North American megadroughts in the Common Era: reconstructions and simulations

Abstract

During the Medieval Climate Anomaly (MCA), Western North America experienced episodes of intense aridity that persisted for multiple decades or longer. These megadroughts are well documented in many proxy records, but the causal mechanisms are poorly understood. General circulation models (GCMs) simulate megadroughts, but do not reproduce the temporal clustering of events during the MCA, suggesting they are not caused by the time history of volcanic or solar forcing. Instead, GCMs generate megadroughts through (1) internal atmospheric variability, (2) sea-surface temperatures, and (3) land surface and dust aerosol feedbacks. While no hypothesis has been definitively rejected, and no GCM has accurately reproduced all features (e.g., timing, duration, and extent) of any specific megadrought, their persistence suggests a role for processes that impart memory to the climate system (land surface and ocean dynamics). Over the 21st century, GCMs project an increase in the risk of megadrought occurrence through greenhouse gas forced reductions in precipitation and increases in evaporative demand. This drying is robust across models and multiple drought indicators, but major uncertainties still need to be resolved. These include the potential moderation of vegetation evaporative losses at higher atmospheric [CO 2 ], variations in land surface model complexity, and decadal to multidecadal modes of natural climate variability that could delay or advance onset of aridification over the the next several decades. Because future droughts will arise from both natural variability and greenhouse gas forced trends in hydroclimate, improving our understanding of the natural drivers of persistent multidecadal megadroughts should be a major research priority. © 2016 Wiley Periodicals, Inc. How to cite this article:WIREs Clim Change 2016Change , 7:411-432. doi: 10.1002 INTRODUCTION R ecurrent droughts are a normal part of climate variability in Western North America, and recent events (e.g., California, the Southwest) have highlighted the vulnerability of people and ecosystems to the capricious nature of water availability in this region. [1][2][3][4] Despite these challenges, however, there is robust evidence from the paleoclimate record that Western North America experienced even worse droughts (megadroughts) over the last two 411millennia. 5,6 These events, many lasting multiple decades, had profound impacts on the contemporary indigenous societies, [7][8][9][10] vegetation, [11][12][13] and landscape. 12,14,15 Megadroughts are not precisely defined in the literature, but typically refer to persistent drought events in the preindustrial period with durations longer than a decade. These droughts stand in sharp contrast to the shorter length of more recent 20th-century events such as the Dust Bowl (1932Bowl ( -1939 and the 1950s drought (1948)(1949)(1950)(1951)(1952)(1953)(1954)(1955)(1956)(1957). Megadroughts are thus primarily differentiated from more recent instrumental-era droughts in terms of their duration and, occasionally, ...

Exacerbation of the 2013–2016 Pan‐Caribbean Drought by Anthropogenic Warming

“…ScPDSI does not account for the effect of increased atmospheric CO 2 concentration on plant physiology, which is hypothesized to diminish PET‐induced soil drying by increasing water use efficiency of plants (Cook et al, 2016; Mankin et al, 2018; Swann et al, 2016). We thus compared scPDSI from CMIP5 with soil moisture from CMIP5 during 1950–2016 over the Caribbean as in Cook, Ault, and Smerdon (2015).…”

Projected drought risk in 1.5°C and 2°C warmer climates

“…Droughts are one of the costliest impacts of climate variability, but the factors controlling their onset and duration are still poorly understood [ Handmer et al ., ]. Paleoclimate proxies and modeling studies indicate that severe multidecadal droughts can arise from internal climate variability alone, in the absence of strong external forcing [ Cook et al ., , ; Hunt , ; Coats et al ., ; Stevenson et al ., ]. At the same time, evidence is accumulating that warming over recent decades may have affected the hydrologic cycle in semiarid regions [ Seager et al ., ; Kelley et al ., ; Lehner et al ., ], potentially amplifying drought risk when precipitation deficits occur [ Diffenbaugh et al ., ; Williams et al ., ].…”

Oceanic Drivers of Widespread Summer Droughts in the United States Over the Common Era

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