Water resources - How severe will freshwater resource shortages be on a regional scale? [Past]


Figure 1: Examples of three megadroughts reconstructed from tree rings that hit the central Mississippi Valley of the United States during early, middle, and late medieval times. See Cook et al. (2010b) for details.

Climate model projections of future hydroclimatic change associated with increasing atmospheric greenhouse gas concentrations are sobering and, depending on where you live, very alarming. For example, southwestern North America is projected to enter into a long-term drying trend in the sub-tropics to mid-latitudes, and this trend in increasing aridity may have already begun (Seager et al. 2007a). Thus, the unprecedented 2011 Texan drought (www.ncdc.noaa.gov/sotc/drought/2011/8) is an example of what might happen with increasing frequency and duration in the future. Independent of whether or not model projected radiatively forced drying is actually happening now, there is abundant paleoclimate evidence for the occurrence of past “megadroughts” in North America (Stine 1994; Woodhouse and Overpeck 1998; Cook et al. 2004, 2007; Stahle et al. 2011), Asia (Buckley et al. 2010; Cook et al. 2010a), and Europe (Helama et al. 2009; Büntgen et al. 2011) that dwarf any periods of drought seen in instrumental climate records over the past century. The seminal property of megadroughts that differentiates them from even the most severe droughts observed today is duration (Herweijer et al. 2007), with the former often lasting several decades to a century or more compared to just a few years to a decade or so for the latter. Figure 1 shows three such megadroughts reconstructed from tree rings (Cook et al. 2010b) that hit the Mississippi Valley of the United States during early, middle, and late medieval times. These megadroughts lasted 46, 148, and 61 years, respectively, and are ominously located in the American “bread basket” where similar droughts in the future would have catastrophic consequences on agricultural production. This also means that water resources planning and infrastructure design based on observed hydroclimatic data are unlikely to be resilient enough to handle the possible return of megadroughts that we now know have happened in the past.

The cause of past megadroughts is still not fully understood, but persistent patterns of cold La Niña-like sea surface temperatures in the eastern equatorial Pacific ENSO region have been strongly implicated in North America (Herweijer et al. 2006; Seager et al. 2007b; Graham et al. 2007), along with the possible influence of the Atlantic Ocean there as well (Feng et al. 2008). Perhaps more importantly, the paleoclimate record indicates that megadroughts occurred more often during an earlier period of generally above average temperatures called the Medieval Warm Period (MWP), approximately 700 to 1,200 years ago. It is not important to know whether or not the MWP was as warm as today (cf. Crowley and Lowery 2000; Bradley et al. 2003; Mann et al. 2009; Ljungqvist et al. 2011). Rather, the paleoclimate record of past megadroughts simply tells us that 1) they are a natural part of the climate system with no need for anthropogenic greenhouse gas forcing to ignite and sustain them, and 2) rather ominously they appear to “like” warmer climates such as that which occurred during the MWP. Given the climate model projections of future drying and the high likelihood that global warming will continue throughout the 21st century (IPCC 2007), we may therefore be entering into a new era of megadroughts with potentially catastrophic consequences to water supplies needed for human consumption, agriculture, energy production, and for maintaining the aquatic environment. The degree to which any future megadroughts caused by human-induced global warming will resemble those in the past is unclear because the climate forcings operating today are different from the past. Regardless, the stage appears to be set now for some possibly radical future changes in hydroclimatic variability if the past is any guide.

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