Thursday, February 12, 2015

Could the western United States survive another “gigadrought” like 6000 BC?

Climate models highlight the possibility of unprecedented and persistent drought across much of the central and southwest U.S. by the end of this century (1), and paleoclimate observations provide empirical examples of comparable climate changes that may be illustrative of potential future impacts. Such analogies are not perfect, but offer case examples grounded in actual changes that affected ecosystems, natural resources such as water, and societies. Many studies have considered the importance of the ‘megadroughts’ of the past millennium, which demonstrate that resource availability may never be stationary and that critical resources, such as water, can decline well below historic levels for decades or longer (2-5). However, long-term changes in climate forcing, such as greenhouse gases, can produce persistent changes that last millennia, such as ice ages (6). The mid-Holocene, ca. 6000-4000 BC, represents the last time that global climate forcing produced sustained temperatures comparable to recent decades (7). Resource changes during this period, therefore, offer insight into potential future changes that will likely sustain or amplify the warmth of the past decade.

If 25-yr episodes of warmth and aridity represent ‘megadroughts’ (2-5),  then the mid-Holocene in central North America could be considered a ‘gigadrought’ comparable to some possible futures (1)(Fig. 1). The region, which is today prairie surrounded by forests and agricultural lands, was transformed as temperatures rose to their maxima of the past 11,000 years (8, 9). The margins of the forest retreated during episodes of abrupt  forest dieoff (10, 11), and the core of the western prairies became fragmented by large areas of active dunes (12). Many lakes across the region fell to extremely low levels and rivers became ephemeral even in their Rocky Mountain headwaters (13-15). The aridity coincided with near total de-population of the central Rocky Mountains, despite substantial human occupation earlier (16).

Notably, all of these changes took place under temperatures similar to those of the past decade (Fig. 1B)(7, 8). The reduction in effective moisture, as defined as the balance of precipitation minus evaporation (P-E), reached >100 mm/yr lower than today in the snow-dominated watersheds of the central Rocky Mountains (14, 15) as well as across much of central and eastern North America (9, 17). The aridity was, therefore, similar in magnitude to that of the AD 1930s “Dust Bowl” drought (18), and also coincided with frequent dust storms, despite the absence of significant human land-use (12, 19). Unlike the Dust Bowl, the ‘gigadrought’ of 6000 BC persisted for centuries to millennia until temperatures declined (Fig. 1C).




Figure 1. Severe aridity affected much of mid-continent North America at ca. 6000 BC. Changes (A) included dieback for forests around the margins of the northern Great Plains (11), extensive declines in lake, river and groundwater levels (13-15), and widespread activation of dune fields (20). Letters on the map indicate the location of time series shown on the right (B-E). The impacts were most severe when annual temperatures (B), as reconstructed for the central Rocky Mountains region (8), rose to their maximum of the last 12,000 years and were similar to regional temperatures of the past decade (dashed lines). Effective moisture (C), human population size (D), and tree cover (E) all fell to their lowest levels during the warmest millennia.  P-E (C) indicates the change from modern in the balance of precipitation minus evaporation as reconstructed from lake-level changes (15);  the human-population reconstruction is a relative index based on the number of radiocarbon-dated archeological sites in the Bighorn Basin, WY (16); tree cover in Alberta and Minnesota (which represent changes across central North America) derives from fossil pollen records calibrated to satellite-based estimates (11). Base map from the Commission for Environmental Cooperation.

Citations
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