Climate extremes likely to drive land mammal extinction during next supercontinent assembly

The research question centers on whether and when Earth's climate will reach a tipping point that threatens the dominance of mammals. While current anthropogenic climate change is concerning, the study focuses on longer-term processes driven by plate tectonics. The formation of the next supercontinent, Pangea Ultima (PU), in approximately 250 million years, is a key driver. PU's formation and subsequent breakup will significantly alter atmospheric CO2 levels due to changes in volcanic activity. Furthermore, the Sun's luminosity will increase by approximately 2.5% over this period. This increase in solar irradiance, combined with the altered geography and increased CO2, will result in a significantly warmer climate. The study's purpose is to assess the impact of these long-term changes on mammalian thermal tolerances and to determine if these changes will drive a mass extinction event. The importance of this research lies in its potential to inform our understanding of long-term climate change, planetary habitability, and the resilience of Earth's dominant terrestrial fauna. Understanding such long-term climate shifts is crucial for contextualizing current anthropogenic climate change and for gaining insights into the habitability of other planets.

The paper reviews previous research on mammalian thermal tolerances and the impact of climate change. Sherwood and Huber's work suggests that current global warming could exceed mammalian physiological limits, rendering certain regions uninhabitable. IPCC reports also project that certain thermal thresholds may be exceeded in limited coastal regions by the mid-to-late 21st century. However, even extreme future warming scenarios, involving the combustion of all fossil fuels, still leave most of the land surface habitable. The literature also highlights the long-term carbon cycle and the impact of supercontinent formation on atmospheric CO2 levels. Large swings in CO2 levels are evident in the geologic record, often correlating with supercontinent assembly and breakup. However, the specific CO2 and climate conditions of past supercontinents are not well understood. Past supercontinent cycles have seen variations in atmospheric CO2 from ~200 ppm to ~2100 ppm, corresponding to periods of both panglacial and greenhouse climates. The formation of supercontinents is associated with increased rates of large igneous province emplacement, further raising CO2 levels and potentially triggering continental breakup. This study builds upon these previous findings to investigate the combined effect of supercontinent formation, increased solar irradiance, and altered CO2 levels on mammalian survival.

The study employs a coupled atmosphere-land-ocean general circulation model (GCM), HadCM3L, with an interactive ozone scheme and a dynamic vegetation model (TRIFFID). The model uses a high-resolution reconstruction of Pangea Ultima's geography. Simulations were run with varying levels of atmospheric CO2 (0, 70, 140, 280, 560, and 1120 ppm) and two solar luminosities (modern and a 2.5% increase expected in 250 million years). The spatial-continuous integration (SCION) biogeochemistry model is used to estimate long-term background atmospheric CO2 concentrations. Mammalian habitability is assessed using physiological metrics: dry-bulb temperature (Td), wet-bulb temperature (Tw), Humidex, and freezing temperature (T0). Habitable regions are defined based on thresholds for these metrics, considering factors like the duration of conditions and the availability of hibernation and aestivation as coping strategies. Desert regions, with limited water and food, are considered uninhabitable. The model's skill is evaluated against a modern-day mammalian species distribution with human impact removed. Energy balance analysis is conducted to understand the contribution of different factors (albedo, emissivity, heat transport) to temperature changes. Finally, an astrophysical habitability index (PHI), combined with the Earth Similarity Index (ESI), is used to assess the overall planetary habitability in each scenario, considering parameters like the planetary mass, radius, temperature, and the presence of liquid water.

The study's simulations project a significant increase in global mean annual temperature (GMAT) under the Pangea Ultima configuration, ranging from 19.9 to 27.3 °C globally (24.5 to 35.1 °C on land). The GMAT anomaly relative to the Pre-industrial control simulation varies between +8.2 and +16.1 °C globally and +12.2 and +29.8 °C on land. The continentality effect from the Pangea Ultima configuration significantly increases land temperatures. Applying mammalian physiological metrics, the study finds that only one scenario (280 ppm CO2) shows comparable habitable land area to the Pre-industrial Earth under increased future solar luminosity (54% vs 66%). With higher CO2 concentrations (560 and 1120 ppm), habitability decreases to 16% and 8%, respectively. Increased CO2 pushes most land areas beyond mammalian thermal tolerance for Td, Tw, and Humidex. Energy balance analysis indicates that the shift to Pangea Ultima geography leads to a significant temperature increase, primarily driven by changes in surface albedo and emissivity at high latitudes. The increase in future solar luminosity further enhances this warming. The doubling of CO2 under increased solar luminosity results in a general increase in equilibrium climate sensitivity. Astrophysical habitability assessment reveals that no future scenario with CO2 levels above 280 ppm remains habitable according to ESI and PHI.

The findings demonstrate that the formation and decay of Pangea Ultima will severely limit, and potentially end, terrestrial mammalian habitability long before a runaway greenhouse effect from increased solar luminosity occurs. The combined effect of altered geography, increased solar radiation, and elevated CO2 levels creates a highly unfavorable climate. Even with the potential for evolutionary adaptation, mammalian thermotolerance limits are likely conserved, suggesting limited capacity for rapid adaptation to such extreme warming. The extensive desert regions predicted under Pangea Ultima further reduce habitability and pose significant biogeographic barriers. While hibernation and aestivation provide some adaptive strategies, their efficacy is limited by the duration and severity of extreme temperature conditions and associated food and water scarcity. The study suggests that only highly specialized and migratory mammals might survive, though even this is doubtful due to extensive aridity. The results highlight the interconnectedness of tectonics, atmospheric composition, and solar energy in determining planetary habitability and the vulnerability of mammalian life to long-term climate change.

This study demonstrates that the formation of Pangea Ultima will likely trigger a mass extinction of terrestrial mammals, exceeding their thermal tolerances well before the Sun's increased luminosity leads to a runaway greenhouse. This conclusion emphasizes the crucial role of plate tectonics and solar radiation in shaping long-term planetary habitability. Future research could focus on refining the accuracy of CO2 degassing estimations in supercontinent scenarios and further exploring the evolutionary adaptations and migration strategies that might enable some mammalian species to survive these extreme conditions. Understanding the limits of life under such extreme scenarios can inform our understanding of exoplanet habitability.

The study relies on a climate model with inherent limitations in predicting long-term climate changes and biological responses. The accuracy of the predicted CO2 levels for Pangea Ultima depends on the assumptions made about tectonic processes and weathering feedbacks. The physiological thresholds used for mammalian habitability are simplified and might not fully capture the complexities of mammalian thermoregulation and adaptation. The study considers only one of several possible supercontinent configurations, and other configurations might yield different results. Additionally, the absence of terrestrial ice sheets in the Pangea Ultima reconstruction might influence the results.