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1. Introduction
Megadroughts are prolonged periods of aridity typically defined by their multidecadal duration (Woodhouse and Overpeck 1998; Ault et al. 2014; Cook et al. 2016) and have been linked to the demise of several preindustrial civilizations (Benson et al. 2002, 2003; Hodell et al. 1995; Buckley et al. 2010; Stahle and Dean 2011). Mounting evidence suggests that the risk of these events is increasing throughout the southwestern United States and other subtropical dry zones due to rising temperatures and dynamic circulation changes (Seager et al. 2007b; Woodhouse et al. 2010; Ault et al. 2014, 2016; Cook et al. 2015, 2016; Kelley et al. 2015). If a megadrought were to unfold in the coming decades, it would require water management decisions to be made on scales that have no historical precedent. Understanding the risks of these events is therefore essential for supporting regional adaptation strategies in the face of a changing climate. Furthermore, quantitative assessments of megadrought risk require us to disentangle the influences of external and internal climate variability in the past because future likelihoods will be governed by both of these factors (Ault et al. 2014, 2016; Cook et al. 2015).
The last millennium affords some insights into the range of forced and unforced preindustrial climate variability, yet it is only one realization of many plausible alternative climate trajectories that could have occurred during that same time period. If a given climate forcing (e.g., enhanced solar output) made megadroughts more probable in the past, but they failed to occur due to internal climate variability, then one might erroneously conclude that the forcing was not important to the statistics of megadrought on the basis of the one realization being examined. Alternatively, if megadroughts occur without any clear external forcing, yet are nonetheless more probable during certain climate regimes, one might correctly conclude that megadroughts do not require exogenous changes to climate boundary conditions, but would miss the critical piece of information that such forcings can make megadroughts more likely. These inherent constraints of having only one realization of reality have largely prevented a consensus view from emerging on whether or not megadroughts are forced [see the review by Cook et al. (2016)].
A number of studies have suggested that megadroughts in the western United...