Take a deep breath in and savor the moment, because it isn't going to last. A new study estimates that Earth’s oxygen will only last for around 1 billion years, spelling catastrophe for much of the life that inhabits the planet.
As reported in the journal Nature Geoscience, two scientists have predicted that Earth’s oxygen-rich atmosphere will return to an oxygen-poor, methane-rich composition – much like the conditions seen in early Earth before the Great Oxidation Event around 2.4 billion years ago. Kazumi Ozaki and Christopher Reinhard reached this conclusion by modeling a number of Earth’s systems, including climate and biological and geological processes, to examine the timescale of the atmospheric conditions on Earth.
Life on Earth is made possible thanks to our planet’s highly oxygenated atmosphere, which is fertile ground for the photosynthetic organisms that act as a cornerstone for much of the planet’s life. However, the new estimates highlight how this oxygenated atmosphere – as well as oxygen-dependent life – is unlikely to be a permanent feature of Earth.
"The atmosphere after the great deoxygenation is characterized by an elevated methane, low-levels of CO2, and no ozone layer. The Earth system will probably be a world of anaerobic life forms," Kazumi Ozaki, study author and Assistant Professor at Toho University in Tokyo, said in a statement.
These changes will take place on a geological timescale of millions and billions of years, so they shouldn’t be confused with the sharp human-driven changes to Earth’s atmosphere we’ve seen in the past century.
One of the main forces thought to drive this long-term change in oxygen levels is the gradual brightening of the sun. As this heats up the planet, the researchers argue that it will eventually decrease the amount of carbon dioxide in the atmosphere, since CO2 absorbs heat and breaks down. Eventually, plants and other photosynthesizing organisms won’t have enough CO2 to produce oxygen, depleting the atmosphere of this vital gas.
"For many years, the lifespan of Earth's biosphere has been discussed based on scientific knowledge about the steadily brightening of the sun and global carbonate-silicate geochemical cycle,” explains Ozaki.
“One of the corollaries of such a theoretical framework is a continuous decline in atmospheric CO2 levels and global warming on geological timescales. Indeed, it is generally thought that Earth's biosphere will come to an end in the next 2 billion years due to the combination of overheating and CO2 scarcity for photosynthesis,” adds Ozaki. "If true, one can expect that atmospheric O2 levels will also eventually decreases in the distant future. However, it remains unclear exactly when and how this will occur.”
This new estimation doesn’t just have implications for life on Earth, but could also guide our understanding of life on other planets. Oxygen and its associated byproducts are some of the key biosignatures used by scientists to sniff out potentially habitable exoplanets elsewhere in the universe. However, as this new study shows, even the habitat home of Earth might not be permanently rich in oxygen, suggesting we may need to widen our perspectives when looking for life beyond our solar system.