What was done
The authors provide complementary evidence for the hydrologic fingerprint of the Medieval Warm Period in central Kenya (Verschuren et al., 2000) via a study of pollen data derived from a 625-cm sediment core taken from Crescent Island Crater, which is a sub-basin of Lake Naivasha.

What was learned
There were good correlations between the lake-level and salinity histories of Verschuren et al. and the authors' pollen-derived histories of both aquatic and surrounding terrestrial vegetation.  The most striking of these correspondences occurred during the Medieval Warm Period, when from AD 980 to 1200 lake-level was at a uniform 1100-year low and woody taxa were significantly underrepresented in the pollen assemblage.

What it means
When independent data sets tell essentially the same story -- in this case, a tale of a two-century-long depression of precipitation and elevated temperature (which leads to lower lake levels) -- one can have increased confidence that the story they tell is true; and in this case, it is that there was indeed a Medieval Warm Period in central Kenya.  Furthermore, when similar stories are told by scientific studies throughout the entire world (see Medieval Warm Period in our Subject Index), it is nigh unto impossible to turn a blind eye or a deaf ear to that evidence.  Nevertheless, there are cadres of climate alarmists who do just that, because if they acknowledge the truth, it becomes evident that the global warming of the past century or so was nothing more than the natural climatic transition that would be expected to occur between the demise of the Little Ice Age (which was due to occur when it did) and the establishment of the Modern Warm Period.  And that means that the historical rise in the air's CO2 content had nothing to do with 20th century global warming and that continuing increases in atmospheric CO2 concentration will pose no threat to the climate of the earth in the 21st century.

Reference
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.