? What observations would help compare two locations?