Rising carbon dioxide concentrations make oceans more acidic

Gradual chemical changes may already be affecting marine ecosystems

This is the first article in a series about the effects of rising atmospheric CO2 concentrations on the world’s oceans.

The rising concentration of atmospheric carbon dioxide (CO2) from human activities is doing more than warming the planet’s climate system. It is being absorbed into the oceans, where changing sea water chemistry could gradually transform ocean ecosystems.

This process, called ocean acidification, most visibly will affect ― some scientists say it is already affecting ― sea animals, small and large, that form their chalky calcium-carbonate shells and other hard parts from chemicals that have been plentiful in the ocean for millions of years.

Ocean acidification could reduce the amount of these chemicals, making it harder for the animals to secrete their shells. It could even, under highly acidic future conditions, dissolve the shells.

Using data gathered through activities like the Climate Variability and Predictability program, the World Ocean Circulation Experiment and ocean-circulation computer models, scientists have confirmed that ocean chemistry is changing as the sea surface absorbs anthropogenic (people-generated) CO2.

Less well understood and studied is the effect this acidification process will have on plants and animals that live in the sea.

“Ocean acidification has the potential to reorganize everything ― the ocean, the food web,” Victoria Fabry, a biological oceanographer in the Department of Biological Sciences at California State University–San Marcos, told America.gov.

“The structure of the marine ecosystem would end up looking very different than it does now,” she added. “It’s definitely going to change; we just don’t know how. This is an experiment we would not want to be doing if we weren’t already doing it.”


Human and ocean life have many things in common and one of them is pH, a scale that indicates whether a solution is acidic, like sulfuric acid (a component of acid rain), juice from citrus fruits and carbonated soft drinks; or basic, like soaps, baking soda and seawater.

The pH scale ranges from 0 to 14. Water has a pH of 7, which is neutral. Below 7, solutions are considered acidic, above 7, basic or alkaline.

The pH range of human blood in the body covers only 0.1 pH unit ― from 7.35 to 7.45. Illness occurs if the blood goes above or below that narrow range. Seawater, which like blood is mildly basic, also has a narrow pH range ― from 8 to 8.3. Anything above or below that range can be harmful.

Ocean pH was a consideration in 1990, when scientists from 30 nations undertook the World Ocean Circulation Experiment, an ambitious, eight-year effort that sought to establish the role of the oceans in Earth’s climate and gather baseline data to use in assessing future changes.


As part of that program, scientists Richard Feely and Christopher Sabine of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle led an effort to study the distribution of anthropogenic CO2 in oceans around the globe.

“Eight countries came together on 99 cruises, collecting about 72,000 samples throughout the world,” senior scientist Feely told America.gov. “We gathered a first detailed global view of the distribution of carbon dioxide and carbon-related [chemical] species in the world oceans. In doing so, Chris was able to map out the amount of anthropogenic CO2 in the oceans.”

“When all was said and done,” Sabine, a supervisory oceanographer, told America.gov, “we found that the oceans had absorbed 118 petagrams of carbon dioxide ― a billion metric tons.” A metric ton is about the size of a small car, he added, so the ocean has absorbed the equivalent of 118 billion small cars.

“Over the last 200 years, that 118 petagrams of carbon has lowered pH by 0.1 pH unit,” Sabine said. “That doesn’t sound like a lot but it represents a 30 percent increase in the acidity of the ocean. And if we continue to add CO2 to the atmosphere at the current rate [the ocean’s uptake is 22 million metric tons per day], by the end of this century, the pH could drop by another 0.3 pH units,” to a 150 percent increase in acidity.

“We are already having a significant and measurable effect on the ocean,” he said, “but projections for the future make the situation quite dire.”


Today, Feely said, everywhere in the surface oceans, from about 200 meters deep to the surface, there is an excess of the chemicals needed for sea life to make their calcium-carbonate shells — the water is supersaturated. In the deep North Pacific Ocean, 200 meters and below, there is a deficit of the chemicals — the water is undersaturated — and calcium-carbonate shells dissolve in these corrosive waters.

Computer models of the open ocean suggest that unless atmospheric CO2 is sharply reduced, acidification could make surface seawater corrosive to these shells by 2020–2030 in the Arctic Ocean, 2040 in the Antarctic Ocean and about 2090 in the North Pacific Ocean.

No one had looked at such conditions in the coastal oceans until 2007, when Feely and Sabine led the international North American Carbon Program West Coast Cruise on the research ship Wecoma. It moved along the continental shelf from Queen Charlotte Sound in Canada, down the coasts of Washington state, Oregon and California and into Baja California in Mexico.

The team found that summer winds were pushing supersaturated surface waters offshore and drawing undersaturated deep water onto the continental shelf and in some cases into the surface waters.

“What was projected to occur in the open ocean models by the end of the century, we found is occurring right now along our entire continental shelf as far as we looked,” Feely said. “This puts the problem into the present instead of into the future.”

More information about ocean acidification is available at the Pacific Marine Environmental Laboratory Web site.

Cheryl Pellerin, America.gov, 1 April 2009. Article.

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