Maximum Lifetime of the Vegetative Biosphere

TL;DR

This study models Earth's future climate to estimate the maximum duration the vegetative biosphere can persist under different atmospheric and thermal scenarios, considering biological and technological adaptations.

Contribution

It introduces a three-dimensional climate model to refine predictions of Earth's biosphere lifetime, highlighting the impact of weathering and alternative photosynthesis pathways.

Findings

Vegetative biosphere could last up to 1.84 Gyr under certain CO₂ starvation limits.

Strong weathering keeps surface temperature stable but reduces CO₂, affecting biosphere longevity.

Weak weathering leads to higher temperatures, limiting plant survival before the greenhouse threshold.

Abstract

We use a three-dimensional model to calculate steady-state climates at various intervals in Earth's future, across a parameter space of increasing insolation and decreasing CO$_2$ mixing ratio. Comparison with prior results shows an overestimation of warming by one-dimensional models when solar constant is increased and CO$_2$ mixing ratio is fixed. We consider two future trajectories as limiting cases: strong weathering, in which surface temperature remains constant but CO$_2$ is drawn down; and weak weathering, in which CO$_2$ remains constant and surface temperature increases. Under strong weathering, we find the conventional 10 ppm CO$_2$ starvation limit for C4 photosynthesis occurs at 1.35 Gyr; however, we suggest that crassulacean acid metabolism (CAM) photosynthesis could persist below this limit and note that aquatic macrophytes can utilize dissolved bicarbonate if atmospheric CO$_2$ is low. If we take the CO$_2$ starvation limit at 1 ppm instead, then the vegetative biosphere could continue until 1.84 Gyr. Thermal limits apply instead under weak weathering, in which Earth would be too hot for most land plants at 1.68 Gyr (>323 K) and too hot for all land plants (>338 K) at 1.87 Gyr. These lifetimes approach the moist and runaway greenhouse limits for Earth. We discuss other possible mechanisms for extending the lifetime of Earth's biosphere, noting that both technological intervention and evolutionary processes could enable life to adapt to a brightening sun.