Cronin et al. (2000) studied the salinity gradient across sediment cores from Chesapeake Bay, the largest estuary in the United Sates, in an effort to examine precipitation variability in the surrounding watershed over the past millennium. Their work revealed the existence of a high degree of decadal and multidecadal variability between wet and dry conditions throughout the 1000-year record, where regional precipitation totals fluctuated by 25 to 30%, often in "extremely rapid [shifts] occurring over about a decade." In addition, precipitation over the last two centuries of the record was found to be generally greater than it was during the previous eight centuries, with the exception of the Medieval Warm Period (AD 1250-1350) when the [local] climate was found to be "extremely wet." Equally significant was the ten researchers' finding that the region had experienced several "mega-droughts" that had lasted for 60 to 70 years, several of which they judged to have been "more severe than twentieth century droughts."

Building upon the work of Cronin et al. were Willard et al. (2003), who examined the last 2300 years of the Holocene record of Chesapeake Bay and the adjacent terrestrial ecosystem "through the study of fossil dinoflagellate cysts and pollen from sediment cores." In doing so, they found that "several dry periods ranging from decades to centuries in duration are evident in Chesapeake Bay records." The first of these periods of lower-than-average precipitation (200 BC-AD 300) occurred during the latter part of the Roman Warm Period, while the next such period (~AD 800-1200), according to Willard et al., "corresponds to the 'Medieval Warm Period'." In addition, they identified several decadal-scale dry intervals that spanned the years AD 1320-1400 and 1525-1650.

In discussing their findings, Willard et al. note that "mid-Atlantic dry periods generally correspond to central and southwestern USA 'megadroughts', which are described by Woodhouse and Overpeck (1998) as major droughts of decadal or more duration that probably exceeded twentieth-century droughts in severity." Emphasizing this important point, they additionally indicate that "droughts in the late sixteenth century that lasted several decades, and those in the 'Medieval Warm Period' and between ~AD 50 and AD 350 spanning a century or more have been indicated by Great Plains tree-ring (Stahle et al., 1985; Stahle and Cleaveland, 1994), lacustrine diatom and ostracode (Fritz et al., 2000; Laird et al., 1996a, 1996b) and detrital clastic records (Dean, 1997)." Hence, their work in the eastern United States, together with the work of other researchers in still other parts of the country, demonstrates that 20th-century global warming has not led to the occurrence of unusually strong wet or dry periods, contradicting climate-alarmist claims that warming will exacerbate extreme climate anomalies.

In concluding our brief review of this issue as it pertains to the eastern United States, we note that Quiring (2004) introduced his study of the subject by describing the drought of 2001-2002, which by June of the latter year had produced anomalously dry conditions along most of the east coast of the country, including severe drought conditions from New Jersey to northern Florida that forced 13 states to ration water. Shortly after the drought began to subside in October of 2002, however, moist conditions returned and persisted for about a year, producing the wettest growing-season of the instrumental record. These observations, in Quiring's words, "raise some interesting questions," including the one we are considering here. As he phrased the call to inquiry, "are moisture conditions in this region becoming more variable?"

Using an 800-year tree-ring-based reconstruction of the Palmer Hydrological Drought Index to address this question, Quiring documented the frequency, severity and duration of growing-season moisture anomalies in the southern mid-Atlantic region of the United States. Among other things, this work revealed, in Quiring's words, that "conditions during the 18th century were much wetter than they are today, and the droughts that occurred during the 16th century tended to be both longer and more severe." Hence, he concluded that "the recent growing-season moisture anomalies that occurred during 2002 and 2003 can only be considered rare events if they are evaluated with respect to the relatively short instrumental record (1895-2003)," for when compared to the 800-year reconstructed record, he notes that "neither of these events is particularly unusual." In addition, Quiring reports that "although climate models predict decreases in summer precipitation and significant increases in the frequency and duration of extreme droughts, the data indicate that growing-season moisture conditions during the 20th century (and even the last 19 years) appear to be near normal (well within the range of natural climate variability) when compared to the 800-year record."

In conclusion, it is clear that climate-alarmist claims of droughts becoming more extreme and erratic in response to global warming are totally incorrect, or so, at least, is the story told by palaeoclimate data from the United States, the eastern portion of which has been highlighted in this summary.

References
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.

Dean, W.E. 1997. Rates, timing, and cyclicity of Holocene aeolian activity in north-central United States: evidence from varved lake sediments. Geology 25: 331-334.

Fritz, S.C., Ito, E., Yu, Z., Laird, K.R. and Engstrom, D.R. 2000. Hydrologic variation in the northern Great Plains during the last two millennia. Quaternary Research 53: 175-184.

Laird, K.R., Fritz, S.C., Grimm, E.C. and Mueller, P.G. 1996a. Century-scale paleoclimatic reconstruction from Moon Lake, a closed-basin lake in the northern Great Plains. Limnology and Oceanography 41: 890-902.

Laird, K.R., Fritz, S.C., Maasch, K.A. and Cumming, B.F. 1996b. Greater drought intensity and frequency before AD 1200 in the Northern Great Plains, USA. Nature 384: 552-554.

Quiring, S.M. 2004. Growing-season moisture variability in the eastern USA during the last 800 years. Climate Research 27: 9-17.

Stahle, D.W. and Cleaveland, M.K. 1994. Tree-ring reconstructed rainfall over the southeastern U.S.A. during the Medieval Warm Period and Little Ice Age. Climatic Change 26: 199-212.

Stahle, D.W., Cleaveland, M.K. and Hehr, J.G. 1985. A 450-year drought reconstruction for Arkansas, United States. Nature 316: 530-532.

Willard, D.A., Cronin, T.M. and Verardo, S. 2003. Late-Holocene climate and ecosystem history from Chesapeake Bay sediment cores, USA. The Holocene 13: 201-214.

Woodhouse, C.A. and Overpeck, J.T. 1998. 2000 years of drought variability in the Central United States. Bulletin of the American Meteorological Society 79: 2693-2714.