One of the criticisms leveled against the climate models used to terrify the world with the unfathomable horror of a change of 1.5 degrees by the turn of the century, relates to how modelers deal with uncertainties.
The entire Earth is a very complex system to model, and our understanding of many of its processes — wind patterns, rainfall, ocean circulation — is incomplete. This means that models cannot claim to be based purely on fundamental physics and well-known laws of nature, the way a simple physics demonstration used in a classroom would.
Instead, each of the uncertain values that is incorporated into the final model has some “wiggle room” in the specific value given to it.
If there are only a few uncertainties, the model as a whole will have only a few adjustment points, and there may be a very small range of setting the uncertain parameters that results in the model accurately producing past data, against which it can be verified.
