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Carbon Dioxide and Earth's Future: Pursuing the Prudent Path

2. More Frequent and Severe Floods and Droughts


The claim: As a result of the global warming and change in weather patterns that climate models predict will occur in response to the ongoing rise in the air's CO2 content, it is claimed that floods and droughts will become both more numerous and severe throughout the world.

With respect to current climate model deficiencies, we note that correctly simulating future extreme weather phenomena such as floods and droughts has proved an extremely difficult task. One reason for the lack of success in this area is inadequate model resolution on both vertical and horizontal spatial scales, which forces climate modelers to parameterize the large-scale effects of processes that occur on smaller scales than their models are capable of handling. This is particularly true of physical processes such as cloud formation and cloud-radiation interactions.

A good perspective on the cloud-climate conundrum was provided by Randall et al. (2003), who stated at the outset of their review of the subject that "the representation of cloud processes in global atmospheric models has been recognized for decades as the source of much of the uncertainty surrounding predictions of climate variability." However, and despite what they called the "best efforts" of the climate modeling community, they had to acknowledge that "the problem remains largely unsolved." What is more, they suggested that "at the current rate of progress, cloud parameterization deficiencies will continue to plague us for many more decades into the future," which has important implications for correctly predicting precipitation-related floods and drought.

In describing some of these deficiencies, Randall et al. stated that "our understanding of the interactions of the hot towers [of cumulus convection] with the global circulation is still in a fairly primitive state," and not knowing all that much about what goes up, it's not surprising that we also don't know all that much about what comes down, as they report that "downdrafts are either not parameterized or crudely parameterized in large-scale models."

With respect to stratiform clouds, the situation is no better, as their parameterizations were described by Randall et al. as "very rough caricatures of reality." As for interactions between convective and stratiform clouds, forget about it ... which is pretty much what the climate modelers themselves did during the 1970s and 80s, when Randall et al. reported that "cumulus parameterizations were extensively tested against observations without even accounting for the effects of the attendant stratiform clouds." Even at the time of their study, in fact, they had to report that the concept of cloud detrainment was "somewhat murky," and that conditions that trigger detrainment were "imperfectly understood." Hence, it should once again come as no surprise that at the time of their review they had to admit that "no existing GCM [included] a satisfactory parameterization of the effects of mesoscale cloud circulations."

Randall et al. additionally noted that "the large-scale effects of microphysics, turbulence, and radiation should be parameterized as closely coupled processes acting in concert," but they reported that only a few GCMs had even attempted to do so. And why? Because, as they described it, "the cloud parameterization problem is overwhelmingly complicated," and "cloud parameterization developers," as they referred to them, were still "struggling to identify the most important processes on the basis of woefully incomplete observations." And to drive this point home, they said "there is little question why the cloud parameterization problem is taking a long time to solve: It is very, very hard." In fact, the four scientists concluded that "a sober assessment suggests that with current approaches the cloud parameterization problem will not be 'solved' in any of our lifetimes."

So is all hope lost with respect to models ever being able to correctly forecast floods and drought if they cannot correctly reproduce clouds and precipitation? Not entirely.

The shining hope of the climate-modeling community resides in something Randall et al. called "cloud system-resolving models" or CSRMs, which can be compared with single-column models or SCMs that can be "surgically extracted from their host GCMs." These advanced models, as they describe them, "have resolutions fine enough to represent individual cloud elements, and space-time domains large enough to encompass many clouds over many cloud lifetimes." Of course, these improvements mean that "the computational cost of running a CSRM is hundreds or thousands of times greater than that of running an SCM." Nevertheless, in a few more decades, according to Randall et al., "it will become possible to use such global CSRMs to perform century-scale climate simulations, relevant to such problems as anthropogenic climate change." In the interim, however, they remain far from ready for prime time, as evidenced in a study conducted four years later by Zhou et al. (2007).

Noting that CSRMs "still need parameterizations on scales smaller than their grid resolutions and have many known and unknown deficiencies," and to help stimulate progress in these areas, Zhou et al. compared the cloud and precipitation properties observed by instruments deployed in the Clouds and Earth's Radiant Energy System (CERES) and Tropical Rainfall Measuring Mission (TRMM) systems against simulations obtained from the three-dimensional Goddard Cumulus Ensemble (GCE) model during the South China Sea Monsoon Experiment (SCSMEX) field campaign of 18 May-18 June 1998. And as a result of that analysis, the nine researchers reported that: (1) "the GCE rainfall spectrum includes a greater proportion of heavy rains than PR (Precipitation Radar) or TMI (TRMM Microwave Imager) observations," (2) "the GCE model produces excessive condensed water loading in the column, especially the amount of graupel as indicated by both TMI and PR observations," (3) "the model also cannot simulate the bright band and the sharp decrease of radar reflectivity above the freezing level in stratiform rain as seen from PR," (4) "the model has much higher domain-averaged OLR (outgoing longwave radiation) due to smaller total cloud fraction," (5) "the model has a more skewed distribution of OLR and effective cloud top than CERES observations, indicating that the model's cloud field is insufficient in area extent," (6) "the GCE is ... not very efficient in stratiform rain conditions because of the large amounts of slowly falling snow and graupel that are simulated," and finally, in summation, that (7) "large differences between model and observations exist in the rain spectrum and the vertical hydrometeor profiles that contribute to the associated cloud field."

Other studies have continued to demonstrate the difficulties models have in simulating precipitation properties and trends. Kiktev et al. (2007), for example, analyzed the abilities of five global coupled climate models that played important roles in the IPCC's Fourth Assessment Report to simulate temporal trends over the second half of the 20th century for five annual indices of precipitation extremes. Their results revealed "low skill" or an "absence" of model skill.

Two years later, Lavers et al. (2009) examined the predictive skill of eight seasonal climate forecast models that were developed at various European climate centers. Specifically, they assessed the predictability of monthly precipitation "retrospective forecasts" or hindcasts, which were composed of multiple nine-month projections initialized during each month of the year over the period 1981-2001, comparing the projections against real-world precipitation values that were obtained from the Global Precipitation Climatology Center data. In addition, they conducted a virtual-world analysis, where the output of one of the models was arbitrarily assumed to be the "truth," and where the average of the rest of the models was assumed to be the "predictor."

The results of these exercises indicated that in the virtual world of the climate models, there was quite good skill over the first two weeks of the forecast, when the spread of ensemble model members was small, but that there was a large drop off in predictive skill in the second 15-day period. Things were even worse in the real world, where they say the models had negligible skill over land at a 31-day lead time, which they described as being "a relatively short lead time in terms of seasonal climate prediction." In light of these findings, therefore, the three researchers concluded that given the real-world skill -- or lack thereof! -- demonstrated by the state-of-the-art models, "it appears that only through significant model improvements can useful long-lead forecasts be provided that would be useful for decision makers," a quest that they quite frankly state "may prove to be elusive."

More of the same was also reported by O'Gorman and Schneider (2009), who assessed "how precipitation extremes change in simulations with 11 different climate models in the World Climate Research Program's (WCRP's) Coupled Model Intercomparison Project phase 3 (CMIP3) archive." Based on their findings, as well as those of others, O'Gorman and Schneider reported that "in simulations with comprehensive climate models, the rate of increase in precipitation extremes varies widely among models, especially in the tropics (Kharin et al., 2007)." They also noted, in this regard, that "the variations among models in the tropics indicate that simulated precipitation extremes may depend sensitively on the parameterization of unresolved and poorly understood processes [italics added]," citing the work of Wilcox and Donner (2007). In fact, they state that "climate models do not correctly reproduce the interannual variability of precipitation extremes in the tropics (Allan and Soden, 2008), or the frequency and intensity distribution of precipitation generally (Wilcox and Donner, 2007; Dai, 2006; Sun et al., 2006)." Thus, the two researchers concluded that "current climate models cannot reliably predict changes in tropical precipitation extremes," noting that "inaccurate simulation of the upward velocities may explain not only the intermodal scatter in changes in tropical precipitation extremes but also the inability of models to reproduce observed interannual variability."

Most recently, Stephens et al. (2010) employed "new and definitive measures of precipitation frequency provided by CloudSat [e.g., Haynes et al., 2009]" to assess the realism of global model precipitation via an analysis that employed five different computational techniques representing "state-of-the-art weather prediction models, state-of-the-art climate models, and the emerging high-resolution global cloud 'resolving' models." The results of this exercise indicated that "the character of liquid precipitation (defined as a combination of accumulation, frequency, and intensity) over the global oceans is significantly different from the character of liquid precipitation produced by global weather and climate models," and that "the differences between observed and modeled precipitation are larger than can be explained by observational retrieval errors or by the inherent sampling differences between observations and models."

More specifically, Stephens et al. reported that for precipitation over the global ocean as a whole, "the mean model intensity lies between 1.3 and 1.9 times less than the averaged observations," while occurrences "are approximately twice the frequency of observations." They also found that the models "produce too much precipitation over the tropical oceans" and "too little mid-latitude precipitation." And they indicate that the large model errors "are not merely a consequence of inadequate upscaling of observations but indicative of a systemic problem of models more generally."

In concluding their study, the nine US, UK and Australian researchers say their results imply that state-of-the-art weather and climate models have "little skill in precipitation calculated at individual grid points," and that "applications involving downscaling of grid point precipitation to yet even finer-scale resolution has little foundation and relevance to the real earth system," which is not too encouraging a result, considering it is the "real earth system" in which we live and for which we have great concern. Therefore, given these findings, as well as the many others previously cited, it is difficult to conceive how today's state-of-the-art computer models can be claimed to produce reliable flood and drought forecasts decades and centuries into the future.

Fortunately, there exists an alternative means by which the claim that global warming will result in more frequent and severe floods and droughts can be evaluated. Since climate alarmists contend the earth has already experienced a warming that has been unprecedented over the past millennium or more, we can assess the validity of their claims about the future of floods and droughts by examining to what extent the planet's emergence from the global chill of the Little Ice Age has -- or has not -- impacted the frequency and magnitude of these two extreme and often-deadly forces of nature. Or for records that are long enough, we can compare the characteristics of these phenomena as they were expressed during the cold of the Little Ice Age and the heat of the prior Medieval Warm Period. And as we do so, we find that there does not appear to have been any warming-driven increase in floods or droughts, as demonstrated by the papers reviewed in the following two subsections of this document.

With respect to prior observed effects of warming on floods, we focus first on Europe, where Nesje et al. (2001) analyzed a sediment core from a lake in southern Norway, attempting to determine the frequency and magnitude of prior floods in that region. The last thousand years of this record, as they describe it, revealed "a period of little flood activity around the Medieval period (AD 1000-1400)," which was followed by a period of extensive flood activity associated with the "post-Medieval climate deterioration characterized by lower air temperature, thicker and more long-lasting snow cover, and more frequent storms associated with the 'Little Ice Age'."

Moving on to France, Pirazzoli (2000) analyzed tide-gauge and meteorological data over the period 1951-1997 for the northern portion of the French Atlantic coast, discovering that "ongoing trends of climate variability show a decrease in the frequency and hence the gravity of coastal flooding." A year later, however, on the 8th and 9th of September 2002, extreme flooding of the Gardon River in southern France claimed the lives of a number of people and caused much damage to towns and villages situated adjacent to its channel. This event elicited much coverage in the press; and Sheffer et al. (2003) wrote that "this flood is now considered by the media and professionals to be 'the largest flood on record'," which record extends all the way back to 1890. Coincidently, however, Sheffer et al. were in the midst of a study of prior floods of the Gardon River when the "big one" hit; and they had data spanning a much longer time period against which to compare its magnitude. Based on their findings, they were able to report that "the extraordinary flood of September 2002 was not the largest by any means," noting that "similar, and even larger floods have occurred several times in the recent past," with three of the five greatest floods they had identified to that point in time occurring over the period AD 1400-1800 during the Little Ice Age.

Five years later, Sheffer et al. (2008) had obtained even more data on the subject. Working in two caves and two alcoves of a 1600-meter-long stretch of the Gardon River, they analyzed geomorphic, sedimentologic and hydrologic data associated with both historical and late Holocene floods, which they had hoped would provide a longer and better-defined perspective on the subject. And so it did, as they discovered that "at least five floods of a larger magnitude than the 2002 flood occurred over the last 500 years," all of which took place, as they describe it, "during the Little Ice Age." In addition, they reported that several other studies had also determined that "the Little Ice Age has been related to increased flood frequency in France," citing the work of Guilbert (1994), Coeur (2003) and Sheffer (2003, 2005).

Also working in France at this time were Renard et al. (2008), who employed four different procedures for assessing field significance and regional consistency with respect to trend detection in both high-flow and low-flow hydrological regimes of French rivers. This they did using daily discharge data obtained from 195 gauging stations having a minimum record length of 40 years; and in doing so, they determined that "at the scale of the entire country, the search for a generalized change in extreme hydrological events through field significance assessment remained largely inconclusive." In addition, they discovered that at the smaller scale of hydro-climatic regions, there were also no significant results for most such areas.

Working in the Myjava Hill Land of Slovakia, Stankoviansky (2003) employed topographical maps and aerial photographs, field geomorphic investigation, and the study of historical documents, including those from local municipal and church sources, to determine the spatial distribution of gully landforms and the temporal history of their creation. These diverse efforts led to his discovery that "the central part of the area, settled between the second half of the 16th and the beginning of the 19th centuries, was affected by gully formation in two periods, the first between the end of the 16th century and the 1730s and the second roughly between the 1780s and 1840s," and he reports that "the triggering mechanism of gullying was extreme rainfalls during the Little Ice Age." More specifically, he writes that "the gullies were formed relatively quickly by repeated incision of ephemeral flows concentrated during extreme rainfall events, which were clustered in periods that correspond with known climatic fluctuations during the Little Ice Age." Subsequently, from the mid-19th century to the present, he reported there has been a decrease in gully growth because of "climatic improvements since the termination of the Little Ice Age."

In Sweden, Lindstrom and Bergstrom (2004) analyzed runoff and flood data from more than 60 discharge stations scattered throughout the country, some of which provided information stretching as far back in time as the early to mid-1800s, when Sweden and the world were still experiencing the cold of the Little Ice Age. This analysis led them to discover that the last 20 years of the past century were indeed unusually wet, with a runoff anomaly of +8% compared with the century average. But they also found that "the runoff in the 1920s was comparable to that of the two latest decades," and that "the few observation series available from the 1800s show that the runoff was even higher than recently." In addition, they determined that "flood peaks in old data [were] probably underestimated," which "makes it difficult to conclude that there has really been a significant increase in average flood levels." Also, they report that "no increased frequency of floods with a return period of 10 years or more, could be determined." And with respect to the generality of their findings, the two researchers state that conditions in Sweden "are consistent with results reported from nearby countries: e.g. [Norway] Forland et al. (2000), [Denmark] Bering Ovesen et al. (2000), [Latvia] Klavins et al. (2002) and [Finland] Hyvarinen (2003)," noting that "it has been difficult to show any convincing evidence of an increasing magnitude of floods (e.g. Roald, 1999) in the near region."

Macklin et al. (2005) developed what they describe as "the first probability-based, long-term record of flooding in Europe, which spans the entire Holocene and uses a large and unique database of 14C-dated British flood deposits," after which they compared their reconstructed flood history "with high-resolution proxy-climate records from the North Atlantic region, northwest Europe and the British Isles to critically test the link between climate change and flooding." As a result of this multifaceted endeavor, they determined that "the majority of the largest and most widespread recorded floods in Great Britain occurred during cool, moist periods," and that "comparison of the British Holocene palaeoflood series ... with climate reconstructions from tree-ring patterns of subfossil bog oaks in northwest Europe also suggests that a similar relationship between climate and flooding in Great Britain existed during the Holocene, with floods being more frequent and larger during relatively cold, wet periods."

Three years later, while noting that "recent flood events have led to speculation that climate change is influencing the high-flow regimes of United Kingdom catchments" and that "projections suggest that flooding may increase in [the] future as a result of human-induced warming," Hannaford and Marsh (2008) used the UK benchmark network of 87 near-natural catchments identified by Bradford and Marsh (2003) to conduct a UK-wide appraisal of trends in high-flow regimes unaffected by human disturbances. This work revealed, in their words, that "significant positive trends were observed in all high-flow indicators ... over the 30-40 years prior to 2003, primarily in the maritime-influenced, upland catchments in the north and west of the UK." However, they say "there is little compelling evidence for high-flow trends in lowland areas in the south and east." They also found that "in western areas, high-flow indicators are correlated with the North Atlantic Oscillation Index (NAOI)," so that "recent trends may therefore reflect an influence of multi-decadal variability related to the NAOI." In addition, they state that longer river flow records from five additional catchments they studied "provide little compelling evidence for long-term (>50 year) trends but show evidence of pronounced multi-decadal fluctuations." Lastly, they add that "in comparison with other indicators, there were fewer trends in flood magnitude," and that "trends in peaks-over-threshold frequency and extended-duration maxima at a gauging station were not necessarily associated with increasing annual maximum instantaneous flow." All things considered, therefore, Hannaford and Marsh concluded that "considerable caution should be exercised in extrapolating from any future increases in runoff or high-flow frequency to an increasing vulnerability to extreme flood events."

"Starting from historical document sources, early instrumental data (basically, rainfall and surface pressure) and the most recent meteorological information," as they describe it, Llasat et al. (2005) analyzed "the temporal evolution of floods in northeast Spain since the 14th century," focusing particularly on the river Segre in Lleida, the river Llobregat in El Prat, and the river Ter in Girona. This work indicated there was "an increase of flood events for the periods 1580-1620, 1760-1800 and 1830-1870," and they report that "these periods are coherent with chronologies of maximum advance in several alpine glaciers." In addition, their tabulated data indicate that for the aggregate of the three river basins noted above, the mean number of what Llasat et al. call catastrophic floods per century for the 14th through 19th centuries was 3.55 ± 0.22, while the corresponding number for the 20th century was only 1.33 ± 0.33. Thus, the four Spanish researchers concluded their paper by saying "we may assert that, having analyzed responses inherent to the Little Ice Age and due to the low occurrence of frequent flood events or events of exceptional magnitude in the 20th century, the latter did not present an excessively problematic scenario."

Five years later, working in southeast Spain, Benito et al. (2010) reconstructed flood frequencies of the Upper Guadalentin River using "geomorphological evidence, combined with one-dimensional hydraulic modeling and supported by records from documentary sources at Lorca in the lower Guadalentin catchment." Their efforts revealed that past floods were clustered during particular time periods: AD 950-1200 (10), AD 1648-1672 (10), AD 1769-1802 (9), AD 1830-1840 (6), and AD 1877-1900 (10), where the first time interval coincides with the Medieval Warm Period and the latter four time intervals all fall within the confines of the Little Ice Age; and calculating mean rates of flood occurrence over each of the five intervals, we obtain a value of 0.40 floods per decade during the Medieval Warm Period, and an average value of 4.31 floods per decade over the four parts of the Little Ice Age, which latter value is more than ten times greater than the mean flood frequency experienced during the Medieval Warm Period.

In Poland, Cyberski et al. (2006) used documentary sources of information (written documents and "flood boards") to develop a reconstruction of winter flooding of the Vistula River all the way back to AD 988; and this work indicated, in their words, that winter floods "have exhibited a decreasing frequency of snowmelt and ice-jam floods in the warming climate over much of the Vistula basin."

Focusing on southwest Germany, Burger et al. (2007) reviewed what was known about flooding in this region over the past three centuries, which takes us back well into the Little Ice Age. The six scientists report that the extreme flood of the Neckar River in October 1824 was "the largest flood during the last 300 years in most parts of the Neckar catchment." In fact, they say "it was the highest flood ever recorded in most parts of the Neckar catchment and also affected the Upper Rhine, the Mosel and Saar." In addition, they report that the historical floods of 1845 and 1882 "were among the most extreme floods in the Rhine catchment in the 19th century," which they describe as truly "catastrophic events." And speaking of the flood of 1845, they say it "showed a particular impact in the Middle and Lower Rhine and in this region it was higher than the flood of 1824." Finally, the year 1882 actually saw two extreme floods, one at the end of November and one at the end of December. Of the first one, Burger et al. say that "in Koblenz, where the Mosel flows into the Rhine, the flood of November 1882 was the fourth-highest of the recorded floods, after 1784, 1651 and 1920," with the much-hyped late-20th-century floods of 1993, 1995, 1998 and 2002 not even meriting a mention.

On a broader multi-country scale, Mudelsee et al. (2003) analyzed historical documents from the 11th century to 1850, plus subsequent water stage and daily runoff records from then until 2002, for two of the largest rivers in central Europe: the Elbe and Oder Rivers. In doing so, they discovered that for the prior 80 to 150 years, which climate alarmists typically describe as a period of unprecedented global warming, there was actually "a decrease in winter flood occurrence in both rivers, while summer floods show[ed] no trend, consistent with trends in extreme precipitation occurrence." Then, shortly thereafter, Mudelsee et al. (2004) wrote that "extreme river floods have had devastating effects in central Europe in recent years," citing as examples the Elbe flood of August 2002, which caused 36 deaths and inflicted damages totaling over 15 billion U.S. dollars, and the Oder flood of July 1997, which caused 114 deaths and inflicted approximately 5 billion dollars in damages.

The researchers then noted that concern had been expressed in this regard "in the Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change," wherein it was stated that "current anthropogenic changes in atmospheric composition will add to this risk." Unconvinced about this contention, however, the four researchers reevaluated the quality of data and methods of reconstruction that had previously produced flood histories of the middle parts of the Elbe and Oder rivers back to AD 1021 and 1269, respectively; and in doing so, they found, for both the Elbe and Oder rivers, "no significant trends in summer flood risk in the twentieth century," but "significant downward trends in winter flood risk," which latter phenomenon -- described by them as "a reduced winter flood risk during the instrumental period" -- they specifically described as "a response to regional warming."

Rounding out the study of Europe, based on information on flood losses obtained from the Emergency Events Database and the Natural Hazards Assessment Network, Barredo (2009) developed a 1970-2006 history of normalized monetary flood losses throughout the continent -- including the member states of the European Union along with Norway, Switzerland, Croatia and the former Yugoslav Republic of Macedonia -- by calculating the value of losses that would have occurred if the floods of the past had taken place under the current socio-economic conditions of the continent, while further removing inter-country price differences by adjusting the losses for purchasing power parities.

This work revealed, in the analyst's words, that "there is no evidence of a clear positive trend in normalized flood losses in Europe," and that "changes in population, inflation and per capita real wealth are the main factors contributing to the increase of the original raw losses." Thus, after removing the influence of the stated socio-economic factors, the European Commission researcher declared "there remains no evident signal suggesting any influence of anthropogenic climate change on the trend of flood losses in Europe during the assessed period."

In summation, the studies described above, from locations scattered throughout all of Europe, contradict the climate-alarmist claim that warming results in more frequent and more severe floods. In addition, there do not appear to have been any increases in either floods or properly-adjusted flood damages throughout all of Europe over the period of time the world's climate alarmists contend was the warmest of the past thousand or more years. And Europe is no anomaly is this regard, for things have been found to be largely the same almost everywhere such studies have been conducted; and in light of this fact -- and to not unnecessarily lengthen our report -- in the following two paragraphs we merely cite the journal references to similar investigations that have produced similar findings on earth's other continents.

For North America, see Ely (1997), Brown et al. (1999), Lins and Slack (1999), Olsen et al. (1999), Haque (2000), Knox (2001), Molnar and Ramirez (2001), Campbell (2002), Garbrecht and Rossel (2002), Ni et al. (2002), Noren et al. (2002), St. George and Nielsen (2002), Fye et al. (2003), Schimmelmann et al. (2003), Shapley et al. (2005), Wolfe et al. (2005), Carson et al. (2007), Pinter et al. (2008), Collins (2009), Cunderlik and Ouarda (2009) and Villarini and Smith (2010).

For Asia, see Cluis and Laberge (2001), Jiang et al. (2005), Zhang et al. (2007), Zhang et al. (2009), and Panin and Nefedov (2010), while for South America, see Wells (1990), Magillian and Goldstein (2001) and Rein et al. (2004), and for Africa, see Heine (2004).

With respect to prior observed effects of warming on drought, we find that the peer-reviewed scientific literature clearly demonstrates that the climate-model-based claim of more frequent and severe droughts being induced by global warming is also false. And we begin our review of the evidence that makes this conclusion very clear by scrutinizing recent research work that has been conducted in North America.

Confining themselves to the continental United States, Andreadis and Lettenmaier (2006) examined 20th-century trends in soil moisture, runoff and drought with a hydro-climatological model forced by real-world data for precipitation, air temperature and wind speed over the period 1915-2003. This work revealed, in their words, that "droughts have, for the most part, become shorter, less frequent, less severe, and cover a smaller portion of the country over the last century." And it would seem to be nigh unto impossible to contemplate a more stunning rebuke of climate-alarmist claims concerning global warming and drought than that provided by this study.

Working within the conterminous United States and part of Mexico (20-50°N, 130-60°W), Van der Schrier et al. (2006) constructed maps of summer moisture availability for the period 1901-2002 with a spatial resolution of 0.5° latitude x 0.5° longitude; and as a result of their efforts, they were able to report that over the area as a whole, "the 1930s and 1950s stand out as times of persistent and exceptionally dry conditions, whereas the 1970s and the 1990s were generally wet." However, they say that "no statistically significant trend was found in the mean summer PDSI over the 1901-2002 period, nor in the area percentage with moderate or severe moisture excess or deficit." In fact, they could not find a single coherent area within the PDSI maps that "showed a statistically significant trend over the 1901-2002 period."

Expanding their scope still further, Cook et al. (2004) developed a 1200-year drought history for the western half of the United States and adjacent parts of Canada and Mexico (hereafter referred to as the 'West'), based on their analysis of annually-resolved tree-ring records of summer-season Palmer Drought Severity Index that were derived for 103 points on a 2.5° x 2.5° grid, 66% of which possessed data that extended back to AD 800. This reconstruction, in their words, revealed "some remarkable earlier increases in aridity that dwarf the comparatively short-duration current drought in the 'West'." Interestingly, they report that the four driest epochs (centered on AD 936, 1034, 1150 and 1253) all occurred during an approximate 400-year interval of overall elevated aridity from AD 900 to 1300, which they describe as being "broadly consistent with the Medieval Warm Period."

Commenting further on their findings, the five researchers stated that "the overall coincidence between our megadrought epoch and the Medieval Warm Period suggests that anomalously warm climate conditions during that time may have contributed to the development of more frequent and persistent droughts in the 'West'," as well as to the megadrought that was discovered by Rein et al. (2004) to have occurred in Peru at about the same time (AD 800-1250); and after citing nine other studies that provide independent evidence of drought during this time period for various sub-regions of the West, they warn that "any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America," noting that "future droughts in the 'West' of similar duration to those seen prior to AD 1300 would be disastrous."

We certainly agree with Cook et al.'s analysis, noting that such an unfortunate fate could well befall the western United States, even in the absence of CO2-induced global warming; for the millennial-scale oscillation of climate that brought the world the Medieval Warm Period (which was obviously not CO2-induced) could well be in process of repeating itself during the possibly still-ongoing development of the Current Warm Period. And if the association between global warmth and drought in the western United States is robust, it additionally suggests that current world temperatures are still far below those experienced during much of the Medieval Warm Period.

At about the same time, Woodhouse (2004) reported what was then known about natural hydroclimatic variability throughout the United States via descriptions of several major droughts that had occurred there over the past three millennia, all but the last century of which had experienced atmospheric CO2 concentrations that never varied by more than about 10 ppm from a mean value of 280 ppm. For comparative purposes, Woodhouse began by noting that "the most extensive U.S. droughts in the 20th century were the 1930s Dust Bowl and the 1950s droughts." The first of these droughts lasted "most of the decade of the 1930s" and "occurred in several waves," while the latter "also occurred in several waves over the years 1951-1956." Far more severe than either of these two droughts, however, was what has come to be known as the 16th-Century Megadrought, which lasted from 1580 to 1600 and included northwestern Mexico in addition to the southwestern United States and the western Great Plains. Then there was what is simply called The Great Drought, which spanned the last quarter of the 13th century and was actually the last in a series of three 13th-century droughts, the first of which may have been even more severe than the last. In addition, Woodhouse notes there was a period of remarkably sustained drought in the second half of the 12th century.

It is evident from these observations, according to Woodhouse, that "the 20th century climate record contains only a subset of the range of natural climate variability in centuries-long and longer paleoclimatic records." It is also obvious that this subset, as it pertains to water shortage, does not even begin to approach the level of drought severity and duration experienced in prior centuries and millennia. This being the case, it is also clear that it would take a drought much more extreme than the most extreme droughts of the 20th century to propel the western United States and adjacent portions of Canada and Mexico into a truly unprecedented state of dryness.

Three years later, Seager (2007) studied the global context of the drought that affected nearly the entire United States, northern Mexico and the Canadian Prairies between 1998 and 2004. On the basis of atmospheric reanalysis data and ensembles of climate model simulations forced by global or tropical Pacific sea surface temperatures over the period January 1856 to April 2005, he compared the climatic circumstances of the recent drought with those of five prior great droughts of North America: (1) the Civil War drought of 1856-65, (2) the 1870s drought, (3) the 1890s drought, (4) the great Dust Bowl drought, and (5) the 1950s drought. And in doing so, he found that the 1998-2002 drought "was most likely caused by multiyear variability of the tropical Pacific Ocean," noting that the recent drought "was the latest in a series of six persistent global hydroclimate regimes, involving a persistent La Niña-like state in the tropical Pacific and dry conditions across the midlatitudes of each hemisphere." In fact, there was no aspect of this study that implicated global warming, either CO2-induced or otherwise, as a cause of -- or contributor to -- the great turn-of-the-20th-century drought that affected large portions of North America. Seager noted, for example, that "although the Indian Ocean has steadily warmed over the last half century, this is not implicated as a cause of the turn of the century North American drought because the five prior droughts were associated with cool Indian Ocean sea surface temperatures." In addition, the five earlier great droughts occurred during periods when the mean global temperature was also significantly cooler than what it was during the last great drought.

Another far-ranging study was that of Cook et al. (2007), who discussed the nature of a number of megadroughts that occurred over the past millennium and clearly exceeded in all aspects all droughts of the instrumental period. Indeed, they state that "these past megadroughts dwarf the famous droughts of the 20th century, such as the Dust Bowl drought of the 1930s, the southern Great Plains drought of the 1950s, and the current one in the West that began in 1999," all of which dramatic droughts fade into almost total insignificance when compared to the granddaddy of them all, which they describe as "an epoch of significantly elevated aridity that persisted for almost 400 years over the AD 900-1300 period."

Of central importance to North American drought formation, in the words of the four researchers, "is the development of cool 'La Niña-like' SSTs in the eastern tropical Pacific." Paradoxically, as they describe the situation, "warmer conditions over the tropical Pacific region lead to the development of cool La Niña-like SSTs there, which is drought inducing over North America." And in further explaining the mechanics of this phenomenon, on which they say both "model and data agree," Cook et al. state that "if there is a heating over the entire tropics then the Pacific will warm more in the west than in the east because the strong upwelling and surface divergence in the east moves some of the heat poleward," with the result that "the east-west temperature gradient will strengthen, so the winds will also strengthen, so the temperature gradient will increase further ... leading to a more La Niña-like state." What is more, they add that "La Niña-like conditions were apparently the norm during much of the Medieval period when the West was in a protracted period of elevated aridity and solar irradiance was unusually high."

In light of these several observations, it would appear that throughout the AD 900-1300 period of what Cook et al. call "significantly elevated aridity" in North America, the tropical Pacific Ocean likely experienced significantly elevated temperature, which may well have been far greater than anything experienced over the course of the 20th century, because there was no period of time over the last several hundred years when North America experienced anything like the seemingly endless aridity of that 400-year megadrought that coincided with the great central portion of the Medieval Warm Period. And in light of this observation, we conclude that much of the Medieval Warm Period had to have been much warmer than even the warmest portion of the 20th century, or any time since. In fact, there is reason to believe that the world as a whole may well have been warmer during the bulk of the Medieval Warm Period than it is currently, for Cook et al. write that "the persistent droughts over North America all arose as part of the response of the global climate to persistent La Niña-like conditions in the tropical Pacific Ocean." And this conclusion contradicts the climate-alarmists' primary but unfounded claim that the world is currently warmer than it has been at any other time over the past two millennia or more.

It is instructive to learn how Native Americans were impacted by different dry phases of the Medieval Warm Period, which was the subject of the study of Benson et al. (2007), who reviewed and discussed possible impacts of early-11th-, middle-12th-, and late-13th-century droughts on three Native American cultures that occupied parts of the western United States (Anasazi, Fremont, Lovelock) plus another culture that occupied parts of southwestern Illinois (Cahokia). They report, in this regard, that "population declines among the various Native American cultures were documented to have occurred either in the early-11th, middle-12th, or late-13th centuries" -- AD 990-1060, 1135-1170, and 1276-1297, respectively -- and that "really extensive droughts impacted the regions occupied by these prehistoric Native Americans during one or more of these three time periods." In particular, they say the middle-12th-century drought "had the strongest impact on the Anasazi and Mississippian Cahokia cultures," noting that "by AD 1150, the Anasazi had abandoned 85% of their great houses in the Four Corners region and most of their village sites, and the Cahokians had abandoned one or more of their agricultural support centers, including the large Richland farming complex." In addition, they write that "the sedentary Fremont appear to have abandoned many of their southern area habitation sites in the greater Unita Basin area by AD 1150 as well as the eastern Great Basin and the Southern Colorado Plateau," so that "in some sense, the 13th century drought may simply have 'finished off' some cultures that were already in decline." Lastly, they state that these "major reductions in prehistoric Native American habitation sites/population" occurred during "anomalously warm" climatic conditions, which characterized the Medieval Warm Period throughout much of the world at that particular time. And the fact that the deadly North American droughts of the MWP have never been equaled throughout all the ensuing years argues strongly that what Benson et al. call the anomalous warmth of that period has also "never been equaled throughout all the ensuing years," which further suggests (since the air's CO2 content was so much less during the MWP than it is now) that the considerably lesser warmth of today need not in any way be related to the much higher CO2 concentration of earth's current atmosphere.

At this point, we have covered large portions of the United States plus other parts of North America; and we will discuss a few additional studies that consider this larger area, beginning with that of Stahle et al. (2000). This team of eight researchers developed a long-term history of drought over North America from reconstructions of the Palmer Drought Severity Index (PDSI), based on analyses of many lengthy tree-ring records; and in doing so, they found that the 1930s Dust Bowl drought in the United States -- which was the nation's most severe, sustained, and wide-spread drought of the past 300 years -- was eclipsed in all three of these categories by a 16th-century "megadrought." Although this drought has been mentioned in some of the prior studies we have reviewed, it is worth noting the additional information that Stahle et al. present.

The 16th-century megadrought, as they describe it, persisted "from the 1540s to 1580s in Mexico, from the 1550s to 1590s over the [U.S.] Southwest, and from the 1570s to 1600s over Wyoming and Montana," and it "extended across most of the continental United States during the 1560s." It also recurred with greater intensity over the Southeast during the 1580s to 1590s; and so horrendous was this climatic event, that the researchers unequivocally stated that "the 'megadrought' of the 16th century far exceeded any drought of the 20th century." In fact, they said that "precipitation reconstruction for western New Mexico suggests that the 16th century drought was the most extreme prolonged drought in the past 2000 years."

To put these various sets of droughts in perspective, we turn to the study of Stahle et al. (2007), who used an expanded grid of tree-ring reconstructions of summer Palmer Drought Severity Indices covering the United States, southern Canada, and most of Mexico to examine the timing, intensity, and spatial distribution of decadal to multidecadal moisture regimes over North America. This work revealed that during the Current Warm Period, "the Dust Bowl drought of the 1930s and the Southwestern drought of the 1950s were the two most intense and prolonged droughts to impact North America," as did the studies of Worster (1979), Diaz (1983) and Fye et al. (2003). During the Little Ice Age, on the other hand, they report the occurrence of three megadroughts, which they define as "very large-scale drought[s] more severe and sustained than any witnessed during the period of instrumental weather observations (e.g., Stahle et al., 2000)." However, they report that still "stronger and more persistent droughts have been reconstructed with tree rings and other proxies over North America during the Medieval era (e.g., Stine, 1994; Laird et al., 2003; Cook et al., 2004)." In fact, they say that these latter megadroughts were so phenomenal that they decided to refer to them as "no-analog Medieval megadroughts."

So with megadroughts occurring at cooler-than-present temperatures and with no-analog megadroughts occurring at warmer-than-present temperatures, one must consider the possibility that something other than temperature is the driving force behind their occurrence. And there are a number of scientists who feel that that "something other" is solar variability, such as Black et al. (1999), who stated that "small changes in solar output may influence Atlantic variability on centennial time scales," Yu and Ito (1999), who felt forced "to consider solar variability as the major cause of century-scale drought frequency in the northern Great Plains," Dean and Schwalb (2000), who concluded "it seems reasonable that the cycles in aridity and eolian activity over the past several thousand years recorded in the sediments of lakes in the northern Great Plains might also have a solar connection," Verschuren et al. (2000), who indicated that variations in solar activity "may have contributed to decade-scale rainfall variability in equatorial east Africa," Hodell et al. (2001), who wrote that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing," Mensing et al. (2004), who concluded that "changes in solar irradiance may be a possible mechanism influencing century-scale drought in the western Great Basin" of the United States, Asmerom et al. (2007), who suggest that a solar link to Holocene climate operates "through changes in the Walker circulation and the Pacific Decadal Oscillation and El Niño-Southern Oscillation systems of the tropical Pacific Ocean," Garcin et al. (2007), who emphasize that the positive correlation of Lake Masoko hydrology with various solar activity proxies "implies a forcing of solar activity on the atmospheric circulation and thus on the regional climate of [a] part of East Africa," and Springer et al. (2008), who say their findings "corroborate works indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes in central and eastern North America,"

In one final and exceptionally perceptive paper dealing with North American droughts, Cook et al. (2009) wrote that "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 that "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." However, they then wrote that "the USA has been in a state of drought over much of the West for about 10 years now," and that "while severe, this turn of the century drought has not yet clearly exceeded the severity of two exceptional droughts in the 20th century," so that "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 that the turn-of-the-century drought had its greatest Drought Area Index value of 59% in the year 2002, while the Great Plains/Southwest drought covered 62% of the US in its peak year of 1954, and the Dust Bowl drought covered 77% of the US in 1934. In terms of drought duration, however, things are not quite as clear. Stahle et al. (2007) estimated that the first two droughts lasted for 12 and 14 years, respectively; Seager et al. (2005) estimated them to have lasted for 8 and 10 years; and Andreadis et al. (2005) estimated them to have lasted for 7 and 8 years, yielding means of 9 and 11 years for the two exceptional droughts, which durations are to be compared to 10 or so years for the turn-of-the-century drought, which again makes the latter drought not unprecedented compared to those that occurred earlier in the 20th century.

Real clarity, however, comes when the turn-of-the-century drought is compared to droughts of the prior millennium. Cook et al. write that "perhaps the most famous example is the 'Great Drouth' (sic) of AD 1276-1299 described by A.E. Douglass (1929, 1935)." Yet this 24-year drought was eclipsed by the 38-year drought that was 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 truly pale in comparison to 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." And each of these megadroughts, as Cook et al. describe them, occurred, in their words, "in the so-called Medieval Warm Period." And they add that "all of this happened prior to the strong greenhouse gas warming that began with the Industrial Revolution [authors' italics]."

In further ruminating about these facts in the "Conclusions and Recommendations" section of their paper, Cook et al. again state that the medieval megadroughts "occurred without any need for enhanced radiative forcing due to anthropogenic greenhouse gas forcing." And, therefore, they go on to 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."

In summation, these and many other studies conducted at various locations throughout North America -- Laird et al. (1998), Woodhouse and Overpeck (1998), Cronin et al. (2000), Fritz et al. (2000), Hidalgo et al. (2000), Benson et al. (2002), Knapp et al. (2002), Ni et al. (2002), Gray et al. (2003), Gedalof et al. (2004), Gray et al. (2004a,b), Mauget (2004), Mensing et al. (2004), Quiring (2004), Daniels and Knox (2005), Forman et al. (2005), Shapley et al. (2005), Rasmussen et al. (2006), Malamud-Roam et al. (2006), Tian et al. (2006), Woodhouse et al. (2006), Woodhouse and Lukas (2006), MacDonald and Tingstad (2007), Meko et al. (2007), MacDonald et al. (2008) and Springer et al. (2008) -- dispute the climate-alarmist claim that warming must always result in more frequent and more severe drought, while studies conducted on other continents have led to the same conclusion. However, to not unnecessarily lengthen this section of our report, we conclude it by merely providing the journal references to some of these studies in the following paragraph.

For Africa, see Holmes et al. (1997), Verschuren et al. (2000), Nicholson et al. (2001), Russell and Johnson (2005), Lau et al. (2006), Therrell et al. (2006) and Esper et al. (2007); for Asia, see Cluis and Laberge (2001), Paulson et al. (2003), Touchan et al. (2003), Kalugin et al. (2005), Davi et al. (2006), Sinha et al. (2007), Kim et al. (2009) and Zhang et al. (2009); for Europe, see Hisdal et al. (2001), Ducic (2005), Linderholm and Chen (2005), Linderholm and Molin (2005), van der Schrier et al. (2006), Wilson et al. (2005), Pfister et al. (2006) and Renard et al. (2008); and for South America, see Marengo (2009).

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