I'm new to climate science - starting to get familiar with the field by reading some published articles. Terminology is always hard when starting into a new domain. 

In this paper, [Dai - Increasing drought under global warming in observations and models](http://www.cgd.ucar.edu/cas/adai/papers/Dai-NatureClimChange-final.pdf), what does the author mean by the term "forcing," in relation to the PDSI?

Here is an example of usage: 

> The PDSI is calculated from a water-balance model forced with observed
> precipitation and temperature...

Another example: 

> Figure 1a,b shows that the broad patterns of the linear trends from
> 1950 to 2010 in observed annual precipitation and calculated
> sc_PDSI_pm using observation-based forcing2

Here is the citation: 

> Dai, Aiguo. "Increasing drought under global warming in observations
> and models." Nature Climate Change 3, no. 1 (2013): 52-58.