Pentti Zetterberg

Fennoscandian summers from AD 500: Temperature changes on short and long timescales

Quantitative estimates of 1480 years of summer temperatures in northern Fennoscandia have previously been derived from continuous treering records from northern Sweden. Here we show the results of spectral analyses of these data. Only a few peaks in the spectra are consistently significant when the data are analyzed over a number of sub-periods. Relatively timestable peaks are apparent at periods of 2.1, 2.5, 3.1, 3.6, 4.8, ∼ 32–33 and for a range between ∼ 55–100 years. These results offer no strong evidence for solar-related forcing of summer temperatures in these regions. Our previously published reconstruction was limited in its ability to represent long-timescale temperature change because of the method used to standardize the original tree-ring data. Here we employ an alternative standardization technique which enables us to capture temperature change on longer timescales. Considerable variance is now reconstructed on timescales of several centuries. In comparison with modern normals (1951–70) generally extended periods when cool conditions prevailed, prior to the start of the instrumental record, include 500–700, 790–870, 1110–1150, 1190–1360, 1570–1750 (A.D.) with the most significant cold troughs centred on about 660, 800, 1140, 1580–1620 and 1640. Predominantly warm conditions occurred in 720–790, 870–1110 and 1360–1570 with peaks of warmth around 750, 930, 990, 1060, 1090, 1160, 1410, 1430, 1760 and 1820.

The supra-long Scots pine tree-ring record for Finnish Lapland: Part 1, chronology construction and initial inferences

This paper reviews the development of the current ‘supra-long’ pine chronology for northern Finnish Lapland. In the forest-tundra ecotone region of northern Finnish Lapland over 250 samples from living Scots pines (Pinus sylvestris L.) and over 1700 samples of subfossil pines have been collected for dendrochronological studies. In addition, over 1400 subfossils have been sampled from the forested area of Finnish Lapland. The goal of the research was to build a more than 7000-year long continuous pine ring-width chronology. The construction of the chronology is now completed. The intensive phase of the data collection and chronology building lasted about 10 years, 1989 to 1999. The major part of the Finnish Lapland master curve was con structed several years ago, but it was extremely difficult to bridge the c. 300-year gap, prior to 165 bc between the ‘absolute’ younger part of the chronology and the “‘ oating’ older part. The crucial samples were identified and assembled in the chronology in early 1999, and there is now an unbroken pine chronology about 7500 years long constructed from the subfossil forest-limit pines of northern Finnish Lapland. The severe growth depression centred on 330 bc is likely to have been caused by increased wetness. A brief summary is presented of inferred tree-line changes from the location of the samples.

Holocene humidity changes in northern Finnish Lapland inferred from lake sediments and submerged Scots pines dated by tree-rings

Conclusive evidence for a rise in water levels has been found in connection with lake-sediment studies undertaken partly in collaboration with the pine megafossil sampling and dendrochronological work in northern Finnish Lapland. The change in lake-level stands is shown by slow sedimentation rate in the early to mid-Holocene and an increase thereafter. These data indicate a regional rise in water levels during the latter part of the Holocene following a relatively dry period between 8000 and 4000 BP. Synchronous changes, also indicating rising water levels, have been observed in the diatom and cladoceran assemblages of the sediment cores. Subfossil Scots pines (Pinus sylvestris, L.) have been preserved in large quantities in small lakes in Lapland, because in many cases the rising water level has inundated the trunks after death. The position of the subfossil trunks and stumps often indicates that the pines have been growing on dry land at the sides of the lakes in which they are now submerged. Traces of a bark beetle (Tomicus minor, Hart.) have been detected in a few old pine logs found far outside the present distribution area of this insect. A total of 1722 samples of pine subfossils have been collected in the forest-tundra ecotone region of Lapland to build a continuous pine ring-width chronology over 7000 years long. The long chronology is almost finished, but its two parts are still separated by a short discontinuity around 250 bc. An absolutely dated, year-by-year chronology over 2000 years long extends from the present close to that time. The older, over 5000 years long continuous floating chronology is fixed to the timescale by several radiocarbon dates. A total of 1212 samples of pine wood have been dated and assembled within the chronologies by tree-ring cross-matching. These substantial data indicate a gradual retreat of pine tree and forest limits with some marked regional differences during the past 5000 years. According to the preliminary interpretations of the tree-ring data the variability of Holocene summer temperatures has increased towards the present time. Shifts in climatic development leading to cooler and more unstable conditions seem to have occurred in mid-Holocene time, and between 2500 and 2000 BP. The increase in humidity is probably in association with these changes in the high-frequency variability of temperature.

Variable Holocene climate, treeline dynamics and changes in natural environments in northern Finnish Lapland

Abstract The forest-tundra ecotone in northern Fennoscandia is a sensitive marginal area where climatic forcing mechanisms change environmental conditions. In this area there is a very distinct zonation of vegetation, including the northern limits of Norway spruce, Scots pine and mountain birch. The palaeoclimatological studies in Finnish Lapland focused on the limit of pine and its changes through time. Subfossil trunks of pines have been found beyond the present occurrences of this species and those findings give concrete evidence of its former wide mid-Holocene distribution area. The pine limit has retreated during the past 5000 yr indicating a 2°C lowering in temperatures. Trunks of old pines have been preserved in large quantities in small lakes of northern Lapland and they constitute a valuable source of information of past climate. Discs cut from the subfossil logs are well suited for dendrochronological studies. It is shown by several studies that the radial growth of present forest-limit pine is strongly determined by the summer temperatures. Thus a long pine chronology can be used to reconstruct time series of past summer temperature variations. A 7600 yr long tree-ring chronology has been constructed, but it still has uncertain correlation between the ring-widths around 400–100 B.C. However, there is a 2000 yr long “absolute” chronology, which will eventually be connected to a 5000 yr long “floating” chronology. The tree rings show the interannual variability of temperatures, but the distinction of the low-frequency variability is problematic. Tree-rings indicate that the interannual variability has increased during the latter part of the Holocene, presumably as a consequence of increased instability of the climatic system. Correlations are made with other climatic proxy data, but they have not allowed a consistent picture of the Holocene short-term temperature variations. It is clear that the humidity has increased in northern Fennoscandia during the past 5000–4000 yr. A period of harsh climatic conditions occurred 2500–2000 yr ago. It was an unfavourable time for the growth of forest-limit pine and in that sense it seems to have been a more severe climatic disturbance than the “Little Ice Age” in historical times a few hundred years ago.

Construction of a 7500-Year Tree-Ring Record for Scots Pine ( Pinus sylvestris , L.) in Northern Fennoscandia and its Application to Growth Variation and Palaeoclimatic Studies

Precisely dated subfossil trees and long chronologies of tree growth provide a unique calendrical record for studies on growth variations in the past and climate history. A total of 1465 subfossil Scots pines (Pinus sylvestris L.) has been sampled from 42 sites in northern Fennoscandia. The sampling sites are mostly small lakes in the timberline zone located between 68°30’ N and 70°00’ N and at elevations ranging from 75 m a.s.l. to 515 m a.s.l. A total of 1023 subfossil trees have been dated by dendrochronology. From the dated trees we have constructed a chronology for each growing site and also a common chronology for the entire area. In this study, subfossil pines are used as “growth reference” of forest site productivity in the past. The abundance of subfossil pines makes it possible to explore the longterm stand history of pine at each site over thousands of years. The unbroken treering record extends from the present back until 165 B.C. and after a 150-200-year gap until about 7500 years before the present time. The continuous part of the master curve is 664 years longer than the earlier published pine chronology for northern Sweden. This 7500-year pine master chronology can be used for dating subfossil pines from a wide area in northern Fennoscandia.

Sea ice, rain-on-snow and tundra reindeer nomadism in Arctic Russia

Sea ice loss is accelerating in the Barents and Kara Seas (BKS). Assessing potential linkages between sea ice retreat/thinning and the regions ancient and unique social–ecological systems is a pressing task. Tundra nomadism remains a vitally important livelihood for indigenous Nenets and their large reindeer herds. Warming summer air temperatures have been linked to more frequent and sustained summer high-pressure systems over West Siberia, Russia, but not to sea ice retreat. At the same time, autumn/winter rain-on-snow (ROS) events have become more frequent and intense. Here, we review evidence for autumn atmospheric warming and precipitation increases over Arctic coastal lands in proximity to BKS ice loss. Two major ROS events during November 2006 and 2013 led to massive winter reindeer mortality episodes on the Yamal Peninsula. Fieldwork with migratory herders has revealed that the ecological and socio-economic impacts from the catastrophic 2013 event will unfold for years to come. The suggested link between sea ice loss, more frequent and intense ROS events and high reindeer mortality has serious implications for the future of tundra Nenets nomadism.

Analysis of the dendroclimatic potential of polylepis pepei, P. subsericans and P. rugulosa in the tropical Andes (Peru-Bolivia)

Abstract This paper reports on investigation of the dendroclimatic potential of three Polylepis species, P. pepei, P. subsericans and P. rugulosa in Peru and Bolivia in the tropical Andes, where they form the worlds highest treeline forests up to 5,000 m a.s.l. In Bolivia, P. pepei trees were sampled close to La Paz City. In Peru, P. pepei and P. subsericans were sampled in the Vilcanota Mountains close to Urubamba City, and P. rugulosa in the Arequipa region on the slope of Coropuna Volcano. Chronologies span the 20th Century and all three species show intermediate values of mean sensitivity, common variance and signal-to-noise ratio. In general, correlation and response-function analyses revealed significant positive relationships with temperature during the rainy season for all three species in Peru and Bolivia. Relationships with precipitation were more difficult to interpret as positive relationships were observed between radial growth and precipitation at the beginning of the rainy season in all three species in Peru, whereas for P. pepei in Bolivia, the relationships with precipitation appeared to be controlled by local conditions including slope and substrate (moraine or scree slope).

On the Eve of Urbanization: Bayesian Model Dating for Medieval Turku

The era of early urbanization in Finland coincides with large variations in the atmospheric radiocarbon concentration around the 13th–14th centuries AD. Therefore, the uncertainties of an individual 14C date are too large to support studies of this time period effectively. In this work, we have employ tree-ring wiggle-matching methodology and used Bayesian modeling of dates to account for stratigraphical a priori information within Aboa Vetus Museum, Turku, Finland. The wiggle-match method helps significantly to overcome the calibration-curve-induced challenges within 14C dating. The results of the dating model agree with the typological dates of the archaeological finds and illustrate a gradual evolution of urbanization in one of the oldest cities in Finland.

Dendrochronological dating of a wooden causeway in Finland

A mean ring‐width chronology for Scots pine in eastern Finland has been compiled based on living trees and dendrochronologically dated historical timber constructions. As an application of this master chronology, a young archaeological site near the town of Hameenlinna, southern Finland, was dated. A previous assumption of the age of this site was that it could even be medieval. The construction was not older than 160 years, the road having been built after 1828, the year of growth of the outermost tree rings of the samples. A new pine chronology for southern Finland extending to the year AD 1539 is presented and it is now applicable for precise dating purposes in this area.