Molnar and Ramirez (2001) conducted a detailed watershed-based analysis of precipitation and streamflow trends for the period 1948-97 in the semiarid region of the Rio Puerco Basin of New Mexico.  They found, in their words, that "at the annual timescale, a statistically significant increasing trend in precipitation in the basin was detected."  This trend was driven primarily by an increase in the number of rainy days in the moderate rainfall intensity range, with essentially no change being observed at the high-intensity end of the spectrum.  In the case of streamflow, however, there was no trend at the annual timescale; but monthly totals increased in low-flow months and decreased in high-flow months.

What are the implications of these observations?  Increasing precipitation in a semiarid region sounds like a huge plus to us; and having most of that increase in the moderate rainfall intensity range also sounds like a plus.  Increasing streamflow in low-flow months sounds good as well, as does decreasing streamflow in high-flow months.  In fact, what more could one possibly desire in terms of changes in precipitation? ... especially in a world that climate alarmists claim is supposed to be experiencing more extreme weather events and increases in both floods and droughts?

We once thought that this prediction was a clever ploy on the part of the world's climate alarmists, i.e., that by predicting more warming-induced floods and droughts at one and the same time, they were making sure there was no way they could possibly miss in their predictions of CO2-induced calamities.  It would appear, however, that we were wrong in that assumption: they did miss!

In a much colder part of the country, Cowles et al. (2002) analyzed snow water equivalent (SWE) data obtained from four different measuring systems -- snow courses, snow telemetry, aerial markers and airborne gamma radiation -- at more than 2000 sites in the eleven westernmost states over the period 1910-1998.  This work revealed that the long-term SWE trend of this entire region was negative, but with some significant within-region differences.  In the northern Rocky Mountains and Cascades of the Pacific Northwest, for example, the trend was decidedly negative, with SWE decreasing at a rate of 0.1 to 0.2 inches per year.  In the intermountain region and southern Rockies, however, there was no change in SWE with time.  Cowles et al. additionally note that their results "reinforce more tenuous conclusions made by previous authors," citing Chagnon et al. (1993) and McCabe and Legates (1995), who studied snow course data from 1951-1985 and 1948-1987, respectively, at 275 and 311 sites.  They too found a decreasing trend in SWE at most sites in the Pacific Northwest but more ambiguity in the southern Rockies.

These findings are particularly interesting in light of the fact that nearly all climate models suggest the planet's hydrologic cycle will be enhanced in a warming world and that precipitation will increase.  This prediction is especially applicable to the Pacific Northwest of the United States, where Kusnierczyk and Ettl (2002) report that climate models predict "increasingly warm and wet winters," as do Leung and Wigmosta (1999).  Over the period of Cowles et al.'s study, however, when there was well-documented worldwide warming, precipitation that fell and accumulated as snow in the western USA did not respond as predicted.  In fact, over the Pacific Northwest, it did just the opposite.

Garbrecht and Rossel (2002) used state divisional monthly precipitation data from the US National Climatic Data Center to investigate the nature of precipitation throughout the US Great Plains from January 1895 through December 1999, finding that regions in the central and southern Great Plains experienced above-average precipitation over the last two decades of the 20th century.  This 20-year span of time was the longest and most intense wet period of the entire 105 years of record, and was primarily the result of a reduction in the number of dry years and an increase in the number of wet years.  What made it even better was the fact that the number of very wet years, in the words of the authors, "did not increase as much and even showed a decrease for many regions."  The northern and northwestern Great Plains also experienced a precipitation increase at the end of this 105-year interval, but it was primarily confined to the final decade of the 20th century; and again, as Garbrecht and Rossel report, "fewer dry years over the last 10 years, as opposed to an increase in very wet years, were the leading cause of the observed wet conditions."

It is interesting to note, in this regard, that during the period of time described by climate alarmists as having been host to the most dramatic global warming of the past two millennia, which according to them should have resulted in devastating droughts and floods, the Great Plains of the United States, like the Rio Puerco Basin of New Mexico, actually experienced the best of both water-resource worlds.  Overall, conditions got wetter, which means less drought; while the constancy - or even decline - in the number of very wet years tended to mitigate against floods.  What could possibly be better?  It looks like the new Modern Warm Period is going to eventually be classified as another Little Climatic Optimum!

Taking a look at the entire conterminous United States from 1895-1999, McCabe and Wolock (2002) evaluated and analyzed (1) values of annual precipitation minus annual potential evapotranspiration, (2) surplus water that eventually becomes streamflow, and (3) the water deficit that must be supplied by irrigation to grow vegetation at an optimum rate.  Their work revealed that for the country as a whole, there was a statistically significant increase in the first two of these three parameters, while for the third there was no change.  In describing the significance of these findings, McCabe and Wolock say "there is concern that increasing concentrations of atmospheric carbon dioxide and other radiatively active gases may cause global warming and ... adversely affect water resources."  The results of their analyses, however, reveal that over the past century of significant (but neither unusual nor unnatural) global warming, just the opposite has occurred, at least within the conterminous United States: moisture has become more available, while there has been no change in the amount of water required for optimum plant growth.

Also studying the conterminous United States were Kunkel et al. (2003), who analyzed a new data base of daily precipitation observations for the period 1895-2000.  This effort indicated, in their words, that "heavy precipitation frequencies were relatively high during the late 19th/early 20th centuries, decreasing to a minimum in the 1920s and 30s, followed by a general increase into the 1990s."  More specifically, they note that "for 1-day duration events, frequencies during 1895-1905 are comparable in magnitude to frequencies in the 1980s and 1990s," while "for 5- and 10-day duration events, frequencies during 1895-1905 are only slightly smaller than late 20th century values."

In commenting on these findings, Kunkel et al. note that since enhanced greenhouse gas forcing of the climate system was very small in the early years of this record, the elevated extreme precipitation frequencies of that time "were most likely a consequence of naturally forced variability," which further suggests, in their words, "the possibility that natural variability could be an important contributor to the recent increases."  This is also the conclusion of Kunkel (2003), who in a review of this and other pertinent studies states that frequencies of extreme precipitation events in the United States in the late 1800s and early 1900s "were about as high as in the 1980s/1990s."  Consequently, he too concludes that "natural variability in the frequency of precipitation extremes is quite large on decadal time scales and cannot be discounted as the cause or one of the causes of the recent increases."

Working with proxy data that extend much further back in time, Haston and Michaelsen (1997) developed a 400-year history of precipitation for 29 stations in coastal and near-interior California between San Francisco Bay and the U.S.-Mexican border using tree-ring chronologies.  Their research revealed that although region-wide precipitation during the 20th century was higher than what was experienced during the preceding three centuries, it was also "less variable compared to other periods in the past," both of which characteristics are huge positive developments for both man and nature in this important region of California.

In a similar study, Gray et al. (2003) examined fifteen tree ring-width series that had been used in previous reconstructions of drought for evidence of low-frequency variation in precipitation in five regional composite chronologies pertaining to the central and southern Rocky Mountains.  In describing what they found, they say that "strong multidecadal phasing of moisture variation was present in all regions during the late 16th century megadrought," and that "oscillatory modes in the 30-70 year domain persisted until the mid-19th century in two regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought."  They also note that "severe drought conditions across consecutive seasons and years in the central and southern Rockies may ensue from coupling of the cold phase PDO [Pacific Decadal Oscillation] with the warm phase AMO [Atlantic Multidecadal Oscillation] (Cayan et al., 1998; Barlow et al., 2001; Enfield et al., 2001)," something they envision happening in both the severe drought of the 1950s and the late 16th-century megadrought.  Hence, there is no particular reason to associate any of the wetter or drier periods of the 20th century with global warming, for both conditions appear to be naturally recurring products of various climate "regime shifts" in the Pacific and Atlantic Oceans that are independent of the mean thermal state of the planet.

Going back even further in time, Ni et al. (2002) developed a 1000-year history of cool-season (November-April) precipitation for each climate division in Arizona and New Mexico from a network of 19 tree-ring chronologies.  With respect to drought, they found that "sustained dry periods comparable to the 1950s drought" occurred in "the late 1000s, the mid 1100s, 1570-97, 1664-70, the 1740s, the 1770s, and the late 1800s."  They also note that the 1950s drought "was large in scale and severity, but it only lasted from approximately 1950 to 1956," whereas the 16th century megadrought lasted more than four times longer.  With respect to the opposite of drought, Ni et al. report that several wet periods comparable to the wet conditions seen in the early 1900s and after 1976 occurred in "1108-20, 1195-1204, 1330-45, the 1610s, and the early 1800s."  They also note that "the most persistent and extreme wet interval occurred in the 1330s."

Speaking of the causes of the different precipitation extremes, Ni et al. say that "the 1950s drought corresponds to La Niña/-PDO [Pacific Decadal Oscillation] and the opposite polarity [+PDO] corresponds to the post-1976 wet period," which leads them to hypothesize that "the prominent shifts seen in the 1000 year reconstructions in Arizona and New Mexico may also be linked to strong shifts of the coupled ENSO-PDO system."  For the particular part of the world covered by their study, therefore, there appears to be nothing unusual about the extremes of both wetness and dryness experienced during the 20th century.

In another equally long study, but on the opposite side of the country, Cronin et al. (2000) measured and analyzed salinity gradients across sediment cores extracted from Chesapeake Bay, the largest estuary in the United Sates, in an effort to examine precipitation variability in the surrounding watershed over the past 1000 years.  They found a high degree of decadal and multidecadal variability between wet and dry conditions throughout the record, where regional precipitation totals fluctuated between 25 to 30%, often in "extremely rapid [shifts] occurring over about a decade."  Precipitation over the last two centuries, however, was on average greater than what it was during the previous eight centuries, with the exception of the Medieval Warm Period (AD 1250-1350), when the climate was judged to have been "extremely wet."  In addition, it was determined that this region, like the southwestern United States, had experienced several "mega-droughts," lasting from 60-70 years in length, some of which Cronin et al. describe as being "more severe than twentieth century droughts."

Also like the study of Ni et al., Cronin et al.'s work reveals nothing unusual about precipitation during the 20th century, the latter two decades of which climate alarmists adamantly claim comprise the warmest such period of the past two millennia.  It indicates, for example, that both wetter and drier intervals occurred repeatedly in the past in the Chesapeake Bay watershed; and there is no reason not to believe they will occur in the future … with or without any further global warming.  Consequently, there is no basis in real-world data for predicting warming-induced droughts and floods in this and other parts of the world; and to imply that humanity has the power to prevent such events, as climate alarmists do, is downright deceitful.  Our efforts should be directed to preparing for these things, not deluding ourselves into thinking we can prevent them, for we can't.

References
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