The Scientist: Are We in the Midst of a Sixth Mass Extinction?

This picture illustrates a scene from northern Laurentia (North America) in the period a few weeks after the Chicxulub impact showing the onset of freezing weather and skies loaded with sulfur aerosols. The focus is on the last surviving dinosaurs – here a pair of T-Rex chicks, which somehow survived the initial impact phenomena, but which will soon succumb to the cold. Credit: ©James McKay – Creative Commons

“You wouldn’t have lasted long, I don’t think, as a puny human swimming around in this ocean,” muses paleontologist and geologist James Witts while viewing an artist’s depiction of marine life during the Cretaceous Period on his computer screen. Starting around 145 million years ago, this was the last age of the dinosaurs. As on land, the food web of these ancient seas was likely dominated by gigantic reptiles. Along with sharks, many of these now-extinct species may have feasted on bottom-dwelling crustaceans and free-swimming cephalopods and fish. 

Today, the fossilized remains of these creatures are buried beneath a conspicuous layer of sediment or rock that geologists call the Cretaceous-Paleogene (K-Pg) boundary. The layer is typically enriched in the element iridium, released by the shattering of the Mount Everest–sized asteroid that smashed into modern-day Mexico one fateful day 66 million years ago and brought the Cretaceous period to a violent end. “It basically happened in the worst possible place,” says Witts, a lecturer at the University of Bristol. Research published earlier this year by him and his colleagues suggests that massive quantities of sulfur from bedrock at the impact site, in the present-day Yucatán peninsula, were blasted into the atmosphere, creating aerosols that blocked sunlight for several months, cooled the climate for decades, and fell down as acid rain—all contributing to the collapse of global ecosystems. The fossil record falls comparatively silent after the K-Pg boundary, as an estimated 76 percent of marine species were wiped out, for instance. And as life recovered over the next million years, only a subset of the lineages that previously roamed the Earth were among its ranks. Like their popular terrestrial counterparts, “the big marine reptiles—they never come back,” says Witts.  

The K-Pg extinction is the most recent of five events in Earth’s history that scientists consider mass extinctions, defined by paleontologists as events where more than 75 percent of species vanish within a geologically short period of time, typically less than two million years. The four previous mass extinctions were also thought to have involved climatic changes—due to large-scale volcanic eruptions, for example—and in one case obliterated all but 5 percent of species. (See illustration below.) In between these events were smaller extinction episodes and periods of relative stability, with new species often arising at rates that compensated for species losses.  

Now, many scientists fear that the next ordeal of this scale is close—this time around, caused by our species, which sprang onto the scene within the last few hundred thousand years. Although we’re still far away from reaching the 75 percent mark, extinction rates are climbing, and many more species appear to be on the brink. Scientists point to the worldwide destruction of natural habitats and the exploitation of wild species, along with climate change, pollution, and ecological disruption caused by the spread of invasive organisms, as driving factors. Indeed, Witts says he reckons that the sheer speed of environmental change today is similar to that caused by the asteroid. 

Whether current biodiversity loss—a crisis by any measure—meets the criteria for another mass extinction is hotly debated. Much of the debate hinges on accurately measuring the scale of modern-day and prehuman extinction, which is complicated by an incomplete understanding of present and past biodiversity. Some scientists also question whether diagnosing a mass extinction is even relevant. 

“We’re in this really unusual position, where, for the first time, we are trying to put our finger on a geologically superlative event while it’s happening,” says Jacquelyn Gill, a paleoecologist at the University of Maine. However, when it comes to biodiversity loss, “just evoking the fact that our influences could even be on the scale of a comet or some of these other big events in the past—I think that should be giving us pause.” 

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