Ocean acidification is the ongoing decrease in the pH value of the Earth's oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere. The main cause of ocean acidification is human burning of fossil fuels. As the amount of carbon dioxide in the atmosphere increases, the amount of carbon dioxide absorbed by the ocean also increases. This leads to a series of chemical reactions in the seawater which has a negative spillover on the ocean and species living below water. When carbon dioxide dissolves into seawater, it forms carbonic acid (H2CO3). Some of the carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, thus increasing ocean acidity. Between 1751 and 1996, the pH value of the ocean surface is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of almost 30% in H+ ion concentration in the world's oceans.

Dissolving CO2 in seawater increases the hydrogen ion (H+

) concentration in the ocean, and thus decreases ocean pH, as follows:

CO2 (aq) + H2O ⇌ H2CO3 ⇌ HCO3− + H+ ⇌ CO32− + 2 H+.

Approximately one-third of the carbon dioxide released into the atmosphere by human activity is dissolved into oceans, rivers, and lakes, resulting in increasing levels of acidification.

Ocean surface acidity levels have increased by 30% since the beginning of the industrial revolution.

Since the industrial revolution began, the ocean has absorbed about a third of the CO2 we have produced since then and it is estimated that surface ocean pH has dropped by slightly more than 0.1 units on the logarithmic scale of pH, representing about a 29% increase in H+. It is expected to drop by a further 0.3 to 0.5 pH units by 2100 as the oceans absorb more anthropogenic CO2, the impacts being most severe for coral reefs and the Southern Ocean. These changes are predicted to accelerate as more anthropogenic CO2 is released into the atmosphere and taken up by the oceans. The degree of change to ocean chemistry, including ocean pH, will depend on the mitigation and emissions pathways taken by society.

Although the largest changes are expected in the future, a report from NOAA scientists found large quantities of water undersaturated in aragonite are already upwelling close to the Pacific continental shelf area of North America. Continental shelves play an important role in marine ecosystems since most marine organisms live or are spawned there, and though the study only dealt with the area from Vancouver to Northern California, the authors suggest that other shelf areas may be experiencing similar effects.

PREVENTIVE MEASURES:

In order to prevent disruption of the calcification of marine organisms and the resultant risk of fundamentally altering marine food webs, the following guard rail should be obeyed: the pH of near surface waters should not drop more than 0.2 units below the pre-industrial average value in any larger ocean region (nor in the global mean).

Limiting global warming to below 2 °C would imply a reduction in surface ocean pH of 0.16 from pre-industrial levels. This would represent a substantial decline in surface ocean pH.

The most effective way to limit ocean acidification is to act on climate change, implementing solutions to dramatically reduce the use of fossil fuels. If we dramatically cut our global warming emissions, and we limit future warming, we can significantly reduce the harm to marine ecosystems