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:As indicated by Bonan (1998), the most severe underestimates occur in the arid Southwest, and in some parts of the central U.S. where maximum differences in the annual average can be about –6 K. Unlike precipitation, the patterns of temperatures have some sharp changes in some cells along the Rock…
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