Runaway climate cooling of ocean planets in the habitable zone: a consequence of seafloor weathering enhanced by melting of high-pressure ice
Pith reviewed 2026-05-25 11:32 UTC · model grok-4.3
The pith
Ocean planets with massive oceans develop extremely cold CO2-poor climates in the habitable zone due to seafloor weathering enabled by high-pressure ice melting.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The climates of terrestrial planets with massive oceans lapse into extremely cold ones or snowball states with CO2-poor atmospheres. This occurs because HP ice melting fixes seafloor temperature at the melting temperature, keeping a high weathering flux regardless of surface temperature. Ocean planets with oceans several tens of the Earth's ocean mass no longer maintain temperate climates.
What carries the argument
High-pressure ice melting near mid-ocean ridges, which enables seafloor weathering by fixing the seafloor temperature at the melting point and sustaining high carbon removal rates independent of surface conditions.
If this is right
- Planets with ocean masses several tens of times Earth's cannot sustain temperate climates.
- Extremely cold climates with low CO2 can exist on water-rich planets inside the habitable zone.
- The carbonate-silicate geochemical carbon cycle is controlled by seafloor weathering fixed by HP ice melting.
- Such cold states become the default outcome for massive ocean planets.
Where Pith is reading between the lines
- Atmospheric observations of water-rich exoplanets could test for low CO2 levels as predicted.
- Models assuming no plate tectonics would need to account for different heat flux distributions.
- Planet formation theories predicting many water-rich planets imply many cold habitable-zone worlds.
Load-bearing premise
The heat flux near the mid-ocean ridge is high enough to melt the high-pressure ice and enable seafloor weathering that dominates the carbon cycle.
What would settle it
Finding a water-rich planet in the habitable zone with a thick CO2 atmosphere would contradict the prediction of rapid CO2 removal.
read the original abstract
Terrestrial planets covered globally with thick oceans (termed ocean planets) in the habitable zone were previously inferred to have extremely hot climates in most cases. This is because ${\rm H_2O}$ high-pressure (HP) ice on the seafloor prevents chemical weathering and, thus, removal of atmospheric CO$_2$. Previous studies, however, ignored melting of the HP ice and horizontal variation in heat flux from oceanic crusts. Here we examine whether high heat fluxes near the mid-ocean ridge melts the HP ice and thereby removes atmospheric ${\rm CO_2}$. We develop integrated climate models of an Earth-size ocean planet with plate tectonics for different ocean masses, which include the effects of HP ice melting, seafloor weathering, and the carbonate-silicate geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge is high enough to melt the ice, enabling seafloor weathering. In contrast to the previous theoretical prediction, we show that climates of terrestrial planets with massive oceans lapse into extremely cold ones (or snowball states) with CO$_2$-poor atmospheres. Such extremely cold climates are achieved mainly because the HP ice melting fixes seafloor temperature at the melting temperature, thereby keeping a high weathering flux regardless of surface temperature. We estimate that ocean planets with oceans several tens of the Earth's ocean mass no longer maintain temperate climates. These results suggest that terrestrial planets with extremely cold climates exist even in the habitable zone beyond the solar system, given the frequency of water-rich planets predicted by planet formation theories.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops integrated climate models for Earth-sized ocean planets with plate tectonics and varying ocean masses. These models incorporate high-pressure (HP) ice melting, seafloor weathering, and the carbonate-silicate geochemical carbon cycle. The central finding is that, unlike previous predictions of hot climates due to HP ice preventing weathering, the heat flux near mid-ocean ridges is sufficient to melt the HP ice. This fixes the seafloor temperature at the melting point, enabling high weathering flux independent of surface temperature, leading to extremely cold climates or snowball states with CO₂-poor atmospheres. The paper concludes that ocean planets with oceans several tens of Earth's ocean mass cannot maintain temperate climates.
Significance. If the results hold, this work has significant implications for understanding the climate diversity of water-rich terrestrial exoplanets in the habitable zone. It challenges the prior view that massive oceans lead to hot climates and instead predicts cold climates, which could affect interpretations of exoplanet observations and habitability assessments. The integration of geophysical heat flux variations with geochemical carbon cycling is a strength, providing a mechanism that is falsifiable through further modeling or observations. The paper explicitly credits the coupling of standard processes without introducing free parameters or ad-hoc entities.
major comments (1)
- [Model description and results summary] The assertion that 'the heat flux near the mid-ocean ridge is high enough to melt the ice' (as stated in the abstract and model description) is central to the claim that seafloor weathering dominates and fixes temperature at the melting point regardless of surface conditions. However, no explicit calculation or verification of the local heat flux, comparison to the pressure-dependent melting curve of HP ice, or threshold across the range of ocean masses is provided; the condition is taken as given rather than derived or tested within the manuscript.
Simulated Author's Rebuttal
We thank the referee for the positive assessment of our work's significance and for the constructive comment. We address the major comment below and will revise the manuscript accordingly.
read point-by-point responses
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Referee: The assertion that 'the heat flux near the mid-ocean ridge is high enough to melt the ice' (as stated in the abstract and model description) is central to the claim that seafloor weathering dominates and fixes temperature at the melting point regardless of surface conditions. However, no explicit calculation or verification of the local heat flux, comparison to the pressure-dependent melting curve of HP ice, or threshold across the range of ocean masses is provided; the condition is taken as given rather than derived or tested within the manuscript.
Authors: We agree that an explicit derivation and verification of this central condition would improve clarity and rigor. In the revised manuscript we will add a dedicated subsection (likely in Section 2 or 3) that (i) recalls the standard mid-ocean-ridge heat-flux scaling from plate-tectonic models, (ii) compares the resulting local heat flux to the pressure-dependent melting curve of HP ice at the relevant seafloor pressures for ocean masses from 1 to ~100 Earth oceans, and (iii) demonstrates that the melting condition is satisfied throughout the parameter range explored. The added material will use the same parameter values already adopted in the model and will not introduce new free parameters. revision: yes
Circularity Check
No significant circularity; standard model coupling with input assumptions
full rationale
The paper integrates geophysical heat flux, HP ice melting, seafloor weathering, and the carbonate-silicate cycle using established processes. The central result (cold climates via fixed seafloor temperature at melting point sustaining high weathering) follows directly from the stated model assumptions for the considered ocean masses, without any quoted equation reducing a prediction to a fitted parameter, self-citation chain, or definitional equivalence. No load-bearing step matches the enumerated circularity patterns; the heat-flux melting condition is presented as an input enabling the carbon-cycle integration rather than a derived output.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Plate tectonics operates on ocean planets with massive oceans
- domain assumption High-pressure ice melting temperature and heat flux near ridges are sufficient to enable weathering
discussion (0)
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