Samuli Helama’s research while affiliated with Natural Resources Institute Finland (Luke) and other places

Summer 2024 was exceptionally warm in northern Fennoscandia, with June-August mean temperatures at several long-term weather stations surpassing the long-standing record set in 1937. In this region, summer mean temperatures have been reconstructed from tree-ring proxies, which provide annually resolved and millennium-long records of past climate. Here we show, using in-situ observations and two different tree-ring reconstructions, that summer 2024 was the warmest summer in 2000 years in northern Fennoscandia. Employing an attribution method based on Coupled Model Intercomparison Project Phase 6 climate models, we further estimate that climate change increased the likelihood of this extreme season by a factor of 93 (5–95% uncertainty range 19–881) and increased the temperature an additional 2.1 °C (1.4–2.8 °C). Atmospheric circulation patterns influencing both summers 1937 and 2024 were largely similar, suggesting a comparable large-scale circulation influence. Our findings highlight the impact of climate change for the contemporary heat extremes in Fennoscandia, indicating that the warming of summer climate is emerging from its range of natural climate variability over the last two millennia.

Northern Fennoscandia is a cradle of tree-ring based climate reconstructions. These Late-Holocene data come from several types of tree-ring proxies and are typically used for discussing the extent of past climate excursions such as the Medieval Climate Anomaly and Little Ice Age, relative to the modern temperatures. This article compares tree-ring records published from the region over the past two decades with those produced here by recalculations, focussing on their low-frequency temperature patterns. Paleoclimate evidence fully independent of trees (sedimentary chironomid data) was used to assess the dendroclimatic records. Chironomid record correlated best with anatomy (maximum latewood radial-cell-wall thickness) data averaged over the region (A-FEN) when the latewood (maximum) anatomy was not tuned by earlywood (minimum) anatomy values. While all the examined tree-ring records are highly sensitive to high-frequency summer temperature variability, non-temperature factors likely play a role in modifying their low-frequency patterns. As a result, the subsets of A-FEN (representing NW and NE Fennoscandian sites i.e. Torneträsk region and Finnish Lapland) did not correlate, unlike the subsets of the maximum density data, for which reason the regionally averaged A-FEN seems to underestimate the temperature amplitudes during the Medieval Climate Anomaly. The findings contribute to the discussion on low-frequency patterns in millennia-long tree-ring chronologies.

Tree growth history was investigated in North Finland to study the recently observed reduction in Scots pine growth. Tree-ring records from the National Forest Inventory (NFI) were compared to changes that have occurred in forest management activities and to variations in the North Atlantic Oscillation (NAO), as similar comparisons have not been previously made with NFI data. A phase of increased growth was dated to the early 1970s when the forest management activities were intensified on a large scale. More recently, growth has notably declined since the early 2000s which coincides with reduced activity to clean the ditches to ensure they are draining the substrate adequately. The growth reduction appeared greater for trees from stands with higher basal area. Trees representing sites where the first cleaning of the stand (thinning) had been carried out did not show reduced growth. NAO indices correlated positively with growth, especially during the cold season. An interpretation on the value of snow conditions in a dynamic interaction with management history/stand density is put forth to explain the growth decline observed in NFI data over the past two decades.

In recent years, much evidence has been presented on the 4.2 ka event. A review of 317 palaeoclimate papers shows that dry conditions were common during the event, especially from Eastern Mediterranean to India. The 4.2 ka event was not, however, a global drought event. Wet conditions were reported especially for central/northern Europe and sub-Saharan Africa. The 4.2 ka event is typically characterized either as short (4.2–4.0 ka) or long (4.4–3.8 ka) episode, possibly developing over an extended interval of time, in keeping with the North Atlantic forcing and correlating with the Bond 3 event of ice-rafted debris. This forcing is understood to drive a southward migration of the Inter-Tropical Convergence Zone (ITCZ), resulting in decreased rainfall over most of the Asian monsoon region, with possibility that an interplay of El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) has modulated the global circulation. Cold conditions were also reported but less frequently, in comparison to other Bond events such as the 8.2 ka event, Dark Ages Cold Period and Little Ice Age. Some high-resolution records show a double peak structure of which two anomalies are tree-ring dated to 4.14–4.05 ka and 3.97 ka. Accurately and precisely dated high-resolution records indicative of various climatic variables, especially outside of the traditional study region (Mediterranean–Middle East–India–China), including reconstructions of the ENSO and NAO histories and ITCZ migrations, are crucially needed for rigorous examination of the global scale characteristics of the 4.2 ka event and its forcings. Such research seems to be just beginning.

Archaeological and living tree data were used to construct tree-ring chronologies over the medieval (AD 1183–1430) and recent (AD 1812–2020) periods in Turku, which is historically an important population centre in Southwest Finland and the country. Comparisons between the two tree-ring assemblages, and between the previously built chronologies from the Åland (historical timber) and Tavastia (lacustrine subfossils and living trees) sites, provided ways of understanding the growth patterns and their linkages to climatic, environmental, and edaphic factors. Tree growth in and around Turku was affected by warm-season precipitation and winter temperature. Similar relationships were previously evident also in the Åland tree rings, whereas the data from a wetter Tavastia site did not exhibit similar precipitation signal. The site conditions influence also the correlations which are higher between Turku and Åland than between Turku and Tavastia chronologies. Construction of long continuous chronology is impaired by human-related activities, the Great Fire of Turku in 1827 and logging, which have diminished the availability of dead and living-tree materials, respectively. These conditions lead to hardships of filling the gap between the medieval and recent periods and updating the archaeological datasets with compatible living-tree data, which are both demonstrated by our results.

Plain Language Summary Strong Earth‐directed solar eruptions can cause ¹⁴C concentration spikes in the atmosphere. Large enough events may leave a signal in the annually grown tree‐rings as they capture the isotopic carbon fingerprint through photosynthesis. Such rapid ¹⁴C increases have been detected, for instance, starting in years 774 and 993 CE. However, no increase has been observed following the Carrington event of 1859, despite it being the largest solar eruption of the modern era. Notably, all prior ¹⁴C measurements covering the Carrington event come from mid‐latitude trees. To achieve a broader geographical coverage, we have measured the event from several high‐latitude locations. After comparing the high‐ and mid‐latitude measurements, we have found a statistically significant difference lasting for several years post‐Carrington. To better understand the difference, we have adopted a ¹⁴C production and atmospheric transport model capable of simulating regional differences. Despite the improved model, we found it unable to reproduce the observational results, which suggests features beyond current understanding. Ultimately, the observation emphasizes the role of subtle ¹⁴C differences in tree‐ring ¹⁴C studies, potentially opening new ways to study past solar phenomena and atmospheric dynamics.

Artikkelissa kuvataan Kuninkaalliseen tiedeakatemian arkistoon sijoitettujen Turun vuosien 1748–1800 päivittäisten säähavaintojen integrointi tietokantaan. Säähavaintojenteon Turussa organisoivat Akatemian professorit Leche, Kalm, Hellenius, Planman ja Mether, joista Leche ja Kalm tekivät säähavaintoja myös itse. Päiväkirjojen tekstit sekä rikastuttavat aikaisemmin koostettuja säämuuttujien kuukausi- ja vuosikeskiarvotilastoja että tarjoavat monipuolisen kuvauksen vuodenkierron vaihtelevista sääoloista sekä moninaisista tapahtumista Turun seudulla 1700-luvun jälkimmäisellä puoliskolla. Tietokanta on tallennettu tutkimuksen yhteyteen, josta se on ladattavissa.