This chapter seeks to answer four questions:

• What is “uncertainty”?

• Why is it that some equate the reduction of scientific uncertainty with an increased likelihood of reaching a political consensus?

• Why is scientific uncertainty fundamentally irreducible in some contexts?

• Why does uncertainty matter in how we think about science in policy and politics?

There are several answers to the first question to be found in the scientific literature, and they share a common characteristic. The answer to the second question lies in a conflation of different types of uncertainty. The answer to the third question lies in an appreciation of the nature of science in the context of a complex world. Efforts to reduce uncertainty through scientific research often have the exact opposite effect of increasing uncertainty. The answer to the fourth question builds on the arguments developed in the previous chapters that information by itself does not compel a particular decision. The reduction of scientific uncertainty does not necessarily compel a political consensus.

What is “uncertainty”?

Uncertainty means that in a particular situation more than one outcome is consistent with our expectations. An “outcome” simply refers to an actual situation (i.e., some realized or true condition) in the past, present, or future, such as the number of whales in the ocean, the current temperature you feel on your skin as you read this sentence, or the roll of a dice.