Laird, K.R., Fritz, S.C. and Cumming, B.F. 1998. A diatom-based reconstruction of drought intensity, duration, and frequency from Moon Lake, North Dakota: a sub-decadal record of the last 2300 years. Journal of Paleolimnology 19: 161-179.
The authors examined a high-resolution sediment core from Moon Lake, North Dakota (46.86°N, 98.16°W), which provided a sub-decadal record of salinity (precipitation/drought) over the past 2300 years. A distinct period of high salinity was noted from AD 1000 to 1200, which they say "provides evidence for an arid 'Medieval Warm Period' in the Northern Great Plains." What is more, the authors note that the peak in salinity during the MWP, and two other peaks prior to it, "have no modern equivalents," which indicates these prior three drought peaks "were of much greater intensity and duration than any in the 20th century" or the past seven centuries before that.