Drowned out by the uproar over global warming, the issue of ocean acidification doesn’t get much press lately, even though its cause (carbon dioxide emissions from burning fossil fuels) is identical and the results are even more disturbing.

Photo: Micah A. Ponce, Creative Commons, Flickr

With acidity levels rising yearly, it’s only a matter of time until earth’s oceans reach what one scientist calls the 7.8 pH "tipping point," after which anything is possible.

When you add carbon dioxide, or CO2, to seawater, you get carbonic acid. It’s very simple chemistry, and can easily be demonstrated in any high school science lab. Or in nature itself, as Jason Hall Spencer and his co-workers discovered in the waters off Ischia, an island in southern Italy, where natural hot vents bubble CO2 into seawater from rocky structures deep in the earth.

At Ischia, sea grasses and invasive algae dominate the ocean floor. Coral, which builds itself from calcium carbonate, is conspicuously absent because higher acidity leads to lower levels of this chemical in the water. Creatures like clams, mollusks and snails are largely absent for the same reason. Where they do occur, their shells are thin as paper, making them infinitely more vulnerable to predators.

Understanding pH, and the narrow range in which the ocean’s creatures survive, makes the danger slightly easier to comprehend. The pH scale itself is very compact, from 0–14, with 0 acidic and 14 alkaline. Neutral – and the internal pH of most living cells – is about 7. This scale is also exponential. A drop in acidity from 7 to 6 is not one degree more acid, but ten times more acid, and pH 2 is one hundred times more acidic than pH 4.

Oceans exist in a range from 8-9 pH, or mildly alkaline. Human blood ranges from 7 to 7.8, and it can buffer mild variations by transferring potassium ions, for example. The ocean does not have this capacity. In some places, like the Atlantic coast off New York and the Carolinas, water has dropped as low as 7.8 as a result of the upwelling of CO2-laden waters from hot vents. Species diversity in these waters has dropped a corresponding 30 percent.

This (7.8 pH) is where scientists predict most ocean water will stand by the year 2100, unless CO2 emissions are curbed. At this level, according to observations, populations of reef-binding coralline algae collapse. Mollusk shells become too thin to protect their wearers, resulting in species devastation. Sea urchins also disappear, and the underwater landscape becomes one long monotony of undulating sea grasses in which invasive species, better able to withstand the acidity, thrive, and native species struggle and finally fail.

This, 7.8, is the tipping point, according to Dr. Carol Turley, senior scientist at the Plymouth Marine Laboratory (PML) and Executive Board member of the European Project on Ocean Acidification, or EPOCA. Once this tipping point is passed, not even scientists can predict the full effect on earth’s oceans.

Oceans create world climate, storing vast quantities of solar energy and transferring heat and cold around the planet via currents that also move nutrients from one place to another. The Gulf Stream, for example, carries more water from the Gulf of Mexico to the eastern Atlantic than all the rivers on earth, moderating the climates of nations like Great Britain, Ireland and even Scotland, which is otherwise too far north to be comfortable. These oceans also provide $85 billion dollars worth of seafood, provide jobs for about 40 million people, and provide coastal habitat and recreation for more than a quarter of the world’s population.

The oceans are also the world’s largest carbon sinks. In a new study, under the joint aegis of the National Oceanic and Atmospheric Association, or NOAA, the National Science Foundation and Department of Energy, measurements now show that – over the past 200 years – oceans have been the only long-term reservoirs of CO2. Land plants, on the other hand, have released more CO2 than they have stored.

According to Christopher Sabine, an oceanographer with the NOAA, these 118 billion tons of CO2 sequestered by oceans between 1800 and 1994 would, if released into the atmosphere, cause atmospheric levels of CO2 to rise to 435 parts per million (compared to 280 ppm prior to the Industrial Revolution, and about 380 ppm now). As it is, their presence in oceans has led to an increasing acidification not seen in the last 400,000 years.

Water that was 8.2 at the turn of the last century is now 8.1. Off Pacific coasts, in Mexico and S. America, pH is approaching 7.95 – as it is around Saudi Arabia and off the coasts of China and India. Scientists don’t know exactly what to expect when and if the tipping point is reached, but they do know that, unlike temperature, ocean acidity will take thousands of years, or longer, to reverse, and that geoengineering this situation by dumping calcium carbonate into the oceans is a process which might result in greater harm than benefit.

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