What Drove the Great Dying?

As long ago as forever and as far away as home, life was withering away wherever you looked. At the end of the Permian, around 250 million years ago, the creatures of Earth were devastated by an extinction that outstripped any seen before or since. Marine species suffered the most — 96% of them died out — but even among their terrestrial cousins, seven out of every ten species were lost. For countless generations, life struggled towards recovery, but it took 10 million years to rebuild the lost diversity. The cause of the catastrophe has long puzzled scientists; global warming, massive volcanos, ocean acidification, and widespread oceanic oxygen depletion have all been implicated. In a paper appearing in Science, researchers from the UK, Germany and Austria showed that increased carbon released into the atmosphere eventually acidified the oceans just as the Permian extinction reached its peak; comparing their findings with how quickly our societies release carbon, they reveal an alarming difference together with a sobering insight.

Changes in the carbon isotope ratio already show an increase in atmospheric carbon dioxide during the late Permian, offering indirect evidence that the oceans may have become more acidic. In this study, the researchers took a more direct look by measuring the boron isotope ratio in marine carbonates, which is more closely connected with ocean pH levels. By tracking these ratios in ancient limestones, the team could assess the ancient ocean's acidity; in combination with a climate model and existing data, this allowed them to piece together the story of the Great Dying.

Their findings tie in with a recent suggestion that the Permian extinction happened in two stages. The first involved a slower carbon pulse over 50,000 years which increased atmospheric and oceanic carbon levels without changing the oceans' pH. The researchers suggest that the ocean may have been buffered by processes like silicate weathering and bacterial sulfur metabolism, which provided enough alkalinity to counteract the acidification. These processes were overwhelmed by the second, more rapid carbon pulse, which released 24,000 gigatons of carbon into the atmosphere in just 10,000 years. Feedback systems buckled, the planet's oceans became four or five times more acidic, and creatures everywhere began to die.

24,000 gigatons is an impressive sounding number, but understanding such large quantities is always difficult. Spread out over 10,000 years, that comes to 2.4 gigatons per year — a smaller number, but still challenging to grasp. However, it has the advantage of being readily compared with the impact of human activities. An annual release of 2.4 gigatons for 10,000 years was enough to drive the largest extinction in Earth's history, so how are we doing by comparison? At the moment, scientists estimate that atmospheric carbon is increasing by about 10 gigatons each year — four times faster than the "rapid" pulse at the height of the Permian extinction! It's an alarming fact, but there's another side to it. Researchers also estimate that the total carbon trapped in fossil fuels (including "unconventional" fossil fuels) is only 5,000 gigatons, far less than the 24,000 gigatons dumped into the atmosphere during the second Permian pulse. It's unclear which will have more of an impact, the rate at which we push carbon into the air and water or the magnitude of the increase, but the Permian extinction offers a clear warning that there is a tipping point — eventually, the balance shifts, and the whole system changes.

Regardless of the specific rates and quantities, I think there's a more important message to be gleaned from this work. The Permian extinction may have been the largest in Earth's history, but it's dwarfed in popular imagination by the Cretaceous extinction, which killed the dinosaurs (and lots of plants, too!). An enormous space rock crashing into Earth and wiping out gigantic, ferocious reptiles is more exciting than slow chemical changes killing off a bunch of sea creatures most people wouldn't even recognize. The story is more dramatic, but there's a deeper issue. We respond readily to discrete, immediate threats (like an impending meteor impact), but we're not as good at recognizing or dealing with gradual threats, just like the famous, but fictional, frog in boiling water. While that may be how we're wired, the difference between the Permian and Cretaceous extinctions should be a stark reminder that gradual threats can be much more dangerous. Thought anthropogenic climate change is a gradual process, lacking the drama to capture people's imagination, its accumulated impact may be overwhelming. As we mark Earth Day this week, it's important to recognize another gradual, but potentially powerful, process — the growing recognition of our impact on the planet and the will to create change through personal choices.

Clarkson, MO, et al. Ocean acidification and the Permo-Triassic mass extinction. Science 348(6231):229-232. (2015) doi: 10.1126/science.aaa0193

Hand, E. Acid oceans cited in Earth's word die-off. Science 348(6231):165-166. (2015) doi: 10.1126/science.348.6231.165

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