We look at classifying extinction risks in three different ways, which affect how we can intervene to reduce risk. First, how does it start causing damage? Second, how does it reach the scale of a global catastrophe? Third, how does it reach everyone? In all of these three phases there is a defence layer that blocks most risks: First, we can prevent catastrophes from occurring. Second, we can respond to catastrophes before they reach a global scale. Third, humanity is resilient against extinction even in the face of global catastrophes. The largest probability of extinction is posed when all of these defences are weak, that is, by risks we are unlikely to prevent, unlikely to successfully respond to, and unlikely to be resilient against. We find that it’s usually best to invest significantly into strengthening all three defence layers. We also suggest ways to do so tailored to the classes of risk we identify. Lastly, we discuss the importance of underlying risk factors – events or structural conditions that may weaken the defence layers even without posing a risk of immediate extinction themselves.
- We can usually best reduce extinction risk by splitting our budget between all defence layers.
- We should include measures that reduce whole classes of risks, such as research uncovering currently unseen risk. We should also address risk factors that would not cause extinction themselves but weaken our defences, for example, bad global governance.
- Future research should identify synergies between reducing extinction and other risks. For example, research on climate change adaptation and mitigation should assess how we can best preserve our ability to prevent, respond to, and be resilient against extinction risks.