What was done
The authors conducted a number of high-resolution analyses -- including varve thickness, relative X-ray density, pollen and diatom assessments, and organic matter loss-on-ignition (LOI) -- on a 3000-year varved sediment sequence obtained from Lake Korttajarvi in central Finland, after which they compared their results with those of other palaeoenvironmental studies conducted in Finland.

What was learned
Tiljander et al. discovered "an organic rich period from AD 980 to 1250" that they say "is chronologically comparable with the well-known 'Medieval Warm Period'."  During this time interval, they report that "the sediment structure changes" and "less mineral material accumulates on the lake bottom than at any other time [our italics] in the 3000 years sequence analyzed and the sediment is quite organic rich (LOI ~20%)."  From these observations they conclude that "the winter snow cover must have been negligible, if it existed at all [our italics], and spring floods must have been of considerably lower magnitude than during the instrumental period (since AD 1881)," which conditions they equate with a winter temperature approximately 2°C warmer than at present.

In support of this conclusion, Tiljander et al. cite much corroborative evidence.  They note, for example, that "the relative lack of mineral matter accumulation and high proportion of organic material between AD 950 and 1200 was also noticed in two varved lakes in eastern Finland (Saarinen et al. 2001) as well as in varves of Lake Nautajarvi in central Finland c. AD 1000-1200 (Ojala, 2001)."  They also note that "a study based on oak barrels, which were used to pay taxes in AD 1250-1300, indicates that oak forests grew 150 km north of their present distribution in SW Finland and this latitudinal extension implies a summer temperature 1-2°C higher than today (Hulden, 2001)."  And they report that "a pollen reconstruction from northern Finland suggests that the July mean temperature was c. 0.8°C warmer than today during the Medieval Climate Anomaly (Seppa, 2001)."

What it means
In these several studies, the authors all conclude that both summer and winter temperatures over much of the Medieval Warm Period throughout many parts of Finland were significantly warmer than they are currently.  In this part of the world, therefore, the climate-alarmist claim that current temperatures are unprecedented over the past one to two millennia rings rather hollow.

References
Hulden, L.  2001.  Ektunnor och den medeltida varmeperioden i Satakunda.  Terra 113: 171-178.

Ojala, A.E.K.  2001.  Varved Lake Sediments in Southern and Central Finland: Long Varve Chronologies as a Basis for Holocene Palaeoenvironmental Reconstructions.  Geological Survey of Finland, Espoo.

Saarinen, T., Tiljander, M. and Saarnisto, M.  2001.  Medieval climate anomaly in Eastern Finland recorded by annually laminated lake sediments.  Monsoon 3: 86-89.

Seppa, H.  2001.  Long-term climate reconstructions from the Arctic tree-line.  A NARP Symposium. The Arctic on Thinner Ice.  10-11 May 2001, Oulu, Finland, Abstracts, p. 29.