JAPAN, June 2: Earth’s atmosphere, currently rich in oxygen and capable of supporting complex life, won’t remain this way forever. Scientists predict that in about a billion years, the planet will undergo a dramatic shift — returning to a state rich in methane and extremely low in oxygen, similar to conditions before the Great Oxidation Event (GOE) roughly 2.4 billion years ago.

Although this transformation is far off, when it begins, the change will happen relatively quickly, according to a 2021 study by environmental scientist Kazumi Ozaki of Toho University in Japan and Earth scientist Chris Reinhard of the Georgia Institute of Technology.

“For many years, the lifespan of Earth’s biosphere has been discussed based on scientific knowledge about the steadily brightening Sun and global carbonate-silicate geochemical cycles,” said Ozaki. “One consequence of this framework is a steady decline in atmospheric CO₂ and eventual global warming over geological timescales.”

Using detailed models of Earth's biosphere — factoring in the Sun’s increasing luminosity and corresponding drops in atmospheric CO₂ — the researchers determined that lower carbon dioxide levels will lead to fewer photosynthesizing organisms. This would significantly reduce the production of oxygen, ultimately pushing atmospheric oxygen levels to a point roughly a million times lower than today.

“The drop in oxygen is very, very extreme,” Reinhard told New Scientist. “We're talking around a million times less oxygen than there is today.”

According to the study, this atmospheric shift is likely to occur before Earth reaches a "moist greenhouse" state, where water vapor dominates the upper atmosphere and the planet begins to lose its surface water. This means that oxygen-dependent life will likely die out long before Earth becomes too hot for oceans to exist.

Despite its long timeline, the study has pressing implications for astrobiology. The findings suggest that atmospheric oxygen may not be a permanent feature of habitable planets. This challenges the current focus on oxygen as a key biosignature in the search for life on exoplanets.

The research, part of NASA’s NExSS (Nexus for Exoplanet System Science) project, highlights the importance of considering alternative biosignatures in the hunt for extraterrestrial life.

“The atmosphere after the great deoxygenation will be characterized by high methane, low CO₂, and no ozone layer,” said Ozaki. “Earth will likely become a world dominated by anaerobic life forms.”

According to the team’s models—based on nearly 400,000 simulations—the oxygen-rich phase of Earth’s habitable era may represent only 20 to 30 percent of the planet’s total lifespan. While human and other oxygen-dependent life will eventually vanish, microbial life adapted to low-oxygen conditions is expected to persist long after.