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Earth's modern atmosphere is highly oxygenated and is a remotely detectablesignal of its surface biosphere. However, the lifespan of oxygen-basedbiosignatures in Earth's atmosphere remains uncertain, particularly for thedistant future. Here we use a combined biogeochemistry and climate model toexamine the likely timescale of oxygen-rich atmospheric conditions on Earth.Using a stochastic approach, we find that the mean future lifespan of Earth'satmosphere with oxygen levels more than 1% of the present atmospheric level is1.08+-0.14 billion years. The model projects that a deoxygenation of theatmosphere, with atmospheric oxygen dropping sharply to levels reminiscnet ofthe Archaean Earth, will most probably be triggered before the inception ofmoist greenhouse conditions in Earth's climate system and before the extensiveloss of surface water from the atmosphere. We find that future deoxygenation isan inevitable consequence of increasing solar fluxes, whereas its precisetiming is modulated by the exchange flux of reducing power between the mantleand the ocean-atmosphere-crust system. Our results suggest that the planetarycarbonate-silicate cycle will tend to lead to terminally CO2-limited biospheresand rapid atmospheric deoxygenation, emphasizing the need for robustatmospheric biosignatures applicable to weakly oxygenated and anoxic exoplanetatmospheres and highlighting the potential importance of atmospheric organichaze during the terminal stages of planetary habitability.

The future lifespan of Earth's oxygenated Atmosphere