 Climate scientists also seek to understand how climate would change in the future if greenhouse gas emissions and other anthropogenic forcing factors were to evolve in particular ways. Such information can be of significant interest to policy makers, who might choose to implement policies that push toward some future scenarios and away from others, and to many other decision makers as well (e.g., insurance companies deciding which policies to offer). Climate models have emerged as an important tool for making projections of future conditions. For a given scenario, it is common to make an ensemble of projections using different climate models or model versions; this is motivated in part by uncertainty about how to construct a climate model that can deliver highly accurate projections (Parker 2006; Betz 2009). This uncertainty in turn stems both from limited theoretical understanding of some processes operating within the climate system as well as from limited computing power, which constrains how existing knowledge can be implemented in models (see the discussion of parameterization in  Section 4.2).