Temperature patterns over the past eight centuries in Northern Fennoscandia inferred from sedimentary diatoms Jan Weckström a, ⁎ , Atte Korhola a , Panu Erästö b , Lasse Holmström c a Environmental Change Research Unit (ECRU), Department of Biological and Environmental Sciences, P.O. Box 65, FIN-00014, University of Helsinki, Finland b Department of Mathematics and Statistics, P.O. Box 68, FIN-00014, University of Helsinki, Finland c Department of Mathematical Sciences, P.O. Box 3000, FIN-90014, University of Oulu, Finland Received 14 June 2005 Available online 9 March 2006 Abstract Establishing natural climate variability becomes particularly important in remote polar regions, especially when considering questions regarding higher than average warming. We present a high-resolution record of temperature variability for the past 800 yr based on sedimentary diatoms from a treeline lake in Finnish Lapland. The BSiZer multiscale smoothing technique is applied to the data to identify significant features in the record at different temporal levels. The overall reconstruction shows relatively large multi-centennial temperature variability with a total range of about 0.6–0.8°C. At millennial scales, the temperatures exhibit a statistically significant long-term cooling trend prior to industrialization (ΔT = -0.03°C/century). At the centennial timescale, three warm time intervals were identified around AD 1200–1300 (terminal phase of the Medieval Warm Period, MWP), 1380–1550 and from 1920 until the present. Pronounced coolness occurred between AD 1600 and 1920, indicative of the Little Ice Age (LIA). At the decadal level, certain shorter-term climate excursions were revealed. The warmest ∼10–30 yr, non- overlapping periods occurred in AD 1220–1250, 1470–1500 and 1970–2000, respectively. The classic events of MWP and LIA are evident in our record, as is also the 20th century warming. © 2006 University of Washington. All rights reserved. Keywords: Diatoms; Paleoclimatology; Climate variability; Temperature; BSiZer; Northern Fennoscandia; Medieval Warm Period; Little Ice Age Introduction Global land surface temperatures have increased by ca. 0.8°C since the late 19th century, and the warming trend has increased to 0.17°C/decade during the past 25-yr period (1979– 2004) (http://lwf.ncdc.noaa.gov/oa/climate/research/trends. html). According to model simulations, large changes in temperatures and precipitation patterns are predicted to continue in the future. Although there are slight differences in the model outcomes regarding the amplitude and speed of the changes, all models agree that the temperature increases will be much greater in the Arctic than in areas of lower latitudes (ACIA, 2004). However, long-term instrumental or historical climate records from high latitudes, which can be used for validation of climate models, are scarce. Instrumental records therefore need to be extended in time using data from proxy sources such as tree rings and marine and lake sediments. Temperature records of the past centuries can be used as analogues of the expected natural variation in the future. The rapid development of quantitative paleoecology has increased the use of sedimentary diatoms as tools in reconstructing past environments. In addition to the common use of diatom algae as indicators of changes in water pH (e.g., Birks et al., 1990a; Weckström et al., 2003), nutrients (e.g., Bennion et al., 1996) and salinity (e.g., Fritz et al., 1991; Laird et al., 1996), they have lately also been used as proxy indicators in paleoclimatological studies (e.g., Korhola et al., 2000; Rosén et al., 2001; Bigler et al., 2002; Wolfe, 2003). Korhola et al. (2000) published the first diatom-based quantitative climate record covering the entire Holocene for northwestern Finnish Lapland. Their results were generally in a good agreement with the known or assumed climate history of the area. Especially for the last ca. 1000 yr, the record seemed to mimic well the known Quaternary Research 66 (2006) 78 – 86 www.elsevier.com/locate/yqres ⁎ Corresponding author. Fax: +358 9 191 57788. E-mail address: jan.weckstrom@helsinki.fi (J. Weckström). 0033-5894/$ - see front matter © 2006 University of Washington. All rights reserved. doi:10.1016/j.yqres.2006.01.005