Expected utility theory tells us how we should make decisions under uncertainty: we should choose the option that leads to the greatest expectation of utility. This may, however, not be the option that is likely to produce the best result – that may be the wrong choice if it also creates a small chance of a great disaster. A small chance of disaster may be the most important consideration in decision making. Climate change creates a small chance of disaster, and some authors believe this to be the most important consideration in deciding our response to climate change. To know whether they are right, we need to make a moral judgement about just how bad the disaster would be.