Climate multistability and the dynamical boundaries of planetary habitability

Planetary habitability is often framed in terms of static boundaries such as the circumstellar habitable zone [1]. However, planetary climates are intrinsically nonlinear and may admit multiple coexisting climate stable states under identical stellar and atmospheric forcing due to feedbacks such as ice–albedo and greenhouse processes [2, 3]. This climate multistability implies that Earth-like planets can occupy fundamentally different climate regimes including temperate, globally glaciated (snowball), and post-runaway greenhouse states, and can undergo abrupt transitions between them at critical forcing thresholds. Such tipping points have profound implications for long-term habitability, as rapid transitions could outpace biological adaptation.

Here, we investigate the multistable climate structure of an Earth-like aquaplanet using a computationally efficient dynamical slab ocean model [4] coupled to the Generic-PCM global climate model (GCM; previously the LMD Generic GCM [5, 6]). The ocean model features sea-ice and snow evolution, wind-driven (Ekman) transport, horizontal eddy diffusion, Gent–McWilliams transport, and convection, while remaining much cheaper than a fully dynamic ocean model. In a modern-Earth configuration, the coupled system reproduces key observed climatic attributes, including the major oceanic heat flows, an annually averaged surface temperature of 13°C, a planetary albedo of 0.32, and sea ice coverage spanning 18 million sq. km [4].

We perform systematic parameter space exploration in stellar forcing to construct bifurcation diagrams and map the stable climate branches of an Earth-like aquaplanet. We identify at least five distinct stable climate regimes, including states previously inaccessible in the Generic-PCM but consistent with results obtained using fully dynamic ocean models [3, 7]. By selectively disabling ocean heat transport at the edge states, we demonstrate which branches are primarily sustained by atmospheric processes (e.g., the post-runaway states seen in [8, 9]), and which rely on ocean dynamics (this work). These results illustrate how climate multistability fundamentally reshapes the mapping between planetary and stellar parameters and the range of conditions under which worlds can be habitable.

Distinct stable climate states imply (radically) different observables, from high-albedo snowball planets to warm, water-rich post-runaway climates with enhanced water vapour columns and cloud cover. We therefore discuss the implications of climate multistability for the interpretation of exoplanet observations, with a particular focus on how combining complementary spectral regimes (reflected light + thermal emission; HWO + LIFE) can provide synergistic constraints on atmospheric and surface properties [e.g., [10]).

References:

[1] Kasting et al. (1993)

[2] Strogatz (2018)

[3] Brunetti et al. (2019)

[4] Bhatnagar et al. (in review)

[5] Hourdin et al. (2006)

[6] Forget et al. (in prep)

[7] Brunetti & Ragon (2023)

[8] Turbet et al. (2021)

[9] Chaverot et al. (2023)

[10] Alei et al. (2024)