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Shrubs shed light on 20th century Greenland Ice Sheet melting
Boreas
Abstract
The Greenland Ice Sheet (GrIS) is a key element of the global climate system and thus knowledge about its melting in
the past is desirable. However, GrIS-melt records only date back until 1979 and climate data based reconstructions
covering the 20th century differ with respect to absolute values. To extend our knowledge about the Greenland Ice
Sheet we examined the potential of shrub ring-widths and wood-anatomy as proxies for GrIS-melt. We found
significant correlations between shrub cell-wall thickness and regional melt derived from passive microwave satellite
brightness for 7% of the total GrIS area. A respective transfer function calibrated over the period 1979 to 2007
successfully passedmodel calibration-verification testsandexplained42%of GrIS-melt variability. Consequently, the
first GrIS-melt reconstruction based on shrub wood-anatomy covering the period 1909 to 2012 is presented and
compared against two temperature-based reconstructions. For the period prior to 1930 the new reconstruction
contrasts with existing literature but generally confirms that most recent record melt rates are amongst the highest
since the early 20th century. We discuss the sensitivity of shrubs to several influencing factors besides summer
temperature as possible reason for the observed differences and highlight the potential of using shrubs as multiparameter
proxies within a network to increase our knowledge about 20th century Greenland Ice Sheet dynamics.
... Second, anatomical traits may reveal shrub responses to growing conditions that are not detectable with just annual ring widths. For example, Buras et al. (2017) found that cell wall thickness, but not ring width, in Juniperus communis ssp. nana in Kobbefjord (southwest Greenland) was significantly correlated with Greenland Ice Sheet melt. ...
... nana in Kobbefjord (southwest Greenland) was significantly correlated with Greenland Ice Sheet melt. It is possible that no significant relationship was found with ring widths due to a higher sensitivity of radial growth to episodically occurring moth outbreaks (Buras et al., 2017). Third, anatomical features can impact the climate-growth relationship. ...
Rapid climate change has been driving changes in Arctic vegetation in recent decades, with increased shrub dominance in many tundra ecosystems. Dendroecological observations of tundra shrubs can provide insight into current and past growth and recruitment patterns, both key components for understanding and predicting ongoing and future Arctic shrub dynamics. However, generalizing these dynamics is challenging as they are highly scale-dependent and vary among sites, species, and individuals. Here, we provide a perspective on how some of these challenges can be overcome. Based on a targeted literature search of dendroecological studies from 2005 to 2022, we highlight five research gaps that currently limit dendroecological studies from revealing cross-scale ecological insight into shrub dynamics across the Arctic biome. We further discuss the related research priorities, suggesting that future studies could consider: 1) increasing focus on intra- and interspecific variation, 2) including demographic responses other than radial growth, 3) incorporating drivers, in addition to warming, at different spatial and temporal scales, 4) implementing systematic and unbiased sampling approaches, and 5) investigating the cellular mechanisms behind the observed responses. Focusing on these aspects in dendroecological studies could improve the value of the field for addressing cross-scale and plant community-framed ecological questions. We outline how this could be facilitated through the integration of community-based dendroecology and dendroanatomy with remote sensing approaches. Integrating new technologies and a more multidisciplinary approach in dendroecological research could provide key opportunities to close important knowledge gaps in our understanding of scale-dependencies, as well as intra-and inter-specific variation, in vegetation community dynamics across the Arctic tundra.
... Arctic shrubs can help fill this gap, because they can be found in large parts of the Arctic, can live to considerable ages and form annual growth rings, like trees. Since temperatures are usually limiting growth at these high latitudes, the link between Arctic shrub growth and climate is often strong (Bär et al., 2008;Beil et al., 2015;Blok et al., 2011;Buchwal et al., 2013;Forbes et al., 2010;Gamm et al., 2017;Hallinger et al., 2010;Hallinger and Wilmking, 2011;Hollesen et al., 2015;Jørgensen et al., 2015;Myers-Smith et al., 2015a;Rozema et al., 2009;Weijers et al., 2017;Young et al., 2016), generally qualifying shrubs for climate and environmental reconstructions (Buras et al., 2012(Buras et al., , 2017aRayback and Henry, 2006;Rayback et al., 2012;Weijers et al., 2010Weijers et al., , 2013. While shrub ring width variability over time has been linked to temperature variability (Havstrom et al., 1995), wood anatomical studies (e.g. using cell wall thickness or vessel lumen area) have recently provided additional insights into the relationship between shrub growth and climate drivers or other environmental parameters such as glacier melt (Buras et al., 2017a;Lehejček et al., 2017;Nielsen et al., 2017), opening up new avenues to interpret the shrub ring record. ...
... Since temperatures are usually limiting growth at these high latitudes, the link between Arctic shrub growth and climate is often strong (Bär et al., 2008;Beil et al., 2015;Blok et al., 2011;Buchwal et al., 2013;Forbes et al., 2010;Gamm et al., 2017;Hallinger et al., 2010;Hallinger and Wilmking, 2011;Hollesen et al., 2015;Jørgensen et al., 2015;Myers-Smith et al., 2015a;Rozema et al., 2009;Weijers et al., 2017;Young et al., 2016), generally qualifying shrubs for climate and environmental reconstructions (Buras et al., 2012(Buras et al., , 2017aRayback and Henry, 2006;Rayback et al., 2012;Weijers et al., 2010Weijers et al., , 2013. While shrub ring width variability over time has been linked to temperature variability (Havstrom et al., 1995), wood anatomical studies (e.g. using cell wall thickness or vessel lumen area) have recently provided additional insights into the relationship between shrub growth and climate drivers or other environmental parameters such as glacier melt (Buras et al., 2017a;Lehejček et al., 2017;Nielsen et al., 2017), opening up new avenues to interpret the shrub ring record. ...
Arctic shrubs have a strong potential for climate and environmental reconstructions in the chronically understudied regions of the high northern latitudes. The climate dynamics of these regions are important to understand because of large-scale feedbacks to the global climate system. However, little is known about other factors influencing shrub ring growth, possibly obscuring their climate signal. For example, as of yet we are not able to differentiate between herbivory or climatically induced growth depressions. Here, we use one of the most common Arctic shrubs, Alnus viridis as a test case to address this question. We sampled Alnus in Kobbefjord, Greenland, measured shrub-ring width and cell wall thickness and built site chronologies of each parameter. We analysed climate-growth relationships, tested their stability over time and employed a pointer-year analysis to detect growth depressions. We employed bootstrapped transfer function stability tests (BTFS) to assess the suitability of our shrub chronologies for climate reconstruction. Correlations with climate data showed strong significantly positive and stable correlations between summer temperature and ring-width with the exception of the recent decade. A climate reconstruction model failed stability tests, when the complete period of record was used for calibration and verification. Wood anatomy analysis uncovered the occurrence of unusual cell structure (very thin cell walls) in the exceptionally narrow ring of 2004, a recorded insect outbreak year in other parts of Greenland. When excluding the affected ring and a recovery period, the reconstruction model passed all tests, suggesting that the unusual 2004 ring was not climate driven, but rather the result of an insect attack. When combining anatomical analysis with traditional ring-width measurements, we move a step further in potentially distinguishing small rings caused by insect attacks from small rings formed in climatically challenging years. While this study does not provide unambiguous evidence, it does provide potential useful methodological combinations to enable more robust climate reconstructions in areas where climatic records are extremely sparse.
... Alpine and arctic shrub sensitivity to contemporary climate change around the circumpolar arctic is well established (Sturm et al., 2001;Danby and Hik, 2007;Forbes et al., 2010;Hallinger et al., 2010;Hallinger and Wilmking, 2011;Myers-Smith et al., 2011, 2015a. Due to this sensitivity, many recent dendrochronological studies use growthring parameters from shrubs for reconstructions of environmental conditions in Arctic and alpine Tundra (Lehejček, 2015) ranging from temperature (Meinardus et al., 2011;Weijers et al., 2013), humidity (Liang and Eckstein, 2009;Liang et al., 2012), and sea surface temperatures (Beil et al., 2015) to understanding ecological processes like permafrost degradation (Blok et al., 2010;Blok et al., 2011), glacial retreat (Buras et al., 2012;Buras et al., 2017) and effects of reducing summer albedo and treeline advances and greening at high latitudes (Raynolds et al., 2006;Tape et al., 2006;Ropars and Boudreau, 2012;Tape et al., 2012;Juszak et al., 2014). This multi-faceted utility of shrubs makes them pivotal for future studies of climate change and vegetation dynamics in the arctic tundra. ...
... We chose Juniperus communis L. as it is a one of the most widely distributed shrub species in the circumpolar arctic with clearly discernible rings and has been commonly used for many dendro-ecological studies (Hantemirov et al., 2011;Liang et al., 2012;Pellizzari et al., 2014;Buras et al., 2017). In accordance with the established sampling protocol, we hypothesized that the climatic signal in ring-width data is consistent along the length of the stem. ...
... The aim is to more accurately estimate the dynamics of shrubi cation in the Arctic tundra. One key factor limiting plant growth in the Arctic is the soil content of biogenic elements (Stimmler et al. 2023), precipitation (Weijers 2022;Buras et al. 2017) and temperature (Weijers et al. 2017; Lehejček et al. 2017). At the same time, soil temperature directly in uences the activity of soil microorganisms and the accessibility of mineral nutrients necessary for plant nutrition (Schmidt et al. 2002). ...
The growth of shrubs in the Arctic is a key factor affecting environmental and vegetation changes framed by the currently accelerated dynamics of global warming. One of the phenomena involved in the shrubification of the Arctic is root exudation, which plays a part in nutrient acquisition by plants and thus the growth of both individual plants and whole communities. It is not known, however, to what extent root exudation differs across different environments in the Arctic tundra, how importantits role is in covering the nutrient demands of plants and what significance this might have for soil properties and vegetation succession. To shed light on the topic, we studied the content of biogenic elements in soils of different maturity following deglaciation, the age structure of the climax species Salix polaris , exudation by its roots,and the concentrations of different nutrients in its roots and leaves in the forefield of the Nordenskiöld glacier in the Svalbard archipelago. Our results reveal that polar willow shrubs have a greater relative propensity towards exudation, and thus larger potential for investing into growth, under more severe environmental conditions, which may be attributable to the stage of their root system development or the level of competition they face. This finding should be taken into consideration when interpreting current vegetation development and plant succession in the Arctic. Nevertheless, to what extent this relationship stays stable in the context of ongoing climate change remains a question to be explored.
... The fairly robust metrics that we computed for shrub anatomical chronologies are very encouraging because shrubs usually exhibit greater inter-individual heterogeneity than trees Buras et al., 2017). We, therefore, posit that the statistics that we find in the case of R. ferrugineum do not only validate the accuracy and representativeness of our measurements and the cross-dating approach (Carrer et al., 2017) but that they presumably point to a common macroenvironmental (i.e. ...
Introduction
Mean xylem vessel or tracheid area have been demonstrated to represent powerful proxies to better understand the response of woody plants to changing climatic conditions. Yet, to date, this approach has rarely been applied to shrubs.
Methods
Here, we developed a multidecadal, annually-resolved chronology of vessel sizes for Rhododendron ferrugineum shrubs sampled at the upper shrubline (2,550 m asl) on a north-facing, inactive rock glacier in the Italian Alps.
Results and Discussion
Over the 1960-1989 period, the vessel size chronology shares 64% of common variability with summer temperatures, thus confirming the potential of wood anatomical analyses on shrubs to track past climate variability in alpine environments above treeline. The strong winter precipitation signal recorded in the chronology also confirms the negative effect of long-lasting snow cover on shrub growth. By contrast, the loss of a climate-growth relation signal since the 1990s for both temperature and precipitation, significantly stronger than the one found in radial growth, contrasts with findings in other QWA studies according to which stable correlations between series of anatomical features and climatic parameters have been reported. In a context of global warming, we hypothesize that this signal loss might be induced by winter droughts, late frost, or complex relations between increasing air temperatures, permafrost degradation, and its impacts on shrub growth. We recommend future studies to validate these hypotheses on monitored rock glaciers.
... Juniperus communis, common juniper, being a long-lived species with distinct annual growth rings and the widest geographical distribution across the northern hemisphere, is ideally suited for quantifying shrubs vegetation responses to climate over broad regions. Several investigations have already highlighted this potential, despite the challenging ring measuring and dating (Buras et al., 2017;Pellizzari et al., 2017;Shetti et al., 2018). ...
... Following a long tradition of dendrochronological research in Greenland (Kraus, 1873;Good, 1927;Molisch, 1938;Parsons, 1994;Büntgen et al., 2015;Young et al., 2016;Buras et al., 2017;Weijers et al., 2017;Gamm et al., 2018), this studyat the interface of wood anatomy and 'dendro'-ecologyprovides another independent line of evidence for low cambial activity and high species longevity in dwarf shrubs at two sites in coastal east Greenland. Moreover, we confirm significant agreement between decadal-scale summer temperature variability and lagged, mid-frequency recruitment intensity. ...
Warming-induced biological and ecological responses have been reported from high-northern latitude sites, where changes in dwarf shrub communities translate into complex vegetation-climate feedbacks. Most of the available Arctic tree-ring evidence is, however, restricted to a limited number of species and locations. A combination of wood anatomical and ‘dendro’-ecological techniques provides insights into past growth rates, recruitment dynamics and even community assemblages of Arctic vegetation. Here, we use thin sectioning and ring counting of 1432 dwarf shrub samples from eight species and two tundra regions in coastal east Greenland to assess community recruitment history and its relation to climate. Site and species-specific annual stem increments, as well as estimated plant ages, range from 0.013-0.720 mm and from 4 to 204 years, respectively. The mean ring width is 0.086 mm, with a mean age of 50 years. Decadal-scale recruitment dynamics of the studied vegetation cover respond to Greenlandic summer temperature variations back to the late 19th century (r = 0.7; 1881–2000).
... Shrubs form annual growth rings like trees and many recent studies have shown that these annual rings in shrubs are sensitive to changes in the environment, especially climate variability, for example Danby and Hik (2007); Hallinger et al. (2010); Blok et al. (2011); Hallinger and Wilmking (2011). Consequently, ring-width data from shrubs can be useful for ecological reconstructions (Lehejček, 2015) like temperature (Liang and Eckstein, 2009;Weijers et al., 2010;Meinardus et al., 2011;Weijers et al., 2012;Weijers et al., 2013), humidity (Liang et al., 2012), glacier melting (Buras et al., 2012;Buras et al., 2017), permafrost degradation (Blok et al., 2010), sea surface temperatures (Beil et al., 2015) and fire history in a landscape (Oddi and Ghermandi, 2015). Therefore, increased use of shrubs in future dendrochronological research is very likely. ...
In recent years an increasing number of studies have shown shrubs to be reliable proxies of environmental conditions in regions where Trees − due to harsh climate conditions − are absent. Although many shrubs are monoecious, some are dioecious, which poses certain questions related to gender-specific growth as observed for trees in previous studies. Here, we address the questions whether dioecious shrubs, similar to trees, show growth differences between male and female plants, and − if so − whether this difference needs to be considered in terms of sample selection. We chose Juniperus communis. L., the most widely distributed woody plant, and a common and well-studied dioecious shrub species in the northern hemisphere, especially in the Boreal, Subarctic tundra and Alpine regions. Our samples were collected from four sites − three from the Ural Mountains and one site from Kirkenes in Norway. To see if there were differences in radial growth between sexes we performed four different analyses. First, we used multivariate explorative statistics to see if there were gender biased sub-populations and generally found no differences. Secondly, to compare growth over the lifetime of shrubs we computed cumulative annual increments of basal area which revealed no gender-specific growth patterns. Thirdly, to test if differences in radial growth between male and female shrubs affect the resulting site-chronology, we compared individual shrub chronologies with the site-chronology and found a significant differentiation between normalized correlations of gender-specific chronologies to the site-chronology. This significant difference was restricted to an overall comparison, but not evident at individual site-level. Lastly, we compared correlations of gender-specific chronologies and a mean site-chronology with monthly climate records to find only very few meaningful differences in their responses. In summary, we could not detect any clear gender-specific growth pattern in Juniperus communis but observed a trend towards more non-climatic signals in female junipers which may affect the resulting site-chronology.
... Unprecedented rates of GrIS mass loss are documented for the 20 th century from climatic re-analyses (Box et al. 2013), but also from e.g. cell-wall thickness of Arctic shrubs (Buras et al. 2017). Further extensive melting of the GrIS may thus lead to decreasing or even terminating the heat transport to higher latitudes of the North Atlantic, with all the climatic consequences for the population of Europe (Rahmstorf et al. 2015, Hansen et al. 2016. ...
Iceland, located in the climatologically sensitive subarctic zone, is one of the key areas for studying climate change and the current Arctic warming. Poor and rich heathland covers approximately 30% of Iceland, with heath vegetation being the area’s most important vegetation type. Prostrate and erect dwarf shrub and shrub species are the primary sources of dendrochronological information on the changes and characteristics of Iceland’s climate. Here, we investigate the dendrochronological potential of seven common Icelandic heathland species (Dryas octopetala, Calluna vulgaris, Salix arctica, Salix herbacea, Empetrum nigrum, Juniperus communis nana, and Betula nana) and explore the impact of climate conditions on their growth, particularly with regard to extreme meteorological events. We conducted comparative analyses among species and observed their climatic responses in a climatically, morphologically, and geologically homogeneous setting in north-eastern Iceland. After sampling, measurements, and cross-dating, we constructed local chronologies for the seven species. Of approximately 200 samples, only 113 were included in the final shrub chronologies. All chronologies covered at least 50 years, with few of them exceeding 100 years. Dendroclimatological analysis indicated that above-average temperatures in June and summer positively influence the growth of D. octopetala (June–September), J. communis (July–August), B. nana (July–August), C. vulgaris (June–August), and S. arctica (August–September). The opposite is observed for S. herbacea and E. nigrum. Climate–growth correlations indicate that the growth of these species is negatively correlated with temperature and positively correlated with winter and summer precipitation. Furthermore, among the long-lived woody species growing in Iceland, seven common species were used to assess the impact of extreme meteorological conditions on their growth. The most pronounced extreme year in the wood anatomy of Icelandic shrubs was 1979, depicted as very narrow or missing rings and as the blue ring in J. communis.
Tree rings have been reliably used as an environmental proxy over the past decades for environmental reconstructions, simulations and forecasting. In our study, we investigated whether tree-ring chronologies are impacted by pollution. We chose sites in the Krušné hory and the Krkonoše Mountains in the Czech Republic which have a known history of pollution. We sampled Norway spruce (Picea abies [L.] Karst) in both ranges and compared their chronologies. We found no significant difference in the overall radial growth in the chronologies from both regions. However, we observed an increased heterogeneity in the growth of trees from the 1970s till the 1990s. Coherently, a severe reduction in tree growth from the late 1970s and a recovery towards the early 1990s was evident. We collected and analysed soil samples for pH and exchangeable element concentrations. All seven sampling sites' soils were strongly acidic (pHCaCl2 = 3.3 ± 0.4). The average soil base saturation at Krušné hory was higher than at Krkonoše (39% versus 12%), likely due to more intensive liming.
Further, we compared these chronologies to other sites in Europe. Analysing 89 sites, we found that most (9 out of 14) of the sites with significantly reduced radial tree growth were located within the former ‘Black Triangle’, an area which was subjected to heavy industrialisation and pollution from the 1960s to the 1990s. Atmospheric sulphur deposition was found to negatively affect radial tree-growth, while limited quantities of oxidised nitrogen appeared to have a positive effect. Our results are consistent with previous research, indicating that atmospheric SO2 pollution and subsequent acid fog and rime have led to a reduction in annual radial tree growth across the Black Triangle.
Old plant communities are of utmost importance for nature conservation, carbon sequestration, as well as gene pool maintenance. Shrub populations occurring in extreme environments beyond abiotic tree-lines provide diverse ecosystem services and have potential as proxy archives because they often inhabit areas with scarce and short instrumental records. We provide dendrochronological insight into one such population made up of prostrate Juniperus communis shrubs in the northern Scandinavian tundra (Norway), growing on a competition-free boulder field. We dated and provided a growth chronology of the oldest living shrub population (190 ± 148 years) from Scandinavia with the oldest individual being 683 years old. This is a conservative estimate, as shrubs in extreme environments do not form even a single row of cells in some years. The cross-dating issues of poorly growing shrubs do not fully allow to rely on climate sensitivity of the juniper population studied, although the species' potential for dendroclimatological reconstructions is generally considered high, also because the common juniper is an abundant woody species distributed globally. Old populations present an important gene source in plant recruitment, particularly in the context of the present environmental change.
(1) Background: Water transport systems play an important role in maintaining plant growth and development. The plasticity responses of the xylem anatomical traits of different species to the environment are different. Studies have shown that there are annual growth rings in the secondary root xylem of perennial herbaceous species. Studies on xylem anatomical traits, however, have mainly focused on woody species, with little attention given to herbaceous species. (2) Methods: We set 14 sampling sites along a rainfall gradient in arid and semiarid regions, and collected the main roots of native (Potentilla) and non-native (Medicago) perennial forbs. The xylem anatomical traits of the plant roots were obtained by paraffin section, and the relationships between the xylem traits of forbs were analyzed by a Pearson correlation. (3) Results: In the fixed measurement area (850 um * 850 um), the vessel number (NV) of Potentilla species was higher than that of Medicago species, while the hydraulic diameter (Dh) and mean vessel area (MVA) of Potentilla species were lower than those of Medicago species. With the increase in precipitation along the rainfall gradient, the Dh (R2 = 0.403, p = 0.03) and MVA (R2 = 0.489, p = 0.01) of Medicago species increased significantly, and NV (R2 = 0.252, p = 0.09) decreased, while the hydraulic traits of Potentilla species showed no significant trend with regard to the rainfall gradient. (4) Conclusions: The hydraulic efficiency of non-native Medicago forbs was higher than that of native Potentilla forbs, and the hydraulic safety of native Potentilla forbs was higher than that of non-native Medicago forbs. With the decrease in precipitation, the hydraulic strategies of non-native Medicago forbs changed from efficiency to safety, while native Potentilla forbs were not sensitive to variations in precipitation.
Les écosystèmes alpins, comme la toundra arctique, subissent un réchauffement accentué par rapport au reste du globe. En Arctique, une approche dendroécologique basée sur l’analyse des relations entre la croissance annuelle radiale et le climat a permis de mettre en évidence les paramètres climatiques qui contrôlent l’expansion des arbustes nains (ligneux bas). Ces changements au sein de la toundra ont de vastes répercussions sur l’albédo, le manteau neigeux, le sol, le microclimat au niveau du sol ainsi que sur le cycle du carbone et le climat à l’échelle planétaire. Les Alpes européennes se sont considérablement réchauffées ces dernières décennies, et ce de manière non-linéaire, avec un point d’inflexion situé à la fin des années 80. Par analogie avec l’Arctique, nous émettons l’hypothèse qu’à haute altitude la diminution des contraintes climatiques (hausse des températures estivales, diminution de l’enneigement) favorise une augmentation de la croissance radiale des ligneux bas. Afin de valider cette hypothèse, huit populations de Rhododendron ferrugineum L. (Ericaceae) ont été prélevées dans des sites, le long de gradients topoclimatiques et dans des massifs caractérisés par des niveaux de continentalité représentatifs des Alpes françaises. Afin de caractériser finement la réponse de R. ferrugineum au changement, les chronologies de largeur de cernes établies pour chaque population ont été comparées aux séries météorologiques en utilisant les approches statistiques courantes en dendrochronologie (fonctions de corrélation) mais également sur la base de modèles empruntés à l’écologie (modèles linéaires mixtes, modèles d’équations structurelles).Le premier chapitre de la thèse met en évidence le potentiel dendroécologique de R. ferrugineum. Sur la base de fonctions de corrélation, il a permis de démontrer que les températures estivales étaient le principal facteur limitant de la croissance radiale d’une population de R. ferrugineum échantillonnée dans le massif du Taillefer (Alpes françaises, 2000 m d’altitude) mais également une très forte sensibilité de cette population aux précipitations hivernales, phénomène non observé sur une population d’arbre (Picea abies) située à proximité. Dans le deuxième chapitre, l’échantillonnage initial a été complété par des prélèvements réalisés sur deux placettes étagées le long d’un gradient altitudinal (1800-2400 m). La comparaison de la croissance radiale de R. ferrugineum et des séries nivo-météorologiques locales issues de la base de données SAFRAN-crocus au moyen de modèles d’équations structurelles démontre une forte hétérogénéité spatiale et temporelle de la réponse des ligneux bas aux fluctuations du climat. La population en limite altitudinale supérieure (2400 m) est très sensible à la durée de saison végétative et à la température estivale. Cette sensibilité décroît fortement à plus basse altitude (1800, 2000 m) en relation avec l’augmentation du stress lié aux gels printaniers depuis la fin des années 1980. De la même manière, l’affaiblissement des relations cernes-climat au cours des deux dernières décennies est attribué à un phénomène de divergence similaire à celui observé dans les populations d’arbres en Arctique. Le troisième chapitre est consacré à l’analyse de populations prélevées dans le massif du Queyras (Alpes françaises du sud) où l’aridité estivale liée à la continentalité est plus marquée que dans le Taillefer. Les résultats obtenus sur deux populations localisées sur des versants ouest et nord montrent le rôle déterminant de la situation topoclimatique sur la réponse de R. ferrugineum au climat et l’impact négatif de l’augmentation de la sévérité des vagues de chaleur et de la sécheresse estivale sur la population exposée à un rayonnement solaire important. Le quatrième chapitre intègre les huit populations échantillonnées dans les Alpes. [...]
The Arctic, one of the most sensitive components of the Earth's climate system, experienced pronounced climatic fluctuations during the Late Holocene. In order to obtain a detailed record of past environmental and climatic variability, sedimentary cores from two interconnected infilled depositional basins in the Kobbefjord area, southwestern Greenland, were retrieved and subjected to a multi‐proxy investigation. The absolute chronostratigraphy was established through radiocarbon (14C) and short‐lived radioisotope (210Pb, 137Cs) dating. We also analysed magnetic susceptibility, grain size, X‐ray fluorescence element composition, total content of carbon, nitrogen and sulphur, biogenic silica and diatom assemblages. The palaeoenvironmental reconstruction presented from the upper basin records ~800 years of sedimentation in a former shallow Arctic lake/pond that was modulated by catchment processes and regional hydroclimate. The record is dominated by alternating periods of increased organic and clastic deposition, suggesting warmer/drier and colder/wetter climate, respectively. Comparison of our records with other proxy‐based lacustrine, marine and glacier records and instrumental measurements from Greenland and the North Atlantic reveals common climatic trends, consistent with the inferred North Atlantic Oscillation pattern. Periods at ~800–600, ~320–220 and ~140–30 cal. a BP correspond to the main cold phases. The earliest climatic deterioration coincides with the demise of the nearby Norse Western Settlement. The most recent cold period represents the culmination of the Little Ice Age with the maximum Holocene advances of Greenland glaciers. Ameliorated conditions predominated at ~550–320 and ~220–140 cal. a BP. The evolution of both water bodies is concluded by transition into oligotrophic peat bogs after complete infilling of the basins at ~30 cal. a BP. Despite differences in proxy signals between the coring sites, this study endorses the usefulness of shallow lake/pond sediments from infilled basins for environmental reconstructions, as long as local depositional processes do not mask the external driving factors.
Deciduous shrubs are expected to rapidly expand in the Arctic during the coming decades due to climate warming. A transition towards more shrub-dominated tundra may have large implications for the regional surface energy balance, permafrost stability and carbon storage capacity, with consequences for the global climate system. However, little information is available on the natural long-term shrub growth response to climatic variability. Our aim was to determine the climate factor and time period that are most important to annual shrub growth in our research site in NE-Siberia. Therefore, we determined annual radial growth rates in Salix pulchra and Betula nana shrubs by measuring ring widths. We constructed shrub ring width chronologies and compared growth rates to regional climate and remotely sensed greenness data. Early summer temperature was the most important factor influencing ring width of S. pulchra (Pearson's r =0.73, p <0.001) and B. nana (Pearson's r =0.46, p <0.001). No effect of winter precipitation on shrub growth was observed. In contrast, summer precipitation of the previous year correlated positively with B. nana ring width ( r =0.42, p <0.01), suggesting that wet summers facilitate shrub growth in the following growing season. S. pulchra ring width correlated positively with peak summer NDVI, despite the small coverage of S. pulchra shrubs (<5% surface cover) in our research area. We provide the first climate-growth study on shrubs for Northeast Siberia, the largest tundra region in the world. We show that two deciduous shrub species with markedly different growth forms have a similar growth response to changes in climate. The obtained shrub growth response to climate variability in the past increases our understanding of the mechanisms underlying current shrub expansion, which is required to predict future climate-driven tundra vegetation shifts.
To study near-surface melt changes over the Greenland ice sheet (GrIS) since 1979, melt extent estimates from two regional climate models were compared with those obtained from spaceborne microwave brightness temperatures using two different remote sensing algorithms. The results from the two models were consistent with those obtained with the remote sensing algorithms at both daily and yearly time scales, encouraging the use of the models for analyzing melting trends before the satellite era (1958–1979), when forcing data is available. Differences between satellite-derived and model-simulated results still occur and are used here to identify (i) biases in the snow models (notably in the albedo parametrization, in the thickness of a snow layer, in the maximum liquid water content within the snowpack and in the snowfall impacting the bare ice appearance in summer) and (ii) limitations in the use of passive microwave data for snowmelt detection at the edge of the ice sheet due to mixed pixel effect (e.g., tundra or rock nearby the ice sheet). The results from models and spaceborne microwave sensors confirm a significant (p-value = 0.01) increase in GrIS surface melting since 1979. The melt extent recorded over the last years (1998, 2003, 2005 and 2007) is unprecedented in the last 50 yr with the cumulated melt area in the 2000's being, on the average, twice that of the 1980's.
The Arctic is one of the most sensitive areas worldwide with respect to climate changes, and recent climate change impacts are evident throughout Arctic ecosystems. In order to put current and projected changes in a palaeoclimatic context, exact information of past conditions and therefore detailed knowledge of proxy archives are crucial. Here, we investigated wood anatomical and other growth-related records of the long-lived and widespread Arctic tundra shrub Juniperus communis L. Annual ring-widths and cell anatomical parameters of 20 individuals from south-west Greenland were measured to test for age-related trends in the proxy time series and to correlate them with past climatic conditions. We documented a clear age trend in the investigated growth parameters (ring-widths, conduit-lumen areas, and cell wall thicknesses), which indicates the need for detrending the time series prior to possible climate correlation analyses and climate or environmental reconstructions. Prostrate growth forms of J. communis do not exhibit the general trend of an exponential widening of the conduit-lumen areas during ontogeny, as has been observed for many tree species, but their conduit lumens stop to increase in size at some point. This is possibly caused by a combination of extreme climate and physiological adaptations, which result in a prostrate growth form. Of the tested growth parameters, only detrended cell wall thickness showed stable correlations with summer temperature and the summer standardized precipitation evapotranspiration index (SPEI). The other growth parameters did not pass cross-calibration verification analyses, although for ring-widths we found the strongest climate correlations. Despite the fact that the ecophysiological processes behind our observations (i.e. reduced cell wall thickness combined with wider rings under dry conditions in the vegetation season) yet are not fully understood, we recognize the potential of J. communis for climate reconstructions. To our knowledge, this is the first demonstration of using cell anatomical parameter of J. communis shrubs as palaeoclimatological proxy for the Arctic.
We present an extended monthly and seasonal Greenland Blocking Index (GBI) from January 1851 to December 2015, which more than doubles the length of the existing published GBI series. We achieve this by homogenizing the Twentieth Century Reanalysis version 2c-based GBI and splicing it with the NCEP/NCAR Reanalysis-based GBI. For the whole time period, there are significant decreases in GBI in autumn, October and November, and no significant monthly, seasonal or annual increases. More recently, since 1981 there are significant GBI increases in all seasons and annually, with the strongest monthly increases in July and August. A recent clustering of high GBI values is evident in summer, when 7 of the top 11 values in the last 165 years - including the two latest years 2014 and 2015 - occurred since 2007. Also, 2010 is the highest GBI year in the annual, spring, winter and December series but 2011 is the record low GBI value in the spring and April series. Moreover, since 1851 there have been significant increases in GBI variability in May and especially December. December has also shown a significant clustering of extreme high and low GBI values since 2001, mirroring a similar, recently identified phenomenon in the December North Atlantic Oscillation index, suggesting a related driving mechanism. We discuss changes in hemispheric circulation that are associated with high compared with low GBI conditions. Our GBI time series should be useful for climatologists and other scientists interested in aspects and impacts of Arctic variability and change.
It is widely believed that deciduous tundra-shrub dominance is increasing in the pan-Arctic region, mainly due to rising temperature. We sampled dwarf birch (Betula nana L.) at a northeastern Siberian tundra site and used dendrochronological methods to explore the relationship between climatic variables and local shrub dominance. We found that establishment of shrub ramets was positively related to summer precipitation, which implies that the current high dominance of B. nana at our study site could be related to high summer precipitation in the period from 1960 to 1990. The results confirmed that early summer temperature is most influential to annual growth rates of B. nana. In addition, summer precipitation stimulated shrub growth in years with warm summers, suggesting that B. nana growth may be co-limited by summer moisture supply. The dual controlling role of temperature and summer precipitation on B. nana growth and establishment is important to predict future climate-driven vegetation dynamics in the Arctic tundra.
The climate of Central and Northern Europe is highly influenced by the North Atlantic Ocean due to heat transfer from lower latitudes. Detailed knowledge about spatio-temporal variability of sea surface temperature (SST) in that region is thus of high interest for climate and environmental research. Because of the close relations between ocean and coastal climate and the climate sensitivity of plant growth, annual rings of woody plants in coastal regions might be used as a proxy for SST. We show here for the first time the proxy potential of the common and widespread evergreen dwarf shrub Calluna vulgaris (heather), using the Faroe Islands as our case study. Despite its small and irregular ring structure, the species seems suitable for dendroecological investigations. Ring width showed high and significant correlations with summer and winter air temperatures and SST. The C. vulgaris chronology from the Faroe Islands, placed directly within the North Atlantic Current, clearly reflects variations in summer SSTs over an area between Iceland and Scotland. Utilising shrubs like C. vulgaris as easy accessible and annually resolved proxies offers an interesting possibility for reconstruction of the coupled climate-ocean system at high latitudes.
[1] We provide an analysis of Greenland temperature records to compare the current (1995–2005) warming period with the previous (1920–1930) Greenland warming. We find that the current Greenland warming is not unprecedented in recent Greenland history. Temperature increases in the two warming periods are of a similar magnitude, however, the rate of warming in 1920–1930 was about 50% higher than that in 1995–2005.
A combined analysis of remote sensing observations, regional climate
model (RCM) outputs and reanalysis data over the Greenland ice sheet
provides evidence that multiple records were set during summer 2012.
Melt extent was the largest in the satellite era (extending up to
∼97% of the ice sheet) and melting lasted up to ∼2
months longer than the 1979-2011 mean. Model results indicate that near
surface temperature was ∼3 standard deviations (σ) above
the 1958-2011 mean, while surface mass balance (SMB) was
∼3σ below the mean and runoff was 3.9σ above the
mean over the same period. Albedo, exposure of bare ice and surface mass
balance also set new records, as did the total mass balance with summer
and annual mass changes of, respectively, -627 Gt and -574 Gt, 2σ
below the 2003-2012 mean. We identify persistent anticyclonic conditions
over Greenland associated with anomalies in the North Atlantic
Oscillation (NAO), changes in surface conditions (e.g., albedo, surface
temperature) and preconditioning of surface properties from recent
extreme melting as major driving mechanisms for the 2012 records. Less
positive if not increasingly negative SMB will likely occur should these
characteristics persist.
This paper describes the construction of an updated gridded climate dataset (referred to as CRU TS3.10) from monthly observations at meteorological stations across the world's land areas. Station anomalies (from 1961 to 1990 means) were interpolated into 0.5° latitude/longitude grid cells covering the global land surface (excluding Antarctica), and combined with an existing climatology to obtain absolute monthly values. The dataset includes six mostly independent climate variables (mean temperature, diurnal temperature range, precipitation, wet-day frequency, vapour pressure and cloud cover). Maximum and minimum temperatures have been arithmetically derived from these. Secondary variables (frost day frequency and potential evapotranspiration) have been estimated from the six primary variables using well-known formulae. Time series for hemispheric averages and 20 large sub-continental scale regions were calculated (for mean, maximum and minimum temperature and precipitation totals) and compared to a number of similar gridded products. The new dataset compares very favourably, with the major deviations mostly in regions and/or time periods with sparser observational data. CRU TS3.10 includes diagnostics associated with each interpolated value that indicates the number of stations used in the interpolation, allowing determination of the reliability of values in an objective way. This gridded product will be publicly available, including the input station series (http://www.cru.uea.ac.uk/ and http://badc.nerc.ac.uk/data/cru/). © 2013 Royal Meteorological Society
In the 21st century, most of the world's glaciers are expected to retreat due to further global warming. The range of this predicted retreat varies widely as a result of uncertainties in climate and glacier models. To calibrate and validate glacier models, past records of glacier mass balance are necessary, which often only span several decades. Long-term reconstructions of glacier mass balance could increase the precision of glacier models by providing the required calibration data. Here we show the possibility of applying shrub growth increments as an on-site proxy for glacier summer mass balance, exemplified by Salix shrubs in Finse, Norway. We further discuss the challenges which this method needs to meet and address the high potential of shrub growth increments for reconstructing glacier summer mass balance in remote areas.
The intention of this research was to explore whether dendroclimatological relationships could be used to reconstruct long-term proxy records of ‘Little Ice Age’ glacier mass balance changes in the southern Coast Mountains of British Columbia. Tree-ring width chronologies from the Mt Waddington area were used in concert with historical glacier records to construct models spanning the past 450 years. The approach was to build models that were based on derived relationships between tree-ring growth and glacier mass balance: (1) warmer temperatures in the summer positively influence tree growth but are detrimental to glacier health; (2) colder temperatures during the winter and deeper snowpack have a negative impact on tree growth, whereas they are related to greater accumulation on the glacier during the winter season. Stepwise regression analyses were applied to tree-ring chronologies to predict glacier mass balance at local and regional scales. The models of net annual balance for the region (regional data set) show that periods of positive mass balance occurred in the AD 1750s, 1820s to 1830s and 1970s. Peaks of winter balance correspond closely to these periods, showing a sharp drop in winter mass balance towards the end of the nineteenth century. Wavelet analyses suggest that glacial mass balance regimes in the region respond synchronously to Pacific Ocean circulation systems such as the El Niño Southern Oscillation and the Pacific Decadal Oscillation.
Meteorological station records and regional climate model output are combined to develop a continuous 168-yr (1840–2007) spatial reconstruction of monthly, seasonal, and annual mean Greenland ice sheet nearsurface air temperatures. Independent observations are used to assess and compensate for systematic errors in the model output. Uncertainty is quantified using residual nonsystematic error. Spatial and temporal temperature variability is investigated on seasonal and annual time scales. It is found that volcanic cooling episodes are concentrated in winter and along the western ice sheet slope. Interdecadal warming trends coincide with an absence of major volcanic eruptions. Year 2003 was the only year of 1840–2007 with a warm anomaly that exceeds three standard deviations from the 1951–80 base period. The annual whole ice sheet 1919–32 warming trend is 33 % greater in magnitude than the 1994–2007 warming. The recent warming was, however, stronger along western Greenland in autumn and southern Greenland in winter. Spring trends marked the 1920s warming onset, while autumn leads the 1994–2007 warming. In contrast to the 1920s warming, the 1994–2007 warming has not surpassed the Northern Hemisphere anomaly. An additional 1.08–1.58C of annual mean warming would be needed for Greenland to be in phase with the Northern Hemispheric pattern. Thus, it is expected that the ice sheet melt rates and mass deficit will continue to grow in the early twenty-first century as Greenland’s climate catches up with the Northern Hemisphere warming trend and the Arctic climate warms according to global climate model predictions. 1.
We present an updated analysis of monthly means of daily mean, minimum and maximum surface air temperature (SAT) data from Greenland coastal weather stations and from a long-running site on the Greenland ice sheet, and analyse these data for evidence of climate change, especially focusing on the last 20 years but using the whole periods of available records (some since 1873). We demonstrate very strong recent warming along the west coast of Greenland, especially during winter (locally >10 • C since 1991), and rather weaker warming on the east Greenland coast, which is influenced by different oceanographic/sea-ice and meteorological synoptic forcing conditions to the rest of Greenland. Coastal Greenland seasonal mean SAT trends were generally 2–6 • C, strongest in winter (5.7 • C) and least in summer and autumn (both 2.2 • C), during 1981–2011/12. Since 2001 Greenland mean coastal SAT increased significantly by 2.9 • C in winter and 0.8 • C in summer but decreased insignificantly by 1.1 • C in autumn and 0.2 • C in spring, during a period when there was little net change (≤ ± 0.1 • C) in northern hemisphere temperatures. SAT means for the latest 2001–11/12 decade were significantly in excess of those for peak decadal periods during the Early Twentieth Century Warm Period only in summer and winter, and not significantly greater in spring and autumn. Summer SAT increases in southern Greenland for the last 20 years were generally greater for maximum than minimum temperatures. By contrast, in winter, the recent warming was greater for minimum than maximum temperatures. The greatest SAT changes in all seasons are seen on Greenland's west coast. SAT changes on the ice sheet and a key marginal glacier closely followed nearby coastal temperatures over the last 20 years.
In a number of areas of applied climatology, time series are either averaged to enhance a common underlying signal or combined to produce area averages. How well, then, does the average of a finite number (N) of time series represent the population average, and how well will a subset of series represent the N-series average. We have answered these questions by deriving formulas for 1) the correlation coefficient between the average of N time series and the average of n such series (where n is an arbitrary subset of N) and 2) the correlation between the N-series average and the population. We refer to these mean correlations as the subsammple signal strength (SSS) and the expressed population signal (EPS). They may be expressed in terms of the mean interseries correlation coefficient r-barm as SSS = (R-bar/sub n/,N)/sup 2/roughly-equaln(1+(N-1)r-bar)/N(1+(n+1)r-bar), EPS = (R-bar/sub N/)/sup 2/roughly-equalNr-bar/1+(N-1)r-bar. Similar formulas are given relating these mean correlations to the fractional common variance which arises as a parameter in analysis of variance. These results are applied to determine the increased uncertainty in a tree-ring chronology which results when the number of cores used to produce the chronology is reduced. Such uncertainty will accrue to any climate reconstruction equation that is calibrated using the most recent part of the chronology. The method presented can be used to define the useful length of tree-ring chronologies for climate reconstruction work.
The discovery of the 2012 extreme melt event across almost the entire
surface of the Greenland ice sheet is presented. Data from three
different satellite sensors - including the Oceansat-2
scatterometer, the Moderate-resolution Imaging Spectroradiometer, and
the Special Sensor Microwave Imager/Sounder - are combined to
obtain composite melt maps, representing the most complete melt
conditions detectable across the ice sheet. Satellite observations
reveal that melt occurred at or near the surface of the Greenland ice
sheet across 98.6% of its entire extent on 12 July 2012, including the
usually cold polar areas at high altitudes like Summit in the dry snow
facies of the ice sheet. This melt event coincided with an anomalous
ridge of warm air that became stagnant over Greenland. As seen in melt
occurrences from multiple ice core records at Summit reported in the
published literature, such a melt event is rare with the last
significant one occurring in 1889 and the next previous one around seven
centuries earlier in the Medieval Warm Period. Given its rarity, the
2012 extreme melt across Greenland provides an exceptional opportunity
for new studies in broad interdisciplinary geophysical research.
We present a reconstruction of the Greenland Ice Sheet surface mass
balance (SMB) from 1870-2010, based on merged Twentieth Century
Reanalysis and European Centre for Medium-Range Weather Forecasts
meteorological reanalyses, and compare our new SMB series with global
and regional climate and atmospheric circulation indices during this
period. We demonstrate good agreement between SMB annual series
constructed from 20CR and ECMWF reanalyses for the common period of
overlap, and show statistically significant agreement of long-term
modelled snowfall with ice-core-based accumulation data. We analyse
variations in SMB for the last 140 years, and highlight significantly
increased runoff and decreased SMB since 1870, which have both been
enhanced in the period since 1990, as well as interannual variations in
SMB linked to Greenland climate fluctuations. We show very good
agreement of our SMB series variations with existing,
independently-derived SMB series (RACMO2) variations for the past few
decades of overlap, but also a significant disparity of up to ~200 km3
yr-1 in absolute SMB values due to poorly constrained modelled
accumulation reflecting a lack of adequate validation data in south-east
Greenland, which could be better addressed as part of the proposed
International Polar Decade. There is no significant correlation between
our SMB time series and a widely-referenced time series of Greenland
icebergs for the past century, which may reflect the complex nature of
the relationship between SMB and ice dynamical changes. Finally, we
discuss how our analysis sheds light on the sensitivity and response of
the Greenland ice sheet to ongoing and future global climate change, and
its contribution to global sea-level rise.
High-resolution (∼11 km) regional climate modeling shows total annual precipitation on the Greenland ice sheet for 1958–2007 to be up to 24% and surface mass balance up to 63% higher than previously thought. The largest differences occur in coastal southeast Greenland, where the much higher resolution facilitates capturing snow accumulation peaks that past five-fold coarser resolution regional climate models missed. The surface mass balance trend over the full 1958–2007 period reveals the classic pattern expected in a warming climate, with increased snowfall in the interior and enhanced runoff from the marginal ablation zone. In the period 1990–2007, total runoff increased significantly, 3% per year. The absolute increase in runoff is especially pronounced in the southeast, where several outlet glaciers have recently accelerated. This detailed knowledge of Greenland's surface mass balance provides the foundation for estimating and predicting the overall mass balance and freshwater discharge of the ice sheet
The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to high-latitude freshwater input is one of the key uncertainties in the climate system. Considering the importance of the AMOC for global heat transports, and the vulnerability of the Greenland Ice Sheet (GrIS) to global warming, assessing this sensitivity is critical for climate change projections. Here we present a unique set of computational experiments to investigate the adjustment of the AMOC to enhanced melt water from the GrIS under present-day conditions. For the first time, the response in a global, strongly-eddying ocean model is systematically compared to that of an ocean model typical of IPCC-class climate models. We find that the overall decline of the AMOC on decadal time scales is quantitatively similar (<10%) in the two configurations. Nonetheless, the transient response is significantly different, as the AMOC decline and reduction in wintertime convection is markedly more gradual and persistent in the strongly-eddying configuration.
Aim Our main aim is to determine if ring‐width variations in Empetrum hermaphroditum reflect regional or local topoclimate signals in an alpine environment. In the case that topoclimate provides the dominant signal, a secondary aim is to link these to spatial distribution patterns of different vegetation types.
Location The study area is situated in the middle alpine belt in the Vågåmo region, Central Norwegian Scandes. Sampling sites cover different topoclimates: ridges, north‐facing slopes and south‐facing slopes.
Methods We constructed ring‐width chronologies of E. hermaphroditum for each type of microsite for the common period 1951–2004. Climate data were prepared on an hourly, daily and growing‐season time scale. Climate–growth relationships were evaluated using bivariate correlations and regression tree methods for continuous time‐series analyses. In addition, extreme growth anomalies (pointer years) were compared with the climate conditions in those years. The impact of water supply on wood anatomy was determined by correlating the conductive area (percentage of vessel per growth ring) with a running mean (sum) of 10‐day intervals for temperature and precipitation.
Results This study indicates that mean summer (June–August) temperatures determine the width of the growth rings of E. hermaphroditum irrespective of topoclimate. The length of the growing season, which is the most differentiating climatic factor between microsites, does not substantially alter the anatomical ring structure. Microsite differences in mean growth rates are attributed to the higher frequency of warm days. Extremely warm days limit ring‐width development at south‐facing slopes, while plants at ridges and north‐facing slopes still benefit from higher temperatures. As a consequence, pointer years are not developed synchronously at all microsites. Vessel formation is affected by available moisture, especially in the later part of the growing season.
Main conclusions Topoclimate induces slight modifications of annual growth‐ring increments of E. hermaphroditum at different microsites. In contrast to the distribution patterns of vegetation types that are determined by snow cover, growth‐ring variations are related to summer temperature conditions, and the prominent regional climate signal is still reflected at all microsites. This offers the opportunity to reconstruct climatic change in alpine regions from dwarf shrub ring‐width chronologies.
1] A three-dimensional Earth system model of intermediate complexity including a dynamic ice sheet component has been used to investigate the long-term evolution of the Greenland ice sheet and its effects on the Atlantic meridional overturning circulation (AMOC) in response to a range of stabilized anthropogenic forcings. Our results suggest that the Greenland ice sheet volume should experience a significant decrease in the future. For a radiative forcing exceeding 7.5 W m À2 , the modeled ice sheet melts away within 3000 years. A number of feedbacks operate during this deglaciation, implying a strong non-linear relationship between the radiative forcing and the melting rate. Only in the most extreme scenarios considered, the freshwater flux from Greenland into the surrounding oceans (of ca. 0.1 Sv during a few centuries) induces a noticeable weakening of the AMOC in the model. (2007), Modeling the influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during the next millennia, Geophys. Res. Lett., 34, L10707, doi:10.1029/2007GL029516.
The dwarf shrub Cassiope tetragona (Arctic bell-heather) is increasingly used for arctic climate reconstructions, the reliability of which depends on the existence of a linear climate-growth relationship. This relationship was examined over a high-arctic to sub-arctic temperature gradient and under multi-year artificial warming at a high-arctic site.
Growth chronologies of annual shoot length, as well as total leaf length, number of leaves and average leaf length per year, were constructed for three sites. Cassiope tetragona was sampled near its cold tolerance limit at Ny-Ålesund, Svalbard, at its assumed climatic optimum in Endalen, Svalbard, and near its European southern limit at Abisko, Sweden. Together these sites represent the entire temperature gradient of this species. Leaf life span was also determined. Each growing season from 2004 to 2010, 17 open top chambers (OTCs) were placed near Ny-Ålesund, thus increasing the daily mean temperatures by 1·23°C. At the end of the 2010 growing season, shoots were harvested from OTCs and control plots, and growth parameters were measured.
All growth parameters, except average leaf length, exhibited a linear positive response (R(2) between 0·63 and 0·91) to mean July temperature over the temperature gradient. Average leaf life span was 1·4 years shorter in sub-arctic Sweden compared with arctic Svalbard. All growth parameters increased in response to the experimental warming; the leaf life span was, however, not significantly affected by OTC warming.
The linear July temperature-growth relationships, as well as the 7 year effect of experimental warming, confirm that the growth parameters annual shoot length, total leaf length and number of leaves per year can reliably be used for monitoring and reconstructing temperature changes. Furthermore, reconstructing July temperature from these parameters is not hampered by divergence.
In dendroclimatology, testing the stability of transfer functions using cross-calibration verification (CCV) statistics is a common procedure. However, the frequently used statistics reduction of error (RE) and coefficient of efficiency (CE) merely assess the skill of reconstruction for the validation period, which does not necessarily reflect possibly instable regression parameters. Furthermore, the frequently used rigorous threshold of zero which sharply distinguishes between valid and invalid transfer functions is prone to an underestimation of instability. To overcome these drawbacks, we here introduce a new approach – the Bootstrapped Transfer Function Stability test (BTFS). BTFS relies on bootstrapped estimates of the change of model parameters (intercept, slope, and r²) between calibration and verification period as well as the bootstrapped significance of corresponding models. A comparison of BTFS, CCV and a bootstrapped CCV approach (BCCV) applied to 42,000 pseudo-proxy datasets with known properties revealed that BTFS responded more sensitively to instability compared to CCV and BCCV. BTFS performance was significantly affected by sample size (length of calibration period) and noise (explained variance between predictor and predictand). Nevertheless, BTFS performed superior with respect to the detection of instable transfer functions in comparison to CCV.
The most recent Intergovernmental Panel on Climate Change assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty analysis, and was limited to the 21st century. Here in a community effort, improved estimates of GrIS mass loss are included in multicentennial projections using eight state-of-the-science climate models, and an AMOC emulator is used to provide a probabilistic uncertainty assessment. We find that GrIS melting affects AMOC projections, even though it is of secondary importance. By years 2090–2100, the AMOC weakens by 18% [−3%, −34%; 90% probability] in an intermediate greenhouse-gas mitigation scenario and by 37% [−15%, −65%] under continued high emissions. Afterward, it stabilizes in the former but continues to decline in the latter to −74% [+4%, −100%] by 2290–2300, with a 44% likelihood of an AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.
The most recent IPCC assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially, but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty analysis, and was limited to the 21st century. Here, in a community effort, improved estimates of GrIS mass loss are included in multi-centennial projections using eight state-of-the-science climate models, and an AMOC-emulator is used to provide a probabilistic uncertainty assessments. We find that GrIS melting affects AMOC projections, even though it is of secondary importance. By years 2090-2100, the AMOC weakens by 18% [-3%, -34%; 90%-probability] in an intermediate greenhouse-gas mitigation scenario, and by 37% [-15%, -65%] under continued high emissions. Afterwards, it stabilizes in the former, but continues to decline in the latter to -74% [+4%, -100%] by 2290-2300, with a 44% likelihood of an AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.
We examined the suitability of two deciduous arctic shrubs (Salix glauca L. and Betula nana L., hereafter Salix and Betula, respectively) for dendroclimatological analysis at two sites in West Greenland. Chronologies were successfully cross-dated, and the oldest covered the period 1954–2010 (Expressed Population Signal [EPS] > 0.85, 1977–2010). Distinctive pointer years, also called micro-rings, including those from a known outbreak of the irruptive moth Eurois occulta L. (Lepidoptera: Noctuidae) that peaked in 2005, assisted in the dating process. Climate-growth analyses were performed in two ways: first, using correlation analysis between residual site-level chronologies and monthly and seasonal climate data, and second, using linear mixed effects models (LMM) with seasonal climate data and standardized chronologies for each individual. We used climate data for the current and previous years for a given growth ring for both analyses. Both analyses revealed differences in climate-growth response among species and among sites of contrasting topography. Salix ring widths from south facing slopes correlated positively with current year’s summer temperatures, while those on gentle slopes associated negatively with current year’s spring precipitation. Betula was only sampled at one site (flat), and displayed associations with temperature and precipitation in spring. Results from the LMM largely corroborated the correlations for Betula and Salix on south facing slopes. Salix at the flat site displayed significant associations with a large number of climate variables, most strongly previous year’s summer and autumn temperatures, though precipitation in multiple seasons of the current and prior year did a better job of accounting for the variation in the data. Many dendrochronological studies in the Arctic illustrate clear summer temperature responses, but the majority were conducted on a single topographic position. Due to the heterogeneity of West Greenland’s landscapes, it is important to examine individuals from varying topographies. We found that samples collected from south facing slopes do appear to respond positively to summer temperatures, while those on shallow slopes respond to a wider array of seasonal temperature and precipitation parameters. Accounting for these species and topographic differences, when sampling, is imperative for improving our understanding of how plant communities in the Arctic will respond to ongoing and expected warming.
Phytogeographically and climatically West Greenland includes parts of the low arctic and high arctic areas. The present vegetation and a summary of its history since the last glaciation based on pollen- and macrofossil analyses are briefly described. The determination of more than 55,000 herbarium sheets of native phanerogams has been checked. Before preparing the dot maps it was necessary to decide at which level taxonomically difficult genera, e.g. Antennaria, Draba, Poa, Puccinellia, and Stellaria would be treated. For all of these the criteria used are given. Dot maps have been prepared for 379 taxa. These maps have been grouped into 11 West Greenland distribution types, which clearly correlate with T. W. Böcher's biological distribution types (Böcher 1963). The collecting intensity in the large area is rather uneven. This is illustrated by a map giving the number of collections at the 305 localities from which 50 or more collections are at hand. Another map giving the number of taxa at 29 well-investigated localities illustrates the species diversity in the region. Most low arctic localities have more than 130 taxa, with 215 at the richest locality (Godhavn/Qeqertarsuaq on Disko), while the number at no high arctic locality exceeds 150. Reference to the corresponding numbers in other parts of Greenland is given. Based on the dot maps a new delimitation of West Greenland in floristic provinces and districts is presented. It differs somewhat from that in Grønlands Flora/The Flora of Greenland (Böcher & al. 1957, and later editions). The major alteration is that the boundary between the low and high arctic phytogeographic provinces is now placed through north Disko - Nuussuaq. As a consequence of this the southernmost district in the high arctic is divided into an outer (NWso) and an inner province (NWsi). Minor alterations are suggested in the southern part of West Greenland.
Here we present a long-term (1850–2200) best estimate of Greenland ice sheet (GrIS) freshwater runoff that improves spatial detail of runoff locations and temporal resolution. Ice discharge is taken from observations since 2000 and assumed constant in time. Surface meltwater runoff is retrieved from regional climate model output for the recent past and parameterized for the future based on significant correlations between runoff and midtropospheric (500 hPa) summer temperature changes over the GrIS. The simplicity of this approach enables assimilation of meltwater runoff into coupled climate models, which is demonstrated here in a case study with the medium-resolution (1°) Community Earth System Model. The model results suggest that the decrease in Atlantic Meridional Overturning Circulation (AMOC) is dominated by warming of the surface ocean and enhanced GrIS freshwater forcing leads to a slightly enhanced (−1.2 sverdrup in the 21st century) weakening of the AMOC.
Rapid climate warming in the tundra biome has been linked to increasing shrub dominance. Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost yet the drivers of shrub growth remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual shrub growth provide an underused resource to explore climate–growth relationships. Here, we analyse circumpolar data from 37 Arctic and alpine sites in 9 countries, including 25 species, and ~42,000 annual growth records from 1,821 individuals. Our analyses demonstrate that the sensitivity of shrub growth to climate was: (1) heterogeneous, with European sites showing greater summer temperature sensitivity than North American sites, and (2) higher at sites with greater soil moisture and for taller shrubs (for example, alders and willows) growing at their northern or upper elevational range edges. Across latitude, climate sensitivity of growth was greatest at the boundary between the Low and High Arctic, where permafrost is thawing and most of the global permafrost soil carbon pool is stored. The observed variation in climate–shrub growth relationships should be incorporated into Earth system models to improve future projections of climate change impacts across the tundra biome.
Meteorological station records, ice cores, and regional climate model output are combined to develop a continuous 171-yr (1840–2010) reconstruction of Greenland ice sheet climatic surface mass balance (Bclim) and its subcomponents including near-surface air temperature (SAT) since the end of the Little Ice Age. Independent observations are used to assess and compensate errors. Melt water production is computed using separate degree-day factors for snow and bare ice surfaces. A simple meltwater retention scheme yields the time variation of internal accumulation, runoff, and bare ice area.
At decadal time scales over the 1840–2010 time span, summer (June–August) SAT increased by 1.6°C, driving a 59% surface meltwater production increase. Winter warming was +2.0°C. Substantial interdecadal variability linked with episodic volcanism and atmospheric circulation anomalies is also evident. Increasing accumulation and melt rates, bare ice area, and meltwater retention are driven by increasing SAT. As a consequence of increasing accumulation and melt rates, calculated meltwater retention by firn increased 51% over the period, nearly compensating a 63% runoff increase. Calculated ice sheet end of melt season bare ice area increased more than 5%.
Multiple regression of interannual SAT and precipitation anomalies suggests a dominance of melting on Bclim and a positive SAT precipitation sensitivity (+32 Gt yr−1 K−1 or 6.8% K−1).
The Bclim component magnitudes from this study are compared with results from Hanna et al. Periods of shared interannual variability are evident. However, the long-term trend in accumulation differs in sign.
This technical note highlights a minor mistake in the calculation of Gleichläufigkeit (glk) suggested by Schweingruber (1983). Namely, if two series that are compared with each other show a synchronous lack of growth change this will not be treated accurately. We present an accurate way of glk calculation based on Gegenläufigkeit (Huber, 1943) as in Eckstein and Bauch (1969) and suggest that software-tools and methodological approaches which make use of glk should be tested for the way they compute glk.
Shrub dendrochronological investigations are recently gaining more and more importance within the dendro-scientific community. As being a rather young discipline, many means of shrub dendrochronology lean on established methods that have been developed for trees. Although shrubs as trees are woody plants, it seems likely that they express differing growth characteristics due to their often multi-stemmed and prostrate stature. Yet, the majority of shrub dendrochronological investigations have measured shrub ring widths along two radii within one (sometimes several) stem disk(s) per individual. To our knowledge only one study so far has undertaken the approach to measure complete area increments (e.g. basal area increments, if applied to the basal stem disk of a shrub), however not focusing in detail on a comparative evaluation of this new approach with respect to radial measurements. To fill this knowledge gap our study focuses on the comparison of stem disk area increment measurements with radial measurements in the context of shrub growth representation and response- and transfer-function analyses. Our results indicate that for eccentric shrubs a minimum of four radial measurements per stem disk should be obtained for a good representation of the average stem disk growth. Inter-stem-disk comparisons showed that growth differences between individuals were often misestimated when only based on one or two radial measurements per stem disk. Response- and transfer-function analyses suggested, that the investigated shrubs reflect different environmental signals within different sectors of stem discs. This implies to carefully select radial measurements and individuals to increase the strength of environmental signals within transfer functions.
Shrub dendrochronological investigations are recently gaining more and more importance within the dendro-scientific community. As being a rather young discipline, many means of shrub dendrochronology lean on established methods that have been developed for trees. Although shrubs as trees are woody plants, it seems likely that they express differing growth characteristics due to their often multi-stemmed and prostrate stature. Yet, the majority of shrub dendrochronological investigations have measured shrub ring widths along two radii within one (sometimes several) stem disc(s) per individual. To our knowledge only one study so far has undertaken the approach to measure complete area increments (e.g. basal area increments, if applied to the basal stem disc of a shrub), however not focusing in detail on a comparative evaluation of this new approach with respect to radial measurements. To fill this knowledge gap our study focuses on the comparison of stem disc area increment measurements with radial measurements in the context of shrub growth representation and response- and transfer-function analyses. Our results indicate that for eccentric shrubs a minimum of four radial measurements per stem disc should be obtained for a good representation of the average stem disc growth. Inter-stem-disc comparisons showed that growth differences between individuals were often misestimated when only based on one or two radial measurements per stem disc. Response- and transfer-function analyses suggested, that the investigated shrubs reflect different environmental signals within different sectors of stem discs. This implies to carefully select radial measurements and individuals to increase the strength of environmental signals within transfer functions.
Supplementing broader-scale dendroecological approaches with high-resolution wood anatomical analyses constitutes a useful technique to assess spatiotemporal patterns of climate-induced growth responses in circumpolar tundra vegetation. A systematic evaluation of dendrochronological and wood anatomical features in arctic dwarf shrubs is, however, still missing. Here, we report on nearly thousand samples from ten major dwarf shrub species that were collected at 30 plot-sites around 70 degrees N and 22 degrees W in coastal East Greenland. Morphological root and stem characteristics, together with intra-annual anatomical variations are outlined and the potential and limitation of ring counting is stressed. This study further demonstrates the possibility to gain annually resolved insight on past dry matter production and carbon allocation in arctic (and alpine) environments well beyond northern (and upper) treelines, where vegetation growth is particularly sensitive to environmental change.
The total extent of ice melt on the Greenland ice sheet has been increasing during the last three decades. The melt extent observed in 2007 in particular was the greatest on record according to several satellite-derived records of total Greenland melt extent. Total annual observed melt extent across the Greenland ice sheet has been shown to be strongly related to summer temperature measurements from stations located along Greenland's coast, as well as to variations in atmospheric circulation across the North Atlantic. We make use of these relationships along with historical temperature and circulation observations to develop a near-continuous 226 year reconstructed history of annual Greenland melt extent dating from 2009 back into the late eighteenth century. We find that the recent period of high-melt extent is similar in magnitude but, thus far, shorter in duration, than a period of high melt lasting from the early 1920s through the early 1960s. The greatest melt extent over the last 2 1/4 centuries occurred in 2007; however, this value is not statistically significantly different from the reconstructed melt extent during 20 other melt seasons, primarily during 1923-1961.
Recognizing that climate influences both annual tree-ring growth and glacier mass balance, changes in the mass balance of Place Glacier, British Columbia, were documented from increment core records. Annually resolved ring-width (RW), maximum (MXD), and mean density (MD) chronologies were developed from Engelmann spruce and Douglas-fir trees sampled at sites within the surrounding region. A snowpack record dating to AD 1730 was reconstructed using a multivariate regression of spruce MD and fir RW chronologies. Spruce MXD and RW chronologies were used to reconstruct winter mass balance (Bw) for Place Glacier to AD 1585. Summer mass balance (Bs) was reconstructed using the RW chronology from spruce, and net balance was calculated from Bw and Bs. The reconstructions provide insight into the changes that snowpack and mass balance have undergone in the last 400years, as well as identifying relationships to air temperature and circulation indices in southern British Columbia. These changes are consistent with other regional mass-balance reconstructions and indicate that the persistent weather systems characterizing large scale climate-forcing mechanisms play a significant glaciological role in this region. A comparison to dated moraine surfaces in the surrounding region substantiates that the mass-balance shifts recorded in the proxy data are evident in the response of glaciers throughout the region.
The melt extent of the snow on the Greenland ice sheet is of considerable importance to the ice sheet's mass and energy balance, as well as Arctic and global climates. By comparing passive microwave satellite data to field observations, variations in melt extent have been detected by establishing melt thresholds in the cross-polarized gradient ratio (XPGR). The XPGR, defined as the normalized difference between the 19-GHz horizontal channel and the 37-GHz vertical channel of the Special Sensor Microwave/Imager (SSM/I), exploits the different effects of snow wetness on different frequencies and polarizations and establishes a distinct melt signal. Using this XPGR melt signal, seasonal and interannual variations in snowmelt extent of the ice sheet are studied. The melt is found to be most extensive on the western side of the ice sheet and peaks in late July. Moreover, there is a notable increasing trend in melt area between the years 1979 and 1991 of 4.4% per year, which came to an abrupt halt in 1992 after the eruption of Mt. Pinatubo. A similar trend is observed in the temperatures at six coastal stations. The relationship between the warming trend and increasing melt trend between 1979 and 1991 suggests that a 1°C temperature rise corresponds to an increase in melt area of 73000 km2, which in general exceeds one standard deviation of the natural melt area variability.
Correlation analysis of Greenland coastal weather station temperatures against the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) indices for the summer season (when Ice Sheet melt and runoff occur) reveals significant temporal variations over the last 100 years, with periods of strongest correlations in the early twentieth century and during recent decades. During the mid-twentieth century, temperature changes at the stations are not significantly correlated with these circulation indices. Greenland coastal summer temperatures and Greenland Ice Sheet (GrIS) runoff since the 1970s are more strongly correlated with the Greenland Blocking Index (GBI) than with the NAO Index (NAOI), making the GBI a potentially useful predictor of ice-sheet mass balance changes. Our results show that the changing strength of NAOI–temperature relationships found in boreal winter also extends to summer over Greenland. Greenland temperatures and GrIS runoff over the last 30–40 years are significantly correlated with AMO variations, although they are more strongly correlated with GBI changes. GrIS melt extent is less significantly correlated with atmospheric and oceanic index changes than runoff, which we attribute to the latter being a more quantitative index of Ice Sheet response to climate change. Moreover, the four recent warm summers of 2007–2010 are characterised by unprecedented high pressure (since at least 1948 – the start of the NCEP/NCAR reanalysis record) in the tropospheric column. Our results suggest complex and changing atmospheric forcing conditions that are not well captured using the NAO alone, and support theories of an oceanic influence on the recent increases in Greenland temperatures and GrIS runoff. Copyright 2012 Royal Meteorological Society
The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a 'heat dome' over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994–2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2 • C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future 'average' summers projected this century.
The instrumental Arctic climate record is both temporally and spatially limited. Therefore, there is a need for reliable climate proxies to increase knowledge of past and future Arctic climate change. Annual shoot length increase of the circumarctic dwarf shrub species Cassiope tetragona represents such a new climate proxy. We measured annual shoot length increase of 32 plant samples of the circumarctic dwarf shrub species C. tetragona using the presence of wintermarksepta within the stems, resulting in a 169 year growth chronology (1840–2008) for a High Arctic site on Svalbard. This is the longest growth chronology for dwarf shrub species produced up to now.
The breeding biology of the Lapland Bunting was studied in an area of central West Greenland during two contrasting seasons. In the delayed thaw of 1984, male birds arrived on the breeding grounds at the same time as in the more typical spring of 1979, but females delayed arrival by two weeks in 1984 compared with 1979. Mean clutch size was depressed in the latter year and declined through the season; breeding productivity was correspondingly reduced in 1984 compared to 1979. Egg and juvenile predation rates were similar for the two years. Nestlings grew as rapidly in 1984 as in 1979 despite an apparent reduction in detected ground-dwelling invertebrates known to form an important constituent of the nestling diet. Because they bred later in 1984, females tended to commence moult later in 1984 than 1979 but completed moult by early September in both years. Males also commenced moult later but completed earlier in 1984.
We review and compare two alternative spatial regression methods used in dendroclimatology to reconstruct climate from tree rings. These methods are orthogonal spatial regression (OSR) and canonical regression (CR). Both the OSR and CR methods have a common foundation in least-squares theory and converge to the same solution when all p candidate tree-ring predictors of climate are forced into the model. However, the perfomance of OSR and CR may differ when only subsets p′ < p predictors are used. Theory cannot predict how either method is likely to perform when best-subset selection is applied, especially with regards to reconstruction accuracy. Consequently, empirical comparisons of OSR and CR are made using three tree-ring and climate networks from western Europe and eastern North America that have been used in previous dendroclimatic studies. These comparisons rely on a suite of regression model verification statistics to validate the accuracy of the climatic reconstructions produced by the best-subset models. The results indicate little real difference between OSR and CR, with each performing equally good or bad depending on the amount of recoverable climatic information in the tree rings. Canonical regression may perform slightly better in high signal-to-noise cases; conversely, OSR may perform slightly better when the signal-to-noise ratio is low. None of these apparent differences are large enough to select one method in preference to the other, however, and many more comparisons would be needed to determine if such indications are generally valid.
Reliable paleoclimate reconstructions are needed to assess if the recent climatic changes are unusual compared to pre-industrial
climate variability. Here, we focus on one important problem in climate reconstructions: Transfer functions relating proxies
(predictors) and target climatic quantities (predictands) can be seriously biased when predictand and predictor noise is not
adequately accounted for, resulting in biased amplitudes of reconstructed climatic time series. We argue for errors-in-variables
models (EVM) for unbiased identification of linear structural relationships between noisy proxies and target climatic quantities
by (1) introducing underlying statistical concepts and (2) demonstrating the potential biases of using the EVM approach, the
most commonly used direct ordinary least squares (OLS) regression, inverse OLS regression, or the reduced major axis method
(‘variance matching’) with a simulation example of artificial noise-disturbed sinusoidal time series. We then develop an alternative
strategy for paleoclimate reconstruction from tree-ring and other proxy data, explicitly accounting for the identified problem.
Some background describing the rationale and early development of regional curve standardization (RCS) is provided. It is
shown how, in the application of RCS, low-frequency variance is preserved in the mean values of individual series of tree
indices, while medium-frequency variance is also preserved in the slopes. Various problems in the use of the RCS approach
are highlighted. The first problem arises because RCS detrending removes the average slope (derived from the data for all
trees) from each individual tree measurement series. This operation results in a pervasive ‘trend-in-signal’ bias, which occurs
when the underlying growth-forcing signal has variance on timescales that approach or exceed the length of the chronology.
Even in a long chronology (i.e., including subfossil data), this effect will bias the start and end of the RCS chronology.
Two particular problems associated with the use of RCS on contemporaneously growing trees, which might represent a typical
(i.e., modern) sample, are also discussed. The first is the biasing of the RCS curve by the residual climate signal in age-aligned
samples and the undesirable subsequent removal of this signal variance in RCS application. The second is the ‘differing-contemporaneous-growth-rate’
bias that effectively imparts a spurious trend over the span of a modern chronology. The first of these two can be mitigated
by the application of ‘signal-free’ RCS. The second problem is more insidious and can only be overcome by the use of multiple
sub-RCS curves, with a concomitant potential loss of some longer-timescale climate variance. Examples of potential biasing
problems in the application of RCS are illustrated by reference to several published studies. Further implications and suggested
directions for necessary further development of the RCS concept are discussed.
KeywordsDendrochronology-Regional curve standardization-Low-frequency variance-Chronology bias-Signal-free regional curve standardization
Das in Mitteleuropa eingesetzte jahrringchronologische Datierungsverfahren wurde im Hinblick auf Rationalisierung und Aussagesicherheit geprüft. Als zeitliche Engpässe erwiesen sich die Probenentnahme, die Jahrringbreitenmessung, das Zeichnen der Zuwachskurven und vor allem der manuelle Kurvenvergleich. Durch Einsatz einer elektronischen Rechenanlage und anderer Hilfsmittel konnte der Ablauf beschleunigt werden. Für eine sichere Synchronisierung von Jahrringfolgen wurde der dafür gewählte Gleichläufigkeitswert, untersucht. Obwohl sich der Streubereich dieses Wertes bei synchroner und unsynchroner Deckungslage weitgehend überschneidet, gestattet der Gleichläufigkeitswert dennoch eine objektive Differenzierung, an Hand derer visuell eine sichere Altersbestimmung möglich ist. Ein Gleichläufigkeitswert resultiert aus den klimatisch geprägten, gleichsinnig verlaufenden Zuwachsschwankungen (Weiserjahre) und aus den zufällig sich deckenden Kurvenabschnitten. Die Werte beider Komponenten streuen. Erst wenn sich Werte au den beiden positiven Streufeldern summieren, liegt der daraus resultierende Gleichläufigkeitswert außerhalb der Zufallsstreuung. Zur Demonstration des modifizierten dendrochronologischen Verfahrens wurden einige Datierungsbeispiele angeführt.