modelling the climate evolution from the last interglacial to the start of the last glaciation the role of arctic ocean freshwater budget 1 the earth s climate has alternated between glacial and interglacial stages but we still do not understand how the changes in insolation have been amplified by the climate system to lead to such a large climatic response here we use a climate model of intermediate complexity to show that the orbitally induced shift from an interglacial to a pre glacial climate is highly sensitive to the high northern latitudes moisture budget using the insolation forcing alone the model produces a first order response of sea surface temperatures and thermohaline circulation evolution which agrees with the available geological data under the same gradual insolation evolution adding a small freshwater input into the arctic ocean induces a rapid climate switch from an interglacial to a pre glacial mode these results suggest a simple connection between orbital forcing and the thermohaline circulation through a freshwater threshold within the ocean atmosphere sea ice system

global ice sheet system interlocked by sea level abstract denton and hughes 1983 quaternary research 20 125 144 postulated that sea level linked a global ice sheet system with both terrestrial and grounded marine components during late quaternary ice ages summer temperature changes near northern hemisphere melting margins initiated sea level fluctuations that controlled marine components in both polar hemispheres it was further proposed that variations of this ice sheet system amplified and transmitted milankovitch summer half year insolation changes between 45 and 75 n into global climatic changes new tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the antarctic ice sheet thus fitting the concept of a globally interlocked ice sheet system but recent atmospheric modeling results manabe and broccoli 1985 journal of geophysical research 90 2167 2190 suggest that factors other than areal changes of the grounded antarctic ice sheet strongly influenced southern hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres atmospheric carbon dioxide linked to high latitude oceans is the most likely candidate shackleton and pisias 1985 atmospheric carbon dioxide orbital forcing and climate in the carbon cycle and atmospheric co2 natural variations archean to present e t sundquest and w s broecker eds pp 303 318 geophysical monograph 32 american geophysical union washington d c but another potential influence was high frequency climatic oscillations 2500 yr it is postulated that variations in atmospheric carbon dioxide acted through an antarctic ice shelf linked to the grounded ice sheet to produce and terminate southern hemisphere ice age climate it is further postulated that milankovitch summer insolation combined with a warm high frequency oscillation caused marked recession of northern hemisphere ice sheet melting margins and the north atlantic polar front about 14 000 14c yr b p this permitted renewed formation of north atlantic deep water which could well have controlled atmospheric carbon dioxide w s broecker d m peteet and d rind 1985 nature london 315 21 26 combined melting and consequent sea level rise from the three warming factors initiated irreversible collapse of the interlocked global ice sheet system which was at its largest but most vulnerable configuration

climate volcanism feedback and the toba eruption of 74 000 years ago abstract a general feedback between volcanism and climate at times of transition in the quaternary climate record is suggested exemplified by events accompanying the toba eruption 74 000 yr ago the largest known late quaternary explosive volcanic eruption the toba paroxysm occurred during the 18o stage 5a 4 transition a period of rapid ice growth and falling global sea level which may have been a factor in creating stresses that triggered the volcanic event toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co ignimbrite ash clouds to heights of at least 32 5 km which may have led to dense stratospheric dust and sulfuric acid aerosol clouds these conditions could have created a brief dramatic cooling or volcanic winter followed by estimated annual northern hemisphere surface temperature decreases of 3 to 5 c caused by the longer lived aerosols summer temperature decreases of 10 c at high northern latitudes adjacent to regions already covered by snow and ice might have increased snow cover and sea ice extent accelerating the global cooling already in progress evidence for such climate volcanic feedback following milankovitch periodicities is found at several climatic transitions

the response of the coupled arctic sea ice atmosphere system to orbital forcing and ice motion at 6 kyr and 115 kyr bp abstract a coupled atmosphere mixed layer ocean gcm genesis2 is forced with altered orbital boundary conditions for paleoclimates warmer than modern 6 kyr bp and colder than modern 115 kyr bp in the high latitude northern hemisphere a pair of experiments is run for each paleoclimate one with sea ice dynamics and one without to determine the climatic effect of ice motion and to estimate the climatic changes at these times at 6 kyr bp the central arctic ice pack thins by about 0 5 m and the atmosphere warms by 0 7 k in the experiment with dynamic ice at 115 kyr bp the central arctic sea ice in the dynamical version thickens by 2 3 m accompanied by a 2 k cooling the magnitude of these mean annual simulated changes is smaller than that implied by paleoenvironmental evidence suggesting that changes in other earth system components are needed to produce realistic simulations contrary to previous simulations without atmospheric feedbacks the sign of the dynamic sea ice feedback is complicated and

the progressive intensification of northern hemisphere glaciation as seen from the north pacific ocean drilling project odp site 882 50 22 n 167 36 e provides the first high resolution grape density magnetic susceptibility carbonate opal and foraminifera planktonic and benthic stable isotopes records between 3 2 and 2 4 ma in the northwest pacific we observed a dramatic increase in ice rafting debris at site 882 at 2 75 ma which is coeval with that found in the norwegian sea suggesting that the eurasian arctic and northeast asia were significantly glaciated from 2 75 ma onwards prior to 2 75 ma planktonic foraminifera 18o records indicate a warming or freshening trend of 4 c or 2 over 80 ka if this is interpreted as a warm pre glacial pliocene north pacific it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet the dramatic drop in sea surface temperatures sst 7 5 c at 2 75 ma ended this suggested period of enhanced sst and thus the proposed moisture pump moreover at 2 79 and 2 73 ma opal mass accumulation rates mar decrease in two steps by five fold and is accompanied by a more gradual long term decrease in caco3 mars evidence from the southern ocean odp site 704 indicates that just prior to 2 6 ma there is a massive increase in opal mars the opposite to what is found in the north pacific this indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget possibly caused by changes in deep ocean circulation the initiation of northern hemisphere glaciation occurred in the late miocene with a significant build up of ice on southern greenland however the progressive intensification did not occur until 3 5 3 ma when the greenland ice sheet expanded to include northern greenland following this stage we suggest that the eurasian arctic and northeast asia glaciated at 2 75 ma approximately 100 ka before the glaciation of alaska 2 65 ma and 200 ka before the glaciation of the north east american continent 2 54 ma

a sea ice climate switch mechanism for the 100 kyr glacial cycles a box model of the ocean atmosphere sea ice land ice climate system is used to study a novel mechanism for self sustained oscillations of the climate system on a time scale of 100 000 years without external forcing the oscillation in land ice volume has the familiar sawtooth shape of climate proxy records the most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice the sea ice acts as a switch of the climate system switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation s sawtooth structure a simple explanation of the 100 kyr timescale is formulated on the basis of the mechanism seen in the model finally rapid sea ice changes such as those seen in our model and their drastic effects on the climate system may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

the pleistocene of northern france a table of stratigraphic correlation has been established between the different provinces of the quaternary paleogeographic regions of northern france extending from brittany to alsace and from the welshbelgium basin to the paris basin initially based on loess and slope deposits in the framework of the aeolian periglacial zone of northwestern europe paepe and sornme 1970 lautridouand sornme 1974 these correlations not only include data from brittany but also provide sequences from achenheim alsace to enable correlation with more continental european areas

large trends in french topsoil characteristics are revealed by spatially constrained multivariate analysis spatially constrained multivariate analysis methods multispati pca and classical principal component analysis are applied for the entire country of france to study the main soil characteristics of topsoil and to assess if their multivariate spatial pattern can provide insight on their extent and origin the results of the multspati pca provided evidence of strong spatial structures attributed to different natural processes the first axis was interpreted as an axis of global soil richness in clay content axis 2 reflected the influence of some parent materials on the geochemical content of k and al axis 3 showed a very large gradient of relative content in coarse silt axis 4 was driven by gradients of maritime influence we show that multispati pca allows better than classical pca to detect and map large regional trends in the distribution of topsoil characteristics the two first axes were expected and the maps obtained by both methods were consistent interestingly the other gradients were not expected and were better shown by multispati pca than by classical pca

the loess and coversands of northern france and southern england loess and coversands are widespread on either side of the english channel in southern england loess is generally thin and discontinuous but locally reaches thickness of about 4 m in east kent coversands occur mainly in areas well north of the channel such as parts of east anglia and the southern side of the bristol channel most of the loess and coversand deposits in southern england date from the late devensian cold stage marine oxygen isotope stage mois 2 but there are also a few localised patches of older mainly mois 6 and 12 loess which are the dissected remnants of originally more extensive covers in france loess is widespread and locally very thick up to 12 m especially in northern and northeastern france in normandy and in the somme basin some long records indicate that accumulation began at the end of the lower pleistocene at about 900 ka nevertheless the main accumulations of typical calcareous loess are related to the upper weichselian and upper saalian stages mois 2 and end of mois 3 and mois 6 indeed before about 160 170 ka loess deposition was generally restricted to special sediment traps such as fluvial and marine terraces exposed to the east or northeast these older deposits are characterised by more sandy facies derived from local fluvial sources mainly by northwest to north northwest winds coversands occur locally along the coast of the mont saint michel bay and over the northern part of the seine estuary near le havre the main differences in thickness and extent of the loess deposits between southern england and northern france are linked to their location in relation to the source areas channel and north sea to ice sheets and to the main wind directions northwest to north northwest in france and western england but northeast in eastern england copyright 2003 john wiley sons ltd

a continuum representation of fracture networks part i method and basic test cases abstract a method to represent fracture networks as grid cell conductivities in a continuum model is described and evaluated the method is developed for a sparsely fractured rock with a conductivity field that is dominated by major fractures and fracture zones for such a fracture system it is believed that an accurate description of the correlation and anisotropy structure is essential the proposed method will capture these features of the fracture system the basic approach of the method is to evaluate accurate cell conductivity values for a staggered grid arrangement i e velocities are stored at cell walls and scalars at cell centres the balance of forces i e the darcy law is evaluated for a control volume centred around the point where the velocity component is stored fractures are assumed to have a thickness and conductivity the conductivity of a velocity control volume is calculated from the intersecting volume between the fracture and the velocity control volume intersecting volumes with their respective conductivities are added up to form the control volume conductivity the general conclusion is that the method results in accurate simulations it is expected that the method can be utilised in many practical groundwater simulations this statement is based on the results from three test cases with analytical solution and one test case where results are compared with those from established theories

self consistency of a heterogeneous continuum porous medium representation of a fractured medium a possible approach to representing fractured rocks is to use heterogeneous continuum porous medium cpm models in which each grid block has an effective permeability appropriate to represent the network of fractures within the block it needs to be demonstrated that the approach is self consistent that is the effective permeability on a length scale larger than the grid blocks derived using the heterogeneous cpm model is close to that derived directly from the underlying network model for models based on the borrowdale volcanic group at sellafield in the united kingdom and the granite at aspo in sweden it is shown that the use of heterogeneous cpm models is self consistent both for models using effective permeabilities determined directly from the underlying network of discrete features model and for models based only on the correlation structure of the effective permeability on the length scale of the grid blocks

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

oxygen isotopes and sea level from the time that detailed oxygen isotope records derived from foraminifera living in the constant temperature environment of the abyssal ocean became available there has been a discrepancy between the ice volume record that these records imply and that derived from the altitude of dated coral terraces around the world here we re examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system

sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model abstract the u k university global atmospheric modeling programme gcm is used to investigate whether the growth of northern hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature two different orbital configurations corresponding to the present day and 115 kyr bp are used the reduced summer solar insulation in the northern hemisphere results in a decrease of the surface temperature by 4 to 10 c in the northern continents and to perennial snow in some high latitude regions therefore the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the northern hemisphere a decreased sea surface temperature northward of 65 n during the northern hemisphere summer may contribute to the maintenance of ice sheets a simple mixed layer ocean model coupled to the gcm indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl

can milankovitch orbital variations initiate the growth of ice sheets in a general circulation model the goddard institute for space studies climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations three different orbital configurations are used corresponding to 116 and 106 kyr b p and a modified insolation field with greater reductions in summer insolation at high northern latitudes the time periods chosen are those in which geophysical evidence suggests ice sheets may have been growing rapidly the reduced summer insolation field characteristic of all the experiments is thought to be a necessary condition for allowing snow to last through the summer and ice sheets to build the results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets despite the reduced summer and fall insolation when 10 m thick ice was inserted in all locations where continental ice sheets existed during the last glacial maximum the model failed to maintain it as well producing energy and mass imbalances which would remove the ice within 5 years only when the ocean surface temperatures were adjusted to their peak ice age values was the model able to keep any of the additional ice and then only in a very restricted region of northern baffin island the experiments indicate there is a wide discrepancy between the model s response to milankovitch perturbations and the geophysical evidence of ice sheet initiation as the model failed to grow or sustain low altitude ice during the time of high latitude maximum solar radiation reduction 120 110 kyr b p it is unlikely it could have done so at any other time within the last several hundred thousand years if the model results are correct it indicates that the growth of ice occurred in an extremely ablative environment and thus demanded some complicated strategy or else some other climate forcing occurred in addition to the orbital variation influence and co2 reduction which would imply we do not really understand the cause of the ice ages and the milankovitch connection if the model is not nearly sensitive enough to climate forcing it could have implications for projections of future climate change

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

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

scale of spatial dependence between chemical properties of topsoil and subsoil over a geologically contrasted area massif central france this study investigates scale spatial specific dependence of some major and trace element contents in topsoil horizons and in alterite type horizons subsoil horizons over a weakly contaminated area the study seeks also to compare the spatial distributions of chemical elements at both depths to the spatial variability of the alterite type in order to examine the effect of the parent material the probability of occurrence of alterite type was estimated using indicator kriging ik factorial kriging analysis fka was used to analyze spatial variability in some soil chemical properties ca mg k fe cr and co measured at two depths over a geologically contrasted area of 10 ha in central of france the coefficients of the coregionalization matrix at different spatial scales reveal the dominant long range autocorrelation and cross correlation in all chemical elements in both depths except for trace elements co and cr where the short range structure dominates the cross correlation the resulting structural correlation coefficients showed strong correlations between variables changing as a function of spatial scale these relationships between chemical properties at different spatial scales were not revealed by the linear correlation coefficients a principal component analysis was performed on the coregionalization matrices at each depth to summarize the relationships among the variables at the different spatial scales cokriging allowed mapping each spatial component for both depths these maps were then compared with the probability map of alterite type estimated using indicator kriging this comparison revealed that the spatial pattern of chemical elements in the subsoil horizons is almost certainly due to the alterite type effect whereas the alterite type effect on the spatial pattern of chemical properties in the topsoil horizons was partly hidden by human activities and erosion

ice age initiation by an ocean atmospheric circulation change in the labrador sea abstract the contrast between the large greenland ice sheet and the nearly ice free area of canada to the west exemplifies the critical role of the moisture supply in glaciation the trigger for new ice sheet growth in canada at the end of the last interglacial was probably an increase in the moisture supply rather than regional cooling triggering is attributed to stronger storm activity in the labrador baffin area caused by warming in the labrador sea and cooling in the greenland norwegian sea the key to this temperature and circulation change may have been an increase in the hydrologic deficit of the mediterranean sea a larger hydrologic deficit would have increased mediterranean salinity and consequently the volume of mediterranean overflow water a resulting increased upwelling of mediterranean overflow water from the lower thermocline off scotland would have diverted additional warm north atlantic drift water westward via the irminger current thus warming the labrador sea the underlying cause of the hydrologic deficit increase was diminishing milankovitch summer insolation in lower latitudes this reduced african monsoon intensity and nile river discharge in modern times nearly all nile discharge to the mediterranean has been lost due to irrigation practices culminating with the completion of the aswan high dam in 1968 the total increase in the hydrologic deficit due to nile discharge loss is now 8 in the next 70 100 years the nile loss added to larger evaporative losses due to co 2 warming may increase the deficit as much as 22 above the inferred deficit at the beginning of the last glaciation thus further warming the labrador sea consequently the triggering of major ice sheet growth in canada 100 years from now is possible

late pleistocene ice age scenarios based on observational evidence ice age scenarios for the last glacial interglacial cycle based on observations of boyle and keigwin concerning the north atlantic thermohaline circulation and of barnola et al concerning atmospheric co sub 2 variations derived from the vostok ice cores are herein analyzed northern hemisphere continental ice sheets are simulated with an energy balance model ebm that is asynchronously coupled to vertically integrated ice sheets models based on the glen flow law the ebm includes both a realistic land sea distribution and temperature albedo feedback and is driven with orbital variations of effective solar insolation with the addition of atmospheric co sub 2 and ocean heat flux variations but not in their absence a complete collapse is obtained for the eurasian ice sheet but not for the north american ice sheet we therefore suggest that further feedback mechanisms perhaps involving more accurate modeling of the dynamics of the mostly marine based laurentide complex appears necessary to explain termination i 96 refs 12 figs 2 tabs

a new stochastic particle tracking approach for fractured sedimentary formations 1 explicit particle tracking simulations in both fractures and surrounding sediments are conceptually complex and difficult to implement this is mainly due to the difference in the nature of the velocity fields in fractures and the matrix the major problems are 1 to avoid particles in the matrix jumping over fractures and 2 to control particle behavior at the interfaces between fractures and the matrix we developed a particle tracking method that models advection and diffusion explicitly in both fractures and the matrix analogously to most flow simulations we conceptualize transport by two separate domains the fracture and the matrix domains which exchange particles between them no a priori assumptions on transport mechanisms in the matrix have to be made each individual particle step stops at an interface between two matrix cells between two fracture cells or between the matrix and a fracture mass exchange at the interface from fractures to the matrix is controlled by the fracture aperture the matrix flux perpendicular to the fracture plane the flux within the fracture and a diffusive component the method is developed for fractured sedimentary rocks where the advective fluxes in the matrix can be as large as the fracture fluxes however it could also be applied to fractured crystalline rocks where the matrix contribution is smaller finally some simple applications to a fractured shale formation in northern switzerland are included to illustrate the usefulness of this method to investigate transport in fractured sedimentary formations

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

orbital signature of interglacials a specific orbital configuration high obliquity combined with the june perihelion marked the beginning of the past three interglacials this suggests that the primary cause of the glacial cycle may be astronomical an astronomical climate index aclin is introduced which combines the three orbital variables in the time lag bivariant model designed to predict the major climate changes in the late and middle pleistocene and in the near future aclin closely correlates with the major climatic events revealed by independently dated proxy climate indicators of the past 130 000 yr it successfully differentiates the interglacials and displays a 100 000 yr periodicity it predicts an early end of the present inter glacial and the start of a new one in 114 000 yr

snowline instability in a general circulation model application to carboniferous glaciation for over twenty years it has been known that energy balance models ebms with snow albedo feedback are characterized by unstable behavior in some areas of parameter space this behavior leads to rapid changes in snow area due to small changes in forcing and has been termed the small ice cap instability sici it has never been clarified whether this behaviour reflects a real feature of the climate system or a limitation in ebms in this study we demonstrate that evidence for similar unstable behavior can also be found in an atmospheric general circulation model gcm using a realistic set of boundary conditions for the carboniferous 300 ma one of the most extensive periods of glaciation in earth history when solar luminosity is sequentially lowered to near values appropriate for the carboniferous there is a discontinuous increase in summer snow area the instability occurs in approximately the same area of parameter space as one previously found in an ebm analysis of selected fields indicates that the circulation is primarily affected in the area of snow increase far field effects are minimal there is good agreement between model generated summer snowcover and one reconstruction of carboniferous ice cover although more work is required on this topic our results provide increased support for the possibility that the snowline instability represents a real feature of the climate system and that it may help explain some cases of glacial inception and abrupt transitions in earth history

orbital forcing of arctic climate mechanisms of climate response and implications for continental glaciation progress in understanding how terrestrial ice volume is linked to earths orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales 10 3 10 5 years a compromise is usually made to rep resent the climate system by models that are averaged over one or more spatial dimensions or by three dimensional models that are limited to simulating par ticular snapshots in time we take advantage of the short equilibration time 310 years of a climate model consisting of a three dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in earths orbital configuration over the past 165 000 years prominent decreases in ice melt and increases in snowfall are sim ulated during three time intervals near 26 73 and 117 thousand years ago ka when aphelion was in late spring and obliquity was low there were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large aph elion was in late spring and obliquity was high or near its long term mean these glaciation friendly time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine d 18 o record both dynamical and thermal effects contribute to the increases in snowfall during these periods through increases in storm activity and the fraction of precipi tation falling as snow the majority of the mid to high latitude response to orbital forcing is organized by the properties of sea ice through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

entering the glaciation with a 2 d coupled climate model a 2 dimensional model which links the atmosphere the mixed layer of the ocean the sea ice the continents the ice sheets and their underlying lithosphere has been used to test the milankovitch theory over the last glacial interglacial cycle sensitivity tests have shown that the orbital variations can induce in such a system feedbacks sufficient to generate the low frequency part of the climatic variations over the last 122 ka bp these variations at the astronomical time scale are broadly in agreement with ice volume and sea level reconstruction independently obtained from geological data at the beginning of the integration during isotopic substage 5e when there was no northern american nor eurasian ice sheets june insolation at high latitudes seemed to correlate well with summer temperature at the surface of the ice sheets and with the ablation rate the relation is more complicated when large ice sheets exist broadly the june july insolation from 60 to 70 degrees n leads the ice volume by roughly 5000 years over the last glacial interglacial cycle the simulated climate was shown to be sensitive to the ice albedo temperature feedback to the precipitation altitude negative feedback over the ice sheets to the ice sheet slope and continentality effects and to the albedo of the ice sheets as a function of the snowfall frequency at their surface the formation and waxing of the ice sheets are particularly sensitive to ablation more than to snowfalls imperfections in the simulated climate were recognized in particular at the last glacial maximum when the cooling was not large enough and the northern hemisphere ice sheets did not extend far enough to the south weaknesses which are partly solved in a further experiment gallee et al 1989 by using the proper co2 atmospheric concentration given by the vostok ice core in addition to the astronomical forcing

fracture size and transmissivity correlations implications for transport simulations in sparse three dimensional discrete fracture networks following a truncated power law distribution of fracture size we characterize how different fracture size transmissivity relationships influence flow and transport simulations through sparse three dimensional discrete fracture networks although it is generally accepted that there is a positive correlation between a fracture s size and its transmissivity aperture the functional form of that relationship remains a matter of debate relationships that assume perfect correlation semi correlation and non correlation between the two have been proposed to study the impact that adopting one of these relationships has on transport properties we generate multiple sparse fracture networks composed of circular fractures whose radii follow a truncated power law distribution the distribution of transmissivities are selected so that the mean transmissivity of the fracture networks are the same and the distributions of aperture and transmissivity in models that include a stochastic term are also the same we observe that adopting a correlation between a fracture size and its transmissivity leads to earlier breakthrough times and higher effective permeability when compared to networks where no correlation is used while fracture network geometry plays the principal role in determining where transport occurs within the network the relationship between size and transmissivity controls the flow speed these observations indicate dfn modelers should be aware that breakthrough times and effective permeabilities can be strongly influenced by such a relationship in addition to fracture and network statistics this article is protected by copyright all rights reserved

coupled energy balance ice sheet model simulations of the glacial cycle a possible connection between terminations and terrigenous dust we apply a coupled energy balance ice sheet climate model in an investigation of northern hemisphere ice sheet advance and retreat over the last glacial cycle when driven only by orbital insolation variations the model predicts ice sheet advances over the continents of north america and eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses the orbital forcing alone however is unable to induce the observed rapid ice sheet retreat and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting in the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination these include marine instability thermohaline circulation effects carbon dioxide variations and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration for the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records of these various influences our analyses suggest that the albedo variations in the ice sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism using our parameterization of dirty snow in the ablation zone we find glacial retreat to be strongly accelerated such that complete collapse of the otherwise stable laurentide ice sheet ensues the last glacial maximum configurations of the laurentide and fennoscandian complexes are also brought into much closer accord with the ice 3g reconstruction of tushingham and peltier 1991 1992 and the ice 4g reconstruction of peltier 1994 when this effect is reasonably introduced

on solar forcing of holocene climate evidence from scandinavia the focus of this study is the possible correlation between changes in the climate of scandinavian and changes in solar irradiation reliable information about holocene climatic change in sweden and norway is currently available from two main sources the 14c dating of pine wood retrieved from above the present pine tree limit and studies of glacier variations based on proglacial lacustrine sediments and on moraines the reconstructed alpine tree limit reveals that summer temperature in general was warmer during the early holo cene than it was during the late holocene superimposed on this general trend are several fluctuations of a few hundred years duration relatively cold periods with a duration of the order of 300 600 years occurred frequently during the holocene in this paper dates of the major climatic events are compared with an index of solar activity the so called 14c anomalies for most of the last 9000 years a good correspondence is demonstrated between the timing of cold events in scandi

modelled nucleation centres of the pleistocene ice sheets from an ice sheet model with subgrid topographic and glaciologic parameterizations abstract ice sheet inception at the onset of the last glacial epoch ca 115 kyr bp is not well understood to cause the northern hemisphere to descend into ice age conditions internal climate system feedbacks must have acted to amplify relatively low amplitude orbital cooling trends while a number of plausible internal mechanisms have been proposed for this internal amplification the dramatic continental ice sheet expansion that is recorded in the marine sediment record is an enduring mystery global sea level lowering of at least 20 m and as much as 75 m over 10 kyr appears typical of the early stages of glacial inception this rate of ice sheet growth has not been freely captured in either glaciologic or climatic models is it because we do not adequately understand the relevant internal feedbacks or fail to capture them in models is it simply related to technical model limitations e g topographic resolution this paper explores the premise that model resolution poses a critical limitation on simulations of ice sheet nucleation i present a series of ice sheet nucleation simulations in north america and eurasia based on an ice sheet model that includes parameterizations of km scale topographic detail snow ice accumulation and melt and glacier dynamics model climate is based on perturbations to modern observational climatology even with this topographic detail and with simulations long enough to allow ice to advect downslope and spread over the terrain it remains difficult to induce a large scale glacierization in both north america and eurasia severe climate perturbations are required cold temperatures need to be accompanied by substantial increases in precipitation in the north atlantic and polar regions in order to accrue continental ice at the rate demanded by sea level reconstructions the simulations presented here are too simple to separate climate perturbations from internal system feedbacks but with modern precipitation rates a combined climate cooling net positive feedback of more than 8 c is required to establish large scale ice sheets in

uranium series ages of marine terrace corals from the pacific coast of north america and implications for last interglacial sea level history abstract few of the marine terraces along the pacific coast of north america have been dated using uranium series techniques ten terrace sequences from southern oregon to southern baja california sur have yielded fossil corals in quantities suitable for u series dating by alpha spectrometry u series dated terraces representing the 80 000 yr sea level high stand are identified in five areas bandon oregon point arena san nicolas island and point loma california and punta banda baja california terraces representing the 125 000 yr sea level high stand are identified in eight areas cayucos san luis obispo bay san nicolas island san clemente island and point loma california punta bands and isla guadalupe baja california and cabo pulmo baja california sur on san nicolas island point loma and punta bands both the 80 000 and the 125 000 yr terraces are dated terraces that may represent the 105 000 sea level high stand are rarely preserved and none has yielded corals for u series dating similarity of coral ages from midlatitude erosional marine terraces with coral ages from emergent constructional reefs on tropical coastlines suggests a common forcing mechanism namely glacioeustatically controlled fluctuations in sea level superimposed on steady tectonic uplift the low marine terrace dated at 125 000 yr on isla guadalupe baja california presumed to be tectonically stable supports evidence from other localities for a 6 m sea level at that time data from the pacific coast and a compilation of data from other coasts indicate that sea levels at 80 000 and 105 000 yr may have been closer to present sea level within a few meters than previous studies have suggested

rapid neoglaciation on ellesmere island promoted by enhanced summer snowfall in a transient climate model simulation of the middle late holocene arctic neoglaciation following the holocene thermal maximum is an important feature of late holocene climate we investigated this phenomenon using a transient 6000 year simulation with the cesm ca

a model study of the role of variable ice albedo in the climate response of the earth to orbital variations abstract using the atmosphere hydrosphere component of a paleoclimate model the model sensitivity on a short time scale to changes in the parameterization of the surface albedo is examined in the context of response to orbitally produced insolation anomalies the model component based on the global zonally averaged model of m suarez and i held 1979 j geophys res 84 4825 4836 is an energy balance model with two atmospheric layers a mixed layer ocean a snow mechanism and sea ice budget a crude approximation of the variation of the ice and snow surface albedo between a melting and nonmelting surface is incorporated into the model the effect of the variable albedo primarily in the summer half year is to approximately double the sensitivity of the model to specific orbital insolation anomalies compared to a model in which the melting effect is omitted

glacial inception on baffin island the role of insolation meteorology and topography abstractgeologic evidence suggests that the last glacial inception 115 kya occurred within the mountains of baffin island global climate models gcms have difficulty simulating this climate transition likely because of their coarse horizontal resolution that smooths topography and necessitates the use of cumulus parameterizations a regional configuration of the weather research and forecasting wrf model is used to simulate the small scale topographic and cloud processes neglected by gcms and the sensitivity of the region to milankovitch forcing topography and meteorology is tested it is found that ice growth is possible with 115 kya insolation realistic topography and slightly colder than average meteorology represented by specific years within the past three decades the simulation with low gcm like topography shows a negative surface mass balance even with the relevant orbital parameter configuration demonstrating the criticality of realistic topography the downslope growth of the ice

post eemian glacial inception part i the impact of summer seasonal temperature bias abstract post eemian glacial inception the transition between marine oxygen isotopic stage mois 5e and mois 5d began approximately 117 000 years before present 117 kyr bp and led to significant northern hemisphere glaciation within the ensuing 5000 yr previous sensitivity studies with atmospheric general circulation models agcms have had difficulty producing glacial nucleation in high northern latitude regions of the globe a base simulation of this process has been conducted using the canadian centre for climate modelling and analysis cccma gcmii with mixed layer slab ocean model constrained so as to ensure that the model reproduces the set of atmospheric model intercomparison project 2 amip2 modern sea surface temperatures ssts under conditions of modern radiative forcing this simulation demonstrates that entry into glacial conditions at 116 kyr bp requires only the introduction of post eemian orbital insolation and standard preindustrial co2 concentrations two additional sensitivity ex

possible role of atmosphere biosphere interactions in triggering the last glaciation we coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system at the time of glacial inception 115 000 years ago orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present atmospheric composition oceans biosphere and cryosphere summers were however cold enough to induce major vegetation shifts in high northern latitudes southward migration of the boreal forest tundra limit helped to create favourable conditions for continental ice sheet growth with increasing snow depth and duration in labrador arctic canada and northern western fennoscandia these results support a role for biogeophysical feedback in initiating glaciations

initiation and development of the laurentide and cordilleran ice sheets following the last interglaciation abstract fossil records from sites overridden by or adjacent to the laurentide ice sheet indicate that the climate of the last interglaciation oxygen isotope substage 5e ca 130 116 ka was warmer than today following the last interglaciation the laurentide ice sheet first developed during stage 5 over keewatin quebec and baffin island along its northern margin the ice sheet reached its maximum extent of the last glaciation during stage 5 the ice sheet advanced across baffin island onto the continental shelf early during stage 5 5d whereas the advance into the western canadian arctic occurred late during stage 5 5b the ice sheet also may have advanced into the st lawrence lowland during substage 5b although this event may be younger stage 4 the hudson bay lowland became ice free during substage 5a retreat of the ice sheet on baffin island occurred during late stage 5 probably substage 5a the exact timing of retreat from the western canadian arctic is unknown but it occurred before 48 ka the southern sector including the st lawrence lowland was ice free during late stage 5 the hudson bay lowland may have remained ice free through stage 4 and much of stage 3 because of conflicting chronologies however it is more likely that this area was glaciated throughout stage 3 and perhaps stage 4 nevertheless the data demonstrate that the lowland was ice free during part of the last glaciation an independent ice cap developed over the appalachian uplands and advanced across nova scotia during stage 4 perhaps as far as the edge of the continental shelf the ice cap remained active over nova scotia as a setellite to the laurentide ice sheet throughout the remainder of the last glaciation the ice sheet advanced into the st lawrence lowland during stage 4 and subsequently overwhelmed the local ice cap in the appalachian uplands advancing perhaps into northern new england but not farther south the lowland remained covered by the ice sheet until late stage 2 the ice sheet may also have advanced into the lake ontario basin during stage 4 the position of the northern margin of the laurentide ice sheet during stage 4 is not known but it remained an unknown distance behind its maximum position reached during stage 5 cores from baffin bay indicate a substantial decrease in high latitude glaciation during stage 4 following retreat the keewatin sector of the ice sheet may have remained over much of northwestern canada as a quasi stable ice mass until it readvanced during stage 2 similarly the baffin island sector of the ice sheet may have remained largely intact the southern margin of the ice sheet may have advanced into the lake ontario basin and upper mississippi valley during the middle of stage 3 ca 50 ka reaching its maximum extent of the last glaciation during stage 2 ca 18 21 ka advance of the northern margin was younger ca 8 13 ka than that of the southern margin this advance was less extensive than the penultimate advance stage 5 paleoenvironmental records indicate that the last interglaciation in areas covered by and near the cordilleran ice sheet was as warm as or warmer than present the cordilleran ice sheet appears to have developed during stage 5 or 4 at that time it advanced over southern british columbia and into the northern puget lowland there is no record of this event in northern areas that were later covered by the ice sheet the ice sheet disappeared before 59 ka at the beginning of a lengthy nonglacial interval paleoenvironmental records indicate that climate was similar to the present during part of this interval the ice sheet was absent and glaciers probably were confined to mountain areas throughout stage 3 climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka by 14 15 ka the ice sheet had achieved its maximum extent of the last glaciation because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted we consider this synthesis as tentative and subject to significant revision as dating methods improve nevertheless this perspective of the north american ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume

a model study of the role of high latitude topography in the climatic response to orbital insolation anomalies abstract the response of a combined global zonal averaged energy balance model and a continental ice sheet model to insulation anomalies produced by orbital perturbations is examined although the role of ice sheet dynamics is included in the model this paper focuses mostly on the response of the atmosphere hydrosphere to the perturbations the model developed from that of suarez and held 1979 explicitly incorporates surface topography surface energy balances snow and sea ice budgets and variable static stability the ice sheet model is based on a power law rheology it is found that the presence of a high latitude plateau significantly increases the sensitivity of the climate to the insolation perturbation the sensitivity is maximized when the elevation of the summer snowline is near the elevation of the plateau as appears to be the case with baffin island today there appears to be a rapid response of the seasonal cycle to an insolation anomaly through albedo temperature feedback and with the

sensitivity of glaciation to initial snow cover co2 snow albedo and oceanic roughness in the ncar ccm we present results from numerical experiments made with a gcm the ncar ccm1 that were designed to estimate the annual balance between snow fall accumulation and ablation for geographically important land regions for a variety of conditions we also attempt to assess the reliability of these results by investigating model sensitivity to changes in prescribed physical parameters experiments were run with an initial imposition of 1 m of midwinter snowcover over all northern hemisphere land points over alaska western canada siberia and the tibetan plateau the model tended to retain this snow cover through the summer and in some cases increase its depth as well we define these regions as glaciation sensitive and note some correspondence between them and source regions for the pleistocene ice sheets an experiment with greatly reduced co2 100 ppm showed a tendency towards spontaneous glaciation i e the model remained snow covered throughout the summer over the same geographic regions noted above with 200 ppm co2 roughly equal to values at the last glacial maximum snow cover over these regions did not quite survive the summer on a consistent basis combining 200 ppm co2 and 1 m of initial northern hemisphere snow cover yielded glaciation sensitive conditions agreeing remarkably well with locations undergoing glaciation during the pleistocene to assess the reliability of these results we have determined minimal model uncertainty by varying two of the empirical coefficients in the model within physically plausible ranges in one case surface roughness of all ocean gridpoints was reduced by an order of magnitude leading to local 10 reductions in precipitation snowfall a change hard to distinguish from inherent model variability in the other case the fraction of a land grid square assumed to be occupied by snow cover for albedo purposes was varied from one half to unity large changes occurred in the degree of summer melting and in some cases the sign of the net balance changed as fractional snow cover was changed we conclude that the model may be able to reveal regions sensitive to glaciation but that it cannot yield a reliable quantitative computation of the magnitude of the net snow accumulation that can be implicitly or explicitly integrated through time

late pliocene to pleistocene sensitivity of the greenland ice sheet in response to external forcing and internal feedbacks the timing and nature of ice sheet variations on greenland over the last 5 million years remain largely uncertain here we use a coupled climate vegetation ice sheet model to determine the climatic sensitivity of greenland to combined sets of external forcings and internal feedbacks operating on glacial interglacial timescales in particular we assess the role of atmospheric pco2 orbital forcing and vegetation dynamics in modifying thresholds for the onset of glaciation in late pliocene and pleistocene the response of circum arctic vegetation to declining levels of pco2 from 400 to 200 ppmv and decreasing summer insolation includes a shift from boreal forest to tundra biomes with implications for the surface energy balance the expansion of tundra amplifies summer surface cooling and heat loss from the ground leading to an expanded summer snow cover over greenland atmospheric and land surface fields respond to forcing most prominently in late spring summer and are more sensitive at lower pleistocene like levels of pco2 we find cold boreal summer orbits produce favorable conditions for ice sheet growth however simulated ice sheet extents are highly dependent on both background pco2 levels and land surface characteristics as a result late pliocene ice sheet configurations on greenland differ considerably from late pleistocene with smaller ice caps on high elevations of southern and eastern greenland even when orbital forcing is favorable for ice sheet growth

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

tracer transport in fractured crystalline rock evidence of nondiffusive breakthrough tailing extended tailing of tracer breakthrough is often observed in pulse injection tracer tests conducted in fractured geologic media this behavior has been attributed to diffusive exchange of tracer between mobile fluids traveling through channels in fractures and relatively stagnant fluid between fluid channels along fracture walls or within the bulk matrix we present a field example where tracer breakthrough tailing apparently results from nondiffusive transport tracer tests were conducted in a fractured crystalline rock using both a convergent and weak dipole injection and pumping scheme deuterated water bromide and pentafluorobenzoic acid were selected as tracers for their wide range in molecular diffusivity the late time behavior of the normalized breakthrough curves were consistent for all tracers even when the pumping rate was changed the lack of separation between tracers of varying diffusivity indicates that strong breakthrough tailing in fractured geologic media may be caused by advective transport processes this finding has implications for the interpretation of tracer tests designed to measure matrix diffusion in situ and the prediction of contaminant transport in fractured rock

the impact of precession changes on the arctic climate during the last interglacial glacial transition three sensitivity experiments using an ocean atmosphere general circulation model oagcm are conducted to simulate the climate impact of precession the relative contributions of components of the hydrological cycle including the albedo of arctic sea ice advection of atmospheric water vapor and sea surface temperature to the summer arctic melt process are evaluated timing of the perihelion is varied in each experiment with meteorological spring sp winter wp and autumn ap perihelion corresponding to conditions at 110 115 and 120 ky bp respectively obliquity is unchanged at the 115 ky level which is lower than today the experiments are assessed relative to the present day control which has been shown to simulate current conditions based on observations in the sp experiment top of the atmosphere toa insolation is weaker than today between the summer solstice and autumnal equinox in the ap case representing the interglacial it is less intense between vernal equinox and summer solstice but stronger during the remainder of the year although the incident solar radiation is reduced in summer in the sp experiment increased melting of snow is found primarily as a result of feedbacks from the delayed seasonal cycle of hydrologic components this is in contrast to both the wp and ap cases in which the perennial snow cover is simulated at the time of the last glacial inception 115 ky bp the wp experiment shows lower insolation to the high northern latitudes in late spring and summer mainly as a result of lower obliquity than today dynamical ocean atmosphere interactions in response to precession maintain the reduced sea ice melting in late spring strengthen the annual equator to pole sea surface temperature sst gradient and increase atmospheric moisture convergence in glaciation sensitive regions in both the wp and

astronomical theory of paleoclimates and the last glacial interglacial cycle in the 19th century croll and pilgrim stressed the importance of severe winters as a cause of quaternary ice ages later on mainly during the first half of the 20th century koppen spitaler and milankovitch regarded high winter and low summer insolation as favoring glaciation after koppen and wegener related the milankovitch new radiation curve to penck and bruckner s subdivision of the quaternary there was a long lasting debate whether or not such changes in the insolation can explain the quaternary glacial interglacial cycles in the 1970s with the improvements of radiometric dating and of acquiring and interpreting the long continuous geological records with the advent of computers and with the development of more sophisticated astronomical and climate models the astronomical theory and in particular its milankovitch version were revived over the last 5 years most geological astronomical and climatological obstacles have been overcome and the causal role of the orbital variations in long term variation of climate is taken almost universally for granted models of different categories of complexity from conceptual ones to 3 d atmospheric general circulation models and 2 d time dependent models of the whole climate system have now been astronomically forced in order to test the physical reality of the astronomical theory for example experiments with both low and high resolution gcms show that the increased seasonality at 9 000 bp leads to intensified summer monsoon circulation of africa and southern asia on the other hand seasonal models for simulating the transient response of the climate system to astronomical forcing are also developed and the output of these most recent modeling efforts compares favorably with data of the past 400 000 years accordingly the model predictions for the next 100 000 years are used as a basis for forecasting how climate would evolve when forced by orbital variations in the absence of anthropogenic disturbances the long term cooling trend which began some 6 000 years ago will continue for the next 5 000 years this first temperature minimum will be followed by an amelioration around 15 000 ap after present by a cold interval centered al 23 000 ap and by a major glaciation at around 60 000 ap

ice sheet inception subgrid hypsometric parameterization of mass balance in an ice sheet model studies of ice sheet inception and mountain glaciation are limited by model resolution in both atmospheric and glaciologic models the accumulation and ablation of snow and ice are controlled by local climate which is sensitive to topographic features that are subgrid in continent scale investigations climate patterns are complex in areas of high relief but the first order influence of elevation on temperature and atmospheric humidity is quantifiable and is central to glacier mass balance we make substantial improvements to the descriptions of accumulation and ablation in a large scale ice sheet model by incorporating subgrid topographic information the distribution of relief in the large scale model is estimated through a hypsometric parameterization based on the minimum maximum and median topographic elevation this permits accounting of high elevation accumulation and melting in deep valleys we combine this with a parameterization of downslope subgrid ice flux simulating but not explicitly describing the role of alpine glaciers the detailed budgeting of mass balance greatly improves our skill in modelling present day ice distribution in north america we anticipate similar improvements in our ability to model nucleation centres of the quaternary ice sheets this approach may also prove useful in projections of glacier and ice field response to climate change

filtering of milankovitch cycles by earth s geography abstract earth s land sea distribution modifies the temperature response to orbitally induced perturbations of the seasonal insolation we examine this modification in the frequency domain by generating 800 000 yr time series of maximum summer temperature in selected regions with a linear two dimensional seasonal energy balance climate model previous studies have demonstrated that this model has a sensitivity comparable to general circulation models for the seasonal temperature response to orbital forcing on land although the observed response in the geologic record is sometimes significantly different than modeled here differences attributable to model limitations and feedbacks involving the ocean atmosphere cryosphere system there are several results of significance 1 in mid latitude land areas the orbital signal is translated linearly into a large 10 c seasonal temperature response 2 although the modeled seasonal response to orbital forcing on antarctica is 6 c the annual mean temperature effect

simulation of the last glacial cycle by a coupled sectorially averaged climate ice sheet model 2 response to insolation and co2 variations a two dimensional climate model which links the northern hemisphere atmosphere ocean mixed layer sea ice and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock the coupled model has been used to test the influence of several factors including snow surface albedo over the ice sheets in producing plausible ice age simulations using astronomically derived insolation and co2 data from the vostok ice core the impact of potentially important processes such as the water vapor transport clouds and deep sea circulation was not investigated in this study after several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr large variations of ice volume are simulated between 122 and 55 kyr b p with a rapid latitudinal extension of the north american and eurasian ice sheets starting at 120 kyr b p the simulated last glacial maximum is at 19 kyr b p the model is able to simulate deglaciation as well the simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions the major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations linked with an underestimation of the ice sheet extent and an excess in the prescribed co2 concentration sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations in the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow which decreases its albedo the other factors which play an important role are in decreasing level of importance the ice sheet altitude insolation taiga cover and ice sheet extent a final set of experiments addresses the effects of co2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr this last experiment is made by forcing the model with both insolation and co2 variations this additional forcing improves the temperature and ice volume results despite the limitations inherent to the present modeling approach the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial interglacial cycle

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

hierarchical modeling of flow in naturally fractured formations with multiple length scales this paper describes hierarchical approach to modeling flow in a naturally fractured formation our model is based on calculating the effective permeability of a fractured formation as a function of grid block size and using the results in a conventional finite difference flow simulator on the basis of their length lf relative to the finite difference grid size lg fractures are classified as belonging to one of three groups 1 short fractures lf lg 2 medium length fractures lf lg and 3 long fractures lf lg the effects of the fractures belonging to each class are computed in a hierarchical manner the permeability contribution from short fractures is derived in an analytical expression and used as an enhanced matrix permeability for the next scale medium length calculation the effective matrix permeability associated with medium length fractures is numerically solved using a boundary element method the long fractures are modeled explicitly as major fluid conduits as numerical examples tracer transport in fractured formations was illustrated the numerical results clearly indicated that effective tensor permeability well represented directional enhanced permeability in fractured formations the fluid conduit formulation captured the efficient fluid transport by long fractures

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

toward the snowball earth deglaciation the current state of knowledge suggests that the neoproterozoic snowball earth is far from deglaciation even at 0 2 bars of co2 since understanding the termination of the fully ice covered state is essential to sustain or not the snowball earth theory we used an atmospheric general climate model agcm to explore some key factors which could induce deglaciation after testing the models sensitivity to their parameterizations of clouds co2 and snow we investigated the warming effect caused by a dusty surface associated with ash release during a mega volcanic eruption we found that the snow aging process its dirtiness and the ash deposition on the snow free ice are key factors for deglaciation our modelling study suggests that under a co2 enriched atmosphere a dusty snowball earth could reach the deglaciation threshold

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

pleistocene climatic variability at astronomical frequencies abstract spectral analysis of geological records show periodicities corresponding to those calculated for the eccentricity 400 and 100 ka the obliquity 41 ka and the climatic precession 23 and 19 ka it is precisely the geological observations of this bi partition of the precessional peak confirmed to be real in astronomical computations which was one of the most impressive of all tests for the milankovitch theory concerning the question of whether or not the observed cycles account for most of the climatic variability having periods in the range predicted by the astronomical theory substantial evidence from cross spectral analysis coherency analysis and modelling is provided that at least near frequencies of variation in obliquity and precession a considerable fraction of the climate variance is driven in some way by insolation changes accompanying changes in the earth s orbit the variance components centered near a 100 ka cycle dominate most ice volume records and seem in phase with the eccentricity cycle although the exceptional strength of this cycle needs a non linear amplification by the glacial ice sheets themselves and associated feedbacks as the insolation spectra change with latitudes and with the type of parameters considered the diversity of the spectra of different climatic proxy data recorded in different places of the world over different periods is used to better understand how the climate system responds to the insolation forcing this study of the physical mechanisms involved is also achieved through the analysis of the log log shape of the geological records and through the comparison in frequency domain between simulated climatic time series and proxy data the evidence both in the frequency and in the time domain that orbital influences are felt by the climate system implies that the astronomical theory may provide a clock with which to date quaternary sediments with a precision several times greater than is now possible

sea ice switches and abrupt climate change we propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea ice cover we explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism we emphasize that because of the significant influence of sea ice on the climate system it seems that high priority should be given to developing ways for reconstructing high resolution in space and time sea ice extent for past climate change events if proxy data can confirm that sea ice was indeed the major player in past abrupt climate change events it seems less likely that such dramatic abrupt changes will occur due to global warming when extensive sea ice cover will not be present

terrestrial conditions at the last glacial maximum and climap sea surface temperature estimates are they consistent climap 1981 seasonal reconstruction of the earth s surface at the last glacial maximum geological society of america map and chart series mc 36 boundary conditions were used as inputs to the giss general circulation model and the last glacial maximum lgm climate was simulated for six model years the simulation was compared with snow line depression and pollen inferred temperature data at low latitudes specifically for hawaii colombia east africa and new guinea the model does not produced as much cooling at low latitudes as is implied by the terrestrial evidence an alternative experiment in which the climap sea surface temperatures were uniformly lowered by 2 c produces a better fit to the land data although in hawaii model temperatures are still too warm the relatively warm climap tropical sea surface temperatures also provide for only a slight decrease in the hydrologic cycle in the model in contrast to both evidence of lgm tropical aridity and the results of the experiment with colder ocean temperatures with the climap sea surface temperatures the lgm global annual mean surface air temperature is 3 6 c colder than at present if the ocean temperatures were allowed to cool in conformity with the model s radiation balance the lgm simulation would be 5 6 c colder than today and in better agreement with the tropical land evidence

initiation of a continental ice sheet in a global climate model genesis the initiation and maintenance of a continental ice sheet are investigated using the genesis global climate model a necessary condition for ice sheet initiation is to have snow cover survive through the summer model simulations examine the sensitivity of the maintenance of summer snow cover to the prescriptions of topography and solar luminosity the time period chosen for this study is the late carboniferous 306 ma when an extensive continental ice sheet as large if not larger than the pleistocene glaciations existed on a supercontinent in the southern hemisphere this ice sheet persisting for over 60 m y was one of the most prolonged periods of continuous glaciation in earth history model simulations indicate that given the geography solar luminosity 3 less than present and atmospheric co 2 same as present estimated for this time period summer snow cover remains as far equatorward as 35 s latitude global mean temperature is 2 4 c 17 2 c cooler than for present probable regions for the initiation of the carboniferous ice sheet are apparent in analysis of the spin up of the model to equilibrium summer snow cover first persists along the polar coastline of the supercontinent during year 4 of the simulation summer snow cover shows the greatest sensitivity to solar luminosity increasing the solar constant to its present value from the late carboniferous value results in a summer warming of southern hemisphere high latitude land areas by as much as 37 c and complete melting of any summer snow cover topography plays a lesser role reducing land elevations to sea level causes persistent summer snow cover to retreat only 8 poleward conversely initializing the model with an elevated land ice sheet has no effect on summer snow cover extent

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

role of orbitally induced changes in tundra area in the onset of glaciation the link between glacial interglacial cycles and changes in insolation due to variations in the earth s orbital parameters is well established1 4 but of the attempts to simulate incipient glaciation using three dimensional general circulation models gcms driven by orbital forcing alone5 10 only one10 has been successful gcm experiments7 11 show that reduced summer insolation 115 000 years ago during an interglacial to glacial climate shift produces sufficient high latitude cooling to cause expansion of tundra at the expense of boreal forest11 which in turn can induce more cooling11 14 here we show using a global climate model that the increase in surface albedo under snow covered conditions that results from a biome model estimate11 of tundra expansion 115 000 years ago is sufficient to induce glaciation over extreme northeastern canada if the additional cooling from this estimated tundra expansion induces further expansion then widespread glaciation occurs at latitudes above 65 n these results suggest that the climate feedback from high latitude tundra expansion might have contributed to the onset of the most recent glaciation

the initiation of ice sheet growth milankovitch solar radiation variations and the 100 ky ice age cycle much work has gone into deciphering the causes of the large scale glacial interglacial variations in the climate system over the last 900 000 years while variations on the 41 thousand year ky and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere milankovitch solar radiation variations the causes of the dominant 100 ky cycle in the geologic record are still unknown one of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky this is done through the use of a coupled energy balance climate thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation and a land surface energy balance which determines the net accumulation of snow and ice results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season

glacial inception and quaternary mountain glaciations in fennoscandia abstract the aim of this review is twofold first the role of the fennoscandian mountains as a nucleation center for glacial inception is investigated the second objective is to review glacial erosion by mountain glaciers throughout the quaternary all available field data and several modelling experiments show that the fennoscandian mountains seeded the last glaciation since the mountains seem to be of key importance for ice sheet inception it is reasonable to assume a similar inception pattern throughout most of the major pleistocene glaciations this pattern also implies prolonged periods of glacial ice in the fennoscandian mountains during late cenozoic time mountain based glaciers thus have had a long time to act on their substratum resulting in a profound geomorphological impact

multiple equilibria in a zonal energy balance climate model the thin ice cap instability from an analysis of observational data and general circulation model gcm simulations a review is given of the empirical parameterizations which lead to the occurrence of multiple equilibria in previously formulated energy balance climate models ebm of the budyko sellers type by drawing an analogy between the present day southern hemisphere high latitude conditions and the northern hemisphere nh high latitude conditions during glacial times we have devised a new parameterization for the zonal planetary albedo in particular we hypothesize that changes in the temperature of the nh polar regions can cause the development of a high latitude planetary albedo anomaly namely the occurrence of continental ice sheets and show that in this case a zonal mean ebm admits multiple equilibria the zonal profiles of the climatic quantities for the warm and cold equilibrium states and their corresponding globally averaged values agree with the observational geological and paleoclimatic evidence and exhibit a poleward amplification of the temperature anomaly the new parameterization of the ice albedo temperature feedback introduces what we have called the thin ice cap instability tici the tici is a more realistic version of the small ice cap instability discussed in earlier versions of ebms the physical meaning of the tici and the sensitivity of the degree of nonlinearity it introduces to changes in the model parameters is then discussed on the basis of modeling experience with a previously developed global ebm to which the current ebm can be reduced by spatial integration the stability of the equilibria is considered in terms of the lyapunov pseudopotential finally we perform time dependent experiments to evaluate the relaxation time scales of the system around its equilibria and to investigate the latitudinal dependence of the response to perturbations

the importance of mesoscale circulations generated by subgrid scale landscape heterogeneities in general circulation models abstract a mesoscale atmospheric model was used to evaluate the impact of subgrid scale landscape discontinuities on the vertical profiles of resolved temperature moisture and moist static energy in the planetary boundary layer pbl of gcms these profiles were produced with a 3d version of the model using a horizontal grid resolution of 7 5 km and 13 vertical layers in the pbl by averaging horizontally the various atmospheric variables over a 180 180 km2 domain about the size of the horizontal domain represented by a single grid element in a gcm they were compared to corresponding vertical profiles produced with a 1 d version of the model which simulates the pbl as in a gcm over a single horizontal grid element differences obtained between the horizontally averaged atmospheric variables produced with the 3d situations and the 1 d simulations emphasize the impact of subgrid scale landscape discontinuities on gcm resolved variables various types of landscape discontinuities characterized by hor

sensitivity of late glacial and holocene climates to the combined effects of orbital parameter changes and lower boundary condition changes snapshot simulations with a general circulation model for 18 9 and 6 ka bp

a coupled climate ice sheet model applied to the quaternary ice ages a coupled climate ice sheet model is used in an attempt to simulate the northern hemispheric ice sheet volume variations implied by deep sea core records over the last several 105 years the climate model consists of a global one level diffusive energy balance equation with seasonal variation and land ocean contrast using a severely truncated series of associated legendre functions the ice sheet model uses a standard vertically averaged flow law and predicts ice thickness in a north south cross section bedrock deformation under the ice is included as linear flow confined to a thin asthenospheric channel the climate model is tuned to the modern climate and tested against results from atmospheric general circulation models for 18 000 and 10 000 yr b p it is suggested that the lack of sea ice variations causes the biggest departures from the gcm results the sensitivities to the orbital perturbations of the coupled model and of the ice sheet model alone with a prescribed snow budget distribution are examined some qualitative differences are found in the latitudinal forms of these sensitivities despite these differences however the two types of models produce similar ice age simulations over the last 700 thousand years in response to the orbital perturbations in particular fairly realistic simulations with strong 100 kyr cycles similar to those obtained by adding an ice calving mechanism to the ice sheet model in pollard 1982a are again produced in the coupled model

is the greenland ice sheet bistable ice core work on greenland has produced dramatic evidence for an instability of the climate system in the north atlantic sector in this paper we provide climate modeling results indicating another possible example of a multiple equilibrium climate state where such behavior might apply to the ice sheet itself baseline sensitivity experiments in which the greenland ice sheet was removed simulate summer temperatures on greenland of about 10 c this number varied between about 6 and 14 c depending on specification of vegetation type elevation of greenland and orbital forcing the implied hysteresis indicates that the present geographic configuration may have been sufficient to maintain an ice free state prior to formation of the ice cap explanations for formation of the ice sheet therefore present a dilemma four possibilities involve undocumented miocene pliocene co2 excursions to values lower than present a high sensitivity of the climate system a variable sensitivity of the climate system or significant problems with the climate model a variable sensitivity is consistent with our results indicating a multiple steady state climate for greenland although most co2 scenarios do not predict a collapse of the greenland ice sheet in the future our results suggest that if it did present boundary conditions may be sufficient to maintain greenland in an ice free environment even after the greenhouse effect has dissipated