As well as rising sea levels, the oceans are getting warmer and more acidic.

Ocean heat

One of the best indicators of changes in the climate system is the amount of heat stored in the oceans. 

This CTD instrument measures water temperature, salinity, and density at different depths.  ©Bob Beattie

The heat content of the world's oceans has increased during recent decades and accounts for more than 90 per cent of the total heat accumulated by the land, air and ocean since the 1970s. This warming increases the volume of ocean waters and is a major contribution to sea-level rise. 

Ocean warming is continuing, especially in the top several hundred metres of the ocean. Sea surface temperatures in the Australian region were very warm during 2010 and 2011, with temperatures in 2010 being the warmest on record. 

Sea surface temperatures averaged over the decades since 1900 have increased for every decade. Sea surface temperature datasets are separate to land temperature datasets, but both land and ocean surface temperatures have shown very similar warming trends over the last century, confirming that temperatures are rising.

Ocean acidification

As well as storing heat, the world's oceans absorb a vast amount of carbon dioxide (CO2).

The ocean currently absorbs about a quarter of the CO2 emitted into the atmosphere each year. As atmospheric CO2 concentration increases, the amount of CO2 absorbed and stored in the ocean also increases. Ocean acidification is a direct result of CO2 absorption. 

pH is a measure of the acidity of a substance. A pH of 0 represents a very acidic solution (high concentration of hydrogen ions), while a pH of 14 represents a very alkaline solution (low concentration of hydrogen ions). A neutral substance has a pH of 7. The pH of seawater is around 8. The CO2 taken up by the ocean reacts in the seawater, reducing its pH. Because the pH scale is logarithmic, a one-unit decrease in pH represents a ten-fold increase in the concentration of hydrogen ions.

Since the beginning of the industrial era, the absorption of the increasing amounts of atmospheric CO2 has decreased ocean surface water pH by 0.1, or a 26 per cent increase in the hydrogen ion concentration, and changes are expected to decrease pH by a further 0.06–0.32 by 2100, depending on the level of CO2 emissions in future. 

Ocean acidification has been shown in laboratory and field studies to reduce the growth of carbonate shells and skeletal material of many key organisms, including reef-building corals. Other effects include causing a change in the development of early life stages of some species, although the response to ocean acidification varies considerably between species. As these organisms span the entire marine food chain, ocean acidification could have far reaching implications for the health and productivity of the world’s oceans.

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