As agents of extinction, comet and asteroid impacts may be losing their punch.
According to a new theory about how mass dyings work, cosmic collisions generally aren't enough to cause a major extinction event. To be truly devastating, they must be accompanied by another event that inflicts long-term suffering, like runaway climate change due to massive volcanic eruptions.
In other words, a comet couldn't have killed the dinosaurs by itself -- unless they were already endangered species.
This kind of one-two punch could explain more than the extinction of dinosaurs, Nan Arens of Hobart and William Smith Colleges said. In a recent paper in the journal Paleobiology, she and colleague Ian West argue that there are two types of events that can cause extinctions -- "pulses" (quick, deadly shocks, like comets) and "presses" (drawn-out stresses that push ecosystems to the brink but may not kill outright, like million-year-long volcanic eruptions).
The chances of mass dyings go way up when both happen together, argues Arens.
But are all mass extinctions created equal? Can researchers come up with a "Grand Unified Theory" of ancient apocalypse?
West and Arens think so. They combed the last 300 million years of geologic record, noting impact craters, massive eruptions, periods of ancient climate change, and then comparing them to extinctions. The rate at which species die off spiked dramatically, they found, when a "pulse"-type event occurred within a million years or so of a "press."
The theory fits well for the dinosaurs. Around the time of their demise 65 million years ago, a comet slammed into the Yucatan Peninsula and a huge volcano, the Deccan Traps, was erupting in what is today India.
But other extinctions are problematic. The greatest dying in geologic history, the Permian-Triassic extinction, killed 90 percent of all life on Earth, but there is no record of an impact. Instead, all signs point to a 200,000-year-long volcanic eruption in Siberia as the murder weapon.
Arens' theory argues that impacts are weaker in effect than is generally thought. But a growing consensus of researchers believes that doesn't go far enough. They believe eruptions, not cosmic impacts, are the real killers.
"I'm not so sure craters really have anything to do with it," Gregory Retallack of the University of Oregon said, adding: "I don't like the 'press' term very much. If you look closely at the isotope record, you can see that flood basalts [large-scale eruptions] are a series of pulses, paving the golden path toward annihilation."
"I'm not saying it's impossible to have an extinction with just a 'press' or a 'pulse' event," Arens admitted. The study states only that it's more likely when the two combine.
Humanity is creating exactly that scenario today, she said. Over the last 6,000 years, subsistence farming began changing the climate and clearing wilderness slowly enough to constitute 'press'-type stresses on the environment. But people began burning fossil fuels in earnest during the Industrial Revolution, and carbon concentrations have skyrocketed while growing population numbers have led to widespread habitat loss around the globe.
Arens argued this constitutes a 'pulse' event, and the sixth great mass extinction may already be underway.