What is ocean acidification?

The ocean absorbs about 25% of atmospheric CO2 derived from burning fossil fuels and land use change. When CO2 dissolves in sea water it forms carbonic acid. As we are emitting increasing amounts CO2 into the atmosphere, the ocean is also absorbing increasing amounts. This is altering the system’s ability to adjust to changes in CO2 that naturally occur over the millennia, significantly changing the chemistry of the seas, and leading to progressive acidification.

Since the beginning of the industrial revolution 250 years ago, sea water acidity has increased by 30%. It should be noted that increasing sea water acidity lowers the ocean’s natural ‘basic’ or ‘alkaline’ status and unnaturally forces the acid-base balance of sea water towards acid. If this accelerates for the next four decades as projected, the increase in ocean acidity will be greater than anything experienced in the past 55 million years. Future projections show that by 2060, seawater acidity could have increased by 120%. To the best of our knowledge, the current rate of change is many times faster than anything previously experienced in the last 55 million years.

How is ocean acidification different from climate change?

Climate change is the consequence of the release of greenhouse gases causing the Earth to absorb more of the sun’s energy, whereas ocean acidification is caused solely by increased levels of atmospheric CO2 dissolving into the ocean. Whilst there remains a degree of uncertainty about the impacts that will arise as a result of climate change, the chemical changes that are occurring in the ocean are certain and predictable. The process of CO2 dissolving in sea water to form carbonic acid is largely independent of climate change, although increasing sea water temperature reduces the solubility of CO2 and therefore very slightly mitigates ocean acidification. Reducing the concentrations of other greenhouse gases will have no effect on ocean acidification. Geoengineering techniques which aim at decreasing the amount of solar radiation reaching the Earth's surface, for example by sulphate particles in the upper atmosphere to generate more clouds, will have no impact on atmospheric CO2 and will not help to alleviate ocean acidification.

Why is it important?

Many of the animals and plants in the ocean have calcium carbonate skeletons or shells. Some of them such as the microscopic plankton at the base of the food chain, the shellfish and molluscs used day-to-day in our diets, and even encrusting plants that cement the coral reefs together, secrete a form of calcium carbonate (aragonite) that will readily dissolve in some areas should the seas continue to become more acidic. A trend towards more acidic conditions will therefore reduce such species’ ability to make their shells. We have taken such plants and animals for granted up to now, but ocean acidification may threaten their very existence.

The ocean not only provides us with food but it indirectly supports us in many other ways; the air we breathe largely depends on a healthy ocean for its production of oxygen, and the productive surface layers of the seas stimulate clouds that help to shade the planet. The ocean provides about half of the Earth’s productivity and humankind takes direct advantage of this through our fisheries and shellfisheries. Ocean acidification is taking us along an uncharted voyage and we only have a few hints about how many of these essential processes will be impacted.

Economic impacts?

The economic impacts of ocean acidification are largely unknown at this stage due to the numerous uncertainties about the response of organisms and ecosystems. Since 2005, there has been progressive decline in the $111 million/yr oyster industry in the US Pacific North-west region as year-after-year oyster larvae have failed to survive. Scientists suspect that the high acidity sea water that rises from deep in the Pacific Ocean is getting pumped into hatcheries – and it may be corrosive enough to kill oyster larvae. Molluscs alone accounted for $748 million (19%) of 2007 US domestic ex-vessel revenues. Fishery losses due to ocean acidification would drive job losses in affiliated industries through economic linkages that are currently difficult to quantify. It is clear, however, that secondary economic losses following decreased fishery harvests would be concentrated in specific regions, many of which have less economic resilience for enduring losses of fishing revenues.

What can we do about it?

Now is the time to act on ocean acidification and the target for action must be to reduce the rapid increase in atmospheric CO2 and limit future levels. Today the impacts remain relatively minor, but the rate of change is accelerating. Furthermore, there is a lag between CO2 emissions and an equilibrium state. Assuming we manage to reduce rather than (implausibly) halt CO2 emissions, acidity in the ocean will increase for many years. This property of the system puts a premium on early emissions cuts and a penalty on delaying making significant cuts in emissions. So whilst both acidification and climate change will impact our lives, the former adds considerable weight to the argument to make immediate and significant cuts in CO2 emissions.