The question of what killed most life on Earth 250 million years ago is a veritable Murder on the Orient Express, with multiple characters all dealing part of the deathblow. Now, scientists have learned how one of the assassins — acid — could have performed its part of the deed.

High levels of atmospheric carbon dioxide would have turned the oceans more than acidic enough to kill off marine critters, a computer simulation indicates.

“This would have been another stressor in the system that might have pushed things toward extinction,” says Alvaro Montenegro, a climate modeler at St. Francis Xavier University in Antigonish, Nova Scotia. He and his colleagues describe the finding in a paper published online August 2 in Paleoceanography.

At the end of the Permian period of geologic time, more than 90 percent of marine species and three-quarters of terrestrial species vanished. Leading suspects in the die-off include oxygen-starved oceans, a belch of hydrogen sulfide from the deep, a shutdown of great marine nutrient cycles, and massive volcanic eruptions.

Using a climate model developed at the University of Victoria, Montenegro and  colleagues set up nine hypothetical worlds — mixing and matching possible continental arrangements, seafloor topographies and levels of atmospheric carbon dioxide. Then the researchers fired up the model and watched how carbon flowed through the ocean and atmosphere.

At atmospheric carbon dioxide levels of 3,000 parts per million — roughly 10 times modern preindustrial levels — much of the gas dissolved in seawater, forming carbonic acid and releasing hydrogen ions. Acidity is measured on the pH scale; the lower the number, the more acidic the waters. Today’s oceans have a pH of around 8.1; those in the modeled end-Permian world dropped to around 7.3 near the equator and 7.1 near the poles. Such acidity would have made it hard for many marine organisms to use calcium carbonate to build protective shells, Montenegro says.

Today’s oceans also are growing more acidic because of carbon dioxide belched into the atmosphere by fossil fuel burning and other sources. Back then, most of the gas probably came from huge volcanic eruptions in Siberia.

How quickly carbon dioxide built up in the atmosphere would have affected how acidic the ocean got, says Jonathan Payne, a paleobiologist at Stanford University. If gas concentrations increased quickly, he says, “then this model may be a reasonable representation of how climate was changing at the time.” If gas built up slowly, the oceans may have been able to buffer the change in other ways.

But the model doesn’t include factors such as carbon weathering off land surfaces and into the oceans — an important player in the carbon cycle, says Lee Kump, a modeler at Pennsylvania State University in University Park. Including such effects, he says, could better show how life’s worst extinction came to pass.