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Precipitation (Variability - North America) -- Summary
Climate alarmists vociferously contend that global warming is responsible for creating more frequent and greater extremes of weather of all types. We here investigate this claim as it pertains to precipitation in North America, focusing on the occurrence of extreme wet and dry periods as described in several papers we have reviewed on our website.

Cronin et al. (2000) studied salinity gradients across sediment cores extracted from Chesapeake Bay, the largest estuary in the United Sates, in an effort to determine precipitation variability in the surrounding watershed over the prior millennium. Their efforts were successful; and they discovered there was a high degree of decadal and multidecadal variability in moisture conditions over the 1000-year period, with regional precipitation totals fluctuating by between 25 to 30%, often in extremely rapid shifts occurring over about a decade. They also determined that precipitation was generally greater over the last two centuries than it was over the eight previous centuries, with the exception of a portion of the Medieval Warm Period (AD 1250-1350), when the climate was extremely wet. In addition, they found that the region surrounding Chesapeake Bay had experienced several "mega-droughts" lasting from 60-70 years in length, some of which the researchers say "were more severe than twentieth century droughts." Likewise, across the continent, 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; and their work also revealed that "region-wide precipitation during the last 100 years has been unusually high and less variable compared to other periods in the past."

Crossing the continent yet again, and dropping down to the Caribbean Sea, Watanabe et al. (2001) analyzed delta 18O/16O and Mg/Ca ratios in cores obtained from a coral in an effort designed to examine seasonal variability in sea surface temperature and salinity there during the Little Ice Age. In doing so, they found that sea surface temperatures during this period were about 2°C colder than they are currently; while sea surface salinity exhibited greater variability than it does now, indicating that during the Little Ice Age "wet and dry seasons were more pronounced."

Up in Canada, Zhang et al. (2001) analyzed the spatial and temporal characteristics of extreme precipitation events for the period 1900-1998, using what they describe as "the most homogeneous long-term dataset currently available for Canadian daily precipitation." This exercise indicated that decadal-scale variability was a dominant feature of both the frequency and intensity of extreme precipitation events; but it provided "no evidence of any significant long-term changes" in these indices during the 20th century. However, their analysis of precipitation totals (extreme and non-extreme) did reveal a slightly increasing trend across Canada during the period of study, but it was found to be due to increases in the number of non-heavy precipitation events. Consequently, the researchers concluded that "increases in the concentration of atmospheric greenhouse gases during the twentieth century have not been associated with a generalized increase in extreme precipitation over Canada."

Dropping down into the Uinta Basin Watershed of northeastern Utah, Gray et al. (2004) used cores extracted from 107 piņon pines at four different sites to develop a proxy record of annual (June to June) precipitation spanning the period AD 1226-2001. Based on their findings, they report that "single-year dry events before the instrumental period tended to be more severe than those after 1900," and that decadal-scale dry events were longer and more severe prior to 1900 as well. In particular, they found that "dry events in the late 13th, 16th, and 18th centuries surpass the magnitude and duration of droughts seen in the Uinta Basin after 1900." At the other end of the spectrum, they report that the 20th century contained two of the strongest wet intervals (1938-1952 and 1965-1987), although the two periods were only the seventh and second most intense wet regimes, respectively, of the entire record. Hence, it would appear that in conjunction with 20th-century global warming, precipitation extremes (both high and low) within the Uinta Basin of northeastern Utah have become attenuated as opposed to amplified.

Last of all, we come to the study of Rasmussen et al. (2006), who had previously demonstrated that "speleothems from the Guadalupe Mountains in southeastern New Mexico are annually banded, and variations in band thickness and mineralogy can be used as a record of regional relative moisture (Asmerom and Polyak, 2004)." In their new study, therefore, they continued this tack, concentrating on "two columnar stalagmites collected from Carlsbad Cavern (BC2) and Hidden Cave (HC1) in the Guadalupe Mountains." So what did they find?

The three researchers report that "both records, BC2 and HC1, suggest periods of dramatic precipitation variability over the last 3000 years, exhibiting large shifts unlike anything seen in the modern record [our italics]." Second, they report that the time interval from AD 900-1300 coincides with the well-known Medieval Warm Period and "shows dampened precipitation variability and overall drier conditions" that are "consistent with the idea of more frequent La Niņa events and/or negative PDO phases causing elevated aridity in the region during this time." Third, they indicate that the preceding and following colder centuries "show increased precipitation variability ... coinciding with increased El Niņo flooding events."

In light of these observations, it is clear that climate alarmists go way overboard when they characterize significant droughts and floods of recent times as "unprecedented" over the past millennium or more, as well as when they attribute them to CO2-induced global warming. Moisture extremes much greater than those observed in the modern era are neither unusual nor manmade; they are simply a normal part of earth's natural climatic variability. Also contradicting climate-alarmist dogma, the three researchers' work, like that of many others, suggests that dramatic El Niņo events are typically less significant during multi-century warm periods than they are during multi-century cold periods.

References
Asmerom, Y. and Polyak, V.J. 2004. Comment on "A test of annual resolution in stalagmites using tree rings." Quaternary Research 61: 119-121.

Cronin, T., Willard, D., Karlsen, A., Ishman, S., Verardo, S., McGeehin, J., Kerhin, R., Holmes, C., Colman, S. and Zimmerman, A. 2000. Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments. Geology 28: 3-6.

Gray, S.T., Jackson, S.T. and Betancourt, J.L. 2004. Tree-ring based reconstructions of interannual to decadal scale precipitation variability for northeastern Utah since 1226 A.D. Journal of the American Water Resources Association 40: 947-960.

Haston, L. and Michaelsen, J. 1997. Spatial and temporal variability of southern California precipitation over the last 400 yr and relationships to atmospheric circulation patterns. Journal of Climate 10: 1836-1852.

Rasmussen, J.B.T., Polyak, V.J. and Asmerom, Y. 2006. Evidence for Pacific-modulated precipitation variability during the late Holocene from the southwestern USA. Geophysical Research Letters 33: 10.1029/2006GL025714.

Watanabe, T., Winter, A. and Oba, T. 2001. Seasonal changes in sea surface temperature and salinity during the Little Ice Age in the Caribbean Sea deduced from Mg/Ca and 18O/16O ratios in corals. Marine Geology 173: 21-35.

Zhang, X., Hogg, W.D. and Mekis, E. 2001. Spatial and temporal characteristics of heavy precipitation events over Canada. Journal of Climate 14: 1923-1936.

Last updated 25 April 2007