The oceans are an important sink for atmospheric CO₂, but as they take up increasing amounts of CO₂ they are becoming more acidic.

This has knock-on effects on some marine organisms and on the ocean's ability to sequester carbon. NIWA oceanographer Dr Cliff Law explains the impacts of ocean acidification on organisms that use carbonates to build their shells, and on bacteria. Effects on bacteria may reduce the ocean's ability to take up atmospheric CO₂, creating a positive feedback loop, whereas organisms that use carbonate may find it more difficult to form and maintain their shells. (07:00) 

Otago water shows how the ocean is changing.

For the last 14 years, in collaboration with the University of Otago's Chemistry Department, Dr Kim Currie has run a time series tracking ocean acidification. Every two months, she collects water samples along a 65-kilometre line from the tip of Otago Harbour out into subantarctic waters.

The series is invaluable because it covers the different subtropical and subantarctic water currents in a one-day trip: the only place in the world where this is possible. Along the line, Otago University scientists measure the pH, while Currie measures the other three parameters - alkalinity, total dissolved inorganic carbon and partial pressure of CO₂. The data helps to model changes in different water masses at different times. For example, pH is linked to temperature, and so varies between summer and winter and from year to year.