Seafloor Weathering and Stochastic Outgassing Unlikely to Significantly Shorten the Future Lifespan of Earth's Terrestrial Biosphere

TL;DR

This paper addresses how long Earth's terrestrial biosphere can persist as the Sun brightens, by incorporating seafloor weathering and stochastic CO$_2$ outgassing into a coupled climate–carbonate-silicate cycle model. It extends the framework of prior work by including seafloor feedback via $W_S$ and an Ornstein–Uhlenbeck process for outgassing, exploring ranges such as $T_{e,S}\in[6.5,62]$ K, $\beta_S\in[0.03,0.36]$, and $\alpha\in[0.027,0.184]$. Key results show that without seafloor weathering the remaining lifespan is about $1.86$ Gyr, while a strong seafloor feedback can shorten this by ~0.3 Gyr and shift the kill mechanism from overheating to CO$_2$ starvation; stochastic outgassing can further shorten lifespans when variability is large, though this is not always the dominant effect. Overall, the findings suggest the terrestrial biosphere’s extended lifespan is robust to many Earth-like planetary parameters, though seafloor weathering and large outgassing fluctuations remain important constraints for exoplanet habitability predictions.

Abstract

Current understanding suggests that as the Sun brightens in the far future, Earth's carbonate-silicate cycle will offset increasing temperatures by drawing CO$_2$ out of the atmosphere, ultimately leading to the extinction of all terrestrial plant life via either overheating or CO$_2$ starvation. Most previous estimates put the future lifespan of Earth's terrestrial biosphere at $\sim$1 billion yr, but recent work used a new coupled climate-continental weathering model with up-to-date parameter constraints to revise this estimate upward to 1.6-1.86 billion yr. In this study, we extend the model to examine the impacts of seafloor weathering and stochastic variations in CO$_2$ outgassing rates on the remaining lifespan of Earth's terrestrial biosphere. We find that if seafloor weathering has a stronger feedback than continental weathering and accounts for a large portion of global silicate weathering, then the remaining lifespan of the terrestrial biosphere can be shortened, but a lifespan of more than 1 billion yr (Gyr) remains likely. Similarly, stochastic fluctuations in outgassing rates can have a significant impact if the size of the fluctuations exceed those observed over the last 1 billion yr. The impact of weak seafloor weathering and lower variability stochasticity are minor. Our work provisionally supports a lengthened lifespan of Earth's terrestrial biosphere, suggests robustness of this lengthened lifespan to planetary parameters that may vary among exoplanets, and identifies seafloor weathering as a key process that requires further study and constraint.

Figures (4)

• Figure 1: The OU stochastic outgassing model output is qualitatively consistent with outgassing reconstructions within the last 1 billion years. The 5 dotted lines in each subplot represent instances of our model output, and the three solid lines represent the reconstructions of mills2021spatial, marcilly2021new, and muller2024degassing. The parameters used for the model are $\tau = 10^{9}$ and $\sigma = 10^{-4}$ on the left hand side, and $\tau = 10^{9}$ and $\sigma = 3 \times 10^{-4}$ on the right hand side. The relative concentration of the simulated outgassing patterns between the reconstructions of mills2021spatial and muller2024degassing on the left suggests that larger variability is likely. The right subplot examines variability more consistent with the reconstructions.
• Figure 2: Strong seafloor weathering shortens the lifespan of the terrestrial biosphere by nearly 300 million years and shifts the biosphere kill mechanism from overheating to CO$_2$ starvation, whereas weak seafloor weathering has little effect. Stochastic CO$_2$ outgassing does not significantly impact results. Time series of surface temperature (A), atmospheric CO$_2$ (B), and weathering rates (C) with a strong seafloor weathering feedback ($T_\mathrm{e,S} = 9.07\ K < T_\mathrm{e,L}$, left column) and a weak seafloor weathering feedback ($T_\mathrm{e,S} = 62\ K > T_\mathrm{e,L}$, right column). Results without seafloor weathering (dotted lines), without stochastic CO$_2$ outgassing (dark colored lines), and for different realizations of stochastic CO$_2$ outgassing (faint colored lines) are shown in each panel. Other parameter values are $\tau = 10^9$, $\sigma = 10^{-4}$, $\beta_\mathrm{S} = 0.20$, $\alpha$ = 0.075, $\beta_\mathrm{L}=0.41$, and $T_\mathrm{e,L}=31$ K.
• Figure 3: Stochastic CO$_2$ outgassing has a major effect on the future lifespan of the terrestrial biosphere when $\sigma \ge 3\times10^{-4}$ Tmol Year$^{-1}$. CDF displaying the proportion of simulations that have reached biosphere death over time. Six combinations of $\tau$ and $\sigma$ are explored, each featuring ensembles of realizations of stochastic CO$_2$ outgassing of size N=10000. With no stochasticity, the expected future lifespan is 1.50 billion years. Other parameter values are $\beta_\mathrm{S} = 0.20$, $T_\mathrm{e,S}=9.07$ K, $\alpha$ = 0.075, $\beta_\mathrm{L}=0.41$, and $T_\mathrm{e,L}=31$ K. This corresponds to the strong seafloor weathering feedback regime that leads to the CO$_2$ starvation kill mechanism.
• Figure 4: The future lifespan of the terrestrial biosphere is only significantly shortened if seafloor weathering accounts for a large portion of global silicate weathering (large $\alpha$) and has a strong feedback (small $T_\mathrm{e,S}$). Contour plots of the future lifespan of the terrestrial biosphere as a function of the temperature (${T\mathrm{_{e,S}}}$) and CO$_2$ ($\beta_{\mathrm{S}}$) dependencies of seafloor weathering. The dark points denote the median values of $T_\mathrm{e,S}$ and $\beta_\mathrm{S}$ from krissansen2017constraining, and the middle panel corresponds to the median value of $\alpha$. The white dotted lines denote the temperature dependence of continental weathering (${T\mathrm{_{e,L}}}$).