The world’s oceans will become dangerously acidic for corals and shellfish this century if carbon dioxide (CO2) levels continue to rise at current rates, adding to the urgency to reduce manmade CO2 emissions, according to an emerging science.
Evidence for ocean acidity may also prove less controversial than the science of global warming, as it seems likely there will be less doubt surrounding the chemical and biological processes involved.
That could add to the impetus to cut manmade carbon emissions, particularly if signs emerge of a threat to the world’s fisheries.
In the past 200 years, people have released more than 2 trillion tonnes of additional CO2 into the atmosphere by fossil fuel burning, cement production and changes in land use such as deforestation.
That has in turn added more than 520 billion tonnes of CO2 to the world’s seas, scientists estimate, or a quarter of all emissions.
CO2 dissolves in and reacts with water to form carbonic acid (H2CO3) which disassociates to form hydrogen and bicarbonate ions. The excess hydrogen ions combine with carbonate ions producing more bicarbonate.
Adding more CO2 to sea water therefore increases aqueous CO2, bicarbonate and hydrogen ions, the latter lowering sea water pH (a measure of acidity or alkalinity) and carbonate ions.
One effect has been a fall in ocean surface pH of 0.1 units compared with pre-industrial levels, to an average of 8.1.
That is equivalent to a 30 percent increase in average acidity of surface ocean waters worldwide.
Faster falls in pH have been observed in the Arctic (as colder water absorbs more CO2), with pH dropping by about 0.02 units per decade since the late 1960s in the Iceland and Barnet’s Seas, according to the “Arctic Ocean Acidification Assessment 2013”, published by the Oslo-based Arctic Monitoring and Assessment Programme.
A second, related effect has been a drop in the level of calcium carbonate, vital for the shells and skeletons of many marine organisms.
As with atmospheric CO2, there is a lag effect where it will take tens of thousands of years for the world’s ocean chemistry to recover from carbon emissions now, even if the latter were to halt today, meaning the effects that do emerge will be felt at least for centuries.