Too Much Carbon Dioxide May Have Caused Earth’s Worst Mass Extinction

A series of colossal volcanic explosions — much larger than this one — is thought to have caused a mass extinction hundreds of millions of years ago via too much carbon dioxide. CREDIT: SHUTTERSTOCK
A series of colossal volcanic explosions — much larger than this one — is thought to have caused a mass extinction hundreds of millions of years ago via too much carbon dioxide. CREDIT: SHUTTERSTOCK

So much for carbon dioxide being a “harmless” gas.

The worst mass extinction in Earth’s history may have been caused by huge amounts of carbon dioxide that accumulated in the atmosphere and the ocean after colossal volcanic eruptions in Siberia 252 million years ago, according to a new study.

In addition to coating ancient Siberia with thick lava, the famed eruptions also released massive amounts of carbon dioxide into the atmosphere, which the study says may have turned the oceans sharply acidic. That acidity is thought to have driven a “global environmental calamity” that killed 90 percent of Earth’s species — also known as the “Great Dying” between the Permian and Triassic periods.

The fact that an ocean acidification event driven by carbon dioxide may have caused a mass extinction provides a “cautionary lesson for today,” wrote Eric Hand in the April issue of the journal Science, where the study was published Friday. Right now, the world’s oceans are acidifying rapidly due to human carbon emissions — in fact, carbon dioxide emitted from power plants, deforestation, manufacturing, and driving, have increased the ocean’s acid levels by a staggering 26 percent over the last 200 years.


“Because of CO2 released by burning fossil fuels,” Hand wrote, “oceans could now be acidifying even faster than they did 250 million years ago, although the process hasn’t yet persisted nearly as long.”

To come to their conclusions, researchers from universities across Europe and New Zealand studied limestone rocks that were hanging out on the sea floor in the United Arab Emirates. Those rocks were so old that they contained data from all the way toward the end of the Permian Period, giving the researchers clues as to what the ocean was like back then, and how it has changed since.

What they found was a huge drop in the pH levels of the ocean following the eruptions. Specifically, by analyzing isotopes of boron in the rocks, they found an isotopic signal that would have corresponded to a drop of 0.6 to 0.7 pH units, “a significant change in seawater chemistry,” according to Hand.

“This is the first really direct evidence of ocean acidification for this mass extinction,” Matthew Clarkson, a geochemist at the University of Otago in New Zealand who led the study, told Hand.

There were some limitations to Friday’s study. It acknowledges that though an acidification event appears to have happened, direct evidence connecting it to the volcanic eruptions — and therefore, the mass extinction — is lacking. One reason is that the rocks the researchers studied were only located in the United Arab Emirates. To make a more conclusive claim, they would have to show that Permian-era ocean rocks in other parts of the world showed the same level of acidification.


Indeed, the exact cause of the “Great Dying” is still a mystery to scientists. While many believe the volcanoes and resulting carbon dioxide were the tipping point, others point out that there are two phases to the extinction — and that the first phase began before the eruptions. Researchers aren’t yet sure what caused that first phase.

“We’ve still got quite a lot of work to do,” Clarkson told the journal Nature. “Everyone always wants the smoking gun for these things.”

It’s unlikely that volcanic eruptions capable of releasing that much carbon dioxide would occur in modern times (though the business of predicting volcanic eruptions can be a bit tricky). The last supervolcano eruption happened 27,000 years ago, according to the U.S. Geological Survey. At the Yellowstone volcano beneath Yellowstone National Park, for example, the yearly probability of a huge explosion is 1 in 730,000, or .00014 percent. So while a huge volcanic explosion could alter the global climate by releasing tons of carbon into the atmosphere, it’s exceedingly unlikely.

What is far more likely to affect the carbon content of the atmosphere is human activity. Since the Industrial Revolution, carbon levels in the atmosphere have increased from an average of around 260 parts per million (ppm) to more than 400 ppm — a level that is widely seen as the highest atmospheric carbon dioxide concentration in human history.

Indeed, the last time carbon dioxide levels were at 400 parts per million was between 800,000 and 15 million years ago. Or, as as Andrew Freedman put it for Climate Central, a time when “[m]egatoothed sharks prowled the oceans, the world’s seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now.”

More than a quarter of all carbon dioxide emitted by humans is absorbed by the ocean. When the carbon enters the ocean, it dissolves, forming carbonic acid. The carbonic acid then dissociates, and forms bicarbonate ions and hydrogen ions. As the hydrogen ions increase, so does the ocean’s acidity.


While the effects of acidifcation are not entirely understood, scientists do know that it can make it harder for coral and some plankton to produce their skeletons and shells. Acidification can also change the behavior of marine fish and some invertebrates, making them more susceptible to predators. This has effects on both marine life and human life — according to one United Nations study, the global economy stands to lose up to $1 trillion in services like coastline maintenance by 2100, just because of acidication’s impacts on coral reefs.

Still, many political leaders consider carbon dioxide to be harmless. Just last month, in a Senate hearing on whether to regulate carbon dioxide in order to slow climate change, Sen. Jeff Sessions (R-AL) told Environmental Protection Agency administrator Gina McCarthy that carbon dioxide is a “plant food [that] doesn’t harm anybody.”