Humans haven’t been great for the health of the planet, but even if we pollute ourselves into extinction, Earth will continue on. It’s survived enormous asteroid impacts and megavolcanoes, after all. A few primates aren’t going to do worse in the long-run. The ultimate fate of life on Earth lies a billion years in the future. A new study supported by NASA’s exoplanet habitability research lays out how the sun will eventually bake the planet, turning Earth from a lush, oxygen-rich world to a dried-up husk with no complex life.
NASA is interested in the future of Earth because it’s the only habitable planet we can study up close. As such, scientists have attempted to extrapolate the properties of Earth-like planets we might be able to detect from great distances. Kazumi Ozaki at Toho University in Japan and Chris Reinhard at the Georgia Institute of Technology created a model of Earth’s climate, biology, and geology to see how it will change.
According to Ozaki and Reinhard, Earth’s oxygenated atmosphere is not a permanent feature. There was very little of it in the atmosphere until 2.4 billion years ago when cyanobacteria evolved to absorb carbon dioxide and expel oxygen — this is known as the Great Oxidation Event. This gave rise to all the forms of multicellular life we see on Earth today. There’s just one problem: the Sun. As stars age, they get hotter, and the Sun is about a billion years from roasting Earth.
The study predicts that in a billion years, the Sun will become so hot that it breaks down carbon dioxide. The levels of CO2 will become so low that photosynthesizing plants will be unable to survive, and that means no more oxygen for the rest of us. When that happens, the changes will be abrupt. Ozaki and Reinhard say in the study, published in Nature Geoscience, that it could take a little as 10,000 years for oxygen levels to drop to a millionth of what it is now. That’s a blink of the eye in geological terms. Methane levels will also begin to rise, reaching 10,000 times the level seen today.
Cyanobacteria like these oxygenated the atmosphere, but the era of oxygen may be fleeting.
This harsh, choking atmosphere will be incompatible with any multicellular life as it exists today. The globe will be given over to bacteria and archaea, the heartiest of living organisms to see the planet through the rest of its existence until it’s swallowed by the Sun. Even if more complex life did survive, it would be irradiated by the increasingly luminous Sun. Without oxygen, the ozone layer will evaporate and expose the surface to more intense UV radiation.
Ozaki and Reinhard conclude that oxygen is an important biomarker, but it may not be a permanent feature of planets with life. That could change how we categorize exoplanets going forward — even without oxygen, there could be plenty of single-celled life.
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