Larry Huldén writes:
The Finnish lake sediments can not be used for temperature interpretations in the 18th to 20th century unless you know exactly the history of the regional lake environment conditions. We have 180,000 lakes in Finland. It is very easy to cherry pick among them and say that it is a random sample. Of all lakes, 1,500 lakes are affected by lowering of water levels. These must be omitted. Many other are affected by agriculture including forestry. This affects the relative components of the sediments. This is well known although somebody can by chance use them for climate trends. Finnish prof. Matti Saarnisto has showed me graphs of lake sediments from Finland which can be used for temperature trends but still show strong deviations in the recent 200 years because of agriculture. These lakes are not always very close to agricultural sites.
In addition, we must remember that the fauna or flora in the sediments do not represent the temperature of the air because long term trends in water temperatures do not correlate with long term trends in air temperatures. I had a poster on this phenomenon in Italy in 2001 (or 2002?).
Finnish Museum of Natural History
Timo Hameranta writes:
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).”
Ref: Tiljander, Mia, Matti Saarnisto, Antti E. K. Ojala and Timo Saarinen, 2003. A 3000-year palaeoenvironmental record from annually laminated sediment of Lake Korttajärvi, central Finland. Boreas, Vol. 26, pp. 566–577. Oslo. ISSN 0300-9483, December 2003
Other references of Finnish climate are e.g.
Haltia-Hovi, Eeva, Timo Saarinen, and Maaret Kukkonen, 2007. A 2000-year record of solar forcing on varved lake sediment in eastern Finland. Quaternary Science Reviews Vol. 26, No 5-6, pp. 678-689, March 2007, online http://www.cfa.harvard.edu/~wsoon/BinWang07-d/HaltiaHovietal07-2000yrSolarActivityEastFinland.pdf
Ogurtsov, Maxim G., Oleg M. Raspopov, Samuli Helama, Marku Oinonen, Markus Lindholm, Hogne Jungner, and Jouko Meriläinen, 2008. Climatic variability along a north-south transect of Finland over the last 500 years: Signature of solar influence or internal climate oscillations? Geografiska Annaler, Series A: Physical Geography Vol. 90, No 2, pp. 141-150, June 2008
Timonen, M., Mielikäinen, K., and Helama, S. 2008. Climate variation (cycles and trends) and climate predicting from tree-rings. Presentation at TRACE 2008: Tree Rings in Archaeology, Climatology and Ecology, April 27-30, Zakopane, Poland.