Graham, Halevy & Abbot (2024) argued, with coupled global-mean models of C3/C4 plant productivity, silicate weathering, and climate, that Earth's land biosphere will not die of CO2_2 starvation in ~1 Gyr as previously thought, but of overheating at ~1.6-1.86 Gyr, potentially persisting until the onset of the moist greenhouse. Their verdict is explicitly conditional: across the full range of planetary weathering parameters tested by Graham, Halevy & Abbot (2024) (e-folding temperature Te≈12T_{\mathrm{e}} \approx 12-48 K, CO2_2 exponent β≈0.05\beta \approx 0.05-0.91) the kill mechanism flips between CO2_2 starvation and overheating, and their zero-dimensional climate with prescribed Bond albedo omits cloud, hydrological, vegetation-albedo, CAM-plant, refugia, soil-respiration, and geodynamic-outgassing feedbacks. A 2025 follow-up by the same group (seafloor weathering and stochastic outgassing) and a 2026 3-D GCM study that brackets strong- and weak-weathering end-member trajectories both corroborate a substantially lengthened lifespan (order 1-1.9 Gyr from now) yet still bracket rather than constrain the actual trajectory. The open problem is therefore to determine the remaining lifespan and extinction mechanism of the complex terrestrial biosphere with a constrained planetary weathering response and/or a self-consistently coupled higher-dimensional climate-carbon-vegetation model, quantifying the sensitivity of the verdict to each unresolved input.

Background

As the Sun brightens along the main sequence (roughly 10% per billion years), Earth's long-term climate is governed by the carbonate-silicate cycle: atmospheric CO2_2 is consumed by chemical weathering of silicate rocks, whose rate increases with temperature and with soil CO2_2, and is returned by volcanic and metamorphic outgassing. This cycle acts as a planetary thermostat on million-year timescales. The canonical view since Lovelock & Whitfield (1982) and Caldeira & Kasting (1992) was that the thermostat becomes the biosphere's executioner: a warmer Earth weathers faster, atmospheric CO2_2 falls below the minimum level required by vascular land plants (the CO2_2 compensation point, the level at which photosynthesis just balances respiration) about 0.5-1 Gyr from now, and land plants - and the animals they feed - die of CO2_2 starvation.

The planetary weathering rate is usually parameterized as W∝exp⁡[(T−Tref)/Te] psoilβW \propto \exp[(T - T_{\mathrm{ref}})/T_{\mathrm{e}}]\,p_{\mathrm{soil}}^{\beta}, where TeT_{\mathrm{e}} is an e-folding temperature (the temperature rise that multiplies the weathering rate by e≈2.7e \approx 2.7) and β\beta is the power-law exponent of the dependence on soil CO2_2 partial pressure psoilp_{\mathrm{soil}}, which plants raise above the atmospheric level by below-ground respiration. Most previous studies assumed strong temperature dependence (Te≈10T_{\mathrm{e}} \approx 10-20 K) and weak CO2_2 dependence (β≈0.25\beta \approx 0.25-0.5), but recent field, proxy, laboratory, and modeling work (e.g., Maher & Chamberlain 2014; Krissansen-Totton & Catling 2017; Winnick & Maher 2018; Brantley et al. 2023) favors much weaker temperature dependence (Te∼30T_{\mathrm{e}} \sim 30-40 K), while one study of clay-alteration degrees (Deng et al. 2022) finds a strong dependence (Te∼12.1T_{\mathrm{e}} \sim 12.1 K); the CO2_2 dependences allowed by the available data span β≈0.2\beta \approx 0.2-0.9.

Graham, Halevy & Abbot (2024) coupled global-mean (zero-dimensional, 0-D) models of C3 and C4 plant productivity (the two dominant photosynthetic metabolisms; C4 plants, which concentrate CO2_2 internally near the Rubisco fixation enzyme, survive to far lower atmospheric CO2_2 than C3 plants), silicate weathering, and a simple radiative-convective climate. With weakly temperature-dependent and/or strongly CO2_2-dependent weathering, falling plant productivity slows, halts, and can temporarily reverse the luminosity-driven CO2_2 decline, averting CO2_2 starvation; land plants instead die of overheating at ~1.6-1.86 Gyr, using a revised upper thermal tolerance for photosynthesis of 338 K (thermophilic C4 desert plants photosynthesize well above the traditionally assumed 323 K limit). This would let the complex land biosphere persist until near the moist greenhouse transition (the climate state in which a warm stratosphere loses water to space) or even the runaway greenhouse (irreversible ocean loss), and it feeds into the 'hard steps' debate about how many improbable transitions produced intelligent life within Earth's habitable window.

The authors themselves flag what keeps the conclusion provisional: (i) the kill mechanism flips between CO2_2 starvation and overheating across the still-permitted TeT_{\mathrm{e}}-β\beta range, and constraining these parameters at the planetary scale is open; (ii) the 0-D climate with prescribed constant Bond albedo (the fraction of incident sunlight reflected to space, fixed at 0.29) omits spatially heterogeneous cloud, albedo, and hydrological feedbacks - and 3-D general circulation models (GCMs) disagree among themselves by tens of kelvin at high insolation (e.g., Leconte et al. 2013 versus Wolf & Toon 2015); (iii) CAM photosynthesis (crassulacean acid metabolism, used by ~6% of plant species including cacti and agaves, which take up CO2_2 at night and refix it by day with stomata closed, an adaptation to aridity) is excluded, as are explicit water cycling and the polar and high-altitude refugia that could host land life after the global-mean temperature passes the plant limit (O'Malley-James et al. 2013); (iv) plant environmental tolerances may be evolutionary optima rather than hard physical limits, and plants could evolve further carbon-concentrating innovations over Gyr timescales; (v) the soil-CO2_2 parameterization assumes organic decay rates that are not strongly temperature-dependent, which may be wrong; and (vi) outgassing and weatherable land fraction were held constant or scanned, not coupled to geodynamic evolution. Related far-future work on the marine and microbial biosphere (e.g., Ozaki & Reinhard 2021 on the decline of atmospheric O2_2) is connected but does not address the weathering-response question for land plants.

Problem Statement

Determine the remaining lifespan and the extinction mechanism of Earth's complex terrestrial biosphere (vascular land plants and the organisms they sustain) as the Sun brightens, replacing the conditional answer of Graham, Halevy & Abbot (2024) with a constrained one. Concretely, resolve either or both of the following: (1) the planetary-scale temperature and CO2_2 dependences of global silicate weathering, TeT_{\mathrm{e}} and β\beta (or an equivalent mechanistic parameterization), sufficient to decide whether the future atmospheric CO2_2 trajectory falls in the CO2_2-starvation regime or the overheating regime, and to state the resulting remaining lifespan (or a defensible quantified bound on it); (2) the trajectory, kill mechanism, and lifespan computed with a coupled climate-carbon-vegetation model whose configuration resolves the following set of processes - 3-D climate with interactive land and dynamic vegetation, an explicit water cycle, C3/C4 and CAM photosynthesis, spatially resolved refugia, temperature-dependent soil respiration and organic decay affecting soil pCO2p_{\mathrm{CO_2}}, and time-evolving geodynamic outgassing and weatherable land fraction - with the sensitivity of the verdict to each remaining uncertainty quantified. Establish the kill mechanism and remaining lifespan - or a defensible quantified bound that improves on the range already implied by the cited prior work - together with how the conclusion depends on the unresolved inputs. Gyr-scale plant evolutionary adaptation may be treated as a one-sided modifier that can only extend the lifespan, bounded above by the onset of ocean loss, rather than as a separately predictable quantity.

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