Mass Extinctions: The Role of Climate and Ocean Acidification

The current theory is that the end-Ordovician extinction(the second most severe mass extinction event in Earth’s history, occurring roughly 445 million years ago) was caused by glaciation. For most of the period, a so-called greenhouse climate prevailed- carbon dioxide levels in the air were high and so, too, were sea levels and temperatures. But right around tie time of the first pulse of extinction- the one that wreaked havoc among the graptolites- CO, levels dropped. Temperatures fell and Gondwana froze. Evidence of the Ordovician glaciation has been found in such far-flung remnants of the supercontinent as Saudi Arabia, Jordan, and Brazil. Sea levels plummeted, and many marine habitats were eliminated, presumably to the detriment of marine organisms. The oceans’ chemistry changed, too; among other things, colder water holds more oxygen. No one is sure whether it was the temperature change or one of the many knock-on effects that killed the graptolites.

The end-Permian extinction also seems to have been triggered by a change in the climate. But in this case, the change went in the opposite direction. Right at the time of extinction, 252 million years ago, there was a massive release of carbon into the air- so massive that geologists have a hard time even imagining where all the carbon could have come from. Temperatures soared – the seas warmed by as much as ten degrees and the chemistry of the oceans went haywire, as if in an out-of-control aquarium. The water became acidified, and the amount of dissolved oxygen dropped so low that many organisms probably, in effect, suffocated. Reefs collapsed.

The end-Permian extinction took place, though not quite in a human lifetime, in geologic terms nearly as abruptly; according to the latest research by Chinese and American scientists, the whole episode lasted no more than two hundred thousand years, and Perhaps less than a hundred thousand. By the time it was over, something like ninety percent of all species on earth had been eliminated. Even intense global warming and ocean acidification seem inadequate to explain losses on such a staggering scale.

The word “Anthropocene” is the invention of Paul Crutzen, a Dutch chemist who shared a Nobel Prize for discovering the effects of ozone-depleting compounds. The importance of this discovery is difficult to overstate; had it not been made and had the chemicals continued to be widely used- the ozone “hole” that opens up every spring over Antarctica would have expanded until eventually it encircled the entire earth. (One of Crutzen’s fellow Nobelists reportedly came home from his lab one night and told his wife, “The work is going well, but it looks like it might be the end of the world.””)

Since the start of the industrial revolution, humans have burned through enough fossil fuels- coal, oil, and natural gas- to add some 365 billion metric tons of carbon to the atmosphere. Deforestation has contributed another 180 billion tons. Each year, we throw up another nine billion tons or so, an amount that’s been increasing by as much as six percent annually. As a result of all this, the concentration of carbon dioxide in the air today–a little over four hundred parts per million- is higher than at any other point in the last eight hundred thousand years. Quite probably it is higher than at any point in the last several million years.

If current trends continue, CO, concentrations will top five hundred parts per million, roughly double the levels they were in preindustrial days, by 2050.

It is expected that such an increase will produce an eventual average global temperature rise of between two and four degrees Celsius, and this will, in turn, trigger a variety of world-altering events, including the disappearance of most remaining glaciers, the inundation of low-lying islands and coastal cities, and the melting of the Arctic ice cap. But this is only half the story.

Ocean covers seventy percent of the earth’s surface, and everywhere that water and air come into contact there’s an exchange. Gases from the atmosphere get absorbed by the ocean and gases dissolved in the ocean are released into the atmosphere. When the two are in equilibrium, roughly the same quantities are being dissolved as are being released. Change the atmosphere’s composition, as we have done, and the exchange becomes lopsided: more carbon dioxide enters the water than comes back out. In this way, humans ‘are constantly adding CO, to the seas, much as the vents do, but from above rather than below and on a global scale. This year alone the oceans will absorb two and a half billion tons of carbon, and next year it is expected they will absorb another two and a half billion tons. Every day, every American in effect pumps three kilograms of carbon into the sea.

Thanks to all this extra CO2, the pH of the oceans’ surface waters has already dropped, from an average of around 8.2 to an average of around 8.1. Like the Richter scale, the pH scale is logarithmic, so even such a small numerical difference represents a very large real-world change. A decline of .1 means that the oceans are now thirty percent more acidic than they were in 1800. Assuming that humans continue to burn fossil fuels, the oceans will continue to absorb carbon dioxide and will become increasingly acidified. Under what’s known as a “business as usual” emissions scenario, surface ocean phi will fall to 8.0 by the middle of this century, and it will drop to 78 by the century’s end. At that point, the oceans will be 150 percent more acidic than they were at the start of the industrial revolution*

Ocean acidification is sometimes referred to as global warming’s “equally evil twin.” The irony is intentional and fair enough as far as it goes, which may not be far enough. No single mechanism explains all the mass extinctions in the record, and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous. There’s strong evidence for ocean acidification during an extinction event known as the Toarcian Turnover, which occurred 183 million years ago, in the early Jurassic, and similar evidence at the end of the Paleocene, 55 million years ago, when several forms of marine life suffered a major crisis.

Source : The Sixth Extinction: An Unnatural History by Elizabeth Kolbert

Goodreads : https://www.goodreads.com/book/show/17910054-the-sixth-extinction

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