What is a climate scenario?

Climate scenarios can take several forms, but generally speaking they are a "what if" narrative about future climate typically derived from climate projections.  They are not a predictive tool, rather, they can be used to think about possible futures. 

Need to add: 

1. example "What if" narrative
2. explain how these narratives are then translated into climate modeling inputs that drive future climate changes in the models (this may help: http://www.ipcc.ch/ipccreports/sres/emission/index.php?idp=97)