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 (1σ). The model projects that a deoxygenation of the atmosphere, with atmospheric O2 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 CO2-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.
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Our Fortran source code is available at https://github.com/kazumi-ozaki/lifespan.
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We thank E. Tajika, Y. Sekine, S. Kadoya, Y. Watanabe, G. Arney, S. D. Domagal-Goldman and E. W. Schwieterman for helpful discussions. K.O. acknowledges support from the NASA Postdoctoral Program at the NASA Astrobiology Program, administered by Universities Space Research Association under contact with NASA. This work was supported by JSPS KAKENHI grant number JP20K04066. C.T.R. acknowledges support from the NASA Astrobiology Institute (grant number 13-13NAI7_2-0027). We acknowledge the NASA Nexus for Exoplanet System Science (NExSS) (grant number 80NSSC19KO461).
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Alexander Krause and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin; Rebecca Neely.
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a, The temperature dependence of terrestrial primary production (VT)48. b, The CO2 dependence of terrestrial primary production (VCO2). c, The UV irradiance dependence of terrestrial primary production (in terms of atmospheric O2) for different assumed values of cUV between 10−2.5 and 10−1.5 (fUV). d, The temperature dependence of marine primary producers (fTocn)24. The grey area represents the ranges for these factors that are explored by the stochastic approach.
a, The outgassing factor, fG. b, The erosion factor, fR. For the Phanerozoic, the time evolution of the original model48 was adopted, whereas the future evolution is explored by changing the amplitude and cycle (equations (34) and (35)). The grey region represents uncertain ranges for those factors which are explored by the stochastic approach.
Uniform prior distributions were assumed, except for uncertainties in OLR and TOA for which Gaussian probability density functions were assumed (1σ is listed here).
Extended Data Fig. 4 The typical simulation results showing the evolution of Earth’s biogeochemistry.
a, Solar luminosity (normalized by modern value). b, The chemical composition of the atmosphere. Blue, green, and orange lines represent O2, CO2 and CH4, respectively. c, Global surface temperature. d, Global net primary production (NPP). Solid line denotes the reference run (n = 0.73, m = 1, aG = aR = Jsorg = Jspy = JsFe-ox = Jscarb = JdCO2 = JdH2S = Jdred = ΔOLR = ΔTOA = 0, G* = 1250 Emol, C* = 5000 Emol, PYR* = 200 Emol, GYP* = 200 Emol, cUV = 0.01, Tref = 1400 K, Trefland = 25 °C, LIFE = 0.15, FERT = 0.4, ACT = 0.09, Pmin = 10 ppmv, RUNsil = 0.038, (Corg/Porg)anoxic = 20 × (Corg/Porg)oxic), whereas dashed (fG = 1, fR = 1), dotted (w/o land plants), dashed-dotted ((Corg/Porg)anoxic = 2 × (Corg/Porg)oxic), and gray (Constant solar luminosity) lines represent the sensitivity experiments (Parameters were set at the reference case, otherwise noted.). Phan = Phanerozoic.
Extended Data Fig. 5 The sensitivity of Earth’s biogeochemical evolution to the terrestrial weatherability.
Same as Extended Data Fig. 4, whereas dashed (ACT = 0.135) and dotted (ACT = 0.05) lines represent the sensitivity experiments (Parameters were set at the reference case, otherwise noted.). The future lifespan is largely insensitive to the uncertainty in the terrestrial weaherability because of the tradeoff between the impact of weatherability on atmospheric CO2 levels, global climate, and biospheric responses. Phan = Phanerozoic.
Extended Data Fig. 6 The default stochastic simulations showing the evolution of Earth’s biogeochemistry.
a, Solar luminosity (normalized by modern value). b, The chemical composition of the atmosphere. Blue, green, and orange lines represent O2, CO2 and CH4, respectively. c, Global surface temperature. d, Global net primary production (NPP). Phan = Phanerozoic.
Extended Data Fig. 7 The default Monte Carlo simulations showing the response of the net primary production (NPP).
Blue and green lines represent oceanic and terrestrial NPP, respectively.
Extended Data Fig. 8 The default Monte Carlo simulations showing the response of model reservoir sizes.
a, Oceanic P concentration, (b) oceanic SO42− level, (c) crustal organic carbon, (d) crustal carbonate carbon, (e) crustal pyrite sulphur, (f) crustal gypsum sulphur.
Extended Data Fig. 9 Evolution of atmospheric chemistry without terrestrial biosphere (color lines) compared with the default analysis (grey).
a, Atmospheric O2. b, CH4. c, CO2. d, CH4/CO2.
Extended Data Fig. 10 The dependency of the future lifespan of Earth’s oxygenated atmosphere (>1% PAL) on a series of key biogeochemical parameters.
a, Temperature factor controlling the activity of marine biosphere, Tref, (b) temperature factor controlling the activity level of terrestrial biosphere, Trefland, (c) minimum CO2 level for land plants, Pmin, (d) runoff factor, RUNsil, (e) activation energy factor, ACT, (f) Corg/Porg ratio of the buried anoxic sediments, (Corg/Porg)anoxic, (g) CO2 fertilization factor for land plants, FERT, (h) weathering factor for no-vegetation area, LIFE, (i) UV factor for terrestrial biosphere, cUV. The future lifespan is largely insensitive to these uncertain parameters (cf. Fig. 3b).
Numerical data for the future lifespan of Earth’s oxygenated atmosphere.
Numerical simulation data for the reference run and sensitivity experiments.
Numerical simulation data for the reference run and sensitivity experiments.
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Ozaki, K., Reinhard, C.T. The future lifespan of Earth’s oxygenated atmosphere. Nat. Geosci. 14, 138–142 (2021). https://doi.org/10.1038/s41561-021-00693-5