moisture and temperature changes associated with the mid holocene tsuga decline in the northeastern united states abstract a decline of hemlock tsuga populations at ca 5 5 ka thousands of calibrated radiocarbon years before 1950 ad stands out as the most abrupt vegetation change of the holocene in north america but remains poorly understood after decades of study recent analyses of fossil pollen have revealed a concurrent abrupt oak quercus decline and increases in the abundance of beech fagus and pine pinus on cape cod in eastern massachusetts but the replacement of drought tolerant oaks by moisture sensitive beeches appears inconsistent with low lake levels in the region at the same time the oak and beech changes are also limited to coastal areas and the coastal inland differences require an explanation here we develop a new lake level reconstruction from deep pond cape cod by using a transect of sediment cores and ground penetrating radar gpr profiles to constrain the past elevations of the sandy littoral zone of the pond the reconstruction shows that a series of multi century episodes of low water coincide with the abrupt hemlock and oak declines and interrupt subsequent phases of hemlock recovery the lake level variations equal precipitation deficits of 100 mm superimposed on a holocene long moisture increase of 400 mm however because moisture deficits do not easily explain the oak and beech changes we also evaluate how the climate preferences of the regional vegetation changed over time by matching the fossil pollen assemblages from deep pond with their modern equivalents reconstructions of the precipitation requirements of the vegetation correlate well even in detail with the lake level record r 0 88 at deep pond and indicate close tracking of effective moisture precipitation minus evapotranspiration by the vegetation despite the abrupt species declines which could have decoupled climate and vegetation trends reconstructions of the temperature preferences of the vegetation indicate that coastal sites may have cooled by 0 5 2 5 c after ca 5 5 ka while inland sites warmed by 0 5 1 c the change in coastal temperature preferences agrees with sea surface cooling in the western atlantic ocean of 1 c consequently the persistence of low hemlock abundance after 5 5 ka in the northeast u s may have resulted from oceanic changes that produced multi century droughts and thus delayed the post decline recovery of hemlock populations

millennial scale temperature variations in north america during the holocene 1 a mean continental july temperature reconstruction based on pollen records from across north america quantifies temperature variations of several timescales for the past 14 000 cal yr bp in north america temperatures increased nearly 4 c during the late glacial reaching maximum values between 6000 and 3000 cal yr bp after which mean july temperatures decreased superimposed on this orbital scale trend are millennial scale temperature variations that appear coherent in structure and frequency with highresolution ice marine and other terrestrial paleoclimate records of the holocene during the holocene climate in north america appears to have varied periodically every 1100 years rather than the 1500 year cycle found during the last glacial period coherence at frequencies between 900 and 1100 years between land ice and ocean records suggests a common forcing associated with widespread surface impacts during the holocene these results provide important insight to the global warming debate as the observed twentieth century temperature increase appears unprecedented compared to our mean north american temperature reconstruction of the past 14 000 years

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

peat brgdgts based holocene temperature history of the altai mountains in arid central asia abstract due to their effects on snow ice melting in mountains and evaporation in basins temperature variations are an important influence on the hydrological cycle and water resources in arid central asia aca we present independently dated peat brgdgts based mbt mbt 5me records from an alpine sahara sand peatland ssp in the southern altai mountains in aca during the past 11 kyr the ssp mbt and mbt 5me profiles exhibit consistently increasing trends and they are significantly positively correlated using a global peat specific mbt 5me tempeature calibration the long term holocene warming trend indicated by the quantitative ssp mean annual temperature matpeat reconstruction is supported by the peat cellulose 13c summer temperature record from the ssp and by an independent peat cellulose 18o winter temperature record from a nearby site i e temporal validation the increasing trends are also evident in holocene peat mbt records from the hongyuan hy peatland in the ne tibetan plateau and in the shuizhuyang szy peatland in se china notably both the mbt data of the surface samples and the averaged mbt data of the holocene peat cores are consistent with the modern spatial temperature gradient at the ssp hy and szy i e spatial validation this evidence demonstrates the temperature significance of peat mbt mbt 5me records the long term holocene warming trend indicated by the ssp mbt mbt 5me records is supported by recently acquired holocene temperature records from the eurasian continent this trend is the opposite to the traditional viewpoint of a long term holocene cooling trend and it indicates that more effort is needed to obtain a reliable holocene temperature history

temporal variability of flooding events of guadiana river iberian peninsula during the middle to late holocene imprints in the shallow marine sediment record abstract sedimentological geochemical and benthic foraminiferal proxies were used to interpret changes of depositional environments in a mud entrapment inside the guadiana river paleo valley northern gulf of cadiz to understand the temporal variability of fluvial flooding events and to detect patterns of latitudinal climatic variability in western iberia the period between ca 5800 and ca 1250 cal yr bp was characterized by slowly accumulating coarse grained sediments high content of biogenic sand components and high abundances of shallow water benthic foraminiferal species after ca 1250 cal yr bp the sedimentary environment was dominated by fine grained sediments with high abundances of opportunistic benthic foraminiferal species and successful colonizers and high values of geochemical ratios indicative of enhanced terrigenous supply sedimentation rates increased drastically over the last ca 500 years the genesis of these environments was mainly driven by the variable frequency of fluvial flooding events driven by the regional climate variability low terrigenous sediment input under dry conditions prevailed in the older sedimentary unit flood frequency and terrigenous sediment supply increased during the younger unit the variability of the north atlantic oscillation at centennial scales highly influenced the sediment supply during the last ca 500 years with high terrigenous supply during negative nao conditions our results corroborate an n s gradient along the atlantic iberia during middle and late holocene with more humid conditions in the northwestern and drier conditions towards the southeast the gradient weakened over the past ca 500 years with increased rainfalls and flood events during the little ice age

the role of advective heat transport in talik development beneath lakes and ponds in discontinuous permafrost 1 regions of warm thin discontinuous permafrost have been observed to be experiencing rapid changes in lake and pond dynamics in recent decades even though surface water and groundwater interactions are thought to play a significant role in heat transport in these regions the effect of these interactions on permafrost remains largely unquantified in order to examine the influence of groundwater flow on permafrost dynamics we modeled the development of a sub lake talik under permafrost conditions similar to those observed in the southern central seward peninsula region of alaska using a numerical solution that couples heat transport and groundwater flow including the effect of water phase changes on soil permeability and latent heat content a comparison of model simulations with and without near surface subpermafrost groundwater flow indicates that stable permafrost thicknesses are 2 to 5 times greater in the absence of groundwater flow simulations examining the thermal influence of lakes on underlying permafrost suggest that a through going talik can develop in a matter of decades and that the incorporation of advective heat transport reduces the time to complete loss of ice beneath the lake by half relative to heat transport by conduction alone this work presents the first quantitative assessment of the rates of sub lake permafrost response to thermal disturbances such as talik development in systems with near surface groundwater flow the results highlight the importance of coupled thermal and hydrologic processes on discontinuous permafrost dynamics

millennial scale climate fire vegetation interactions in a mid elevation mixed coniferous forest mission range northwestern montana usa mixed coniferous forests are widespread at middle elevations in the northern rocky mountains yet relatively little is known about their long term vegetation and fire history pollen and charcoal records from twin lakes in the mission range of northwestern montana provide information on mixed coniferous forest development and fire activity over the last 15 000 years these data suggest an open parkland and minimal fire activity before 13 500 cal yr bp consistent with cold dry conditions increases in pinus pollen after 13 500 cal yr bp indicate a transition to closed forests and a slight rise in fire activity as conditions warmed and fuel biomass increased high levels of artemisia pollen between 10 000 and 6000 cal yr bp suggest an open forest during the early holocene when conditions were warmer and drier than present low severity fires likely maintained open forest shrublands but produced little charcoal during this interval present day mixed coniferous forests were established in the last 6 ka and included forest taxa characteristic of low pseudotsuga larix pinus ponderosa and high severity fire regimes picea abies increased climate variability and anthropogenic burning may have helped maintain heterogeneous mixed coniferous forests during the last several millennia when climate conditions became cooler and wetter

orbitally driven insolation forcing on holocene climate trends evidence from alkenone data and climate modeling a global spatial pattern of long term sea surface temperature sst trends over the last 7000 years is explored using a comparison of alkenone derived sst records with transient ensemble climate simulations with a coupled atmosphere ocean circulation model under orbitally driven insolation forcing the spatial trend pattern both in paleo sst data and in model results shows pronounced global heterogeneity generally the extratropics cooled while the tropics experienced a warming during the middle to late holocene we attribute these divergent holocene climate trends to seasonally opposing insolation changes furthermore climate mode changes similar to the arctic north atlantic oscillation are superimposed on the prevalent pattern it is concluded that nonlinear changes in the entire seasonal cycle of insolation played a dominant role for the temporal evolution of holocene surface temperatures for understanding of marine proxy data apart from the dominance of summer insolation in high latitudes a notable shift in the maximum insolation of the year in low latitudes has to be taken into account which may influence timing of phytoplankton production and thus alters the seasonal origin of temperature signals recorded in the proxies

younger dryas and allerod summer temperatures at gerzensee switzerland inferred from fossil pollen and cladoceran assemblages linear and unimodal based inference models for mean summer temperatures partial least squares weighted averaging and weighted averaging partial least squares models were applied to a high resolution pollen and cladoceran stratigraphy from gerzensee switzerland the time window of investigation included the allerod the younger dryas and the preboreal characteristic major and minor oscillations in the oxygen isotope stratigraphy such as the gerzensee oscillation the onset and end of the younger dryas stadial and the preboreal oscillation were identified by isotope analysis of bulk sediment carbonates of the same core and were used as independent indicators for hemispheric or global scale climatic change in general the pollen inferred mean summer temperature reconstruction using all three inference models follows the oxygen isotope curve more closely than the cladoceran curve the cladoceran inferred reconstruction suggests generally warmer summers than the pollen based reconstructions which may be an effect of terrestrial vegetation not being in equilibrium with climate due to migrational lags during the late glacial and early holocene allerod summer temperatures range between 11 and 12 c based on pollen whereas the cladoceran inferred temperatures lie between 11 and 13 c pollen and cladocera inferred reconstructions both suggest a drop to 9 10 c at the beginning of the younger dryas although the allerod younger dryas transition lasted 150 160 years in the oxygen isotope stratigraphy the pollen inferred cooling took 180 190 years and the cladoceran inferred cooling lasted 250 260 years the pollen inferred summer temperature rise to 11 5 12 c at the transition from the younger dryas to the preboreal preceded the oxygen isotope signal by several decades whereas the cladoceran inferred warming lagged major discrepancies between the pollen and cladoceran inference models are observed for the preboreal where the cladoceran inference model suggests mean summer temperatures of up to 14 15 c both pollen and cladoceran inferred reconstructions suggest a cooling that may be related to the gerzensee oscillation but there is no evidence for a cooling synchronous with the preboreal oscillation as recorded in the oxygen isotope record for the gerzensee oscillation the inferred cooling was ca 1 and 0 5 c based on pollen and cladocera respectively which lies well within the inherent prediction errors of the inference models

on the definition of seasons in paleoclimate simulations with orbital forcing 1 orbital forcing is a major driver of climate variability on timescales of 10 000 to 100 000 years changes in the orbital parameters cause variations in the length of the seasons by several days consequently models using a fixed present day calendar result in biased paleoseasons especially in boreal autumn when the vernal equinox is used as an anchor point the bias is estimated for temperatures and precipitation in a transient model simulation over the last 21 000 years and an accelerated simulation over the last 129 000 years the largest differences of up to 4 k occur over the continents in high latitudes precipitation estimates are mostly affected in the low latitudes the time dependent bias is large enough to modify the temporal characteristics of temperature and precipitation indices it is discussed to what extent the bias in one season is distorting comparisons between models and paleoproxies the bias has minor implications for proxy model comparisons in general however proxies of monsoon activity should be compared with fixed angular seasons for process studies and climate sensitivity studies the use of fixed angular seasons is imperative

pollen based continental climate reconstructions at 6 and 21 ka a global synthesis subfossil pollen and plant macrofossil data derived from 14c dated sediment profiles can provide quantitative information on glacial and interglacial climates the data allow climate variables related to growing season warmth winter cold and plant available moisture to be reconstructed continental scale reconstructions have been made for the mid holocene mh around 6 ka and last glacial maximum lgm around 21 ka allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth assessment report ar5 of the intergovernmental panel on climate change the synthesis of the available mh and lgm climate reconstructions and their uncertainties obtained using modern analogue regression and model inversion techniques is presented for four temperature variables and two moisture variables reconstructions of the same variables based on surface pollen assemblages are shown to be accurate and unbiased reconstructed lgm and mh climate anomaly patterns are coherent consistent between variables and robust with respect to the choice of technique they support a conceptual model of the controls of late quaternary climate change whereby the first order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance

quantifying regional vegetation changes in china during three contrasting temperature intervals since the last glacial maximum abstract the need to reduce the uncertainty of predictions of vegetation change under global warming highlights the importance of understanding the vegetation patterns and possible mechanisms of vegetation response during past warming intervals here we present quantitative regional vegetation reconstructions for china during the last glacial maximum lgm 18 2 14c kyr b p early holocene eh 8 5 0 5 14c kyr b p and mid holocene mh 6 0 5 14c kyr b p using the biomization method and based on 249 pollen records in addition we used an inverse vegetation modeling approach to investigate the effect of climate change and co2 concentration on the observed vegetation changes the results demonstrate that during the lgm steppe expanded southeastwards reaching the present day temperate deciduous forest tede zone in contrast the forest in eastern china underwent a substantial southward retreat and its percentage reached a minimum the warm mixed forest wamf and tede shifted southwards of 10 n relative to today and tropical seasonal rain forest tsfo was almost absent in addition the forest steppe boundary shifted southwards to near the middle and lower reaches of the yangtze river during the eh and mh the tsfo wamf and tede exhibited respective northward shifts of 2 4 and 5 relative to today in mh the percentage of forest sites increased and reached a maximum and the forest steppe boundary had shifted northwestwards to near the present day 300 mm isohyet our palaeovegetation reconstructions and model sensitivity experiments suggest that temperature was the dominant factor controlling the vegetation distribution during the lgm while precipitation became increasingly more important during the holocene throughout china we further show that precipitation was the primary factor controlling palaeovegetation distribution in northern china while temperature became increasingly more important in southern china our findings are potentially important for understanding the evolution of vegetation in china in response to ongoing global warming

pollen weighs in on a climate conundrum simulations by climate models show that earth warmed during the holocene epoch whereas ocean sedimentary cores suggest that global cooling occurred an analysis of fossil pollen samples now sides with the models simulations by climate models show that earth warmed during the holocene epoch whereas ocean sedimentary cores suggest that global cooling occurred an analysis of fossil pollen samples now sides with the models

the magnitude of error in conventional bulk sediment radiocarbon dates from central north america abstract although the carbon reservoir problem with bulk sediment radiocarbon dates from lakes has long been recognized many synoptic studies continue to use chronologies derived from such dates for four sites in central north america we evaluate chronologies based on conventional radiocarbon dates from bulk sediment versus chronologies based on accelerator mass spectrometry ams radiocarbon dates from terrestrial plant macrofossils the carbon reservoir error varies among sites and temporally at individual sites from 0 to 8000 yr an error of 500 2000 yr is common this error has important implications for the resolution of precise event chronologies

reconciling the westerlies and monsoon models a new hypothesis for the holocene moisture evolution of the xinjiang region nw china abstract the holocene evolution of moisture conditions in the arid xinjiang region in inland nw china is increasingly debated analysis of climatic reconstructions from lake sediments loess sand dunes and peats reveals a long term wetting trend westerlies model whereas other studies mainly of lake sediments and stalagmites have indicated a long term drying trend monsoon model two observations can be made about these conflicting interpretations i the monsoon model records are mainly from high altitude mountain areas with higher precipitation whereas the westerlies model records are mainly from low altitude piedmont areas and basins with lower precipitation ii an increasing number of studies of the xinjiang region and its surroundings demonstrate a long term warming trend during the holocene based on these observations we propose the following simple hypothesis which is a compromise between the westerlies and monsoon models against the background of long term holocene warming the increased meltwater supply from snow and ice in high altitude mountain areas fed the low altitude lakes which finally resulted in higher lake levels expanded oasis areas and enhanced local moisture recycling this potentially explains why the westerlies model evidenced in the moisture records from lakes and adjacent loess and peat deposits comprises a long term wetting trend this hypothesis is largely consistent with the parts of the region that are currently experiencing climatic warming more importantly with the likely acceleration of climatic warming in the near future the further ablation of ice and snow in the high altitude mountain regions will present a major and growing threat to the sustainable socioeconomic development of xinjiang consequently it is important that both the government and the public seriously consider the implications of an ice free environment in the region

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

warm mediterranean mid holocene summers inferred from fossil midge assemblages reconstructions of holocene summer temperatures differ between models and vegetation based proxies a quantitative reconstruction for the mediterranean region based on fossil midge assemblages suggests warm summers in line with climate models

sensitivity of paleoclimate simulation results to season definitions according to the milankovitch theory slow variations of the earth s orbital parameters change the amplitude of the seasonal cycle of insolation and are considered to be the main forcing mechanism of glacial interglacial cycles because of the precession and changes in eccentricity the length of seasons also varies no absolute phasing is then possible between the insolation curves of two different periods various solutions to compare different periods have been given either for astronomical computations e g berger and loutre 1991 laskar et al 1993 or for model simulations e g kutzbach and otto bliesner 1982 mitchell et al 1988 but the sensitivity of model results to the different possible solutions has never been quantified our results based on simulations of the last interglacial climate 126 kyr b p where changes in the length of the seasons are large clearly show that phase leads or lags between the various solutions used introduce biases in the analysis of insolation and climate change of the same order of magnitude as the milankovitch forcing our main conclusions are that 1 when comparing various model simulations the date of the vernal equinox i e the phasing of the seasonal cycle of insolation as well as the definition of seasons must be the same for all models in order to avoid artificial differences 2 seasons based on astronomical positions are preferred to seasons defined with the same lengths as today since they better account for the phasing of insolation curves however insolation is not the only forcing in most atmospheric general circulation model simulations we also discuss the impact of the calendar hidden behind the definition of the seasonal cycle of the other boundary conditions such as sea ice or sea surface temperatures

two long and pronounced cold periods 3 000 5 000 and 6 600 8 400 years b p in east asia and the southward migration of the westerly jet abstract to reconstruct the variations in sea surface temperature sst during the holocene the alkenone unsaturation index of marine sediments from two deep drilled cores and 81 different core top surface sediments recovered from the heuksan mud belt located off the southwestern coast of the korean peninsula was measured first comparison of the alkenone temperature estimates of 81 core top sediments with in situ temperatures indicated that the alkenone temperatures corresponded to the average sst in april to october the spatial distribution pattern of the core top alkenone sst showed a north south temperature gradient which represented the in situ temperatures well this indicated that the effects of the resuspension and lateral transport of sediments on the past temperature estimation might be insignificant based on the two deep drilled cores variations in the alkenone sst during the holocene were reconstructed in general the alkenone sst decreased by less than 1 c from the early to late holocene an interesting feature was the presence of two long and pronounced cold approximately 2 c periods which occurred at 3 5 kyr and 6 6 8 4 kyr b p these cold periods were also observed in other mid latitude regions in east asia indicating that the cooling pattern was regional the cold periods appeared to be associated with the southward migration of the westerly jet at that time investigations into the effects of local sea level changes and resultant changes in the tidal regime on changes in the sst during the holocene period suggested that they were insignificant in the study area

degradation of permafrost beneath a road embankment enhanced by heat advected in groundwater for the past few decades northwestern north america has been affected by climate warming leading to permafrost degradation and instability of the ground this is problematic for all infrastructure built on permafrost especially roads and runways thaw settlement and soil consolidation promote embankment subsidence and the development of cracks potholes and depressions in road pavement in this study we investigate highway stability in permafrost terrain at an experimentally built road embankment near beaver creek yukon a network of 25 groundwater monitoring wells was installed along the sides of the road to estimate groundwater flow and its thermal impact on the permafrost beneath the road data on topography water table elevation ground temperature and stratigraphy of the soil were collected at the site the geotechnical properties of each soil layer were determined by laboratory analysis and used to calibrate a two dimensional groundwater flow model field observations showed that water was p

complex human vegetation climate interactions in the late holocene and their significance for paleotemperature reconstructions using 3 980 14c ages and 43 669 pollen data from 632 globally distributed lakes 88 of them in north america and europe fig 1 a jenny et al 1 demonstrate that increased soil erosion rates fig 1 b and decreased tree coverage fig 1 c have occurred in lake watersheds across a substantial portion of earth s surface since 4 000 y bp besides using climate model results fig 1 d they propose that the late holocene tree coverage decreases were caused mainly by anthropogenic deforestation 1 notably the model deduced late holocene slight cooling trends in north america and europe 1 fig 1 d are the opposite of the slight global warming trends indicated by other models 2 fig 1 e arguably it is problematic to use model results to determine a climate vegetation relationship moreover the model deduced slight cooling trends since 4 000 y bp 1 fig 1 d are supported by a synthesized global mean temperature record 3 fig 1 f however only 4 out of the 73 selected records fig 1 a comprising the synthesized record are terrestrial records 3 fig 1 a locations of the study sites cited herein the blue squares represent the 1 1to whom correspondence may be addressed email raozhg at hunnu edu cn 1 xref corresp 1 1

the climate of europe 6000 years ago palaeoclimates across europe for 6000 y bp were estimated from pollen data using the modern pollen analogue technique constrained with lake level data the constraint consists of restricting the set of modern pollen samples considered as analogues of the fossil samples to those locations where the implied change in annual precipitation minus evapotranspiration p e is consistent with the regional change in moisture balance as indicated by lakes an artificial neural network was used for the spatial interpolation of lake level changes to the pollen sites and for mapping palaeoclimate anomalies the climate variables reconstructed were mean temperature of the coldest month tc growing degree days above 5 c gdd moisture availability expressed as the ratio of actual to equilibrium evapotranspiration and p e the constraint improved the spatial coherency of the reconstructed palaeoclimate anomalies especially for p e the reconstructions indicate clear spatial and seasonal patterns of holocene climate change which can provide a quantitative benchmark for the evaluation of palaeoclimate model simulations winter temperatures tc were 1 3 k greater than present in the far n and ne of europe but 2 4 k less than present in the mediterranean region summer warmth gdd was greater than present in nw europe by 400 800 k day at the highest elevations and in the alps but 400 k day less than present at lower elevations in s europe p e was 50 250 mm less than present in nw europe and the alps but was 10 15 greater than present in s europe and p e was 50 200 mm greater than present in s and e europe

millennial scale increase in winter precipitation in the southern rocky mountains during the common era we employed the modern analog technique to quantitatively reconstruct temperature and precipitation over the past 2500 yr based on fossil pollen records from six high elevation lakes in northern colorado reconstructed annual temperatures for the study area did not deviate significantly from modern over the past 2500 yr despite hemispheric expressions of medieval climate anomaly warmth and little ice age cooling annual precipitation however shifted from lower than modern rates from 2500 to 1000 cal yr bp to higher than modern rates after 1000 cal yr bp a greater than 100 mm increase in precipitation winter precipitation accounts for the majority of the change in annual precipitation while summer precipitation rates did not change significantly over the past 2500 yr the large change in winter precipitation rates from the first to second millennium of the common era is inferred from a shift in fossil pollen assemblages dominated by subalpine conifers which have southern sites as modern analogs to assemblages representing open subalpine vegetation with abundant artemisia spp sagebrush which have more northern modern analogs the change helps to explain regional increases in lake levels and shifts in some isotopic and tree ring data sets highlighting the risk of large reductions in snowpack and water supplies in the intermountain west

long term thermal and settlement characteristics of air convection embankments with and without adjacent surface water ponding in permafrost regions abstract surface water is an important local factor in engineering geological conditions in permafrost regions in this paper the impacts of surface water ponding on the long term thermal and settlement characteristics of an air convection embankment ace were investigated based on 13 years of data records from field observations along the qinghai tibet railway qtr the time series of the ground temperatures and the settlement of two aces with and without adjacent surface water ponding were investigated and compared the results indicated that surface water ponding could significantly affect the thermal performance of the ace without surface water ponding the ace showed a satisfactory cooling effect the underlying permafrost table rose and the shallow permafrost layer cooled significantly after construction of the embankment however with surface water ponding the great heat capacity and latent heat of the ponded water and related capillary action impeded the thermal interaction between the ace and the underlying permafrost before the drainage of the ponded water no permafrost cooling occurred beneath the embankment the settlement of the two aces also differed significantly the creep of the ice rich permafrost layer immediately beneath the permafrost table was inferred as the main contribution of the settlement the temperature of the layer was the main factor determining the rate of settlement the results can help the hydro thermal analysis of engineering foundations built over permafrost and provide a reference for embankment construction and maintenance in permafrost regions with rich surface water

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

in situ monitoring of settlement at different layers under embankments in permafrost regions on the qinghai tibet plateau abstract ground temperature and settlement at different soil layers are monitored at four typical sections of the qinghai tibet highway qth and one section of a newly built test road nbtr in beilu river basin for the purpose of extensive investigation on the rather complicated mechanism of embankment settlement owing to several physical and mechanical processes i e settlement sources taking place in different soil layers in permafrost regions combining the ground temperature with borehole drilling exploration thawing temperatures of the permafrost layers in the five sections are determined and then characteristics of permafrost degradation are analyzed three types of sections are summarized to clarify different settlement sources in different soil layers on these bases characteristics of embankment settlement under different geological conditions are analyzed the influence of the seasonal changes of air temperature on embankment settlement is discussed since the two sections one on qth and the other on nbtr are both located in beilu river basin they are compared with respect to the variation of the main settlement source in the process of permafrost degradation the results help to understand the mechanism of embankment settlement

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

the spatial and temporal complexity of the holocene thermal maximum the holocene thermal maximum a period of relatively warm climate between 11 000 and 5 000 years ago1 2 is most clearly recorded in the middle and high latitudes2 3 of the northern hemisphere where it is generally associated with the local orbitally forced summer insolation maximum however proxy based reconstructions have shown that both the timing and magnitude of the warming vary substantially between different regions2 3 4 suggesting the involvement of extra feedbacks and forcings here we simulate the holocene thermal maximum in a coupled global ocean atmosphere vegetation model we find that before 7 000 years ago summers were substantially cooler over regions directly influenced by the presence of the laurentide ice sheet whereas other regions of the northern hemisphere were dominated by orbital forcing our simulations suggest that the cool conditions arose from a combination of the inhibition of labrador sea deep convection by the flux of meltwater from the ice sheet which weakened northward heat transport by the ocean and the high surface albedo of the ice sheet we thus conclude that interglacial climate is highly sensitive to relatively small changes in ice sheet configuration

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

holocene temperature history of northwest greenland with new ice cap constraints and chironomid assemblages from deltaso abstract arctic temperature shifts drive changes in carbon cycling sea ice extent and greenland ice sheet mass balance all of which have global ramifications paleoclimate data from past warm periods provide a unique means for assessing the sensitivity of these systems to warming climate but the magnitude and timing of past temperature changes in many parts of the arctic are poorly known here we assess orbital scale holocene temperature change in northwest greenland near the margin of the ice sheet using subfossil insect assemblages from lake deltaso based upon sedimentation history in this currently proglacial lake we also place constraints on holocene extents of the adjacent north ice cap a large independent ice cap reconstructed summer temperatures were warmer than present at the onset of lacustrine sedimentation following regional deglaciation by the greenland ice sheet sometime between 10 8 and 10 1 ka bp deltaso experienced the warmest summer temperatures of the holocene between 10 and 6 2 ka bp followed by progressive cooling that continued through the late holocene as summer insolation declined culminating in the lowest temperatures during the pre industrial last millennium deltaso chironomids indicate peak early holocene summer temperatures at least 2 5 3 c warmer than modern and at least 3 5 4 c warmer than the pre industrial last millennium we infer based upon lake sediment organic and biogenic content that in response to declining temperatures north ice cap reached its present day size 1850 ad having been smaller than present through most of the preceding holocene our synthesis of paleoclimate evidence from northwest greenland ellesmere island and northern baffin bay supports the timing of temperature trends inferred at deltaso and suggests that quantitative temperature reconstructions from deltaso may represent a minimum bound on regional early holocene warming collectively records from the region indicate 4 c summer cooling through the holocene intense early holocene warmth around northwest greenland argues against delayed onset of warmer than present conditions due to the influence of the nearby waning laurentide ice sheet and has implications for understanding the greenland ice sheet s sensitivity to climate change

permafrost warming under the earthen roadbed of the qinghai tibet railway this paper investigates the stability of the earthen roadbed built in the warm and ice rich permafrost region the varying thermal regime of the subgrade and the ongoing settlement of the roadbed were observed at field the temperature data demonstrate that in warm and ice rich permafrost regions adoption of earthen roadbed results in warming of the underlying permafrost it is primarily because the earthen roadbed traps the warm season absorbed heat in the natural ground in addition the carried heat of the earthen roadbed that was constructed in warm season propagates downward to warm the underlying soil the warming permafrost layer promotes the roadbed settlement which was mostly linearly developed in the past five service years a comprehensive analysis for the varying thermal regime and the ongoing settlement shows that the unfrozen water liberated from the warming undrained layer experiences consolidation the deformation of the undrained soils is mainly responsible for settlement of the roadbed in comparison the temperature variation of this warming permafrost layer is found to be less beneath roadbeds protected by thermosyphons or crushed rock revetments the installation of thermosyphons into the earthen roadbed is recommended to prevent the further degradation of the underlying permafrost

holocene and last interglacial climate of the faroe islands from sedimentary plant wax hydrogen and carbon isotopes abstract the last interglacial period lig is earth s most recent globally warm period and is analogous in some ways to projected future global warming however questions remain regarding the state of the climate during the lig in the north atlantic a region that is extremely sensitive to changes in oceanic and atmospheric circulation here we present hydrogen and carbon isotope d and 13c records from a suite of plant wax biomarkers preserved in holocene and lig lacustrine sediments from the north atlantic faroe islands and interpret them as qualitative proxies for temperature and hydroclimate variability these data are used to directly compare lig and holocene climate using the same proxy approaches from the same terrestrial location measuring multiple isotopes on multiple types of waxes elucidates the sources of homologous plant waxes we deduce that the d values of long chain n alkanes c27 c33 and mid chain n alkanes c23 c25 in these sedimentary archives reflect leaf water and lake water d values respectively while the d values for both long chain c28 c30 and mid chain n alkanoic acids c24 c26 primarily represent lake water d values plant wax inferred d values of precipitation during the early holocene 10 100 to 8 200 cal yr bp are 35 more positive than late holocene values and decline over the holocene d inferred hydrologic change and 13c inferred plant water use efficiency both indicate that the faroe islands became drier throughout the holocene comparison with measurements from lig plant waxes indicates that late lig in the faroe islands was hydrologically similar to the early to mid holocene 8 200 to 4 000 cal yr bp with enriched precipitation isotopes and reduced evapotranspiration indicating a warmer wetter environment

reconciling divergent trends and millennial variations in holocene temperatures cooling during most of the past two millennia has been widely recognized and has been inferred to be the dominant global temperature trend of the past 11 700 years the holocene epoch however long term cooling has been difficult to reconcile with global forcing and climate models consistently simulate long term warming the divergence between simulations and reconstructions emerges primarily for northern mid latitudes for which pronounced cooling has been inferred from marine and coastal records using multiple approaches here we show that temperatures reconstructed from sub fossil pollen from 642 sites across north america and europe closely match simulations and that long term warming not cooling defined the holocene until around 2 000 years ago the reconstructions indicate that evidence of long term cooling was limited to north atlantic records early holocene temperatures on the continents were more than two degrees celsius below those of the past two millennia consistent with the simulated effects of remnant ice sheets in the climate model community climate system model 3 ccsm3 ccsm3 simulates increases in growing degree days a measure of the accumulated warmth above five degrees celsius per year of more than 300 kelvin days over the holocene consistent with inferences from the pollen data it also simulates a decrease in mean summer temperatures of more than two degrees celsius which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records despite the differing trends pollen and marine based reconstructions are correlated at millennial to centennial scales probably in response to ice sheet and meltwater dynamics and to stochastic dynamics similar to the temperature variations produced by ccsm3 although our results depend on a single source of palaeoclimatic data pollen and a single climate model simulation they reinforce the notion that climate models can adequately simulate climates for periods other than the present day they also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11 000 years

formal subdivision of the holocene series epoch a discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommission on quaternary stratigraphy international commission on stratigraphy this discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommision on quaternary stratigraphy sqs of the international commission on stratigraphy ics considers the prospects for a formal subdivision of the holocene series epoch although previous attempts to subdivide the holocene have proved inconclusive recent developments in quaternary stratigraphy notably the definition of the pleistocene holocene boundary and the emergence of formal subdivisions of the pleistocene series epoch mean that it may be timely to revisit this matter the quaternary literature reveals a widespread but variable informal usage of a tripartite division of the holocene early middle or mid and late and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology we propose an early middle holocene boundary at 8200 a bp and a middle late holocene boundary at 4200 a bp each of which is linked to a global stratotype section and point gssp should the proposal find a broad measure of support from the quaternary community a submission will be made to the international union of geological sciences iugs via the sqs and the ics for formal ratification of this subdivision of the holocene series epoch copyright 2012 john wiley sons ltd

quantitative interpretation of fossil pollen spectra dissimilarity coefficients and the method of modern analogs abstract dissimilarity coefficients measure the difference between multivariate samples and provide a quantitative aid to the identification of modern analogs for fossil pollen samples how eight coefficients responded to differences among modern pollen samples from eastern north america was tested these coefficients represent three different classes 1 unweighted coefficients that are most strongly influenced by large valued pollen types 2 equal weight coefficients that weight all pollen types equally but can be too sensitive to variations among rare types and 3 signal0to noise coefficients that are intermediate in their weighting of pollen types the studies with modern pollen allowed definition of critical values for each coefficient which when not exceeded indicate that two pollen samples originate from the same vegetation region dissimilarity coefficients were used to compare modern and fossil pollen samples and modern samples so similar to fossil samples were found that most of three late quaternary pollen diagrams could be reconstructed by substituting modern samples for fossil samples when the coefficients indicated that the fossil spectra had no modern analogs then the reconstructed diagrams did not match all aspects of the originals no modern analogs existed for samples from before 9300 yr b p at kirchner marsh minnesota and from before 11 000 yr b p at wintergreen lake michigan but modern analogs existed for almost all holocene samples from these two sites and brandreth bog new york

climate change vegetation history and landscape responses on the tibetan plateau during the holocene a comprehensive review abstract numerous studies of changes in the climate vegetation and landscape responses of the tibetan plateau tp during the holocene have been published in recent decades however several conflicting interpretations are evident within these studies and a comprehensive review of the topic is needed to reconcile them and thus to provide an improved understanding of the history and forcing mechanisms of holocene environmental changes within the region here we review the evidence for changes in climate temperature precipitation moisture vegetation and landscape attributes glacier and eolian activity on the tp during the holocene our main conclusions are as follows 1 the patterns of holocene mean annual temperature change on the tp remain controversial however an early to middle holocene summer temperature maximum is evident which coincided with high summer insolation and a strengthening of the indian summer monsoon ism this suggests that the summer land sea thermal gradient impacted by the temperature of the tp may be an important factor driving orbital scale ism changes 2 lake level records indicate an optimum in moisture conditions on the southwestern sw tp in the early holocene while on the northeastern ne tp the optimum occurred in the middle to late holocene this out of phase or anti phased relationship may be linked to the role of the ism in dominating precipitation on the sw tp while the westerlies and east asian summer monsoon dominated the precipitation on the ne tp 3 most of the lake sediment based stable isotope records 18o and leaf wax d exhibit a pattern similar to that of stalagmite 18o records from monsoonal asia reflecting the dominance of large scale patterns of atmospheric circulation rather than localized moisture changes reflected by lake level records 4 a synthesis of pollen records reveals that the extent of forest was maximal in the mid holocene while from the early holocene onwards alpine steppe shrunk and alpine meadow and desert gradually expanded 5 evidence of glacier advance increased slightly during the early holocene decreased during the mid holocene and then increased substantially after 3 ka bp 6 the integrated ages of eolian sand and loess from the ne tp reveal that eolian activity intensified during the early and late holocene site inter comparisons reveal that regional climate change played a major role in modulating changes in vegetation and other landscape attributes overall producing holocene climate reconstructions for the entire tp remains a major challenge even though much progress has been made further improvements in terms of the spatial coverage of high resolution paleoclimatic records with robust chronological control and a deeper understanding of the specific climatic significance of several climatic proxies will facilitate an improved understanding of how the monsoon and the westerlies interacted and their impacts on vegetation and landscape changes on the tp

calendar effect on phase study in paleoclimate transient simulation with orbital forcing several studies have shown that the use of different calendars in paleoclimate simulations can cause artificial phase shifts on insolation forcing and climatic responses however these important calendar corrections are still often neglected in this paper the phase shifts at the precession band is quantitatively assessed by converting the model data of the transient gcm climate simulation of kutzbach et al clim dyn 30 567 579 2008 from the fixed day calendar to the fixed angular calendar with a new and efficient approach we find that insolation has a big phase shift in september october november son when the vernal equinox ve is fixed to march 21 at high latitude the phase bias is up to 60 about 3650 years the insolation phase bias in son in southern hemisphere sh is especially important because it can influence the timing of the sh summer monsoon response due to the large heat capacity of ocean the calendar correction has minor effect 2 on the phase relationships between forcing and precipitation responses of the six global summer monsoons studied in kutzbach et al 2008 after correcting the calendar effect especial on sh ocean temperature the new phase wheel results are more similar for both hemispheres the results suggest that the calendar effect should be corrected before discussing the dynamics between orbital forcing and climatic responses in phase studies of transient simulations

both differential and equatorial heating contributed to african monsoon variations during the mid holocene the sahara was significantly greener 11 5 kya and during multiple earlier interglacial periods but the mechanisms related to the greening of the sahara remain uncertain as most climate models severely underestimate past wet conditions over north africa the variations in the african monsoon related to the greening of the sahara are thought to be associated with the variations in the inter hemispheric differential heating of earth caused by orbital variations however how orbital variations affect regional climate is not well understood using recent theory that relates the position of the tropical rain belt to the atmospheric energy budget we study the effect of orbital forcing during the mid holocene on the african monsoon in simulations provided by the third phase of the paleo model intercomparison project pmip3 we find that energy fluxes in the african sector are related to orbital forcing in a complex manner contrary to generally accepted theory orbital modulation of seasonal differential heating alone is shown to be a weak driver of african monsoon variations instead net atmospheric heating near the equator which modulates the intensity and extent of seasonal migrations of the tropical rain belt is an important but overlooked driver of african monsoon variations a conceptual framework that relates african monsoon variations to both equatorial and inter hemispheric differential solar heating is presented

causes of pile foundation failure in permafrost regions the case study of a dry bridge of the qinghai tibet railway abstract pile foundations are widely used to support dry bridges crossing large extents of warm and ice rich permafrost zones along the qinghai tibet railway qtr the performance of these pile foundations are related to permafrost conditions surrounding the piles however the impacts of permafrost degradation on the stability of pile foundations have been rarely investigated in this study permafrost degradation has been assessed around several pile foundations in the tanggula mountain area along the qtr 15 years after the first field investigation in 2001 for the construction of the qtr this assessment is mainly based on drilling geophysical surveys and monitoring of the settlements affecting the pile foundations the permafrost in contact with the piles has significantly thinned after the piles were casted in place and put into service about 8 years ago moreover the thickness of residual permafrost is less than the embedment length for some piles and therefore the adfreeze bond between the piles and permafrost has significantly decreased in addition artesian sub permafrost groundwater has been observed around the middle lower and below the piles due to this sub permafrost aquifer the end bearing capacity of the piles and the friction between the piles and thawed soils have also probably decreased the applied load on the piles is now supported by the residual permafrost resulting in large settlements of the piles the thaw consolidation and settlement of degrading permafrost have also potentially contributed to the total settlement according to the investigation presented herein the occurrence of sub permafrost aquifer which was induced by permafrost degradation is the primary cause of pile settlements the failure mechanisms of the piles as revealed by this assessment are useful for the design and maintenance of piles in warm and ice rich permafrost regions moreover the use of geophysical methods for investigating pile foundation failure due to permafrost degradation has proved effective

assessment of frozen ground conditions for engineering geology along the qinghai tibet highway and railway china abstract the qinghai tibet highway and railway the corridor across the qinghai tibet plateau traverses 670 km of permafrost and seasonally frozen ground in the interior of the plateau which is sensitive to climatic and anthropogenic environmental changes the frozen ground conditions for engineering geology along the corridor is complicated by the variability in the near surface lithology and the mosaic presence of warm permafrost and talik in a periglacial environment differential settlement is the major frost effect problem encountered over permafrost areas the traditional classification of frozen ground based on the areal distribution of permafrost is too generalized for engineering purposes and a more refined classification is necessary for engineering design and construction a proposed classification of 51 zones sub zones and sections of frozen ground has been widely adopted for the design and construction of foundations in the portion of the corridor studied the mean annual ground temperature magt near surface soil types and moisture content and active faults and topography are most commonly the primary controlling factors in this classification however other factors such as local microreliefs drainage conditions and snow and vegetation covers also exert important influences on the features of frozen ground about 60 of the total length of the corridor studied possesses reasonably good frozen ground conditions which do not need special mitigative measures for frost hazards however other sections such as warm and ice rich or saturated permafrost particularly in the sections in wetlands ground improvement measures such as elevated land bridges and passive or proactive cooling techniques need to be applied to ensure the long term stability of thermally unstable thick permafrost subsoils and or refill with non frost susceptible soils due to the long history of the construction and management of the corridor by various government departments adverse impacts of construction and operation on the permafrost environment have been resulted it is recommended that an integrated executable plan for the routing of major construction projects within this transportation corridor be established and long term monitoring networks installed for evaluating and mitigating the impact from anthropogenic and climatic changes in frozen ground conditions

proterozoic basalt dikes in the nain anorthosite complex labrador two sets of proterozoic between 1290 and 1042 ma alkali to transitional basaltic dikes occur sparsely in an area of at least 5000 km2 within the nain anorthosite complex both are compositionally similar in terms of major elements but the set that trends roughly north south hp dikes contains much higher concentrations of incompatible elements than the more abundant dikes that trend roughly east northeast west southwest lp dikes both types have low normative clinopyroxene relative to olivine a characteristic shared with widespread troctolitic rocks in the nain complex low mg numbers and low ni contents suggest that the magmas producing these dikes have undergone extensive fractional crystallization the low normative di content of these dikes suggests that this fractional crystallization occurred at depths equivalent to at least 10 kbar 1000 mpa high equilibration temperatures for coexisting fe ti oxides in the matrices of these dikes indicate that the dikes cooled rapidly and were emplaced near

cooling of northwest atlantic slope waters during the holocene 2 greenland ice cores depict a holocene climate that was warm and stable compared to the preceding 100 kyr yet most elements of the climate system were undergoing large secular changes during the last 11 kyr summer insolation in the northern hemisphere declined to last glacial maximum lgm levels atmospheric co2 increased 20 ppmv and the carbonate ion concentration of the deep sea decreased 5 10 mmol kg broecker et al 1999 signaling a large shift in the global carbon cycle the oxygen isotope composition of atmospheric o2 an indicator of global productivity and hydrology increased 0 5 retracing roughly 50 of the non ice volume deglacial decrease bender et al 1999 the monsoons of asia and

in situ creep of frozen soil fenghuo shan tibet plateau china field measurements of frozen soil creep in the upper 3 0 m of permafrost indicate that creep occurs in both winter and summer between 1992 and 1993 the mean rate of creep ranged from 0 44 cm at 1 6 m depth to 0 16 cm at 2 8 m depth but there was extreme variability creep parameters n and a as defined by the power flow law were calculated from field data parameter n ranged between 1 96 and 2 29 and increased with depth while a decreased with depth comparisons of creep rates for different permafrost environments suggest that ground temperature largely controls the magnitude of permafrost creep key words permafrost creep parameters tibet plateau

intercomparison of simulated global vegetation distributions in response to 6 kyr bp orbital forcing abstract the response of ten atmospheric general circulation models to orbital forcing at 6 kyr bp has been investigated using the biome model which predicts equilibrium vegetation distribution as a diagnostic several common features emerge a reduced tropical rain forest as a consequence of increased aridity in the equatorial zone b expansion of moisture demanding vegetation in the old world subtropics as a consequence of the expansion of the afro asian monsoon c an increase in warm grass shrub in the northern hemisphere continental interiors in response to warming and enhanced aridity and d a northward shift in the tundra forest boundary in response to a warmer growing season at high northern latitudes these broadscale features are consistent from model to model but there are differences in their expression at a regional scale vegetation changes associated with monsoon enhancement and high latitude summer warming are consistent with palaeoenvironmental observations but the simulated sh

continental scale temperature variability during the past two millennia past global climate changes had strong regional expression to elucidate their spatio temporal pattern we reconstructed past temperatures for seven continental scale regions during the past one to two millennia the most coherent feature in nearly all of the regional temperature reconstructions is a long term cooling trend which ended late in the nineteenth century at multi decadal to centennial scales temperature variability shows distinctly different regional patterns with more similarity within each hemisphere than between them there were no globally synchronous multi decadal warm or cold intervals that define a worldwide medieval warm period or little ice age but all reconstructions show generally cold conditions between ad 1580 and 1880 punctuated in some regions by warm decades during the eighteenth century the transition to these colder conditions occurred earlier in the arctic europe and asia than in north america or the southern hemisphere regions recent warming reversed the long term cooling during the period ad 1971 2000 the area weighted average reconstructed temperature was higher than any other time in nearly 1 400 years

time transgressive holocene maximum of temperate and mediterranean forest development across the iberian peninsula reflects orbital forcing abstract we describe a new pollen and sea surface temperature sst record with sub centennial to centennial resolution from a well dated sediment core d13882 obtained from the atlantic margin of the southwest iberian peninsula the core provides new insights into the dynamics of five ecological pollen based groups temperate forests mediterranean forests pinewoods heathlands and steppe since the late pleistocene last 13 260 cal yr bp the record shows that the early holocene was marked by an increase of temperate and mediterranean taxa accompanied by a reduction of steppic plants reflecting warm and relatively moist conditions when compared with the previous younger dryas episode the timing of this temperate and mediterranean forest tmf maximum which reflects optimum conditions of temperature and precipitation is regionally specific as detected through a comparison of three marine cores across the iberian peninsula maximum tmf development occurs between 9760 and 7360 cal yr bp in d13882 whereas at a site to the northwest md03 2697 the maximum occurs some centuries earlier and at a southeast site md95 2043 the maximum is evident later the duration of the forest maximum is also progressively longer from northwest to southwest to southeast a transition from forested ecosystems to heathland over western iberia characterises the middle holocene starting from around 7360 cal yr bp in d13882 suggesting reduced seasonality and a more atlantic climate character with more moisture evenly available through the year this contrasts with the aridity recorded in southeast iberian sites and reflects a stable precipitation gradient between the wetter west and drier east of the iberian peninsula against this backdrop pinewoods had a continuous presence albeit with a trend of steady decline through the holocene the time transgressive pattern in the tmf maximum is best explained by a less favourable moisture balance in the drier southeast sector which was particularly important during the early holocene despite the relatively short duration of the holocene interglacial one half precession cycle our observations support a key role for orbital forcing in the temporal expression of major vegetation groups specifically the decline in summer insolation and the precession related passage of perihelion season from summer to winter

creep behaviour of undisturbed clay permafrost an in situ analysis of naturally occurring creep has been carried out at the proposed canadian arctic gas pipeline crossing of great bear river in the northwest territories this is the last of fou

the structure of holocene climate change in mid latitude north america abstract a sequence of long term and rapid changes during the holocene appears in a network of 40 well resolved paleoclimate datasets from mid latitude north america including records of pollen inferred temperatures alkenone derived sea surface temperatures ssts lake level changes dust accumulation and lake isotopes from idaho to maine statistical analyses reveal that changes in insolation and the laurentide ice sheet explain 51 7 of the variance in the records especially multi millennial trends but peak rates of change indicate additional rapid changes at ca 10 8 9 4 8 3 7 0 5 5 5 2 4 7 2 1 and 0 9 ka step changes between 9 4 and 8 3 ka relate to ice sheet dynamics that warmed much of the region and changes at 5 5 ka were the largest since the demise of the ice sheet the shift at 5 5 ka initiated widespread cooling and increases in effective moisture which culminated in the coolest wettest millennia in most areas after 2 1 ka replicated evidence from multiple records also shows a spatially varied set of multi century fluctuations including 1 low temperatures and high effective moisture at 5 5 4 8 ka in the mid continent and 2 repeated phases of low ssts cool summers and drought superimposed upon long cooling moistening trends in eastern north american since 5 5 ka

typical embankment settlement heave patterns of the qinghai tibet highway in permafrost regions formation and evolution abstract the qinghai tibet highway qth in china has been experiencing a serious embankment settlement heave for more than half a century the settlement heave is resulted from several mechanical processes taking place in different soil layers in this paper four typical settlement heave patterns of road embankments in permafrost regions namely steady low rate fluctuate down step and steady high rate are proposed based on 3 year monitored data of embankment settlement heave and ground temperature at four road sections as well as previous published data of the qth their formation mechanisms and evolution trend in the process of permafrost degradation are discussed from the viewpoints of engineering geology and mechanical process based on the clarification of the deformation characteristics of different soil layers of a typical road embankment the results show that the four patterns are caused by the negligible settlement heave freeze thaw cycling in an open condition consolidation of the thawed permafrost layers and creep of warm permafrost layers respectively the pattern will change from one to another during the process of permafrost degradation which will become very complex with the occurrence of frost heave the results can help revealing the deformation mechanism of road embankments in permafrost regions and can benefit road construction and maintenance in the future

the climate of europe during the holocene a gridded pollen based reconstruction and its multi proxy evaluation we present a new gridded climate reconstruction for europe for the last 12 000 years based on pollen data the reconstruction is an update of davis et al 2003 using the same methodology but with a greatly expanded fossil and surface sample dataset and more rigorous quality control the modern pollen dataset has been increased by more than 80 and the fossil pollen dataset by more than 50 representing almost 60 000 individual pollen samples the climate parameters reconstructed include summer winter and annual temperatures and precipitation as well as a measure of moisture balance and growing degree days above 5 degrees c confidence limits were established for the reconstruction based on transfer function and interpolation uncertainties the reconstruction takes account of post glacial isostatic readjustment which resulted in a potential warming bias of up to 1 2 degrees c for parts of fennoscandia in the early holocene as well as changes in palaeogeography resulting from decaying ice sheets and rising post glacial sea levels this new dataset has been evaluated against previously published independent quantitative climate reconstructions from a variety of archives on a site by site basis across europe the results of this comparison are generally very good only chironomid based reconstructions showed substantial differences with our values our reconstruction is available for download as gridded maps throughout the holocene on a 1000 year time step the gridded format makes our reconstructions suitable for comparison with climate model output and for other applications such as vegetation and land use modelling our new climate reconstruction suggests that warming in europe during the mid holocene was greater in winter than in summer an apparent paradox that is not consistent with current climate model simulations and traditional interpretations of milankovitch theory c 2015 elsevier ltd all rights reserved

effect of sunny shady slopes and strike on thermal regime of subgrade along a high speed railway in cold regions china abstract ground temperature and deformation of embankment were monitored at typical sections of the harbin qiqihar high speed railway hsr to investigate complex causes of differential embankment deformation due to sunny shady slopes and route strike in cold regions maximum frozen depth ground temperature frost heave and thaw settlement of the embankment in the sunny and shady sides were analyzed relative to measurement dates and then a numerical model for the embankment temperature filed was also established on these bases the influence of route strike on the embankment thermal regime and its mechanism were then revealed and proper engineering countermeasures were proposed to mitigate the transverse embankment thermal difference the formation mechanism of the differential deformation due to sunny shady slopes was discussed the results showed that both temperature and maximum frozen depth of the sunny side were greater than the shady side whereas the cumulative frost heave of the sunny side was less than the shady side with opposite trends found for the cumulative thaw settlement the temperature difference between the sunny and shady shoulders decreased gradually with increasing strike angle the findings are potentially helpful for better understanding differential embankment deformation in cold regions as well as for improving hsr embankment quality and operational safety

damage and mitigation of railway embankment drainage trench in warm permafrost a case study abstract warm permafrost sites such as those extensively existing on the qinghai tibet plateau present a challenging geological environment for infrastructure engineering a reliable drainage system is crucial for long term stability of railway embankment in such sites this paper presents a case study of the drainage trench along the qinghai tibet railway embankment the damage patterns to the precast u shaped concrete lining originally installed in 2008 is described a coupled thermal hydro mechanical finite element model and its results including temperature and stress distributions during ground freezing are presented parametric study was also conducted to assess the sensitivity of these results to soil water content and the stiffness of the u shaped concrete lining based on the modeling results a new design including a coarse grained transition layer and impermeable geotextile was proposed for field experiment and its field performance six years after its construction is presented the damage to the original trench was characterized by disjointed lining segments and extensive longitudinal cracks mostly located below the mid height of the linings factors contributing to the damage include challenging construction conditions uneven thaw settlement and frost heave stress modeling results show that significant horizontal frost heave stress can develop on the outside surface of the u shaped lining and hence results in large tensile stress at the outside edge of the u shaped lining the tensile stress may exceed the lower bound of the concrete flexural strength and induce longitudinal cracks at depths below the mid height of the lining which agrees well with field observation field performance of the new trench design during a period of six years shows that the improved design performed well and can be adopted for future engineering practice in similar challenging geology conditions

long term summer warming trend during the holocene in central asia indicated by alpine peat α cellulose δ13c record abstract the long term cooling or warming trend is one of the most important aspects of holocene paleoclimatic research however there are currently significant discrepancies among climatic simulation results sedimentary and synthesized holocene temperature records these discrepancies call for new holocene temperature records with a robust chronology and unambiguous indicative significance here we present a ca 45 year resolution alpine peat cellulose carbon isotopic 13c record from the altai mountains in central asia the chronology of the record has been well constrained by 22 ams 14c dates of the peat cellulose samples based on detailed modern process study results the indicative significance of the record has been carefully determined as summer temperature indicator the record reveals an overall trend of increasing summer temperature for the past ca 11 ka with a warmer early from ca 8 6 ka bp and late from ca 4 0 ka bp holocene and colder middle holocene from ca 6 4 ka bp the general pattern of a warmer early and late holocene and colder middle holocene is consistent with previously reported temperature records from central asia the tibetan plateau and north china the long term warming trend in our summer temperature record is consistent with recently reported winter temperature records from high latitudes of continental eurasia all these results further support the reliability of our summer temperature record from central asia the better understandings about the holocene temperature history and its possible driven mechanism s can be benefited potentially from more reliable holocene temperature records including our record reported in this paper

relative sea level change in newfoundland canada during the past 3000 years abstract several processes contributing to coastal relative sea level rsl change in the north atlantic ocean are observed and or predicted to have distinctive spatial expressions that vary by latitude to expand the latitudinal range of rsl records spanning the past 3000 years and the likelihood of recognizing the characteristic fingerprints of these processes we reconstructed rsl at two sites big river and placentia in newfoundland from salt marsh sediment bayesian transfer functions established the height of former sea level from preserved assemblages of foraminifera and testate amoebae age depth models constrained by radiocarbon dates and chronohorizons estimated the timing of sediment deposition during the past 3000 years rsl rose by 3 0 m at big river and by 1 5 m at placentia a locally calibrated geotechnical model showed that post depositional lowering through sediment compaction was minimal to isolate and quantify contributions to rsl from global regional linear regional non linear and local scale processes we decomposed the new reconstructions and those in an expanded global database using a spatio temporal statistical model the global component confirms that 20th century sea level rise occurred at the fastest century scale rate in over 3000 years p 0 999 distinguishing the contributions from local and regional non linear processes is made challenging by a sparse network of reconstructions however only a small contribution from local scale processes is necessary to reconcile rsl reconstructions and modeled rsl trends we identified three latitudinally organized groups of sites that share coherent regional non linear trends and indicate that dynamic redistribution of ocean mass by currents and or winds was likely an important driver of sea level change in the north atlantic ocean during the past 3000 years