Death by deoxygenation
For a more likely glimpse of an Earth-altering cataclysm, we need to look to the distant past.
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, sometimes called blue-green algae, filled our 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.
What could have caused such an extreme event? During the Ordovician period, the continents were one jumbled mass called Gondwana. 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.
But then a second pulse of the extinction ramped up as oxygen levels plummeted. Scientists see evidence of this shift in seafloor samples collected from around the world. Some researchers think that the glaciers were responsible for fundamentally changing the layers of the oceans, which have unique temperatures and specific concentrations of elements like oxygen. Yet, the exact cause of the oxygen drop is still up for debate.
Whatever the cause, the end result is that more than 80 percent of life on Earth died during the Late Ordovician mass extinction, according to some estimates.
So, it may have happened before, but could a deoxygenation event happen again? In an eerie comparison to today, researchers involved in the recent Nature Communications study say that climate change is already reducing oxygen levels in our oceans, potentially killing off marine species.