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| Earth on Mar 02, 2012

Oceans acidifying faster today than in past 300 million years

Scientists say that too much carbon dioxide in the atmosphere is making the oceans more acidic and imperiling key parts of the marine food chain.

The oceans might be acidifying faster today than they did in the last 300 million years, according to scientists publishing a paper in the journal Science in March 2012.

The common sea fan is but one of the species being affected by acidifying oceans. Image Credit: NOAA

Scientists say that too much carbon dioxide in the atmosphere is making the oceans more acidic and imperiling key parts of the marine food chain.

As the quantity of carbon dioxide in the atmosphere increases, more of it is absorbed by the world’s oceans. Carbon dioxide and water bind together to create carbonic acid, which is used to make soft drinks bubbly – but also makes water more acidic.

Earth scientists representing 18 institutions worldwide have united to examine the geologic record of the past 300 million years for clues about what the future holds if atmospheric carbon dioxide levels continue to increase.

Bärbel Hönisch, a paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory, said:

Marine scientists study how corals and other species respond to more acid seas. Image Credit: NSF Moorea Coral Reef Long-Term Ecological Research Site

We know that life during past ocean acidification events was not wiped out–new species evolved to replace those that died off. But if industrial carbon emissions continue at the current pace, we may lose organisms we care about–coral reefs, oysters, salmon.

The oceans act like a sponge to draw down excess carbon dioxide from the air. The gas reacts with seawater to form carbonic acid, which over time is neutralized by fossil carbonate shells on the seafloor.

If too much carbon dioxide enters the ocean too quickly, it can deplete the carbonate ions that corals, mollusks and some plankton need for reef and shell-building.

In a review of hundreds of paleoceanographic studies, the researchers found evidence for only one period in the last 300 million years when the oceans changed as fast as today: the Paleocene-Eocene Thermal Maximum, or PETM.

About 56 million years ago, a mysterious surge of carbon into the atmosphere warmed the planet and turned the oceans corrosive. In about 5,000 years, atmospheric carbon doubled to 1,800 parts per million (ppm), and average global temperatures rose by about 6 degrees Celsius.

The carbonate plankton shells littering the seafloor dissolved, leaving the brown clay layer that scientists see in sediment cores today.

Corals form the backbone of a reef ecosystem that supports many other creatures.. Image Credit: NSF Moorea Coral Reef Long-Term Ecological Research Site

As many as half of all species of benthic foraminifera, a group of one-celled organisms that live at the ocean bottom, went extinct, suggesting that deep-sea organisms higher on the food chain may have also disappeared, said paper co-author Ellen Thomas, a paleoceanographer at Yale University. She said:

It’s really unusual that you lose more than 5 to 10 percent of species.

Scientists estimate that ocean acidity–its pH–may have fallen as much as 0.45 units as the planet vented stores of carbon into the air.

Candace Major is program officer in the National Science Foundation’s (NSF) Division of Ocean Sciences, which funded the research. She said:

The ocean acidification we’re seeing today is unprecedented, even when viewed through the lens of the past 300 million years, a result of the very fast rates at which we’re changing the chemistry of the atmosphere and oceans.

In the last hundred years, rising carbon dioxide from human activities has lowered ocean pH by 0.1 unit, an acidification rate at least 10 times faster than 56 million years ago, says Hönisch.

Oceans may be acidifying faster today than in the last 300 million years. Image Credit: NOAA

The Intergovernmental Panel on Climate Change (IPCC) predicts that pH will fall another 0.2 units by 2100, raising the possibility that we may soon see ocean changes similar to those observed during the PETM.

In lab experiments, scientists have tried to simulate modern ocean acidification, but the number of variables currently at play–high carbon dioxide and warmer temperatures, and reduced ocean pH and dissolved oxygen levels–make predictions difficult.

An alternative to investigating the paleo-record has been to study natural carbon seeps from offshore volcanoes that are producing the acidification levels expected by the year 2100.

In a recent study of coral reefs off Papua New Guinea, scientists found that during long-term exposure to high carbon dioxide and pH 0.2 units lower than today–at a pH of 7.8 (the IPCC projection for 2100)–reef biodiversity and regeneration suffered.

Bottom line: According to a March 2012 paper in the journal Science, Earth’s oceans might be acidifying faster today than they did in the last 300 million years. Scientists say that too much carbon dioxide in the atmosphere is making the oceans more acidic and imperils key parts of the marine food chain.

Read more from the National Science Foundation