Top PDF Impact of Holocene climate variability on Arctic vegetation

Impact of Holocene climate variability on Arctic vegetation

Impact of Holocene climate variability on Arctic vegetation

An interest in predicting the impact of global warming on Arctic eco- systems has led to considerable research using many approaches. Plant physiological studies have long been popular in Arctic regions, as this is a clear example of an area where temperature stress is important, al- though other environmental factors also affect plant growth, reproduc- tion, and survival ( Crawford, 1989 ). Population and community level studies, for example from the ITEX project, have attempted to estimate how the current warming is affecting the vegetation (e.g., Elmendorf et al., 2012 ). Remote sensing is used to document larger scale changes in plant production ( Epstein et al., 2013 ), and models of the vegetation can summarize our understanding of Arctic vegetation response to cli- mate variability (e.g., Epstein et al., 2000 ). Among the many conclusions of the above studies is the importance of scale, in that short-term tran- sient changes cannot be used to predict longer-term development of terrestrial plant communities, and that considerable spatial variability is present, especially from the Low to the High Arctic. The current warming is affecting the Low Arctic more than the High Arctic and changes differ depending on plant functional type ( Elmendorf et al., 2012 ). Although there are occasional exceptions (e.g., from Svalbard,
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A Geobotanical Analysis Of Circumpolar Arctic Vegetation, Climate, And Substrate

A Geobotanical Analysis Of Circumpolar Arctic Vegetation, Climate, And Substrate

30 2.2 Introduction Arctic land use is undergoing rapid change: expanding resource extraction and changing cultural practices are predicted to seriously impact over half o f the Arctic within the next 50 years (Nellemann et al., 2001). In addition, the climate is changing; some areas are cooling while most are warming (Hassol, 2004; Comiso, 2003). As a result vegetation in the Arctic is changing (Goetz et al., 2005; Stow et al., 2004), including characteristics such as phytomass (aboveground plant biomass) (Jia et al., 2003). In order to determine the scale and importance o f these changes and to evaluate any actions that might be taken in response, it is necessary to understand the present distribution o f Arctic vegetation and phytomass. To meet this need, an international group o f vegetation scientists collaborated to produce the Circumpolar Arctic Vegetation Map (CAVM) (CAVM Team, 2003; Walker et al., 2005). This paper summarizes vegetation characteristics indicated by spatial trends in the Normalized Difference Vegetation Index (NDVI) and phytomass shown on the CAVM. This analysis provides data for assessing change on global and regional levels and is useful for modeling climate change, for land-use planning, resource development, education and conservation studies.
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Holocene climate variability in the winter rainfall zone of South Africa

Holocene climate variability in the winter rainfall zone of South Africa

Over the last 700 yr, the GeoB8332-4 is marked by a gradual increase of fine-grained fluvial sediment (EM3). Nd and Sr isotope analyses suggest that ephemeral rivers of Namaqua- land and probably tributaries of the Lower Orange River sig- nificantly contributed to the continuous rise of fluvial sedi- ments between 600 and 100 cal years BP (Fig. 4c and d). Robust climate records from the main catchment of the Or- ange River covering the last 700 yr are rare. δ 18 O records of cave deposits in the Makapansgat Valley have been inter- preted to reflect cold and relatively dry conditions (Tyson et al., 2000). Pollen in a sediment sequence from Lake Eteza located in easternmost South Africa (SRZ) indicates low tree percentages and an increase of grass accompanied by high sedimentation rate during the last 700 yr, and has been inter- preted as reflecting dry conditions and anthropogenic impact (Neumann et al., 2010). The interpretation of dry climate in eastern South Africa is corroborated by measurements of tree ring thickness suggesting two prolonged dry phases within the last 700 yr (Vogel et al., 2001). Overall, available records from eastern South Africa suggest that climate during the last 700 yr was relatively dry. In contrast, an increasing flood oc- currence in the banks of the Buffels River (Figs. 1 and 7j) (Benito et al., 2011) and in the Lower Orange River (Herbert and Compton, 2007, and references therein) as well as pulses of freshening events evident in the Lake Verlorenvlei record (Stager et al., 2012) (Figs. 1 and 7k) over the last 700–600 yr lend credence to our Nd and Sr isotope-based inference of increased sediment contribution from the local rivers.
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Mid- to late Holocene climate variability of the Maritime Continent

Mid- to late Holocene climate variability of the Maritime Continent

Abstract Precipitation reconstructions from Indonesia reveal a complex rainfall history over the Maritime Continent during the Holocene. Previous studies highlighted the strong connection of Indonesian climate variability to high northern latitude forcing via latitudinal shifts in the Intertropical Convergence Zone (ITCZ) in symphony with changes in the Australian- Indonesian monsoon system on glacial-interglacial timescales as well as during the past centuries. Here we present variations in grain size and element distribution in gravity cores from the Java Sea in order reconstruct changes in precipitation in response to the controlling climate phenomena El Niño-Southern Oscillation and the Australian-Indonesian monsoon system. The cores were taken off major river mouths, the Seruyan Delta off Borneo and the Solo Delta off Java, reflecting changes in precipitation-related runoff for the past 5.5 kyr. The records suggest a long-term decrease in rainfall culminating at ~2 ka in Borneo and relatively unchanged rainfall in Java. We suggest that drier conditions around ~2 ka are related to prolonged dry seasons in Borneo owing to intense El Niño events. These conditions are masked by a stronger Australian-Indonesian summer monsoon (AISM) in Java. Based on observational data from the past decades, we infer a strong teleconnection of the Australian- Indonesian monsoon system to North Atlantic sea surface temperature anomalies known as the Atlantic Multidecadal Oscillation (AMO). A southward shift in the austral summer ITCZ and the ensuing increase in the sensitivity of the Indonesian rainfall to AMO warm phases might have resulted in intensified AISM in Java leveling out the impact of intensified and/or more frequent El Niño events during the late Holocene.
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Holocene climate variability in the North-Western Mediterranean Sea (Gulf of Lions)

Holocene climate variability in the North-Western Mediterranean Sea (Gulf of Lions)

Six CRs of different duration and amplitude were identified in the Gulf of Lions SST record (Table 2). The occurrence of CRs has been previously described in global compilations (Mayewski et al., 2004; Wanner et al., 2011) and seems to be associated with glacier advances in Europe (Denton and Karlén, 1973). They reflect either polar cooling or tropical aridity that likely express atmospheric circulation changes (Mayewski et al., 2004). The influence of the AMV has also been suggested (Kushnir and Stein, 2010). There are, how- ever, discrepancies on the spatio-temporal distribution and amplitude of these events (Wanner et al., 2011, 2014). Each CR does not necessarily impact everywhere with the same intensity due to local responses to climate changes. The sen- sitivity of proxies or particular sediment settings (e.g. coastal areas), their seasonal character, may also be another reason for not detecting CRs in all records. For example, it is inter- esting to note that the 8200 yr BP, well expressed in Green- land ice cores (Johnsen et al., 2001) is not found in the core
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Late Pleistocene to Mid-Holocene climate variability in Ireland : evidence from Ostracod geochemistry

Late Pleistocene to Mid-Holocene climate variability in Ireland : evidence from Ostracod geochemistry

This research is justified because concerns over anthropogenic forcing of climate due to increased greenhouse gas emissions require novel efforts to better understand the impact of global warming on earth’s ecosystems. Due to the relatively short duration of instrumental records, preanthropogenic climate and environmental changes must be characterized to compare to modern trends. Therefore, high-resolution environmental reconstructions must be prioritized that preserve centennial variability. Records such as this study are critical to further our understanding of natural regional climate dynamics. The cosmopolitan distribution of freshwater ostracods permits characterization of lacustrine environments throughout Western Europe by faunal assemblages of ostracods (e.g. Meisch 2000, Absolon 1973). The composition and relative abundance of species have been used as a qualitative measure for freshwater ecosystem health (Boomer 2002; Meisch 2000). The primary objective of the research herein is to generate a carbon isotope record of carbon cycling for Western Ireland tuned with species-specific ostracod ecological requirements.
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Simulating the climate response to atmospheric oxygen variability in the Phanerozoic: a focus on the Holocene, Cretaceous and Permian

Simulating the climate response to atmospheric oxygen variability in the Phanerozoic: a focus on the Holocene, Cretaceous and Permian

HadCM3-BL (Valdes et al., 2017) is an AOGCM coupled to an interactive vegetation model. The model was originally developed by the United Kingdom Met Office Hadley Cen- tre (Pope et al., 2000) but has since been substantially de- veloped further by the University of Bristol. The atmosphere component of the model employs a regular Cartesian grid of 3.75 ◦ longitude by 2.5 ◦ latitude. In the vertical, 19 hy- brid height vertical levels are employed. A 30 min time step is used. The primitive equation set of White and Bromley (1995) is solved to conserve energy and angular momentum, solved on a staggered Arakawa B grid (Arakawa and Lamb, 1977) in the horizontal. Radiation is represented using the Edwards and Slingo (1996) scheme with six shortwave and eight longwave bands, accounting for the radiative effects of water vapour, carbon dioxide and ozone, amongst other ra- diative active species. The ocean component of the model employs the same horizontal grid as the atmosphere compo- nent of the model, 3.75 ◦ longitude by 2.5 ◦ latitude. In the
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Reconstruction of glacier variability from lake sediments reveals dynamic Holocene climate in Svalbard

Reconstruction of glacier variability from lake sediments reveals dynamic Holocene climate in Svalbard

glacier growth on Svalbard remains elusive and deserves future investigation. 6. Conclusions This study presents a continuous reconstruction of Holocene glacier variability on Svalbard at centennial timescales, combining physical, geochemical and magnetic proxies with multivariate statistics. Integrating our findings from Lake Hajeren in a regional paleoclimatic context, we observe a three-staged Holocene climate history. Following deglaciation around 11,300 cal BP, glaciers remained present in the catchment throughout the Early Holocene, culminating in a glacier maximum around 9.5 cal BP. During this period, glacier activity appears to be affected by meltwater pulses from the melting LIS ( Jennings et al., 2015 ). Following a late onset of the HTM around 6.7 ka cal BP, in line with modeled cooling effects from the melting LIS ( Renssen et al., 2009 ), glaciers disappear from the catchment. Instead, changing redox conditions driven by strati fication of the water column governed lacustrine sedimenta- tion during most of the Middle and Late Holocene. Two advances between 4205e4050 and 3380e3220 cal BP mark the onset of the Neoglacial and coincide with brief episodes of North Atlantic cooling ( Bond et al., 2001; Debret et al., 2007; Renssen et al., 2007 ). A gradual decline in summer insolation progressively lowered the glaciation threshold towards the Late Holocene, resulting in pro- longed glacier activity around 700 cal BP. This local onset of the LIA is in line with advances reported at nearby sites ( Furrer, 1991; Røthe et al., 2015; Svendsen and Mangerud, 1997; Werner, 1993 ). The rapid response of the small Hajeren glaciers improves our under- standing of climate variability on Svalbard, suggesting that the Holocene was punctuated by major centennial-scale perturbations. As such, this study underlines the value of glacier-fed lake sedi- ments in contextualizing Arctic climate dynamics.
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7300 years of vegetation history and climate for NW Malta: a Holocene perspective

7300 years of vegetation history and climate for NW Malta: a Holocene perspective

4500 cal BP, followed by declining winter and summer wet- ness. This is largely consistent with findings from Burmar- rad. These changing moisture levels during the Holocene have been linked to significant societal changes. Sadori et al. (2015b) propose that periods of increased humidity, over the last 2000 years, coincided with both agricultural and de- mographic expansions. While Weiss and Bradley (2001) sug- gest that, around 4250 BP, a number of cultures were at their economic peak, such as Mesopotamia’s Akkadian empire, Egypt’s Old Kingdom civilization and Palestine, Greece, and Crete’s Early Bronze societies; however, these once flourish- ing areas declined rapidly after 4150 BP possibly due to se- vere drought and cooling. The event has been recorded else- where in the world and seems to have acted at a global scale (Booth et al., 2005). The impact of drought events on the human socio-economy, and the consequent impacts on the landscape, should thus not be underestimated, as has been recently suggested by Sharifi et al. (2015) for the continen- tal Middle East. These increases in aridity not only affect the vegetation communities directly but also indirectly by altering the anthropogenic pressure on the local landscape, both directly in those regions, as well as wherever the dis- placed people migrate. This combined effect is not confined to the eastern Mediterranean at this time. Closer to the Mal- tese archipelago, Noti et al. (2009) suggest that at Gela di Biviere, Sicily, between 5000 and 4000 cal BP, the anthro- pogenic impact occurring on the landscape is probably influ- enced by the climatic changes.
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Contribution of oceanic and vegetation feedbacks to Holocene climate change in monsoonal Asia

Contribution of oceanic and vegetation feedbacks to Holocene climate change in monsoonal Asia

Paleoreconstructions reveal significant changes of the monsoon intensity during the Holocene. Around 6000 years before present (6 k) the African as well as the Asian monsoon likely penetrated further inland, implying a wetter climate (e.g. Winkler and Wang, 1993; Kohfeld and Harrison, 2000; Ge et al. 2007; Maher, 2008) and a change in the vegetation distribution (e.g. Jolly et al., 1998; Yu et al., 2000). Early modelling studies related this enhancement of the monsoons to the impact of changing insolation on climate (e.g. Kutzbach and Otto-Bliesner, 1982; Harrison et al., 1998). Due to the variations in the Earth’s orbit (mainly the precessional cycle), perihelion was reached in September (6 k) instead of January at present time, yielding an increase of summer and decrease of winter insolation in the Northern Hemisphere relative to today (Berger, 1978). The seasonal cycle was enhanced in the Northern and reduced in the Southern Hemisphere, affecting the cross-equatorial ocean-land temperature gradient and thereby the monsoon flows. However, modelling studies just focusing on the direct response to orbital forcing tend to underestimate the monsoon expansion and associated increase of rainfall, at least over North Africa (Joussaume et al., 1999). More recently, research studies also consider ocean and/or vegetation interactions by performing coupled model simulations. Ocean-atmosphere interaction seems to further enhance the North African monsoon (Kutzbach and Liu, 1997; Braconnot et al., 2000; Liu et al., 2004). Regarding the Asian monsoon ambiguous results can be found. Several climate modelling studies report an increase of precipitation to be attributed to the ocean coupling (Hewitt and Mitchell, 1998; Braconnot et al. 2000, Wei and Wang, 2004). Other model results suggest an attenuation of Asian monsoon precipitation due to interactive ocean (Voss and Mikolajewicz, 2001; Liu et al., 2004 (for 11 k); Ohgaito and Abe-Ouchi, 2007; Li and Harrison, 2008, Marzin and Braconnot, 2009b).
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Arctic shrubification mediates the impacts of warming climate on changes to tundra vegetation

Arctic shrubification mediates the impacts of warming climate on changes to tundra vegetation

Variation across landscape The effects of warming and shrubification on vegeta- tion show spatial variation, with the strongest in flu- ence at low and high elevations (figure 6). All spatial patterns hold in general for all species groups despite some fine-scale spatial variability (see figures S3–S8), yet the variation between predictions of the gains and losses of arctic and boreal species is noteworthy (figure S3 ). The spatial predictions indicate increases in vascular plant species richness at high elevations and decreases at low elevations subsequent to warming (figures 6(a) and (b)). Comparing the spatial predic- tions of plant species richness produced by abiotic and biotic models shows that shrubification can both reverse and amplify the changes generated by warming (figures 6(c)–(f)). At high elevations, the biotic models predict lower increases in species richness than the abiotic models, while at low elevations, shrubi fication amplifies the decrease in species richness. At mid- elevations, shrubi fication has locally heterogeneous impacts, both amplifying and decreasing the predicted warming-induced changes in species richness. Both the turnover and extinction risk caused by warming are strongly ampli fied by shrubification at the highest and lowest elevations of the test area (figures 6(g)–(r)).
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Bunker Cave stalagmites: an archive for central European Holocene climate variability

Bunker Cave stalagmites: an archive for central European Holocene climate variability

Abstract. Holocene climate was characterised by variabil- ity on multi-centennial to multi-decadal time scales. In cen- tral Europe, these fluctuations were most pronounced dur- ing winter. Here we present a record of past winter climate variability for the last 10.8 ka based on four speleothems from Bunker Cave, western Germany. Due to its central Eu- ropean location, the cave site is particularly well suited to record changes in precipitation and temperature in response to changes in the North Atlantic realm. We present high- resolution records of δ 18 O, δ 13 C values and Mg/Ca ratios. Changes in the Mg/Ca ratio are attributed to past meteoric precipitation variability. The stable C isotope composition of the speleothems most likely reflects changes in vegetation and precipitation, and variations in the δ 18 O signal are inter- preted as variations in meteoric precipitation and tempera- ture. We found cold and dry periods between 8 and 7 ka, 6.5 and 5.5 ka, 4 and 3 ka as well as between 0.7 and 0.2 ka. The proxy signals in the Bunker Cave stalagmites compare well with other isotope records and, thus, seem representative for central European Holocene climate variability. The promi- nent 8.2 ka event and the Little Ice Age cold events are both recorded in the Bunker Cave record. However, these events show a contrasting relationship between climate and δ 18 O, which is explained by different causes underlying the two climate anomalies. Whereas the Little Ice Age is attributed
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Holocene climate variability on millennial scales recorded in Greenland ice cores

Holocene climate variability on millennial scales recorded in Greenland ice cores

American) bioactivity, and serves as an indicator of biomass burning events (Taylor et al., 1996). High ammonium con- centrations in the ice core indicate stronger atmospheric load- ing over the North American continent, meaning improved transport conditions in terms of increased storminess in this region and/or rather dry climatic conditions. High-frequent storm events are caused by a strengthening of the polar vortex that evokes growing pressure gradients between the pole and lower latitudes and a cooling in the North and at middle lat- itudes, respectively. Besides of some spread centennial vari- ability the wavelet analysis reveals an almost continuous cy- cle on the 1.5 kiloyears scale, namely for ages between 2 and 10.5 kiloyears B.P., and, with very high amplitudes from 12.5 to 15.7 kiloyears B.P. (Fig. 3). Further, continuous cycles with periods of 3.0–3.5 kiloyears, 4.5 kiloyears and around 7 kiloyears are visible, the first two of them are close to mul- tiples of the famous period of 1.47 kiloyears. Rather similar results are found for the variations in Potassium (Witt and Oberhaensli, 2005) 1 , which is related to the vegetation cover of the soil.
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Interglacial climate variability during MIS 15 to Holocene Insight from Coupled climate modelling

Interglacial climate variability during MIS 15 to Holocene Insight from Coupled climate modelling

3.5 Discussion While most time slices presented in this study were simulated for the first time using a com- prehensive CGCM, the 6, 115 and 125 ka time slices have been studied extensively in previous model studies. In general, the CCSM3 results are in line with these previous stud- ies in terms of large-scale temperature and precipitation patterns. Warm boreal summer conditions (relative to PI) over most parts of the continents and the Arctic are a general fea- ture in paleoclimatic simulations of the mid-Holocene (6 ka), while the North African and South Asian monsoon regions are anomalously cold due to enhanced rainfall (Braconnot et al., 2007). Though evidenced by proxy records (e.g., McClure, 1976; Hoelzmann et al., 1998; Fleitmann et al., 2003), several models fail to simulate wetter mid-Holocene condi- tions over the Arabian Peninsula (cf. https://pmip3.lsce.ipsl.fr/database/ maps/), while CCSM3 simulates not only enhanced rainfall but also greening of the Ara- bian Desert. The 125 ka surface temperature pattern shows similar features than the 6 ka pattern, but much more pronounced due to the larger eccentricity and hence stronger pre- cessional forcing. However, compared to other simulations of the last interglaciation, our CCSM3 simulation produces a relatively cold MIS 5e surface climate as shown by Lunt et al. (2013). At 115 ka, surface temperature anomalies show the opposite sign with dramatic cooling over the Arctic and the northern continental regions providing ideal conditions for glacial inception (e.g., Khodri et al., 2005; Kaspar and Cubasch , 2007; Jochum et al., 2012). A retreat of the vegetation at high northern latitudes tends to amplify the insolation- induced cooling (cf. Gallimore and Kutzbach, 1996; Meissner et al., 2003).
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Diatom response to mid-Holocene climate in three small Arctic lakes in northernmost Finnmark

Diatom response to mid-Holocene climate in three small Arctic lakes in northernmost Finnmark

The diatom floristic shifts at c. 5,270 and 5,350 cal. yr BP within these three lake sites occur between two notable short-lived peaks in Pinus:Betula ratios. Pollen records from OGF and LCJ have higher Pinus:Betula ratios during the period examined, in comparison to OKV where the treeline response to climate appears more muted. The results highlight significant differences in the responses to climatic changes between aquatic ecosystems (freshwater diatoms) and terrestrial vegetation (tree limit fluctuations). There are clear similarities between the diatom records, however site-specific differences in the diatom response can make it more difficult to interpret a clear climate signal, as diatoms respond to a variety of environmental factors. Climatic signals from aquatic systems can be problematic, due to the complexity of climate-driven changes within high-latitude lake ecosystem functioning, with many factors affecting the diatom community composition. The diatom assemblage records from these lake sites appear to be responding to regional climatic changes, and are modulated by their catchment and lake site conditions. The abrupt diatom assemblage shifts found within these lakes between c. 5,270 and 5,350 cal. yr BP are indicative of the onset of warm conditions. This is where Cyclotella species and large pennate diatoms (Navicula radiosa and Cymbella descripta) increase in OGF and OKV. However, pollen data suggest that the warming trend did not occur until c. 4,525 cal. yr BP. This could be a result of lags in the response of trees to climatic changes, or it is possible that the diatoms within these three Arctic lakes are responding to a separate climatic event to which the vegetation was not sensitive. Thus, our results did not find synchronous responses of tree limits and diatom assemblages. Given, however, that these biological systems are experiencing and responding to different climatic or climate-related variables, such a result should perhaps not be a surprise, but should encourage further research that aims to clarify the climatic variables to which each proxy are responding (Huntley, 2012).
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Glacier response to North Atlantic climate variability during the Holocene

Glacier response to North Atlantic climate variability during the Holocene

coarser, clastic intervals. However, the XRF data have a greater sensitivity to minerogenic changes and were mea- sured at higher resolution (0.2 mm) (Fig. 3). We focused our analysis on the following elements: K, Ca, Ti, Mn, Fe, Zn, Rb, and Sr, which are common in siliciclastic sedi- ments. Changes in the concentrations of these elements re- flect changes in the contribution of minerogenic material eroded from catchment bedrock and delivered to the lake. Statistical analysis of the scanning XRF data indicates that all of the elements are highly correlated and that there is a strong primary trend in the data. Correlation coefficients show the strong significant relationships among the majority of the elements (Table 2). Rather than relying on a single ele- ment (e.g., Ti), we used principal component analysis (PCA) to define the leading mode of variability (PC1) among the elemental data. PCA allows for a multidimensional exami- nation of the data set in order to identify the primary sig- nal(s). PCA results indicate that there is one strong primary trend in the elemental data with the first eigenvector (PC1) accounting for 76 % of the total variance. The factor load- ings reveal the high correlations between individual element profiles and PC1 (Table 2). The trends in PC1 are similar to those in the lower-resolution magnetic susceptibility and or- ganic matter content records, justifying use of PC1 data to infer past minerogenic changes (Fig. 3). The choice to use PC1 rather than a single representative element (e.g., Ti) to represent changes in sedimentary minerogenic content has no impact on any of our conclusions.
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Pollen-based reconstruction of Holocene vegetation and climate in southern Italy: the case of Lago Trifoglietti

Pollen-based reconstruction of Holocene vegetation and climate in southern Italy: the case of Lago Trifoglietti

From ca. 6100 to ca. 5200 cal BP, the first shallow wa- ter phase corresponds to alder expansion reflecting evolu- tion towards the terminal phase of lake infilling, but the re- gression of fir to the benefit of beech also suggests a dry episode, this corresponds to a ca. 100 mm summer precip- itation decrease inferred by quantitative climate reconstruc- tion (Peyron et al., 2012). Alder development is also reported at Canolo Nuovo at ca. 5000 uncal BP (i.e., ca. 5800 cal BP; Schneider, 1985). This time interval is also characterised by a cooling associated with the short cold event (SCE4) reported in marine cores from the Tyrrhenian (annual SST; BS7937; Sbaffi et al., 2004) and Adriatic Seas (alkenone SST; AD91- 17; Sangiorgi et al., 2003). Magny (2004) and Magny et al. (2012) have already discussed the possible impact in Eu- rope of a cold event related with a Rapid Climate Change (RCC) between 6000 and 5000 cal BP defined by Mayewski et al. (2004). Local effect of these RCC may, thus, have af- fected the rain regime in the Alps where successive episodes of higher lake level between 5550 and 5300 cal yr BP are ob- served at Lake Constance, coinciding with glacier advance (Magny and Haas, 2004). The wet and cool climate oscilla- tion (wet winter but dry summer) observed in southern Italy contrasts with higher lake levels reconstructed in central and northern Italy (Magny et al., 2007a, 2012), and is in accor- dance with wet and cool conditions from 6000 to 5400 cal BP reported in the eastern Mediterranean by Finn´e et al. (2011). From ca. 5200 to 4300 cal BP, a more humid phase is in- ferred from deeper water reconstructed at Trifoglietti. This is supported by increases in annual precipitation (Fig. 9c, C106, Di Donato et al., 2008) and summer precipitation (Peyron et al., 2012). This phase also coincides with a reduction in for- est cover at Trifoglietti, which can also be affected by a bias in AP wa percentages.
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The accuracy of climate variability and trends across Arctic Fennoscandia in four reanalyses

The accuracy of climate variability and trends across Arctic Fennoscandia in four reanalyses

In this analysis, we have focussed on annual and sea- sonal data but extreme SAT and PPN events are likely to be of greater importance to planning authorities given they can be potential triggers for natural hazards that may impact socio-economic activity (Dyrrdal et al., 2012). The spatial scale of modern global reanalyses, although considerably improved over previous versions, is still too coarse to accu- rately represent such events. This is especially the case in regions of steep and complex orography, such as the Scan- dinavian Mountains, where both SAT and PPN can vary markedly over short distances (e.g., Aalto, Riihimäki, Mei- neri, Hylander, & Luoto, 2017; Johansson & Chen, 2003; Pike, Pepin, & Schaefer, 2013; Yang et al., 2012). The examples when the interquartile ranges of SAT and/or PPN from the gridded reanalysis data and observations fail to even overlap provide clear evidence of the spatial mismatch between the two data sets. Thus, in such regions, global rea- nalyses are probably most usefully employed in providing boundary conditions for either dynamical downscaling stud- ies, using regional atmospheric models (e.g., Bieniek et al., 2016; Heikkilä et al., 2011; Lenaerts et al., 2013), or higher-resolution regional-scale reanalyses, such as the Arc- tic System Reanalysis (Bromwich et al., 2016) and the Uncertainties in Ensembles of Regional Reanalyses (UERRA) project that encompasses Europe (www.uerra.eu).
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The impact of Arctic sea ice change on midlatitude climate

The impact of Arctic sea ice change on midlatitude climate

The results in the previous paragraph suggest an equatorward jet shift in cold past periods such as the LGM. Model experiments have further investigated this hypothesis. Model simulations of the LGM with specified SST (either from reconstructions or from model runs) create storm tracks which are very much aligned along the zonal ice edge on the North Atlantic (Kageyama et al., 1999; Dong and Valdes, 2000). Dong and Valdes (1998) also attributed part of the response in LGM experiments with fixed SSTs versus those with a slab ocean to the change in sea ice. Pausata et al. (2011) forced a coupled climate model with isolated components of LGM forcing (greenhouse gases, ice sheet albedo and ice sheet topography). The response to topography, in particular the Lauren- tide ice sheet over the North American continent, dominated the response in mean SLP, the leading mode of SLP variability, and the North Atlantic jet stream. This suggests that it is the mechanical forcing from topography driving the downstream response, rather than local surface temperatures. This contrasts with the conclusions of Kageyama et al. (1999) andDong and Valdes (2000); however, the runs in Pausata et al. (2011) did have less sea ice than the atmosphere only models of these two earlier studies. The dominance of the Laurentide ice sheet in driving the LGM stationary wave patterns and Atlantic eddy driven jet was also found in the atmosphere- and land- only simulations of Merz et al. (2015).
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Late Holocene vegetation changes in relation with climate fluctuations and human activity in Languedoc (southern France)

Late Holocene vegetation changes in relation with climate fluctuations and human activity in Languedoc (southern France)

Abstract. Holocene climate fluctuations and human activ- ity since the Neolithic have shaped present-day Mediter- ranean environments. Separating anthropogenic effects from climatic impacts to better understand Mediterranean pale- oenvironmental changes over the last millennia remains a challenging issue. High-resolution pollen analyses were un- dertaken on two cores from the Palavasian lagoon system (Hérault, southern France). These records allow reconstruc- tion of vegetation dynamics over the last 4500 years. Re- sults are compared with climatic, historical and archeological archives. A long-term aridification trend is highlighted dur- ing the late Holocene, and three superimposed arid events are recorded at 4600–4300, 2800–2400 and 1300–1100 cal BP. These periods of high-frequency climate variability coin- cide in time with the rapid climatic events observed in the Atlantic Ocean (Bond et al., 2001). From the Bronze Age (4000 cal BP) to the end of the Iron Age (around 2000 cal BP), the spread of sclerophyllous taxa and loss of forest cover result from anthropogenic impact. Classical An- tiquity is characterized by a major reforestation event re- lated to the concentration of rural activity and populations in coastal plains leading to forest recovery in the mountains. A major regional deforestation occurred at the beginning of the High Middle Ages. Around 1000 cal BP, forest cover is mini- mal while the cover of olive, chestnut and walnut expands in relation to increasing human influence. The present-day veg- etation dominated by Mediterranean shrubland and pines has been in existence since the beginning of the 20th century.
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