What causes megadroughts?

The severe and prolonged drought that beset North America in the 12th century likely caused the collapse of the Ancestral Puebloan and some other civilizations on the continent. Given that no megadroughts of comparable magnitude have yet recurred, evaluating their likelihood is challenging as well as prudent. Rare events can arise either from purely statistical fluctuations or as the result of rare exogenous events, such as a prolonged yet transient uptick in solar activity. Toby Ault of Cornell University and his collaborators set out to determine the causes of megadroughts using a so-called linear inversion model (LIM), which presumes that a system’s dynamics consist of a linear deterministic component and a nonlinear component of random fluctuations. Feeding the LIM with data yields the two components. Recent, well-sampled measurements of Earth’s sea-surface temperatures and other climate variables can be used to investigate past climates—provided the underlying dynamics have remained the same. Indeed, Ault and his collaborators explicitly assumed that was the case. They found that their LIM could reproduce the 12th-century megadrought, but not the four other megadroughts that clustered in the preceding five centuries during the Medieval Climate Anomaly. The researchers conclude that the megadroughts were caused by external forcing or by a type of internal climate behavior that was quiescent in the recent past and therefore not manifested in the data fed to the LIM. (T. Ault et al., J. Climate 31, 3, 2018, doi:10.1175/JCLI-D-17-0154.1 .)