Starting in North America, 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.-Mexico border using tree-ring chronologies.  Over the past century, which was the warmest hundred years of this period, they found that region-wide precipitation in this area was not more variable but less variable "compared to other periods in the past."

Across the continent on the east coast of the United States, Cronin et al. (2000) found much the same thing. In analyzing salinity profiles of sediment cores extracted from Chesapeake Bay in an effort to determine past precipitation variability in the surrounding watershed, they found that the region had experienced several 60- to 70-year "mega-droughts" over the past thousand years, many of which were not less severe but more severe than droughts of the twentieth century, when climate alarmists claim the world was far warmer than it had been over the prior 900 years.

Dropping down to the Caribbean Sea, Watanabe et al. (2001) analyzed ð¹⁸O and Mg/Ca ratios in cores obtained from a Montastrea faveolata coral in an effort to reconstruct a long-term history of seasonal variability in sea surface temperature and salinity there.  They found that mean sea surface temperatures during the Little Ice Age were about 2°C colder than they are currently; while sea surface salinity exhibited not less variability but more variability than it does now, specifically noting 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."  In harmony with the results of the other studies we have reviewed, but in contrast to the climate-alarmist claim that weather phenomena are more extreme in a warmer world, they report 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."

Studies from other parts of the world report findings similar to those from North America.  In Mongolia, Pederson et al. (2001) used tree-ring chronologies to reconstruct annual precipitation and streamflow histories for the period 1651-1995, discovering that "variations over the recent period of instrumental data [i.e., the warmest part of the record] are not unusual relative to the prior record."  In a review of African studies, Nicholson (2001) likewise determined that the protracted aridity that has affected nearly all of the continent, particularly since the 1980s, is not in itself "evidence of irreversible global change," because an even longer period of similar conditions occurred between 1800 and 1850, when the earth was in the clutches of the Little Ice Age, a period of cold that was without precedent in at least the last 6500 years, even in Africa (Lee-Thorp et al., 2001), while similar findings and sentiments have been reported by Nicholson and Yin (2001) with respect to climatic and hydrologic conditions in equatorial East Africa.  Last of all, Kripalani and Kulkarni (2001) analyzed summer monsoon (June-September) rainfall data from 120 east Asia stations for the period 1881-1998, finding no significant long-term trends in the data and noting that the decadal variability inherent in the record "appears to be just a part of natural climate variations."

In summation, all of these studies from various parts of the world attest to the fact that the warming of the past century has not increased the variability of precipitation and, therefore, extremes of wetness and dryness.  Hence, there is no basis in real-world data to believe that any further warming of the world would bring the planet any greater extremes of wetness and dryness in the future.

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.

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.

Kripalani, R.H. and Kulkarni, A.  2001.  Monsoon rainfall variations and teleconnections over south and east Asia.  International Journal of Climatology 21: 603-616.

Lee-Thorp, J.A., Holmgren, K., Lauritzen, S.-E., Linge, H., Moberg, A., Partridge, T.C., Stevenson, C. and Tyson, P.D.  2001.  Rapid climate shifts in the southern African interior throughout the mid to late Holocene.  Geophysical Research Letters 28: 4507-4510.

Nicholson, S.E.  2001.  Climatic and environmental change in Africa during the last two centuries.  Climate Research 17: 123-144.

Nicholson, S.E. and Yin, X.  2001.  Rainfall conditions in equatorial East Africa during the Nineteenth Century as inferred from the record of Lake Victoria.  Climatic Change 48: 387-398.

Pederson, N., Jacoby, G.C., D'Arrigo, R.D., Cook, E.R. and Buckley, B.M.  2001.  Hydrometeorological reconstructions for northeastern Mongolia derived from tree rings: 1651-1995.  Journal of Climate 14: 872-881.

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.