Time series analysis of a diatom-inferred drought record suggests that Holocene hydroclimateof the northern Rocky Mountains has been characterized by oscillation betweentwo mean climate states. The dominant climate state was initiated at the onset ofthe Holocene (ca. 11 ka); under this climate state, drought was strongly cyclic, recurringat frequencies that are similar to twentieth century multidecadal phase changes of thePacific Decadal Oscillation. This pattern remained consistent throughout much of the mid-Holocene, continuing until ca. 4.5 ka. After this time the mean climate state changed, anddrought recurrence became unstable; periods of cyclic drought alternated with periods ofless predictable drought. The timing of this shift in climate was coincident with widespreadsevere drought in the mid-continent of North America. Overall, the strongest periodicityin severe drought occurred during the mid-Holocene, when temperatures in the northernRocky Mountains were warmer than today.

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