Presently, Earth thrives as a bastion of life, nourished by an atmosphere abundant in oxygen. However, historical geological evidence suggests that this is a transient condition—scientific projections indicate that, given the inexorable passage of time, our planet’s atmosphere may once again become enriched in methane while dwindling in oxygen concentration.

Though this eventuality is projected to occur in approximately a billion years, analyses conducted in 2021 indicate that such a transformation is likely to transpire with alarming rapidity.

This atmospheric shift could revert Earth to a primordial state reminiscent of the conditions preceding the Great Oxidation Event (GOE), occurring roughly 2.4 billion years ago.

Environmental scientist Kazumi Ozaki of Toho University elucidates, “For many years, discussions regarding the lifespan of Earth’s biosphere have been anchored in scientific insights concerning the Sun’s gradual luminosity increase and the global carbonate-silicate geochemical cycle.”

Ozaki further reflects, “A significant corollary of this theoretical framework posits a consistent decline in atmospheric CO₂ levels, concomitant with global warming over geological epochs.”

The researchers assert that atmospheric oxygen likely does not constitute a permanent fixture of habitable worlds. This realization bears considerable implications for our endeavors to discern potential biosignatures of extraterrestrial life.

The researchers posit that the deoxygenation of the atmosphere—wherein oxygen levels plummet to those analogous to the Archaean eon—will likely precede the onset of moist greenhouse conditions within Earth’s climate system, as well as the extensive evaporation of surface water.

At that juncture, the extinction of humans and the vast majority of aerobic life forms will be inevitable, underscoring the urgency of our need to explore beyond our planetary confines.

To arrive at these conclusions, the research team meticulously simulated Earth’s biosphere, incorporating variables such as solar luminosity and subsequent decreases in carbon dioxide levels resulting from escalating temperatures.

This reduction in carbon dioxide will diminish the proliferation of photosynthetic organisms, causing a subsequent decline in atmospheric oxygen.

While previous studies suggested that heightened solar radiation would lead to the evaporation of Earth’s oceans within approximately two billion years, the current model, which amalgamated nearly 400,000 simulations, forecasts that oxygen depletion will precipitate mass extinctions long before such climate extremes manifest.

“The impending decrease in oxygen levels is extraordinarily drastic,” asserts Chris Reinhard, an Earth scientist at the Georgia Institute of Technology. “We anticipate levels could plummet to around one-millionth of current concentrations.”

This research holds profound relevance for contemporary astrobiological studies. As telescopes become increasingly advanced, discerning the myriad data collected will prove critical.

The researchers advocate for the exploration of alternative biosignatures beyond oxygen, positing that this approach may enhance our prospects for identifying extraterrestrial life. Their study contributes to NASA’s NExSS (Nexus for Exoplanet System Science) initiative, which strives to unravel the complexities of planetary habitability beyond our own.

Calculations suggest that the epoch of an oxygen-rich Earth could span merely 20-30 percent of the planet’s overall lifespan, with microbial organisms persevering long after more complex life forms have perished.

Ozaki poignantly remarks, “Following the impending great deoxygenation, the atmosphere will be characterized by elevated levels of methane, diminished CO₂, and the absence of a protective ozone layer.”

He foresees a world predominantly inhabited by anaerobic life forms.

This significant research was published in Nature Geoscience.

An earlier version of this article was first published in March 2021.