Life on Earth Could Have a Suffocating End If Oxygen Levels Drop by a Factor of a Million

Scientists have projected a dramatic shift in Earth’s atmosphere that could see our planet revert to a state similar to the one before the Great Oxidation Event (GOE) around 2.4 billion years ago. This event, which saw a significant increase in atmospheric oxygen, fundamentally altered our planet’s environment and enabled the rise of aerobic life forms, including humans.

But researchers now say oxygen levels will drop by a factor of a million to just one-third of what they are today. That would kill most aerobic lifeforms, including humans. The study, led by University of Washington professor Ilya Bindeman, appears this week in Nature.

When oxygen is present in the air, it reacts with nitrogen and carbon dioxide to produce carbonic acid that dissolves in water. This chemical reaction is the basis of photosynthesis, in which plants transform sunlight, water, and carbon dioxide into carbohydrates, energy, and oxygen. All plants on Earth incorporate symbiotic cyanobacteria to conduct this reaction for them.

The Great Oxidation Event, as it’s known, happened when oxygen production overwhelmed the amount of nitrogen and carbon dioxide released into the atmosphere. This change in the planetary balance of oxygen producers and oxygen sinks also changed the land’s chemistry, allowing for the rapid growth of surface plants.

Oxygen levels rose and fell many times before this, but the most recent long-term spike—and the most successful—occurred about 2.4 billion years ago in the Archean era. Until then, the planet’s surface was covered by a thick layer of frozen methane called Snowball Earth.

But a layer of cyanobacteria—or blue-green algae—covered much of the frozen surface, producing the oxygen that kept the Earth warm and alive. It took a while for these tiny organisms to become so abundant that they began to dominate the global ecosystem.

After a billion years, they had reduced methane to oxygen and other gases that trapped heat. This helped the climate to cool, allowing for the formation of oceans and terrestrial environments where life eventually took hold.

But that life-supporting process needs more time. In the new study, scientists used computer simulations to predict how rapidly atmospheric oxygen levels drop. The team also compared their results to measurements from the past two hundred million years.

The research has implications for the search for life on other planets. As powerful telescopes scour the cosmos for potential signs of extraterrestrial life, these results will help keep astronomers from unnecessarily ruling out habitable planets that may not appear to have oxygen because their life forms rely on different mechanisms.

For example, bacteria can consume oxygen but can’t generate it themselves, so their presence in an environment can lead to a buildup of toxic nitrogen and phosphorous levels—often called eutrophication—which depletes the oxygen level of a body of water. These nutrient-rich environments can be challenging to clear of oxygen. But other signs of life can survive in such an environment.

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