Fifty-six million years ago, a surge of carbon dioxide in the atmosphere raised the acidity of the world's oceans substantially. Many single-celled organisms, and most likely larger creatures farther up the food chain, went extinct.
The carbon dioxide that humans are pumping into the atmosphere now is causing a similar acidification effect -- only 10 times faster.
In a study published today in the journal Science, researchers compared the current rates of ocean change to other major acidification events going back 300 million years, and what they found is shocking: never in that long period did the ocean pH fall as rapidly as it is falling right now (lower pH means higher acidity). Ocean pH has already dropped 0.1 units to 8.1 -- it is a logarithmic scale, meaning the drop represents about a 30 percent change in acidity. Within another hundred years, it could drop to 7.8.
At this level, coral, mollusks, and many other creatures will be unlikely to survive. Increased CO2 entering the oceans depletes the carbonate ions that these animals need to make their shells and reefs.
The new study, led by Bärbel Hönisch of Columbia University's Lamont-Doherty Earth Observatory, found that only the event 56 million years ago, known as the Paleocene-Eocene Thermal Maximum period, even approached the current rates of acidification. Other mass extinction events also involved rises in atmospheric as well as oceanic CO2, but rates were slower (or the fossil record is not strong enough to firmly establish those rates).
For example, about 252 million years ago, a surge in volcanic activity killed off more than 95 percent of marine life. Fifty million years later, another increase in volcanic activity doubled atmospheric CO2, and coral reefs and other marine life largely disappeared. For the record, humans have pushed CO2 levels from 280 parts-per-million to about 392 ppm since the start of the Industrial Revolution, with concentrations due to keep rising at more than two percent each year until we decide to do something drastic to cut emissions.
When it comes to the oceans, though, "something drastic" might not even be enough. The rise in acidity is likely to continue apace for some time even if we were to stop emitting all CO2 right now. The fact that we're outpacing even the great extinction events of the last 300 million years suggests that we are indeed in the midst of the sixth such event in the planet's history. As disturbing as it is, there is a decent chance that for huge chunks of the world's marine life, it is already too late.
"I do not think there is much hope to reduce the rate of acidification," Hönisch said in an email. "Even if we stopped producing CO2 now, the concentration in the atmosphere would remain high for a long time, unless we find an efficient and cost-effective way to reduce atmospheric CO2." This is not, though, a reason to give up trying, she added. "While we may not be able to slow the current rate of calcification, reducing emissions would help to maintain this rate and prevent it from growing even higher."
Christopher Langdon, a biological oceanographer at the University of Miami who was not involved in the new Science study, agreed that we have entered unprecedented territory. "It's not a problem that can be quickly reversed," he said in a press release from Columbia. "We're playing a very dangerous game."
Image: Bleached coral on the Great Barrier Reef, via Acropora/Wikimedia