Chapter 3 discusses the spectrum of natural and cosmic threats to humanity and the likelihood that such events would lead to human extinction. It first considers the catastrophic potential of supervolcanic eruptions, which have historically resulted in climatic upheavals like “volcanic winters,” and the awe-inspiring yet potentially lethal phenomena of supernovae and gamma-ray bursts that can disrupt Earth’s atmosphere.

While supervolcanic eruptions are incredibly destructive, their frequency is relatively low, with the probability of a civilization-threatening eruption estimated at about 1 in 200 per century for eruptions of magnitude 8-9 and 1 in 800 for those above magnitude 9. This contrasts with the even lower risks from asteroid impacts, as the likelihood of a supervolcanic eruption in the next century is significantly higher than that of asteroid or comet collisions.

The discussion then transitions to stellar events. Supernovae are cataclysmic stellar explosions that can outshine galaxies and have the potential to damage Earth’s ozone layer if occurring nearby. These can lead to gamma-ray bursts, immensely energetic events thought to arise from supernovae or neutron star collisions. Despite their power, the probability of either event causing a catastrophic effect on Earth in the next century is minimal.

In addition to discussing catastrophic natural events, the chapter explores the concept of “background extinction rate” and how current natural risks compare to historical data.