Acidification

Dissolving CO
₂ in seawater increases the hydrogen ion (H+
) concentration in the ocean, and thus decreases ocean pH, as follows:

CO_{2 (aq)} + H₂O ⇌ H₂CO₃ ⇌ HCO₃− + H+ ⇌ CO₃2− + 2 H+.

Caldeira and Wickett (2003) placed the rate and magnitude of modern ocean acidification changes in the context of probable historical changes during the last 300 million years.

Since the industrial revolution began, the ocean has absorbed about a third of the CO
₂ 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 (an additional doubling to tripling of today's post-industrial acid concentrations) by 2100 as the oceans absorb more anthropogenic CO
₂, the impacts being most severe for coral reefs and the Southern Ocean. These changes are predicted to accelerate as more anthropogenic CO
₂ is released to 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.

Average surface ocean pHfailed verification

Time	pH	pH change relative to pre-industrial	Source	H+ concentration change relative to pre-industrial
Pre-industrial (18th century)	8.179		analysed fieldfailed verification
Recent past (1990s)	8.104	−0.075	field	+ 18.9%
Present levels	~8.069	−0.11	field	+ 28.8%
2050 (2×CO 2 = 560 ppm)	7.949	−0.230	modelfailed verification	+ 69.8%
2100 (IS92a)	7.824	−0.355	modelfailed verification	+ 126.5%

Rateedit

If we continue emitting CO₂ at the same rate, by 2100 ocean acidity will increase by about 150 percent, a rate that has not been experienced for at least 400,000 years.

— UK Ocean Acidification Research Programme, 2015

One of the first detailed datasets to examine how pH varied over 8 years at a specific north temperate coastal location found that acidification had strong links to in situ benthic species dynamics and that the variation in ocean pH may cause calcareous species to perform more poorly than noncalcareous species in years with low pH and predicts consequences for near-shore benthic ecosystems. Thomas Lovejoy, former chief biodiversity advisor to the World Bank, has suggested that "the acidity of the oceans will more than double in the next 40 years. He says this rate is 100 times faster than any changes in ocean acidity in the last 20 million years, making it unlikely that marine life can somehow adapt to the changes." It is predicted that, by the year 2100, If co-occurring biogeochemical changes influence the delivery of ocean goods and services, then they could also have a considerable effect on human welfare for those who rely heavily on the ocean for food, jobs, and revenues. A panel of experts who had previously participated in the IPCC reports have determined that it is not yet possible to determine a threshold for ocean acidity that should not be exceeded.

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Current rates of ocean acidification have been compared with the greenhouse event at the Paleocene–Eocene boundary (about 55 million years ago) when surface ocean temperatures rose by 5–6 degrees Celsius. No catastrophe was seen in surface ecosystems, yet bottom-dwelling organisms in the deep ocean experienced a major extinction. The current acidification is on a path to reach levels higher than any seen in the last 65 million years, and the rate of increase is about ten times the rate that preceded the Paleocene–Eocene mass extinction. The current and projected acidification has been described as an almost unprecedented geological event. A National Research Council study released in April 2010 likewise concluded that "the level of acid in the oceans is increasing at an unprecedented rate". A 2012 paper in the journal Science examined the geological record in an attempt to find a historical analog for current global conditions as well as those of the future. The researchers determined that the current rate of ocean acidification is faster than at any time in the past 300 million years.

A review by climate scientists at the RealClimate blog, of a 2005 report by the Royal Society of the UK similarly highlighted the centrality of the rates of change in the present anthropogenic acidification process, writing:

"The natural pH of the ocean is determined by a need to balance the deposition and burial of CaCO
₃ on the sea floor against the influx of Ca2+
and CO2−
₃ into the ocean from dissolving rocks on land, called weathering. These processes stabilize the pH of the ocean, by a mechanism called CaCO
₃ compensation...The point of bringing it up again is to note that if the CO
₂ concentration of the atmosphere changes more slowly than this, as it always has throughout the Vostok record, the pH of the ocean will be relatively unaffected because CaCO
₃ compensation can keep up. The present fossil fuel acidification is much faster than natural changes, and so the acid spike will be more intense than the earth has seen in at least 800,000 years."

In the 15-year period 1995–2010 alone, acidity has increased 6 percent in the upper 100 meters of the Pacific Ocean from Hawaii to Alaska. According to a statement in July 2012 by Jane Lubchenco, head of the U.S. National Oceanic and Atmospheric Administration "surface waters are changing much more rapidly than initial calculations have suggested. It's yet another reason to be very seriously concerned about the amount of carbon dioxide that is in the atmosphere now and the additional amount we continue to put out."

A 2013 study claimed acidity was increasing at a rate 10 times faster than in any of the evolutionary crises in Earth's history. In a synthesis report published in Science in 2015, 22 leading marine scientists stated that CO
2 from burning fossil fuels is changing the oceans' chemistry more rapidly than at any time since the Great Dying, Earth's most severe known extinction event, emphasizing that the 2 °C maximum temperature increase agreed upon by governments reflects too small a cut in emissions to prevent "dramatic impacts" on the world's oceans, with lead author Jean-Pierre Gattuso remarking that "The ocean has been minimally considered at previous climate negotiations. Our study provides compelling arguments for a radical change at the UN conference (in Paris) on climate change".

The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because the chemical equilibria that govern seawater pH are temperature-dependent. Greater seawater warming could lead to a smaller change in pH for a given increase in CO₂.