What was done
Based on data obtained from hundreds of moisture-sensitive Scots pine tree-ring records originating in Finland, and using regional curve standardization (RCS) procedures, the authors developed what they describe as "the first European dendroclimatic precipitation reconstruction," which "covers the classical climatic periods of the Little Ice Age (LIA), the Medieval Climate Anomaly (MCA), and the Dark Ages Cold Period (DACP)," running all the way from AD 670 to AD 1993.

What was learned
Helama et al. say their data indicate that "the special feature of this period in climate history is the distinct and persistent drought, from the early ninth century AD to the early thirteenth century AD," which interval "precisely overlaps the period commonly referred to as the MCA, due to its geographically widespread climatic anomalies both in temperature and moisture." In addition, they report that "the reconstruction also agrees well with the general picture of wetter conditions prevailing during the cool periods of the LIA (here, AD 1220-1650) and the DACP (here, AD 720-930)."

What it means
In discussing their findings, the three Finnish scientists note that "the global medieval drought that we found occurred in striking temporal synchrony with the multicentennial droughts previously described for North America (Stine, 1994; Cook et al., 2004, 2007), eastern South America (Stine, 1994; Rein et al., 2004), and equatorial East Africa (Verschuren et al., 2000; Russell and Johnson, 2005, 2007; Stager et al., 2005) between AD 900 and 1300." Noting further that "the global evidence argues for a common force behind the hydrological component of the MCA," they report that "previous studies have associated coeval megadroughts during the MCA in various parts of the globe with either solar forcing (Verschuren et al., 2000; Stager et al., 2005) or the ENSO (Cook et al., 2004, 2007; Rein et al., 2004; Herweijer et al., 2006, 2007; Graham et al., 2007, Seager et al., 2007)," stating that "the evidence so far points to the medieval solar activity maximum (AD 1100-1250), because it is observed in the Δ¹⁴C and ¹⁰Be series recovered from the chemistry of tree rings and ice cores, respectively (Solanki et al., 2004)."

And so the evidence continues to mount for a global and solar-induced Medieval Warm (and Dry!) Period, which likely eclipsed (in both categories) what the world has so far experienced during the Current Warm Period.

References
Cook, E.R., Woodhouse, C.A., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Cook, E.R., Seager, R., Cane, M.A. and Stahle, D.W. 2007. North American droughts: Reconstructions, causes and consequences. Earth Science Reviews 81: 93-134.

Graham, N., Hughes, M.K., Ammann, C.M., Cobb, K.M., Hoerling, M.P., Kennett, D.J., Kennett, J.P., Rein, B., Stott, L., Wigand, P.E. and Xu, T. 2007. Tropical Pacific-mid-latitude teleconnections in medieval times. Climatic Change 83: 241-285.

Herweijer, C., Seager, R. and Cook, E.R. 2006. North American droughts of the mid to late nineteenth century: History, simulation and implications for Medieval drought. The Holocene 16: 159-171.

Herweijer, C., Seager, R., Cook, E.R. and Emile-Geay, J. 2007. North American droughts of the last millennium from a gridded network of tree-ring data. Journal of Climate 20: 1353-1376.

Rein, B., Luckge, A. and Sirocko, F. 2004. A major Holocene ENSO anomaly during the Medieval period. Geophysical Research Letters 31: 10.1029/2004GL020161.

Russell, J.M. and Johnson, T.C. 2005. A high-resolution geochemical record from Lake Edward, Uganda Congo and the timing and causes of tropical African drought during the late Holocene. Quaternary Science Reviews 24: 1375-1389.

Russell, J.M. and Johnson, T.C. 2007. Little Ice Age drought in equatorial Africa: Intertropical Convergence Zone migrations and El Niño-Southern Oscillation variability. Geology 35: 21-24.

Seager, R., Graham, N., Herweijer, C., Gordon, A.L., Kushnir, Y. and Cook, E. 2007. Blueprints for Medieval hydroclimate. Quaternary Science Reviews 26: 2322-2336.

Solanki, S.K., Usoskin, I.G., Kromer, B., Schussler, M. and Beer, J. 2004. Unusual activity of the sun during recent decades compared to the previous 11,000 years. Nature 431: 1084-1087.

Stager, J.C., Ryves, D., Cumming, B.F., Meeker, L.D. and Beer, J. 2005. Solar variability and the levels of Lake Victoria, East Africa, during the last millennium. Journal of Paleolimnology 33: 243-251.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during medieval time. Nature 369: 546-549.

Verschuren, D., Laird, K.R. and Cumming, B.F. 2000. Rainfall and drought in equatorial East Africa during the past 1,100 years. Nature 403: 410-414.