Droughts
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Definition
From Wikipedia, the free encyclopedia
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage and harm the local economy.
This global phenomenon has a widespread impact on agriculture. The United Nations estimates that an area of fertile soil the size of Ukraine is lost every year because of drought, deforestation, and climate instability.[3] Lengthy periods of drought have long been a key trigger for mass migration and played a key role in a number of ongoing migrations and other humanitarian crises in the Horn of Africa and the Sahel.
As in the case of floods, the IPCC foresees drought as one of the many dangers of CO2-induced global warming. An examination of the pertinent scientific literature, however, demonstrates droughts are not becoming more frequent, more severe, or longer-lasting.
Springer et al. (2008) constructed a multidecadal-scale history of east-central North America’s hydroclimate over the past 7,000 years, based on Sr/Ca ratios and δ13C data obtained from a stalagmite in West Virginia, USA. Their results indicated the presence of seven significant mid- to late-Holocene droughts that “correlate with cooling of the Atlantic and Pacific Oceans as part of the North Atlantic Ocean ice-rafted debris [IRD] cycle, which has been linked to the solar irradiance cycle,” as demonstrated by Bond et al. (1997, 2001). In addition, they found “the Sr/Ca and δ13C time series display periodicities of ~200 and ~500 years,” and “the ~200-year periodicity is consistent with the de Vries (Suess) solar irradiance cycle,” and that the ~500-year periodicity is likely “a harmonic of the IRD oscillations.” They also reported “cross-spectral analysis of the Sr/Ca and IRD time series yields statistically significant coherencies at periodicities of 455 and 715 years,” noting the latter values “are very similar to the second (725-years) and third (480-years) harmonics of the 1450 ± 500-years IRD periodicity.”
The five researchers concluded these findings “corroborate works indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes in central and eastern North America (Viau et al., 2002; Willard et al., 2005; Denniston et al., 2007)” and that their high-resolution time series “provide much stronger evidence in favor of solar-forcing of North American drought by yielding unambiguous spectral analysis results.”
Writing in the Journal of Quaternary Science, Cook et al. (2009) note “IPCC Assessment Report 4 model projections suggest that the subtropical dry zones of the world will both dry and expand poleward in the future due to greenhouse warming” and “the US southwest is particularly vulnerable in this regard and model projections indicate a progressive drying there out to the end of the 21st century.” They then note “the USA has been in a state of drought over much of the West for about 10 years now,” but “while severe, this turn of the century drought has not yet clearly exceeded the severity of two exceptional droughts in the 20th century.” Therefore, they conclude, “while the coincidence between the turn of the century drought and projected drying in the Southwest is cause for concern, it is premature to claim that the model projections are correct.”
We begin to understand this fact when we compare the turn-of-the-century-drought with the two “exceptional droughts” that preceded it by a few decades. Based on gridded instrumental Palmer Drought Severity indices for tree-ring reconstruction that extend back to 1900, Cook et al. calculated the turn-of-the-century drought had its greatest Drought Area Index value of 59 percent in the year 2002, whereas the Great Plains/Southwest drought covered 62 percent of the United States in its peak year of 1954 and the Dust Bowl drought covered 77 percent of the United States in 1934.
In terms of drought duration, things are not quite as clear. Stahle et al. (2007) estimated the first two droughts lasted for 12 and 14 years, respectively; Seager et al. (2005) estimated them to have lasted for eight and ten years; and Andreadis et al. (2005) estimated periods of seven and eight years. That yields means of nine and 11 years for the two exceptional droughts, compared to ten or so years for the turn-of-the-century drought. This, too, makes the latter drought not unprecedented compared with those that occurred in the twentieth century.
Real clarity, however, comes when the turn-of-the-century drought is compared to droughts of the prior millennium. Cook et al. write, “perhaps the most famous example is the ‘Great Drouth’ [sic] of AD 1276–1299 described by A.E. Douglass (1929, 1935).” This 24-year drought was eclipsed by the 38-year drought found by Weakley (1965) to have occurred in Nebraska from AD 1276 to 1313, which Cook et al. say “may have been a more prolonged northerly extension of the ‘Great Drouth’.” But even these multi-decade droughts pale in comparison with the “two extraordinary droughts discovered by Stine (1994) in California that lasted more than two centuries before AD 1112 and more than 140 years before AD 1350.” Each of these megadroughts, as Cook et al. describe them, occurred, in their words, “in the so-called Medieval Warm Period.” They add, “all of this happened prior to the strong greenhouse gas warming that began with the Industrial Revolution.”
In further ruminating about these facts in the “Conclusions and Recommendations” section of their paper, Cook et al. again state the medieval megadroughts “occurred without any need for enhanced radiative forcing due to anthropogenic greenhouse gas forcing”—because, of course, there was none at that time—and therefore, they say, “there is no guarantee that the response of the climate system to greenhouse gas forcing will result in megadroughts of the kind experienced by North America in the past.”
Reinforcing the findings of Cook et al. two years later, Stambaugh et al. (2011) “used a new long tree-ring chronology developed from the central U.S. to reconstruct annual drought and characterize past drought duration, frequency, and cycles in the U.S. Corn Belt during the last millennium.” This new record, in their words, “is the first paleoclimate reconstruction achieved with subfossil oak wood in the U.S.,” and they indicate it “increases the current dendroclimatic record in the central U.S. agricultural region by over 500 years.”
Of great significance among their findings is the fact that the new drought reconstruction indicates “drought conditions over the period of instrumental records (since 1895) do not exhibit the full range of variability, severity, or duration of droughts during the last millennium.” As an example, the six scientists compared the 1930s-era Dust Bowl drought with other prior severe events, finding “three years in the last millennium were drier than 1934, a classic Dust-Bowl year and the driest year of the instrumental period,” and “three of the top ten most severe droughts occurred within a 25-year period corresponding to the late 16th century.” Likewise, they state “the four longest droughts occurred prior to Euro-American settlement of the region (ca. AD 1850),” the longest of which occurred in the middle of the Medieval Warm Period and, as the authors describe it, “lasted approximately 61 years (AD 1148–1208).”
Other studies in North America also point to a large and persistent Medieval drought unequaled in modern times. Working in the Sierra de Manantlan Biosphere Reserve (SMBR) in west-central Mexico, Figueroa-Rangel et al. (2010) constructed a 1,300-year history of cloud forest vegetation dynamics via analyses of fossil pollen, microfossil charcoal, and organic and inorganic sediment data obtained from a 96-cm core of black organic material retrieved from a small forest hollow (19°35’32”N, 104°16’56”W). Their results showed oscillating intervals of humidity, including a major dry period that lasted from approximately AD 800 to 1200 in the SMBR, a dry period that corresponds with those of other locations in the region.
Quoting the four researchers, “results from this study corroborate the existence of a dry period from 1200 to 800 cal years BP in mountain forests of the region (B.L. Figueroa-Rangel, unpublished data); in central Mexico (Metcalfe and Hales, 1994; Metcalfe, 1995; Arnauld et al., 1997; O’Hara and Metcalfe, 1997; Almeida-Lenero et al., 2005; Ludlow-Wiechers et al., 2005; Metcalfe et al., 2007); lowlands of the Yucatan Peninsula (Hodell et al., 1995, 2001, 2005a,b) and the Cariaco Basin in Venezuela (Haug et al., 2003).” In addition, they write, “the causes associated to this phase of climate change have been attributed to solar activity (Hodell et al., 2001; Haug et al., 2003), changes in the latitudinal migration of the Intertropical Convergence Zone (ITCZ, Metcalfe et al., 2000; Hodell et al., 2005a,b; Berrio et al., 2006) and to ENSO variability (Metcalfe, 2006).”
In one final study from Mexico, Escobar et al. (2010) analyzed sediment cores from Lakes Punta Laguna, Chichancanab, and Peten Itza on the Yucatan Peninsula. With respect to drought, they report “relatively dry periods were persistently dry, whereas relatively wet periods were composed of wet and dry times.” Their findings also “confirm the interpretations of Hodell et al. (1995, 2007) and Curtis et al. (1996) that there were persistent dry climate episodes associated with the Terminal Classic Maya Period.” In fact, they find “the Terminal Classic Period from ca. AD 910 to 990 was not only the driest period in the last 3,000 years, but also a persistently dry period.” In further support of this interpretation, they note “the core section encompassing the Classic Maya collapse has the lowest sedimentation rate among all layers and the lowest oxygen isotope variability.”
Moving to South America, Marengo (2009) examined the hydrological history of the Amazon Basin in an effort “to explore long-term variability of climate since the late 1920s and the presence of trends and/or cycles in rainfall and river indices in the basin.” These analyses were based on northern and southern Amazonian rainfall data originally developed by Marengo (1992) and Marengo and Hastenrath (1993) and subsequently updated by Marengo (2004).
In describing the results of the analysis, the Brazilian researcher reports, “no systematic unidirectional long-term trends towards drier or wetter conditions have been identified since the 1920s.” Instead, he found “the rainfall and river series show variability at inter-annual scales.” Marengo states the patterns he uncovered are “characteristic of decadal and multi-decadal modes,” which he describes as “indicators of natural climate variability” that are linked to the El Niño Southern Oscillation, “rather than any unidirectional trend towards drier conditions (as one would expect, due to increased deforestation or to global warming).”
In Europe, based on data obtained from hundreds of moisture-sensitive Scots pine tree-ring records originating in Finland, and using regional curve standardization (RCS) procedures, Helama et al. (2009) developed what they describe as “the first European dendroclimatic precipitation reconstruction,” which “covers the classical climatic periods of the Little Ice Age (LIA), the Medieval Climate Anomaly (MCA), and the Dark Ages Cold Period (DACP),” running from AD 670 to AD 1993.
The authors state their data indicate “the special feature of this period in climate history is the distinct and persistent drought, from the early ninth century AD to the early thirteenth century AD,” which “precisely overlaps the period commonly referred to as the MCA, due to its geographically widespread climatic anomalies both in temperature and moisture.” In addition, they report, “the reconstruction also agrees well with the general picture of wetter conditions prevailing during the cool periods of the LIA (here, AD 1220–1650) and the DACP (here, AD 720–930).”
The three Finnish scientists note “the global medieval drought that we found occurred in striking temporal synchrony with the multicentennial droughts previously described for North America (Stine, 1994; Cook et al., 2004, 2007), eastern South America (Stine, 1994; Rein et al., 2004), and equatorial East Africa (Verschuren et al., 2000; Russell and Johnson, 2005, 2007; Stager et al., 2005) between AD 900 and 1300.” Noting further “the global evidence argues for a common force behind the hydrological component of the MCA,” they report “previous studies have associated coeval megadroughts during the MCA in various parts of the globe with either solar forcing (Verschuren et al., 2000; Stager et al., 2005) or the ENSO (Cook et al., 2004, 2007; Rein et al., 2004; Herweijer et al., 2006, 2007; Graham et al., 2007, Seager et al., 2007).” They state, “the evidence so far points to the medieval solar activity maximum (AD 1100–1250), because it is observed in the Δ14C and 10Be series recovered from the chemistry of tree rings and ice cores, respectively (Solanki et al., 2004).”
Moving next to Asia, Sinha et al. (2011) write of “the potential consequences that would be associated with a drought lasting years to decades, or even a century (megadrought).”They state such a phenomenon “constitutes one of the greatest threats to human welfare,” noting it would be “a particular serious threat for the predominantly agrarian-based societies of monsoon Asia, where the lives of billions of people are tightly intertwined with the annual monsoon cycle.”
In exploring this ominous subject in great detail, Sinha et al. review what is known about it as a result of numerous pertinent studies, relying heavily on the work of Sinha et al. (2007) and Berkelhammer et al. (2010), based on the δ18O record of a speleothem from Dandak Cave in central-eastern India, which documents Indian monsoon rainfall variations between AD 600 and 1500.
The eight researchers, from China, Germany, and the United States, report “proxy reconstructions of precipitation from central India, north-central China [Zhang et al., 2008], and southern Vietnam [Buckley et al., 2010] reveal a series of monsoon droughts during the mid 14th–15th centuries that each lasted for several years to decades,” and they say “these monsoon megadroughts have no analog during the instrumental period.” They also note “emerging tree ring-based reconstructions of monsoon variability from SE Asia (Buckley et al., 2007; Sano et al., 2009) and India (Borgaonkar et al., 2010) suggest that the mid 14th–15th century megadroughts were the first in a series of spatially widespread megadroughts that occurred during the Little Ice Age” and that they “appear to have played a major role in shaping significant regional societal changes at that time.” Among these upheavals, they make special mention of “famines and significant political reorganization within India (Dando, 1980; Pant et al., 1993; Maharatna, 1996), the collapse of the Yuan dynasty in China (Zhang et al., 2008), Rajarata civilization in Sri Lanka (Indrapala, 1971), and the Khmer civilization of Angkor Wat fame in Cambodia (Buckley et al., 2010),” noting the evidence suggests “monsoon megadroughts may have played a major contributing role in shaping these societal changes.”
In light of the fact that there were, in the words of Sinha et al., “at least five episodes of monsoon megadroughts during the Little Ice Age (nominally, AD 1350–1850),” we should be extremely thankful the Earth has emerged from this unique period of global coolness—which is universally recognized as having been the coldest interval of the current interglacial—especially because “the present-day water-resource infrastructure and planning are barely sufficient to meet the welfare of billions of people during a single season of anomalous weak monsoon, let alone a protracted failure,” such as what occurred repeatedly during the global chill of the Little Ice Age.
Another paper from Asia, Kim et al. (2009), was previously summarized in Chapter 4. The only major multiyear deviation from long-term normalcy they found were a decadal-scale decrease in precipitation and ensuing drought, both of which achieved their most extreme values (low in the case of precipitation, high in the case of drought) around AD 1900. The warming of the twentieth century had essentially no effect on the long-term histories of either precipitation or drought at Seoul, Korea.
Closing out this section on drought, we highlight a study published in Science by Zhao and Running (2010), who raised some concerns that global warming was affecting global net primary production of biomass due to the increased frequency of drought. In introducing their work, the two authors note “previous studies have shown that climate constraints [on global production of biomass] were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP [net primary production] from 1982–1999,” but over the past decade (2000–2009), satellite data “suggest a reduction in the global NPP.” Closer examination of this study, however, shows little reason for concern.
Zhao and Running state their work shows “a reduction in the global NPP of 0.55 petagrams of carbon” over the period 2000–2009. But in viewing a graphical representation of their results (see Figure 5.3.1 below), it can be seen that apart from the starting point of the initial year (2000) of their study, there is only one other year (2004) in which the global NPP was greater than what it was at the end of the study (2009). And since global NPP was on the rise from 1982 to 1999, what the more recent data show would more accurately be described as a leveling off from that prior upward trend.
Zhao and Running say the leveling off of global NPP over the past decade was induced by drought, and that “NPP in the tropics explains 93% of variations in the global NPP” and “tropical rainforests explain 61% of global NPP variations.” These findings also serve to undermine whatever concerns that selective reporting of their study’s results might have raised, since the recent work of Coelho and Goddard (2009) shows climate models forecast fewer tropical droughts in a warming world.
Coelho and Goddard write, “the majority of drought-related hazards and the attendant economic losses and mortality risks reside in the tropics,” citing Dilley et al. (2005). They write, “El Niño brings widespread drought (i.e., precipitation deficit) to the tropics,” and “stronger or more frequent El Niño events in the future” would “exacerbate drought risk in highly vulnerable tropical areas.”
The two researchers evaluated “the patterns, magnitude, and spatial extent of El Niño-induced tropical droughts during a control period in the twentieth century in climate simulations, which have realistic evolution of greenhouse gases,” after which they examined “the projected changes in the characteristics of El Niño and in the strength of the identified patterns of El Niño-induced tropical drought in the twenty-first century.” That allowed them to examine patterns of mean precipitation changes in order to “assess whether those changes exacerbate or ameliorate the risk of El Niño-induced drought conditions in the twenty-first century.”
Coelho and Goddard report “a possible slight reduction in the spatial extent of droughts is indicated over the tropics as a whole,” and they report “all model groups investigated show similar changes in mean precipitation for the end of the twenty-first century, with increased precipitation projected between 10°S and 10°N.” So it would appear—at least from a climate modeling perspective—that we can probably expect tropical drought to decrease throughout the remainder of the twenty-first century, which should enable the historical “greening of the Earth” to continue.
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