The coming extinction of land mammals - The next great mass extinction

Mammals have dominated the Earth for the last ~55 Myr. Mammals have shown remarkable adaptation and resilience to climate change. However, it is unknown how long the Earth will be able to continue to sustain mammalian life. Estimates suggest the ultimate demise of all life will be in a ‘Venusian’ style runaway greenhouse climate ~1.5 billion years where increasing solar luminosity (L_☉) will raise temperature beyond that able to sustain life. However, conditions may develop sooner that will render the Earth naturally inhospitable to mammals. In ~250 million years all the continents of the world come together to form the Earth's fourth supercontinent, Pangea-Ultima. A natural consequence of the creation and decay of Pangea-Ultima will be extremes in pCO₂, both low (silicate weathering) and high (volcanic degassing). Here we show that variations in pCO₂, increased solar luminosity (~2% greater than now), and extreme continentality will lead to extreme climate states that are inhospitable to mammalian life. We assess the impact of these climate states on mammalian physiological limits using dry-bulb, wet-bulb, and Humidex stress indicators as well as planetary habitability index. Although low pCO₂ states will increase habitability, snowball Earth conditions may occur if the silicate weathering-pCO₂ burial feedback becomes too strong (resulting in low pCO₂ values <280ppm) under increased L_☉. Likewise, small short-term spikes in pCO₂ (≥1120ppm) outgassing will lead to extremes in heat. Under such conditions, thermal tolerances of endotherms will exceed physiological limits leading to mass extinction. The results reported here also show that global landmass configuration, pCO₂, and solar luminosity play a critical role in planetary habitability.