Writing as background for their study, Pederson et al. (2012) note that drought is "a pervasive phenomenon throughout much of North America with profound ecological and societal implications," as has been suggested by the work of Hursh and Haasis (1931), Breshears et al. (2005), Manuel (2008) and Allen et al. (2010). And getting more specific, while citing Knight (2004) and Seager et al. (2009), they add that recent moisture deficits in the southeastern US have renewed water management challenges that underscore the need to "better understand drought processes in humid, subtropical regions," which is what they thus set out to do. More specifically, Pederson et al. (2012) set out "to put the region's recent drought variability in a long-term perspective," by reconstructing historic drought trends in the Apalachicola-Chattahoochee-Flint river basin over the period 1665 to 2010 using a dense and diverse tree-ring network, which network, as they describe it, "accounts for up to 58.1% of the annual variance in warm-season drought during the 20th century and captures wet eras during the middle to late 20th century." So what did they find?

The twelve researchers say the Palmer Drought Severity Index reconstruction for their study region shows that "recent droughts are not unprecedented over the last 346 years," and that "droughts of extended duration occurred more frequently between 1696 and 1820," when most of the world was entrenched in the midst of the Little Ice Age. And they also state that their results "confirm the findings of the first reconstruction of drought in the southern Appalachian Mountain region, which indicates that the mid-18th and early 20th centuries were the driest eras since 1700, citing Stahle et al. (1988), Cook et al. (1998) and Seager et al. (2009).

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 considered 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." He therefore 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."

Moving the time window of analysis back slightly further, 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 model-based claims that warming will exacerbate extreme climate anomalies.

In one final study covering most all of the Holocene, Springer et al. (2008) constructed a multi-decadal-scale history of east-central North America's hydroclimate over the past 7,000 years, based on Sr/Ca ratios and δ¹³C data obtained from stalagmite BCC-002 of Buckeye Creek Cave (BCC) in West Virginia, USA. In doing so, the authors detected seven significant mid- to late-Holocene droughts that "correlate with cooling of the Atlantic and Pacific Oceans as part of the North Atlantic Ocean ice-rafted debris [IRD] cycle, which has been linked to the solar irradiance cycle," as per Bond et al. (1997, 2001). In addition, they found that "the Sr/Ca and δ¹³C time series display periodicities of ~200 and ~500 years," and that "the ~200-year periodicity is consistent with the de Vries (Suess) solar irradiance cycle," and that the ~500-year periodicity is likely "a harmonic of the IRD oscillations." They also report that actual "cross-spectral analysis of the Sr/Ca and IRD time series yields statistically significant coherencies at periodicities of 455 and 715 years," which latter values "are very similar to the second (725-years) and third (480-years) harmonics of the 1450 ± 500-years IRD periodicity." Such findings, in the words of the five researchers, "corroborate works indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes in central and eastern North America (Viau et al., 2002; Willard et al., 2005; Denniston et al., 2007)," and that their high-resolution time series "provide much stronger evidence in favor of solar-forcing of North American drought by yielding unambiguous spectral analysis results."

In conclusion, it is clear that 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
Allen, C.D., Macalady, A.K., Chenchouni, H., Bachelet, D., McDowell, N., Vennetier, M., Kitzbereger, T., Rigling, A., Breshears, D.D., Hogg, E.H. (Ted), Gonzalez, P., Fensham, R., Zhang, Z., Castro, J., Demidova, N., Lim, J., Allard, G., Running, S.W., Semerci, A. and Cobb, N. 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259: 660-684.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278: 1257-1266.

Breshears, D.D., Cobb, N.S., Rich, P.M., Price, K.P., Allen, C.D., Balice, R.G., Romme, W.H., Kastens, J.H., Floyd, M.L., Belnap, J., Anderson, J.J., Myers, O.B. and Meyer, C.W. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences USA 102: 15,144-15,148.

Cook, E.R., Kablack, M.A. and Jacoby, G.C. 1988. The 1986 drought in the southeastern United States - how rare an event was it? Journal of Geophysical Research 93: 14,257-14,260.

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.

Denniston, R.F., DuPree, M., Dorale, J.A., Asmerom, Y., Polyak, V.J. and Carpenter, S.J. 2007. Episodes of late Holocene aridity recorded by stalagmites from Devil's Icebox Cave, central Missouri, USA. Quaternary Research 68: 45-52.

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.

Hursh, C.R. and Haasis, F.W. 1931. Effects of 1925 summer drought on southern Appalachian hardwoods. Ecology 12: 380-386.

Knight, T. 2004. Reconstruction of Flint River Streamflow Using Tree-Rings. Water Policy Working Paper 2004/2005. Georgia Water Policy and Planning Center, Albany, Georgia, USA.

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.

Manuel, J. 2008. Drought in the southeast: lessons for water management. Environmental and Health Perspectives 116: A168-A171.

Pederson, N., Bell, A.R., Knight, T.A., Leland, C., Malcomb, N., Anchukaitis, K.J., Tackett, K., Scheff, J., Brice, A., Catron, B., Blozan, W. and Riddle, J. 2012. A long-term perspective on a modern drought in the American Southeast. Environmental Research Letters 7: 10.1088/1748-9326/7/1/014034.

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

Seager, R., Tzanova, A. and Nakamura, J. 2009. Drought in the southeastern United States: causes, variability over the last millennium, and the potential for future hydroclimate change. Journal of Climate 22: 5021-5045.

Springer, G.S., Rowe, H.D., Hardt, B., Edwards, R.L. and Cheng, H. 2008. Solar forcing of Holocene droughts in a stalagmite record from West Virginia in east-central North America. Geophysical Research Letters 35: 10.1029/2008GL034971.

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.

Stahle, D.W., Cleaveland, M.K. and Hehr, J.G. 1988. North Carolina climate changes reconstructed from tree rings: AD 372-1985. Science 240: 1517-1519.

Viau, A.E., Gajewski, K., Fines, P., Atkinson, D.E. and Sawada, M.C. 2002. Widespread evidence of 1500 yr climate variability in North America during the past 14,000 yr. Geology 30: 455-458.

Willard, D.A., Bernhardt, C.E., Korejwo, D.A. and Meyers, S.R. 2005. Impact of millennial-scale Holocene climate variability on eastern North American terrestrial ecosystems: pollen-based climatic reconstruction. Global and Planetary Change 47: 17-35.

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.