The future lifespan of Earth’s oxygenated atmosphere

• Published in: Nature geoscience 2021-03, Vol.14 (3), p.138-142
• Main Authors: Ozaki, Kazumi, Reinhard, Christopher T.
• Format: Article
• Language: English
• Subjects: 704/445/209; 704/445/824; 704/445/862; 704/47/4112; 704/47/4113; Anoxia; Atmosphere; Atmospheric conditions; Biogeochemistry; Biosphere; Carbon dioxide; Carbonates; Climate; Climate models; Climate system; Deoxygenation; Earth; Earth and Environmental Science; Earth mantle; Earth Sciences; Earth System Sciences; Extrasolar planets; Geochemistry; Geology; Geophysics/Geodesy; Habitability; Haze; Life span; Longevity; Oceanic crust; Oxygen; Oxygenation; Planetary atmospheres; Silicates; Solar flux; Stochasticity; Surface water
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-021-00693-5

Earth’s modern atmosphere is highly oxygenated and is a remotely detectable signal of its surface biosphere. However, the lifespan of oxygen-based biosignatures in Earth’s atmosphere remains uncertain, particularly for the distant future. Here we use a combined biogeochemistry and climate model to examine the likely timescale of oxygen-rich atmospheric conditions on Earth. Using a stochastic approach, we find that the mean future lifespan of Earth’s atmosphere, with oxygen levels more than 1% of the present atmospheric level, is 1.08 ± 0.14 billion years (1 σ ). The model projects that a deoxygenation of the atmosphere, with atmospheric O 2 dropping sharply to levels reminiscent of the Archaean Earth, will most probably be triggered before the inception of moist greenhouse conditions in Earth’s climate system and before the extensive loss of surface water from the atmosphere. We find that future deoxygenation is an inevitable consequence of increasing solar fluxes, whereas its precise timing is modulated by the exchange flux of reducing power between the mantle and the ocean–atmosphere–crust system. Our results suggest that the planetary carbonate–silicate cycle will tend to lead to terminally CO 2 -limited biospheres and rapid atmospheric deoxygenation, emphasizing the need for robust atmospheric biosignatures applicable to weakly oxygenated and anoxic exoplanet atmospheres and highlighting the potential importance of atmospheric organic haze during the terminal stages of planetary habitability. Earth’s oxygen-rich atmosphere will probably persist for only one billion more years before it sharply deoxygenates to low-level oxygen similar to those of the Archaean, according to a combined biogeochemistry and climate model.