Positive feedbacks make earth uninhabitable

The second mode is human destruction of the natural environment, which could leave the planet uninhabitable for humanity (Ward 2008; Brannen 2017; Steffen et al. 2018; Spratt and Dunlop 2019; Wallace-Wells 2019; Ripple et al. 2020). Humans have become the driving force behind environmental change, which is why this geological epoch is increasingly called “the Anthropocene” (Crutzen 2002), to distinguish it from the unusually temperate period since the end of the last Ice Age roughly 11,700 years ago, the Holocene, when human civilization—probably not coincidentally—began and flourished (Steffen, Crutzen, and McNeill 2007).9 The decline of some of Earth’s environmental subsystems already exceeds the “planetary boundaries” believed to constitute a “safe operating space for humanity” (e.g., climate change, biodiversity loss, and the nitrogen cycle), while others are under serious strain (e.g., ocean acidification and the phosphorus cycle). The outcome of human-driven environmental decline could be “irreversible and, in some cases, abrupt environmental change, leading to a state less conducive to human development” (Rockstrom et al. 2009, 472). The species extinction crisis—what some are calling the “sixth mass extinction” (Kolbert 2014; Brannen 2017)—is a sign of Earth’s declining habitability in the Anthropocene.10

The biggest environmental danger may be climate change (or “global warming”). The major anthropogenic drivers of climate change are the burning of fossil fuels (e.g., coal, oil, and gas), combined with deforestation for agriculture (e.g., livestock and monocultures) and resource extraction (e.g., mining and oil), plus ocean warming undermining the planet’s capacity for absorbing carbon dioxide. Anthropogenic climate change could soon pass certain “tipping points,” whereby positive feedback loops in Earth’s climate system could lead to potentially irreversible and self-reinforcing “runaway” global warming (Kump, Kasting, and Crane 2003; Steffen et al. 2018). For example, the melting of Arctic “permafrost” could produce additional warming, as glacial retreat reduces the refractory effect of the ice and releases huge quantities of methane currently trapped beneath it. Earth could then move towards a “Hothouse Earth” climate, which would make the planet a less-hospitable place for humanity (Steffen et al. 2018; Spratt and Dunlop 2019; Wallace-Wells 2019; Ripple et al. 2020).