Matti Eronen

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.

Trends in recent temperature and radial tree growth spanning 2000 years across northwest Eurasia

This paper describes variability in trends of annual tree growth at several locations in the high latitudes of Eurasia, providing a wide regional comparison over a 2000-year period. The study focuses on the nature of local and widespread tree-growth responses to recent warming seen in instrumental observations, available in northern regions for periods ranging from decades to a century. Instrumental temperature data demonstrate differences in seasonal scale of Eurasian warming and the complexity and spatial diversity of tree-growing-season trends in recent decades. A set of long tree-ring chronologies provides empirical evidence of association between inter-annual tree growth and local, primarily summer, temperature variability at each location. These data show no evidence of a recent breakdown in this association as has been found at other high-latitude Northern Hemisphere locations. Using Kendalls concordance, we quantify the time-dependent relationship between growth trends of the long chronologies as a group. This provides strong evidence that the extent of recent widespread warming across northwest Eurasia, with respect to 100- to 200-year trends, is unprecedented in the last 2000 years. An equivalent analysis of simulated temperatures using the HadCM3 model fails to show a similar increase in concordance expected as a consequence of anthropogenic forcing.

New ice core evidence for a volcanic cause of the A.D. 536 dust veil

In New Zealand human cryptosporidiosis demonstrates spring and autumn peaks of incidence with the spring peak being three times greater in magnitude than the autumn peak. The imbalance between the two peaks is notable, and may be associated with the high livestock density in New Zealand. In the summer and autumn the cryptosporidiosis rate was positively associated with temperatures in the current and previous month, highlighting the importance of outdoor recreation to transmission. No associations between spring incidence and weather were found providing little support for the importance of drinking-water pathways. Imported travel cases do not appear to be an important factor in the aetiology of cryptosporidiosis in New Zealand.

Sub-Milankovitch solar forcing of past climates: Mid and late Holocene perspectives

A comparison was performed of solar activity and terrestrial temperature records, both derived from tree rings (i.e., without dating uncertainties), with identification of detailed and highly quantified time- and timescale-dependent characteristics of solar forcing on climate through the current interglacial in the context of oceanic variability. The tree-ring–derived temperature record from high latitudes of Europe (Lapland) exhibits persistent annual-to-millennial–scale variations, with multidecadal to multicentennial periodicities reminiscent of the Sun9s periodicities. At millennial scales, cool temperatures coincided with large-scale glacial maxima. Moreover, millennial and bimillennial modes of climate variability were correlative with variations in sunspot numbers on similar scales, with near-century and near-zero lags, respectively. Although they were subtle in amplitude, the sub-Milankovitch–scale changes in the reception of the Sun9s energy could thus suffice to noticeably modulate interglacial climate variations. The relative significance of timescale-dependent, Sun-climate linkages has likely varied during the mid and late Holocene times, respectively. Thus, the warmer and cooler paleotemperatures during the Medieval Climate Anomaly and Little Ice Age were better explained by solar variations on a millennial rather than bimillennial scale. The observed variations may have occurred in association with internal climate amplification (likely, thermohaline circulation and El Nino–Southern Oscillation activity). The near-centennial delay in climate in responding to sunspots indicates that the Sun9s influence on climate arising from the current episode of high sunspot numbers may not yet have manifested itself fully in climate trends. If neglected in climate models, this lag could cause an underestimation of twenty-first–century warming trends.

MULTICENTENNIAL RING-WIDTH CHRONOLOGIES OF SCOTS PINE ALONG A NORTH-SOUTH GRADIENT ACROSS FINLAND

Four regional Scots pine ring-width chronologies at the northern forest-limit, and in the northern, middle and southern boreal forest belts in Finland cover the last fourteen centuries. Tree-ring statistics and response functions were examined, and tree-ring width variation was also compared to North Atlantic Oscillation (NAO) and volcanic forcing. The tree-ring statistics show evidence of an ecogeographical gradient along a north–south transect. The three northernmost regional chronologies share a positive response to mid-summer temperature, and all four chronologies show positive and significant correlation to early-summer precipitation. Moreover, a positive and significant relationship to winter NAO was detected in three out of four regional chronologies. NAO also drives the common (inter-regional) growth variability. Years of known cool summers caused by volcanic forcing exhibit exceptionally narrow tree rings in the three northernmost regional chronologies.

A Reconstruction of Mid–summer Temperatures from Ring–widths of Scots Pine since ad 50 in Northern Fennoscandia

We have reconstructed mid–summer (July) temperatures using a master ring–width chronology of Scots pine (Pinus sylvestris L.) for northern Fennoscandia, covering nearly the last two millennia. The chronology is constructed from 93 living trees and 275 dead trees collected between 68 ∘ and 70 ∘ N, 20 ∘ and 30 ∘ E. In standardization, negative exponential functions and, alternatively, regression lines were applied. Because of a strong autocorrelation in the data, we used a model structure including 2–year lagging and a 3–year leading predicter along with the master chronology in the transfer function. Over one–half of the dependent climate variance was retrieved in our final reconstruction model. We indicate the largest temperature anomalies of individual summers as well as longer–term temperature variability starting from ad 50.

Subfossil pine dates and pollen diagrams from northern Fennoscandia

Abstract As a continuation of earlier work on the Holocene history of the pine forests of northern Fennoscandia, new finds of subfossil pine (Pinus silvestris) are reported from sites in Enontekio, northwestern Finnish Lapland, and at one location in the Lyngenfjorden area in North Norway. Two pollen diagrams from lake sediments are also presented, one from near Kilpisjarvi in Finland, the other from close to Skibotn in Norway. The megafossil, pollen, and radiocarbon data provide a more or less consistent picture of the history of the pine forests of northern Fennoscandia. Pine spread to the Kilpisjarvi and Lyngen area around 7500–7000 B.P., and the pine forests were at their maximum there from about 7000 to 4000 B.P. Their subsequent gradual decline meant a retreat of the pine limit by some 70 km or so. Discussions are provided on climatic variations in the area, and on possible further research into these and other palaeo-ecological changes.

The dating of the Holocene relative sea-level changes in Finnmark, North Norway

The Holocene land/sea-level changes in Finnmark, North Norway, were determined with the help of radiocarbon dates of shells from raised beaches. The regression of the sea level below 25 m could be dated in the Varangerfjord area for the time interval 5500–3600 B.P:, and below 15 m for the outer coast of the Varanger peninsula after about 4500 B.P. In addition, dates from two lakes and a former dated peat at Tomaselv were used to determine the lowering of the sea level after 9000 B.P. It was shown that radiocarbon dates of charcoal samples from archaeological sites cannot directly be compared with the other dates and also that lichenometry cannot be used in the study of the Iand/sea-level changes of the area. The influence of corrected radiocarbon ages is also discussed.