Climate models usually end up in the news because of projections of future climate, but many researchers use the models to study other planets or the Earth's past. They can help test hypotheses about past climate events by comparing model simulations to estimates of past climates obtained from things like ice and sediment cores.

One climatic event that looms large in Earth's history is the end-Permian mass extinction about 252 million years ago—the worst mass extinction event on record. A volcanic event seems to have been at least partly to blame. Tremendously vast eruptions in Siberia coughed up lava flows and ash that may have covered an area nearly as large as Australia—a feature known as the Siberian Traps. During this event, some 90 percent of marine species disappeared, and species on land didn’t fare well, either.

Apart from the warming caused by all the carbon dioxide emitted by the eruptions, many researchers have explored the problems that volcanic gases might have caused for land-dwelling organisms. Currently, the pH of the ocean is dropping as we increase atmospheric CO 2 , but at high enough levels of carbon dioxide, acidification of rain can become a problem as well. Add in volcanic emissions of sulfur dioxide—the same compound that we control in coal emissions to prevent acid rain—and the atmosphere would get even worse.

Among the Siberian rocks in the vicinity of the eruptions were salts deposited by evaporating seawater. The heat of the magma feeding such eruptions can metamorphose surrounding rocks, causing those salts to release chloride-containing gas. Along with some methane cooked out of organic compounds in the rocks, that could have depleted the ozone layer, allowing more dangerous UV radiation through to the surface.

In order to investigate how these effects could have played out during the eruption of the Siberian Traps, a group led by MIT’s Benjamin Black used a global climate model. To simplify things, atmospheric CO 2 was held constant at a little less than 10 times today’s concentration—roughly the level it likely reached during that time. A number of plausible Siberian eruptions were then simulated using what we know about the timing and size of individual events during that time. Other than CO 2 , the gases released by the eruptions only last a few years in the atmosphere before breaking down, so rainfall acidity and ozone depletion worsened in pulses that abated soon after each eruption died down.

The average pH of rain water today is about 5.0 to 5.5, on the acidic side of a neutral pH of 7.0. The higher CO 2 in the model alone lowered the average pH to about four—a significant increase in acidity since pH is measured on a logarithmic scale. During the simulated eruptions, the pH dropped even more. Because the Siberian Traps were almost as far north then as they are now, the effects were more intense in the Northern Hemisphere. From the equator to about 50 degrees north latitude, rainwater pH in the model was as low as two—about as acidic as lemon juice. That’s acidic enough to harm many types of organisms.

Simulations of ozone loss covered a wider range due to uncertainty about the amount of ozone-depleting gases that might have been released. Still, the impact was significant, with average global ozone concentration decreasing by as much as 70 percent. Near the poles, where ozone depletion would be strongest, the amount of harmful UV radiation reaching the surface would increase by a factor of 49.

These climate model simulations can now be compared to records from the mass extinction to see if evidence consistent with the impacts of acid rain and ozone depletion matches the patterns in the model. The simulations describe a terrestrial environment swinging into and out of relative extremes of nastiness as eruptions thrashed and subsided. They don’t call it “the Great Dying” for nothing.

Geology, 2014. DOI: 10.1130/G34875.1 (About DOIs).