In a lengthy introductory section, Wallace-Wells broadly sketches out the magnitude of the threat facing humanity as a result of climate change. While total human extinction is unlikely even under the most dire climate projections, most major extinction events on Earth were caused not by asteroid collisions like the one that killed the dinosaurs but by elevated levels of greenhouse gases in the atmosphere. This is true of the so-called Great Dying 250 million years ago, which corresponded with an increase in global temperatures of five degrees Celsius. A comparable temperature increase by 2100 is firmly within the parameters of scientific projection, and it could occur at a rate ten times faster than it did during the Great Dying.

Global temperature increase since the start of the Industrial Revolution in the late 18th century is the primary metric by which scientists measure the threat of climate change. At present, that metric sits at around one degree Celsius. At two degrees, scientists predict deadly heat waves, devastating droughts, coastal cities rendered uninhabitable by flooding, and daily hurricanes and monsoons of such size and severity that they used to occur only once every 500 years or longer. As recently as 1997, experts pegged two degrees as a worst-case scenario that must be avoided at all costs.