Quantitative summer-temperature reconstructions for the last 2000 years based on pollen-stratigraphical data from northern Fennoscandia

bjune, a.e., seppä, h., and birks, h.j.b.

Abstract

Study Location

We studied sediments from 11 small lakes located in the middle boreal, northern boreal, low-alpine, or low-arctic zones of northern Norway, northern Sweden, northern Finland, and north-west Russia. The lakes were selected to have as little human influence as possible, and have only small, if any, inflows and outflows.

Climate Proxy

Pollen stratigraphy :

• Detailed pollen-stratigraphical analyses were performed using consistent sampling, preparation, counting, and identification procedures. Pollen and spore values were calculated as percentages of total pollen and spores excluding all obligate aquatic taxa. We used a previously published pollen-climate transfer function for the region to reconstruct mean July temperatures. The pollen transfer function has a root-mean-square error of prediction (RMSEP) of 0.99°C and is sensitive to low frequency patterns, which can shed light on broad-scale climate changes.

Results

The temperature reconstructions from our 11 sites indicate similar patterns as those documented by most other proxy data from the region during the last two millennia. In general, a decreasing trend in temperature is inferred during this time period, but with a higher-than-present temperatures between 0 and 1100 AD (2000-850 cal yr BP), and temperatures below the present day between 1100 and 1900 AD (850-50 cal yr BP). There is no indication for a temperature peak during the Medieval Warm Period 900 – 1200 AD (1050–750 cal yr BP), whereas the cooler period between 1200 and 1900 AD (750-50 cal yr BP) corresponds in general with the ‘Little Ice Age’.

References

• Grudd H (2008) Torneträsk tree-ring width and density AD 500-2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Clim Dyn DOI 10.1007/s00382-007-0358-2
• Rosqvist GC, Leng MJ and Jonsson C (2007) North Atlantic region atmospheric circulation dynamics inferred from a late-Holocene lacustrine carbonate isotope record, northern Swedish Lapland. Holocene 17:867-873.