# GCM Simulations of Unstable Climates in the Habitable Zone

## TL;DR

This study confirms that Earth-like planets in the outer habitable zone can experience unstable climate cycles, using advanced 3D climate models and a novel hybrid integrator to validate previous low-dimensional model results.

## Contribution

It introduces a hybrid 0D-3D climate modeling approach to verify climate cycle phenomenology in complex GCM simulations, extending prior idealized models.

## Key findings

- GCM climate cycles align with low-dimensional model results
- Outer habitable zone planets may often be in transient glaciated states
- Hybrid integrator effectively simulates long-term climate evolution

## Abstract

It has recently been proposed that Earth-like planets in the outer regions of the habitable zone experience unstable climates, repeatedly cycling between glaciated and deglaciated climatic states (Menou 2015). While this result has been confirmed and also extended to explain early Mars climate records (Haqq-Misra et al. 2016; Batalha et al. 2016), all existing work relies on highly idealized low-dimensional climate models. Here, we confirm that the phenomenology of climate cycles remains in 3D Earth climate models with considerably more degrees of freedom. To circumvent the computational barrier of integrating climate on Gyr timescales, we design a hybrid 0D-3D integrator which uses a general circulation model (GCM) as a short relaxation step along a long evolutionary climate sequence. We find that GCM climate cycles are qualitatively consistent with reported low-dimensional results. This establishes on a firmer ground the notion that outer habitable zone planets may be preferentially found in transiently glaciated states.