The surge of carbon dioxide released by humankind’s use of fossil fuels may overwhelm the world’s oceans with an unprecedented rate of acidification, scientists said this week as they released a study showing historic rates of acidification.
Specifically, the researchers looked at pulse of CO2 that happened about 56 million years ago, when global temperatures soared and carbon sediments in the oceans simply dissolved, while some marine organisms went extinct. They traced the rate of acidification during that era by analyzing fossil chemistry, finding that ocean acidity may have increased by a 100 percent in just a few thousand years — and today’s rate is even higher.
Publishing the results of their study in the journal Paleoceanography, the scientists explained that the sudden increase kept acidity levels high for about 70,000 years and radically changed the ocean environment.
“This could be the closest geological analog to modern ocean acidification,” said study coauthor Bärbel Hönisch, a paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory. “As massive as it was, it still happened about 10 times more slowly than what we are doing today.”
The oceans have absorbed about a third of the carbon humans have pumped into the air during the industrial age, but that uptake of carbon has come at a price. Chemical reactions caused by that excess CO2 have made seawater grow more acidic, depleting it of the carbonate ions that corals, mollusks, and calcifying plankton need to build their shells and skeletons.
In the last 150 years or so, the pH of the oceans has dropped substantially, from 8.2 to 8.1 – equivalent to a 25 percent increase in acidity. By the end of the century, ocean pH is projected to fall another 0.3 pH units, to 7.8. While the researchers found a comparable pH drop during the PETM – 0.3 units – the shift happened over a few thousand years.
“We are dumping carbon in the atmosphere and ocean at a much higher rate today — within centuries,” said study coauthor Richard Zeebe, a paleoceanographer at the University of Hawaii. “If we continue on the emissions path we are on right now, acidification of the surface ocean will be way more dramatic than during the PETM.”
It’s not clear what caused the ancient burst of carbon, but the fossil record shows the consequences — up to half of the tiny animals that live in mud on the seafloor died, probably affecting other organisms higher up the food chain.
Other species thrived in this changed environment and new ones evolved. In the oceans, dinoflagellates extended their range from the tropics to the Arctic, while on land, hoofed animals and primates appeared for the first time. Eventually, the oceans and atmosphere recovered as elements from eroded rocks washed into the sea and neutralized the acid.
Today, signs are already emerging that some marine life may be in trouble. In a recent study led by Nina Bednaršek at the U.S. National Oceanic and Atmospheric Administration, more than half of the tiny planktic snails, or pteropods, that she and her team studied off the coast of Washington, Oregon and California showed badly dissolved shells. Ocean acidification has been linked to the widespread death of baby oysters off Washington and Oregon since 2005, and may also pose a threat to coral reefs, which are under additional pressure from pollution and warming ocean temperatures.
“Seawater carbonate chemistry is complex but the mechanism underlying ocean acidification is very simple,” said study lead author Donald Penman, a graduate student at University of California at Santa Cruz. “We can make accurate predictions about how carbonate chemistry will respond to increasing carbon dioxide levels. The real unknown is how individual organisms will respond and how that cascades through ecosystems.”