The Miocene Climatic Optimum (MCO) from ~17 to 14 million years ago (Ma) represents an enigmatic reversal in Cenozoic cooling. A synthesis of marine paleotemperature records shows that the MCO was a local maximum in global sea surface temperature superimposed on a period from at least 19 Ma to 10 Ma, during which global temperatures were on the order of 10°C warmer than at present. Our high-resolution global reconstruction of ocean crustal production, a proxy for tectonic degassing of carbon, suggests that crustal production rates were ~35% higher than modern rates until ~14 Ma, when production began to decline steeply along with global temperatures. The magnitude and timing of the inferred changes in tectonic degassing can account for the majority of long-term ice sheet and global temperature evolution since 20 Ma.

The potential habitability of tidally locked planets orbiting M-dwarf stars has been widely investigated in recent work, typically with a nondynamic ocean and without continents. On Earth, ocean dynamics are a primary means of heat and nutrient distribution. Continents are a critical source of nutrients, strongly influence ocean dynamics, and participate in climate regulation. In this work, we investigate how the size of a substellar land mass affects the oceans ability to transport heat and upwell nutrients on the tidally locked planet Proxima Centauri b using the ROCKE-3D coupled ocean-atmosphere general circulation model (GCM). We find that dayside ice-free ocean and nutrient delivery to the mixed layer via upwelling are maintained across all continent sizes. We also find that Proxima Centauri b’s climate is more sensitive to differences among atmospheric GCMs than to the inclusion of ocean dynamics in ROCKE-3D. Finally, we find that Proxima Centauri b transitions from a “lobster” state where ocean heat transport distributes heat away from the substellar point to an “eyeball” state where heat transport is restricted and surface temperature decreases symmetrically from the substellar point when the continent size exceeds ∼20% of the surface area. Our work suggests that both a dynamic ocean and continents are unlikely to decrease the habitability prospects of nearby tidally locked targets like Proxima Centauri b that could be investigated with future observations by the James Webb Space Telescope.

Planets orbiting within the habitable zones of M stars are prime targets for future observations, which motivates a greater understanding of how tidal locking can affect planetary habitability. In this Letter we will consider the effect of tidal locking on limit cycling between snowball and warm climate states, which has been suggested could occur for rapidly rotating planets in the outer regions of the habitable zone with low CO2 outgassing rates. Here, we use a 3D Global Climate Model that calculates silicate-weathering to show that tidally locked planets with an active carbon cycle will not experience limit cycling between warm and snowball states. Instead, they smoothly settle into “Eyeball” states with a small unglaciated substellar region. The size of this unglaciated region depends on the stellar irradiation, the CO2 outgassing rate, and the continental configuration. Furthermore, we argue that a tidally locked habitable zone planet cannot stay in a snowball state for a geologically significant time. This may be beneficial to the survival of complex life on tidally locked planets orbiting the outer edge of their stars, but might also make it less likely for complex life to arise.

Stratocumulus clouds cover about a fifth of Earth’s surface, and due to their albedo and low-latitude location, they have a strong effect on Earth’s radiation budget. Previous studies using Large Eddy Simulations have shown that multiple equilibria (both stratocumulus-covered and cloud-free/scattered cumulus states) exist as a function of fixed-SST, with relevance to equatorward advected air masses. Multiple equilibria have also been found as a function of atmospheric CO2, with a subtropical SST nearly 10 K higher in the cloud-free state, and with suggested relevance to warm-climate dynamics. In this study, we use a mixed-layer model with an added surface energy balance, and the ability to simulate both the stratocumulus (coupled) and cloud-free/scattered cumulus (decoupled) states using a “stacked” mixed-layer approach to study both types of multiple equilibria and the corresponding hysteresis. The model’s simplicity and computational efficiency allow us to qualitatively explore the mechanisms critical to the stratocumulus cloud instability and hysteresis as well as isolate key processes that allow for multiple equilibria via mechanism denial experiments not possible with a full-complexity model. For the hysteresis in fixed-SST, we find that decoupling can occur due to either enhanced entrainment warming or a reduction in cloud-top longwave cooling. The critical SST at which decoupling occurs is highly sensitive to precipitation and entrainment parameterizations. In the CO2 hysteresis, decoupling occurs in the simple model used even without the inclusion of SST-cloud cover feedbacks, and the width of the hysteresis displays the same sensitivities as the fixed-SST case. Overall, the simple model analysis and results motivate further studies using higher complexity models.