Discover Magazine: January/ February 2022
Asteroid strikes, supernova blasts and other calamities that could take out humanity
Life is resilient
The first living things on Earth appeared as far back as 4 billion years ago, according to some scientists. Throughout Earth’s history, it’s seen all manner of cataclysms. These disparate doomsdays have killed countless like-forms. And at times, those mass extinctions have even eliminated most species on Earth.
Yet life has always rebounded. New species emerge. The cycle repeats.
So, what would it take to kill off
life in full? Here are just a few possible doomsday events that could
permanently extinguish all life on Earth- and the last one is likely
unavoidable.
Asteroid-Impact Apocalypse
When a city-sized asteroid struck the Golf of Mexico 66 million years ago, it was game over for the dinosaurs, as well as most other species on Earth at the time. A future asteroid could just as easily take out every person on Earth. Fortunately, that’s unlikely to happen any time soon. Based on the geological record of cosmic impacts, Earth gets hit by a large asteroid roughly every 100 million years, according to NASA. However, smaller asteroid impacts do happen all the time. There’s even evidence that some people may have been killed by small meteorite impacts within the past few thousand years.
But what are the chances that our planet will ever be struck by an asteroid massive enough to wipe out all life on Earth? Simulations published in Nature back in 2017 suggest it would take a truly gigantic space rock to accomplish such a feat. Killing all life on Earth would require an impact that literally boils away the oceans. And only asteroids like Pallas and Vesta- the solar system’s largest- are big enough to do that. While there is evidence that infant Earth was struck by a large planetoid called Theia, these days collisions of such large objects are extremely unlikely.
Death by Deoxygenation
Nearly 2.5 billion years ago, a period called the Great Oxidation Event gave us the breathable atmosphere we all now depend on. An eruption of cyanobacteria filled out atmosphere with oxygen, creating a world where multicellular life-forms could take hold, and where creatures like humans could ultimately breathe.
However, one of Earth’s great die-offs, an event 450 million years ago called the Late Ordovician mass extinction, likely happened because the inverse took place. The planet saw a sudden drop in oxygen levels that lasted for several million years. During the Ordovician period, most life on Earth still lived in the oceans, but plants were beginning to emerge on land. Then, near the end of the Ordovician, a sweeping climate shift left the supercontinent covered with glaciers. That global cooling alone was enough to start killing off species. And a second pulse of the extinction ramped up as oxygen levels plummeted. Some researchers think that the glaciers were responsible for fundamentally changing the layers of the oceans, which have unique temperatures and specific concentration of elements like oxygen.
More than 80 percent of life on Earth died during the Late Ordovician mass extinction, according to some estimates. Could a similar deoxygenation event happen again? In an eerie parallel, researchers in a recent Nature Communications study say that climate change is already reducing oxygen levels in our oceans, potentially killing off marine species.
Gamm-Ray-Burst Extinction
Even if a sudden spate of global cooling sparked the Late Ordovician mass extinction, what set that in motion in the first place? Over the years, numerous astronomers have suggested the culprit might have been a gamma-ray burst, or GRB. GRBs are mysterious events that seem to be the most violent and energetic explosions in the cosmos, and astronomers suspect they’re tied to extreme supernovas. So far, GRBs have only been spotted in other galaxies.
But if one did happen in the Milky Way, as has likely happened in the past, it could cause a mass extinction on Earth. A GRB pointed in our direction might last just 10 seconds or so, but it could still destroy at least half Earth’s ozone in that short period of time. Even a relatively small amount of ozone depletion is enough to chip away at our planet’s natural sunscreen. Wiping out the ozone on a large enough scale could wreak havoc on food chains, killing off huge numbers of species.
A GRB would wipe out the life-forms that live in the upper levels of the ocean, which currently contribute significant amounts of oxygen to our atmosphere. And, it turns out, gamma rays also break apart atmospheric oxygen and nitrogen. These gases get converted into nitrogen dioxide, more commonly known as the smog that blocks out the sun above heavily polluted cities. Having this smog blanketing the entire Earth would kickstart a global ice age.
End of the Sun
Gamma-ray burst or not, most life on Earth will eventually die anyway due to a lack of oxygen, according to a study published last March in Nature Geoscience. The researchers suggest that our oxygen-rich atmosphere is not a permanent feature of the planet. Instead, in about a billion year, solar activity will cause atmospheric oxygen to plummet back down to the level it was at before the Great Oxidation Event.
To determine this, the authors simulated what will happen to the atmosphere as the Sun ages and puts out more energy. They found that, eventually, Earth reaches a point where atmospheric carbon dioxide breaks down. At that point, oxygen-producing plants and organisms that rely on photosynthesis will die out. Our planet won’t have enough life-forms to sustain the oxygen-rich atmosphere humans and other animals require.
The precise timing of when that starts and how long it takes- the deoxygenation process could take as few as 10,000 years- depends on a broad range of factors. But, in the end, the authors say that cataclysm is an unavoidable one for the planet.
Luckily, humanity still has another billion years to figure out other plans.- Eric Betz
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