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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