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Extreme Weather Events: Are they Influenced by Rising Atmospheric CO2?

3.2.1. Drought Trends of the Past Century


The first step in evaluating claims that rising CO2 is causing more frequent and severe droughts begins with a rather simple analysis of drought events over the past few decades, during which time the bulk of Earth's anthropogenic-produced CO2 accumulated in the atmosphere. If the observational data show no concurrent trends in drought events, or if they are shown to decline over this period, the hypothesis that rising CO2 is increasing the frequency and/or magnitude of these events can be falsified, as such findings contradict the hypothesis. This section thus examines the results of several scientific studies that have performed this initial phase of drought uniqueness evaluation.

Andreadis and Lettenmaier (2006) examined 20th-century trends in soil moisture, runoff and drought over the conterminous United States with a hydro-climatological model forced by real-world measurements of 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," in a rebuke of model-based claims concerning global warming and its effects on drought.

Quiring and Papakyriakou (2005) used an agricultural drought index (Palmer's Z-index) to characterize the frequency, severity and spatial extent of June-July moisture anomalies for 43 crop districts from the agricultural region of the Canadian prairies over the period 1920-1999. This work revealed that for the 80-year period of their study, the single most severe June-July drought on the Canadian prairies occurred in 1961, and that the next most severe droughts, in descending order of severity, occurred in 1988, 1936, 1929 and 1937, for little net overall trend. At the same time, however, they say there was an upward trend in mean June-July moisture conditions. In addition, they note that "reconstructed July moisture conditions for the Canadian prairies demonstrate that droughts during the 18th and 19th centuries were more persistent than those of the 20th century (Sauchyn and Skinner, 2001)."

Focusing on Asia, Cluis and Laberge (2001) analyzed streamflow records stored in the databank of the Global Runoff Data Center at the Federal Institute of Hydrology in Koblenz (Germany) to see if there were any changes in Asian river runoff of the type predicted by climate alarmists to lead to more frequent and more severe drought. More specifically, their study was based on the streamflow histories of 78 rivers said to be "geographically distributed throughout the whole Asia-Pacific region." The mean start and end dates of these series were 1936 5 years and 1988 1 year, respectively, representing an approximate half-century time span. Results of their analysis indicate that in the case of the annual minimum discharges of these rivers, which are the ones associated with drought, 53% of them were unchanged over the period of the study; and where there were trends, 62% of them were upward, indicative of a growing likelihood of both less frequent and less severe drought.

Noting "the media often reflect the view that recent severe drought events are signs that the climate has in fact already changed owing to human impacts," Hisdal et al. (2001) examined pertinent data from many places in Europe. Specifically, they performed a series of statistical analyses on more than 600 daily streamflow records from the European Water Archive to examine trends in the severity, duration and frequency of drought over the four time periods 1962-1990, 1962-1995, 1930-1995, and 1911-1995. This work revealed, in their words, that "despite several reports on recent droughts in Europe, there is no clear indication that streamflow drought conditions in Europe have generally become more severe or frequent in the time periods studied." Quite to the contrary, they found that "overall, the number of negative significant trends pointing towards decreasing drought deficit volumes or fewer drought events exceeded the number of positive significant trends (increasing drought deficit volumes or more drought events)."

In another paper, van der Schrier et al. (2006) constructed monthly maps of the Self-Calibrating Palmer Drought Severity Index (SC-PDSI, a variant put forward by Wells et al. (2004) of the more common PDSI) for the period 1901-2002 for Europe (35°N-70°N, 10°W-60°E), which index, in their words, "improves upon the PDSI by maintaining consistent behavior of the index over diverse climatological regions," which "makes spatial comparisons of SC-PDSI values on continental scales more meaningful." In doing so, they found that "over the region as a whole, the mid-1940s to early 1950s stand out as a persistent and exceptionally dry period, whereas the mid-1910s and late 1970s to early 1980s were very wet." Over the entire study period, however, they found that trends in the continent's summer moisture availability "fail to be statistically significant, both in terms of spatial means of the drought index and in the area affected by drought." In addition, they say that "evidence for widespread and unusual drying in European regions over the last few decades [as suggested by the work of Briffa et al. (1994) and Dai et al. (2004)] is not supported by the current work," in that "values for the total percentage area subject to extreme moisture conditions in the years 1996-99 returned to normal levels at ~2% from a maximum of nearly 10% in 1990." And in further support of their findings, the four researchers note that "the absence of a trend toward summer desiccation has recently also been observed in soil moisture records in the Ukraine (Robock et al., 2005) and supports conclusions in the current study."

Working in the Southern Hemisphere, Minetti et al. (2010) evaluated the annual occurrence of droughts and their persistence in what they describe as "an attempt to determine any aspects of the impact of global warming." This was accomplished by examining a regional inventory of monthly droughts for the portion of South America located south of approximately 22°S latitude-which was divided into six sections (the central region of Chile plus five sections making up most of Argentina). The results of this effort indicated, in the words of the authors, "the presence of long favorable tendencies [1901-2000] regarding precipitations or the inverse of droughts occurrence are confirmed for the eastern Andes Mountains in Argentina with its five sub-regions (Northwest Argentina, Northeast Argentina, Humid Pampa, West-Centre Provinces and Patagonia) and the inverse over the central region of Chile." From the middle of 2003 to 2009, however, they report "an upward trend in the occurrence of droughts with a slight moderation over the year 2006." However, they additionally note that the driest single year periods were 1910-11, 1915-16, 1916-17, 1924-25 and 1933-34, suggesting that 20th-century global warming has not promoted an abnormal increase in droughts over the southern third of South America.

Svensson et al. (2005) examined 20th-century river flow data for a group of 21 stations distributed around the globe. Individual record lengths of the 21 stations varied from 44 to 100 years, with an average of 68 years; and the three researchers' analyses of the data consisted of computing trends in both high flows and low flows using Mann-Kendall and linear regression methods. This work revealed, in the case of low-flows, nearly all stations showed increasing trends, approximately half of which were significant at the 90% level, indicative of a general trend of decreasing drought throughout the world.

Narisma et al. (2007) analyzed "global historical rainfall observations to detect regions that have undergone large, sudden decreases in rainfall [that] are statistically significant at the 99% level, are persistent for at least ten years, and ... have magnitudes that are [mostly] 10% lower than the climatological normal (1901-2000 rainfall average)." And working with the gridded high-resolution (0.5 x 0.5 degrees of latitude and longitude) global precipitation data set of Mitchell et al. (2004), which covers the period 1901-2000, they identified 30 drought episodes throughout the world that satisfied these stringent criteria during the 20th century, among which were the sudden and prolonged Sahel drought of Africa in the late 1960s, the United States Dust Bowl of the 1930s and Southwest drought of the 1950s (which also affected parts of Mexico), the strong and persistent droughts that occurred in northeast China in the 1920s, in Kazakhstan and regions of the former Soviet Union in the late 1930s, in southeast Australia in the late 1930s, and in southern Africa and eastern Europe in the 1980s, as well as the World War II droughts of 1937-1945 and the droughts that occurred over large regions of East India and Bangladesh in the 1950s.

With respect to the temporal distribution of the 30 severe and persistent droughts identified by Narisma et al., seven of them occurred during the first two decades of the 20th century (1901-1920), seven occurred during the next two decades (1921-1940), eight during the middle two decades of the century (1941-1960), but only five during the next two decades (1961-1980), and a mere three during the final two decades of the century (1981-2000), which is not at all what one would have expected if the model-based thesis propounded by climate alarmists is correct.

In commenting on their findings, the authors note that the 30 major droughts they identified were "mostly located in semi-arid and arid regions" that "are naturally prone to large fluctuations." Thus, the 30 major droughts of the 20th century were likely natural in all respects; and, hence, they are "indicative of what could also happen in the future," as Narisma et al. state in their concluding paragraph. And sooner or later they will happen, but independently of the air's CO2 concentration or the globe's temperature, just as they have done in the past.

In one final global-scale analysis, Sheffield et al. (2009) used "observation-driven simulations of global terrestrial hydrology and a cluster algorithm that searches for spatially connected regions of soil moisture," to identify "296 large scale drought events (greater than 500,000 km2 and longer than 3 months) globally for 1950-2000." And in doing so, they found that "the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity."

The results of the several studies discussed above all fail to support the hypothesis that CO2-induced global warming increased the frequency and severity of drought over the past few decades. The next section reviews work that was conducted over even longer time periods; and it also reveals no systematic change in drought in response to what climate alarmists refer to as the unprecedented warming of the late 20th and early 21st centuries.

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