Substantial extension of the lifetime of the terrestrial biosphere (2409.10714v1)

Abstract: Approximately one billion years (Gyr) in the future, as the Sun brightens, Earth's carbonate-silicate cycle is expected to drive CO$_2$ below the minimum level required by vascular land plants, eliminating most macroscopic land life. Here, we couple global-mean models of temperature- and CO$_2$-dependent plant productivity for C$_3$ and C$_4$ plants, silicate weathering, and climate to re-examine the time remaining for terrestrial plants. If weathering is weakly temperature-dependent (as recent data suggest) and/or strongly CO$_2$-dependent, we find that the interplay between climate, productivity, and weathering causes the future luminosity-driven CO$_2$ decrease to slow and temporarily reverse, averting plant CO$_2$ starvation. This dramatically lengthens plant survival from 1 Gyr up to $\sim$1.6-1.86 Gyr, until extreme temperatures halt photosynthesis, suggesting a revised kill mechanism for land plants and potential doubling of the future lifespan of Earth's land macrobiota. An increased future lifespan for the complex biosphere may imply that Earth life had to achieve a smaller number of ``hard steps'' (unlikely evolutionary transitions) to produce intelligent life than previously estimated. These results also suggest that complex photosynthetic land life on Earth and exoplanets may be able to persist until the onset of the moist greenhouse transition.

• The paper demonstrates that alternative silicate weathering feedbacks may extend terrestrial plant life by up to 1.86 billion years.
• The study employs global-mean models to simulate interactions between temperature, CO2 levels, and plant metabolism, emphasizing both C3 and C4 types.
• It contrasts the traditional CO2 starvation hypothesis with an overheating mechanism that temporarily sustains photosynthesis before thermal limits are reached.

Extension of the Lifetime of the Terrestrial Biosphere

The paper presented by Graham, Halevy, and Abbot explores the impact of temperature and atmospheric CO$_2$ on the future longevity of terrestrial plant life, specifically considering the interaction between the silicate weathering feedback and the carbon cycle. Approximately one billion years ahead, as solar luminosity increases, the typical expectation is a reduction in atmospheric CO$_2$ concentrations below levels necessary for plant survival due to the carbonate-silicate cycle. This paper challenges that notion by proposing alternative climatic scenarios that may extend the persistence of land-based macrobiota.

Core Analysis and Methodology

The authors utilize global-mean models to predict the relationship between climate variables, CO$_2$ levels, and plant productivity. This comprehensive modeling considers C$_3$ and C$_4$ plant metabolism, silicate weathering, and climate dynamics. Notably, the paper posits two potential outcomes based on the varied dependencies of weathering rates on temperature and CO$_2$: the traditional hypothesis of CO$_2$ starvation and a novel mechanism involving eventual overheating.

1. CO$_2$ Starvation: Traditionally accepted as a likely endpoint for macroscopic plant life, this scenario arises from a continued decline of atmospheric CO$_2$, driven by silicate weathering as temperatures rise. The hypothesis suggests a decrease in plant productivity leading to biotic crises and eventual extinction.
2. Overheating Hypothesis: Under this scenario, the decreased rate of silicate weathering at high temperatures may stabilize or even increase atmospheric CO$_2$ levels temporarily. This effect could halt or reverse CO$_2$ decreases, keeping photosynthesis viable until temperatures become prohibitively high for plant metabolism.

Key Results

• Extended Lifespan: The paper suggests that if silicate weathering proves to be weakly temperature-dependent or strongly CO$_2$-dependent, terrestrial plant life could extend its future lifespan by up to 1.86 billion years, substantially longer than previous estimates.
• Shift in Extinction Methodology: Overheating is proposed as a probable kill mechanism, an assertion based on revised assumptions on plant thermal limits and emerging data on silicate weathering kinetics.
• Importance of C$_4$ Plants: The authors emphasize the role of C$_4$ plants in enduring low CO$_2$ levels, pivotal in determining the timeline of future biospheric functionality.

Implications and Speculations

Practically, the potential extension of complex life scenarios suggests a range of conditions under which terrestrial plant life might adapt or be sustained. Theoretically, this challenges long-held predictions about planetary lifespan limits and the interactions governing Earth's biosphere. It invites further inquiries into planetary climate models and a reevaluation of life's resilience on Earth-like exoplanets.

The paper prompts curiosity regarding astrobiological pursuits, particularly concerning life detection and understanding of habitability thresholds on exoplanets. The notion that Earth's biosphere's persistence might pivot on precise temperature and CO$_2$ dynamics inevitably leads to broader questions about life's adaptability under variable planetary conditions, offering a fresh perspective on the probability and nature of intelligent life in the universe.

Future Directions

This work beckons a closer look at biogeochemical models integrating advanced climate projections and enhanced silicate weathering mechanisms to predict planetary futures. Such interdisciplinary integration will be crucial in refining our understanding of biospheric longevity as contingent on juxtaposed climate factors. Additionally, expanding inquiry into how terrestrial thresholds, if adaptive or inherent, are mirrored or recalibrated on exoplanets can further elucidate astrobiological paradigms on life's tenacity.

In conclusion, Graham et al. provide a meticulously modeled, data-backed reevaluation of Earth's biotic resilience under a brightening sun, deserving of further investigation across scientific domains.