Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…role, with local effects of dust and land surface changes having potentially intensified drought conditions during the Dust Bowl era, although the importance of these factors is still under discussion (McCrary and Randall 2010; Broennimann et al. 2009; Cook et al. 2009; Hoerling et al. 2009).
Category: