timing of abrupt climate change a precise clock 1 many paleoclimatic data reveal a 1 500 year cyclicity of unknown origin a crucial question is how stable and regular this cycle is an analysis of the gisp2 ice core record from greenland reveals that abrupt climate events appear to be paced by a 1 470 year cycle with a period that is probably stable to within a few percent with 95 confidence the period is maintained to better than 12 over at least 23 cycles this highly precise clock points to an origin outside the earth system oscillatory modes within the earth system can be expected to be far more irregular in period

conceptual model for millennial climate variability a possible combined solar thermohaline circulation origin for the 1 500 year cycle dansgaard oeschger and heinrich events are the most pronounced climatic changes over the last 120 000 years although many of their properties were derived from climate reconstructions the associated physical mechanisms are not yet fully understood these events are paced by a 1 500 year periodicity whose origin remains unclear in a conceptual model approach we show that this millennial variability can originate from rectification of an external solar forcing and suggest that the thermohaline circulation through a threshold response could be the rectifier we argue that internal threshold response of the thermohaline circulation thc to solar forcing is more likely to produce the observed do cycles than amplification of weak direct 1 500 year forcing of unknown origin by thc one consequence of our concept is that the millennial variability is viewed as a derived mode without physical processes on its characteristic time scale rather the mode results from the linear representation in the fourier space of nonlinearly transformed fundamental modes

cassini imaging science initial results on phoebe and iapetus the cassini imaging science subsystem acquired high resolution imaging data on the outer saturnian moon phoebe during cassini s close flyby on 11 june 2004 and on iapetus during a flyby on 31 december 2004 phoebe has a heavily cratered and ancient surface shows evidence of ice near the surface has distinct layering of different materials and has a mean density that is indicative of an ice rock mixture iapetus s dark leading side cassini regio is ancient heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief local albedo variations within and bordering cassini regio suggest mass wasting of ballistically deposited material the origin of which remains unknown

radar detection of iapetus saturn s satellite iapetus has the largest albedo asymmetry of any natural satellite with the optical albedo of its trailing hemisphere as much as 10 times that of its leading hemisphere 1 1 the sharp contrast on this synchronously rotating satellite suggests that exogenic materials such

relaxation oscillators in concert a framework for climate change at millennial timescales during the late pleistocene 1 using a box model of the north atlantic ocean and a parameterization of heinrich events we suggest that self sustained oscillations of the large scale oceanic circulation provide a framework to accommodate crucial elements of late pleistocene climate variability i dansgaard oeschger style oscillations with varying interstadial length ii synchronization between dansgaard oeschger stadials and heinrich events and iii younger dryas type events the latter result from the restart of the oscillations after a glacial maximum and can be regarded as dansgaard oeschger stadials overprinted by rapidly changing boundary conditions

an evaluation of seismic decoupling and underground nuclear test monitoring using high frequency seismic data an effective solution to the problem of the detection and identification of low yield coupled and fully decoupled underground nuclear explosions appears available via use of high frequency seismic data ranging up to 30 or 40 hz in order to evaluate detection identification capabilities when using such data it is necessary to estimate 1 spectral characteristics and relative amplitudes of both p and s waves from explosions and earthquakes over the frequency band from 5 to 40 hz 2 signal transmission characteristics over this band through pertinent types of earth structure and 3 recording system and ground noise characteristics over this frequency band in this study each of these topics is considered in turn as they relate to detection and discrimination of the signals from low yield coupled and decoupled explosions in the regional and teleseismic distance ranges estimates of the capabilities of specific hypothetical networks to detect and identify insofar as signal to noise ratio is an important factor in identification explosions within the ussr are then considered these estimates of signal detection capability provide the central focus for the study as they serve to translate diverse and rather complex sets of observational data and theory into concrete predictions of monitoring capability following the assessment of detection capabilities the problem of identification of small events is considered with particular emphasis on discrimination at regional distances where the network is calculated to provide signals of high signal to noise ratio the principal results and conclusions of this study are as follows 1 seismic system noise can be suppressed to levels well below ground noise at quiet sites up to frequencies at least as high as 30 40 hz when using presently available hardware 2 average amplitudes of high frequency noise in a variety of geological environments are very low and change little with time or season 3 transmission of high frequency p and s wave signals in the regional distance range in stable continental areas and shields is nearly as efficient as at 1 hz with effective q factors in shield areas being about 9000 and 4000 for high frequency pn and sn respectively while the effective q for pn waves in tectonic areas is about 1000 4 a properly designed and deployed network of 25 simple three component nonarray stations internal to the ussr and 15 similar stations surrounding the ussr is predicted to be capable of multistation detection at high signal to noise ratio of fully decoupled 1 kt explosions located at all potential decoupling sites within the ussr 5 by inference from the quantitative agreement of empirical observations and theoretical predictions when using lower frequency data over a great range of explosion yields we conclude that procedures based on the use of both detectable p and s waves will serve to identify explosion generated seismic signals at least as small as those expected from a fully decoupled 1 kt explosion

spectral discrimination of nts explosions and earthquakes in the southwestern united states using high frequency regional data a high frequency seismic station has been operating since march 1987 near nelson nv records of earthquakes at regional distances from the station show substantially more energy at frequencies above 10 hz than records of nuclear tests of comparable magnitude on the nevada test site this contradicts the predictions made by investigators who have advocated high frequency monitoring for nuclear test detection and identification measuring the slopes of the pg velocity spectra between 4 and 30 hz successfully discriminates 11 nts explosions from 36 earthquakes recorded in the first seven months of operation the spectral differences may result from the respective source depths with the earthquakes occurring deeper and in faster material than the explosions

the tempo of climate change during dansgaard oeschger interstadials and its potential to affect the manifestation of the 1470 year climate cycle 1 the oxygen isotope record from the gisp2 ice core serves a air temperature proxy to estimate cooling rates during dansgaard oeschger interstadials between 15 53 thousand years before present the duration of individual dansgaard oeschger interstadials scales inversely with the cooling rate and high cooling rates are associated with large continental ice volume and vice versa cooling rates determine the extent to which the postulated 1470 year pacing cycle of the dansgaard oeschger interstadials is visible in paleoclimatological records only cooling rates associated with intermediate ice volume approx 84 20 m relative sea level equivalent allow the climate system to respond to succeeding trigger events of the 1470 year cycle and result in stadials interstadials of nearly the same duration

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

synchronous high frequency oscillations in tropical sea surface temperatures and north atlantic deep water production during the last glacial cycle stable isotopic measurements of g sacculifer and c wuellerstorfi in a core from the western equatorial atlantic imply that there are parallel suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3 low values of g sacculifer 18o accompany high values of c wuellerstorfi 13c linking warmer sea surface temperatures ssts in the tropics with increased production of lower north atlantic deep water nadw the amplitude of the 18o oscillations is 0 6 or 2 3 c which is superimposed on a glacial interglacial amplitude of about 2 1 using the g sacculifer 18o data we calculate that surface waters were colder during stage 2 than calculated by climap 1976 1981 the longer period 2 kyr oscillations in air temperature recorded in the greenland and antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial atlantic the magnitude of these oscillations in tropical sst is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone the apparent synchroneity of equatorial sst and polar air temperature changes as well as the amplitude of the sst changes at the equator are consistent with the climate effects expected from changes in the atmosphere s greenhouse gas content h2ovapor co2 and ch4

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

hysteresis behavior of the atlantic ocean circulation identified in observational data abstract on the basis of the multidecadal component of a fram strait sea ice export reconstruction and on a sea surface temperature proxy for changes in the ocean circulation a bifurcation diagram for two cycles of the atlantic ocean multidecadal oscillation is constructed it suggests a hysteresis behavior that is consistent with nonlinear convective adjustment to changes in deep water formation in the north atlantic and with the meridional overturning circulation bistability associated with two distinct configurations of the north atlantic subpolar gyre simulated in response to idealized carbon dioxide increase the nonlinear dynamics of the atlantic ocean circulation emphasized here on the basis of observational data adds to the hysteresis behavior shown in theoretical and numerical studies

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

southern ocean control of glacial amoc stability and dansgaard oeschger interstadial duration glacial periods exhibit abrupt dansgaard oeschger do climatic oscillations that are thought to be linked to instabilities in the atlantic meridional overturning circulation amoc great uncertainty remains regarding the dynamics of the do cycle as well as controls on the timing and duration of the individual events using ice core data we show that the duration of the warm interstadial phase is strongly correlated with antarctic climate and presumably with southern ocean so temperature and the position of the southern hemisphere sh westerlies we propose a so control on amoc stability and interstadial duration via the rate of antarctic bottom water formation meridional density pressure gradients agulhas leakage and so adiabatic upwelling this hypothesis is supported by climate model experiments that demonstrate so warming leads to a stronger amoc that is less susceptible to freshwater perturbations in the amoc stability diagram so warming and strengthening of the sh westerlies both shift the vigorous amoc branch towards higher freshwater values thus raising the threshold for amoc collapse the proposed mechanism could provide a consistent explanation for several diverse observations including maximum do activity during intermediate ice volume sh temperature and successively shorter do durations within each bond cycle it may further have implications for the fate of the amoc under future global warming

slopes of p to s wave spectral ratios a broadband regional seismic discriminant and a physical model concerns about nuclear proliferation and negotiations toward a comprehensive test ban treaty ctbt emphasize the need for robust seismic monitoring and regional discrimination capabilities especially at low magnitudes standard regional discrimination techniques have many shortcomings for example they cannot always discriminate shallow low magnitude earthquakes or mining related events i show that slopes of p to s wave spectral ratios can be used to discriminate shallow and normal depth earthquakes from explosions and possibly to discriminate explosions from mining related rockburst or collapse events p to s wave slopes and conventional high frequency p to s wave spectral amplitude ratios discriminate even better in combination than alone a comparison of pn and lg spectra from nevada test site nts explosions and earthquakes shows that this method discriminates concentrated explosions from earthquakes and other shallow seismic events such as mine collapses because s waves from explosions decay more rapidly at high frequencies than do s waves from earthquakes spectra of one dimensional reflectivity synthetic seismograms show that frequency dependent attenuation of explosion generated p to s wave converted phases in the near surface layers may be responsible for the differences in lg s wave spectral falloff these simulations also show that absolute amplitudes of earthquake p to s wave spectral ratios are very sensitive to mechanism and depth which may help explain the variable performance of conventional methods

a new mechanism for dansgaard oeschger cycles 1 we present a new hypothesis to explain the millennial scale temperature variability recorded in ice cores known as dansgaard oeschger do cycles we propose that an ice shelf acted in concert with sea ice to set the slow and fast timescales of the do cycle respectively the abrupt warming at the onset of a cycle is caused by the rapid retreat of sea ice after the collapse of an ice shelf the gradual cooling during the subsequent interstadial phase is determined by the timescale of ice shelf regrowth once the ice shelf reaches a critical size sea ice expands driving the climate rapidly back into stadial conditions the stadial phase ends when warm subsurface waters penetrate beneath the ice shelf and cause it to collapse this hypothesis explains the full shape of the do cycle the duration of the different phases and the transitions between them and is supported by proxy records in the north atlantic and nordic seas

abrupt north atlantic circulation changes in response to gradual co2 forcing in a glacial climate state glacial climate is marked by abrupt millennial scale climate changes known as dansgaard oeschger cycles the most pronounced stadial coolings heinrich events are associated with massive iceberg discharges to the north atlantic these events have been linked to variations in the strength of the atlantic meridional overturning circulation however the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear here we show that in a fully coupled atmosphere ocean model gradual changes in atmospheric co2 concentrations can trigger abrupt climate changes associated with a regime of bi stability of the atlantic meridional overturning circulation under intermediate glacial conditions we find that changes in atmospheric co2 concentrations alter the transport of atmospheric moisture across central america which modulates the freshwater budget of the north atlantic and hence deep water formation in our simulations a change in atmospheric co2 levels of about 15 ppmv comparable to variations during dansgaard oeschger cycles containing heinrich events is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode because changes in the atlantic meridional overturning circulation are thought to alter atmospheric co2 levels we infer that atmospheric co2 may serve as a negative feedback to transitions between strong and weak circulation modes

hyperion analysis of voyager observations abstract a total of 82 images of hyperion was returned by the voyager spacecraft the most detailed views have a nominal resolution of 8 7 km line pair hyperion had a rotation period of about 13 days and a spin vector lying close to its orbital plane at the time of the voyager 2 encounter in 1981 the satellite s shape is very irregular and cannot be approximated suitably by an ellipsoid the largest cross section a c is about 370 225 km the b c cross section is approximately 280 225 km most prominent among the surface features is a 120 km diameter crater with an estimated depth of 10 km and a series of arcuate scarps 300 km long that have relief in excess of 5 km the density of large craters of hyperion is smaller than that on other small saturnian satellites and suggests the possibility that the last significant fragmentation of hyperion occurred near the end of or after initial heavy bombardment voyager photometry yields an average normal reflectance of the surface material of 0 21 in the clear filter 0 47 m and evidence of slight albedo mottling over the surface the disk integrated phase coefficient between phase angles of 22 and 82 is 0 018 mag de there is little indication of a strong opposition effect in voyager data down to phase angles of 3 hyperion s average color is definitely redder than that of phobe but matches that of the dark material on the leading hemisphere of iapetus quite well the satellite s albedo and color are consistent with those of contaminated water ice but since no mass determinations of hyperion exist we do not know whether the bulk composition is icy or rocky

spectral evidence for source multiplicity in explosions application to regional discrimination of earthquakes and explosions abstract spectral and cepstral analysis were performed on regional array noress recordings of mining explosions in scandinavia and russia in order to detect the effects of delayed explosions or ripple firing commonly used in mine blasting practice noress array stacked spectra corrected for instrument and noise were computed in windows on pn pg sn and lg waves from the mine blasts by averaging individual channel spectra frequency by frequency across the array for comparison spectra were also computed for six western norway earthquakes located near the blasjo and titania norwegian mines also stack spectra of p coda sn and lg were computed using norsar seismograms for presumed peaceful nuclear explosions pnes located at near regional distances 10 in western russia the norsar spectra were determined in the same manner as the noress spectra except that an individual source correction was applied to the spectra comparison of western norway mine blast spectra and nearby earthquakes shows that the earthquake pn spectra are peaked in the 8 to 16 hz band with little energy below 6 hz whereas the explosion spectra have flatter spectra from 3 to 16 hz this difference in the pn spectra may be caused by the earthquakes occurring at greater depths than the explosions differences in the near source media or source mechanism effects in the earthquake spectra the main difference between earthquake and explosion spectra is that the explosion spectra exhibit marked scalloping or modulation patterns not observed in the earthquake spectra the modulation patterns are identical in spectra of all phases indicating that they are caused by multiple shot sequences or ripple firing simulated multiple source spectra determined using the noress seismograms for one of the western norway earthquakes resulted in spectra that resembled many of the explosion modulation patterns based on the simulation studies and cepstral analysis of the spectra the mine explosions appear to be composed of two to three separate explosions delayed by between 80 to 150 msec moreover the norsar spectra of pnes indicate that these signals are also produced by multiple explosions although they have larger delay times of between 0 9 to 1 5 sec these results show that spectral modulations in seismogram spectra of economic explosions including mine explosions and nuclear explosions fired for peaceful economic purposes can be used to distinguish them from earthquakes and nuclear explosion weapons tests

coherent resonant millennial scale climate oscillations triggered by massive meltwater pulses the role of mean and stochastic freshwater forcing on the generation of millennial scale climate variability in the north atlantic is studied using a low order coupled atmosphere ocean sea ice model it is shown that millennial scale oscillations can be excited stochastically when the north atlantic ocean is fresh enough this finding is used in order to interpret the aftermath of massive iceberg surges heinrich events in the glacial north atlantic which are characterized by an excitation of dansgaard oeschger events based on model results it is hypothesized that heinrich events trigger dansgaard oeschger cycles and that furthermore the occurrence of heinrich events is dependent on the accumulated climatic effect of a series of dansgaard oeschger events this scenario leads to a coupled ocean ice sheet oscillation that shares many similarities with the bond cycle further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic linear and nonlinear deterministic components a schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data

antarctic climate signature in the greenland ice core record a numerical algorithm is applied to the greenland ice sheet project 2 gisp2 dust record from greenland to remove the abrupt changes in dust flux associated with the dansgaard oeschger d o oscillations of the last glacial period the procedure is based on the assumption that the rapid changes in dust are associated with large scale changes in atmospheric transport and implies that d o oscillations in terms of their atmospheric imprint are more symmetric in form than can be inferred from greenland temperature records after removal of the abrupt shifts the residual dejumped dust record is found to match antarctic climate variability with a temporal lag of several hundred years it is argued that such variability may reflect changes in the source region of greenland dust thought to be the deserts of eastern asia other records from this region and more globally also reveal antarctic style variability and suggest that this signal is globally pervasive this provides the potential basis for suggesting a more important role for gradual changes in triggering more abrupt transitions in the climate system

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

southern ocean origin for the resumption of atlantic thermohaline circulation during deglaciation during the two most recent deglaciations the southern hemisphere warmed before greenland at the same time the northern atlantic ocean was exposed to meltwater discharge which is generally assumed to reduce the formation of north atlantic deep water yet during deglaciation the atlantic thermohaline circulation became more vigorous in the transition from a weak glacial to a strong interglacial mode here we use a three dimensional ocean circulation model to investigate the impact of southern ocean warming and the associated sea ice retreat on the atlantic thermohaline circulation we find that a gradual warming in the southern ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation triggered by increased mass transport into the atlantic ocean via the warm indian ocean and cold pacific ocean water route this effect prevails over the influence of meltwater discharge which would oppose a strengthening of the thermohaline circulation a southern ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of southern and northern hemispheric climate change at times of deglaciation in agreement with the available proxy records

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

cassini radar observations of enceladus tethys dione rhea iapetus hyperion and phoebe cassini 2 2 cm radar and radiometric observations of seven of saturn s icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy galilean satellites average radar albedos decrease in the order enceladus tethys hyperion rhea dione iapetus and phoebe this sequence most likely corresponds to increasing contamination of near surface water ice which is intrinsically very transparent at radio wavelengths plausible candidates for contaminants include ammonia silicates metallic oxides and polar organics ranging from nitriles like hcn to complex tholins there is correlation of our targets radar and optical albedos probably due to variations in the concentration of optically dark contaminants in near surface water ice and the resulting variable attenuation of the high order multiple scattering responsible for high radar albedos our highest radar albedos for enceladus and tethys probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex i e mature enough for there to be high order multiple scattering within it at the other extreme phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering iapetus 2 2 cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side whereas 13 cm results reported by black et al black g j campbell d b carter l m ostro s j 2004 science 304 553 show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2 2 cm these iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths and if the leading side s optically dark contaminant is present to depths of at least one to several decimeters as argued by lanzerotti et al lanzerotti l j brown w l marcantonio k j johnson r e 1984 nature 312 139 140 a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of saturn s icy satellites given the hypersensitivity of water ice s absorption length to ammonia concentration an increase in ammonia with depth could allow efficient 2 2 cm scattering from within the top one to several decimeters while attenuating 13 cm echoes which would require a six fold thicker scattering layer if so we would expect each of the icy satellites average radar albedos to be higher at 2 2 cm than at 13 cm as is the case so far with rhea black g campbell d 2004 bull am astron soc 36 1123 as well as iapetus

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

the albedo dichotomy of iapetus measured at uv wavelengths the dramatic hemispheric dichotomy in albedo displayed by saturn s moon iapetus has intrigued astronomers for centuries here we report on far ultraviolet observations of iapetus bright and dark terrains from cassini we compare the reflectance spectra of iapetus s dark terrain hyperion and phoebe and find that both phoebe and hyperion are richer in water ice than iapetus dark terrain spectra of the lowest latitudes of the dark terrain display the diagnostic water ice absorption feature water ice amounts increase within the dark material away from the apex at 90 w longitude the center of the dark leading hemisphere consistent with thermal segregation of water ice the water ice in the darkest warmest low latitude regions is not expected to be stable and may be a sign of ongoing or recent emplacement of the dark material from an exogenic source

an interhemispheric mechanism for glacial abrupt climate change the last glacial period was punctuated by abrupt climate changes that are widely considered to result from millennial scale variability of the atlantic meridional overturning circulation amoc however the origin of these amoc reorganizations remains poorly understood the climatic connection between both hemispheres indicated by proxies suggests that the southern ocean so could regulate this variability through changes in winds and atmospheric co 2 concentration here we investigate this hypothesis using a coupled climate model forced by prescribed co 2 and so wind stress variations we find that the amoc exhibits an oscillatory behavior between weak and strong circulation regimes which is ultimately caused by changes in the meridional density gradient of the atlantic ocean the evolution of the simulated climatic patterns matches the amplitude and timing of the largest events that occurred during the last glacial period and their widespread climatic impacts our results suggest the existence of an internal interhemispheric oscillation mediated by the bipolar seesaw that could promote glacial abrupt climate changes through variations in atmospheric co 2 levels the strength of the so winds and amoc reorganizations and provide an explanation for the pervasive antarctic like climate signal found in proxy records worldwide

spectral characteristics of regional phases recorded at noress abstract the spectral characteristics of pn and lg are studied for regional events recorded at the noress array in norway the emphasis is on the potential value of spectral ratios for identifying events as earthquake chemical explosions and nuclear explosions the events studied include a suite of explosions from the titania mine in southwest norway a suite of events of unknown source type from a site offshore about 90 km from this mine suites of explosions from several mines in the soviet union about 1000 km from noress events from several locations along the 90 azimuth from noress an apparent earthquake in the north atlantic and a nuclear explosion pne at a range of 1560 km the event identification issues addressed are as follows 1 can earthquakes and explosions be identified based on the ratio of high and low frequency energy in their signal spectra 2 do spectral ratios separate mine blasts from earthquakes for all source areas 3 will spectral ratio discriminants be effective for identifying decoupled underground nuclear explosions we conclude that spectral ratios can sometimes separate events an example is separation of the titania mine blasts from the events at the nearby offshore locations although we cannot be sure how much this is due to path differences however in general spectral ratios vary as much within classes of events as they do among different classes the pne in our data set allows the simulation of spectra from a decoupled nuclear explosion by simply applying the frequency dependent decoupling factor to the observed pn spectrum for this event after applying a distance correction the spectrum for this simulated decoupled nuclear explosion is similar to those for the soviet union mine blasts in our data set

discrimination using spectral slopes at frequencies up to 50 hz abstract the spectral characteristics of regional pn waves recorded at the high frequency seismic element hfse at the noress array in southern norway are investigated to determine if spectral slope can be used to discriminate between earthquakes and chemical explosions displacement spectra are calculated for 134 events in southern norway that include 36 known earthquakes and 36 known chemical explosions events range in distance from 230 to 620 km and most have signal above the noise at frequencies over 30 hz the average frequency at the peak signal to noise ratio snr is 15 5 hz spectral slopes are measured on a displacement spectrum calculated with a 2 sec window the slopes range between 1 8 and 5 1 and there is no strong evidence that the slope steepens with distance for this data set according to one theory earthquakes should have steeper slopes than explosions but when this data set is separated into earthquakes and explosions we do not see separation between sources with respect to the slope of the spectrum both source types have events with the same range of spectral slopes and the mean slopes and standard deviations are similar spectra of events at the same locations are often similar however explosions from the blasjo mine in western norway show variability in the spectral slope since the path and distance to hfse are the same for these events the variability must be due to differences in the source mechanism for a set of small earthquakes however the observed variability in spectral slopes appears to be more related to distance and path effects therefore we observe as much variation in spectral slope for one source type as there is between source types the time window over which the spectrum is calculated is varied from 1 to 8 sec to determine if a time window consisting of a pure pn wave was required to discriminate between source types spectral slope usually decreases as the window extends deeper into the coda and the energy content of the signal changes the slopes also become more stable and more reliable as longer windows are used however source discrimination is not possible with any time window

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

phase relationships between antarctic and greenland climate records comparison of climate records from antarctic and greenland ice cores shows that the two regions respond asynchronously during millennial scale climate changes the apparent out of phase relationship between the records has been described as a climate seesaw in which cooling in the northern hemisphere is balanced by warming in the southern hemisphere the same relationship has also been attributed to the initiation of climate change events in the southern hemisphere rather than the north atlantic as is conventionally assumed a simple statistical approach band pass filtering combined with lag correlation tests used to examine the phase relationships in more detail shows that neither an anti phase nor a phase lag relationship adequately describes the observations whereas antarctic and greenland climate records do exhibit approximate anti phase behavior about 50 of the time they are generally in phase during cooling a phase lead of southern hemisphere climate of 1000 1600 years is statistically indistinguishable from a lag of 400 800 years whether for dansgaard oeschger heinrich or longer duration events the seesaw or southern lead appearance of the data arises from the fundamentally different characteristics of the climate time series most importantly the absence of rapid warming events in antarctica comparable to those in greenland to be consistent with the observations climate models will need to capture these characteristics in addition to reproducing the correct phase relationships

the role of southern ocean processes in orbital and millennial co2 variations a synthesis abstract recent progress in the reconstruction of atmospheric co 2 records from antarctic ice cores has allowed for the documentation of natural co 2 variations on orbital time scales over the last up to 800 000 years and for the resolution of millennial co 2 variations during the last glacial cycle in unprecedented detail this has shown that atmospheric co 2 varied within natural bounds of approximately 170 300 ppmv but never reached recent co 2 concentrations caused by anthropogenic co 2 emissions in addition the natural atmospheric co 2 concentrations show an extraordinary correlation with southern ocean climate changes pointing to a significant direct or indirect influence of climatic and environmental changes in the southern ocean region on atmospheric co 2 concentrations here we compile recent ice core and marine sediment records of atmospheric co 2 temperature and environmental changes in the southern ocean region as well as carbon cycle model experiments in order to quantify the effect of potential southern ocean processes on atmospheric co 2 related to these orbital and millennial changes this shows that physical and biological changes in the so are able to explain substantial parts of the glacial interglacial co 2 change but that none of the single processes is able to explain this change by itself in particular changes in the southern ocean related to changes in the surface buoyancy flux which in return is controlled by the waxing and waning of sea ice may favorably explain the high correlation of co 2 and antarctic temperature on orbital and millennial time scales in contrast the changes of the position and strength of the westerly wind field were most likely too small to explain the observed changes in atmospheric co 2 or may even have increased atmospheric co 2 in the glacial also iron fertilization of the marine biota in the southern ocean contributes to a glacial drawdown of co 2 but turns out to be limited by other factors than the total dust input such as bioavailability of iron or macronutrient supply

spectrophotometry of the small satellites of saturn and their relationship to iapetus phoebe and hyperion abstract we have obtained broadband spectrophotometric observations of four of the recently discovered small satellites of saturn gladman et al 2001 nature 412 163 166 the new data enable an understanding of the provenance composition and interrelationships among these satellites and the other satellites of saturn particularly iapetus phoebe and hyperion temporal coverage of one satellite s21 tarvos was sufficient to determine a partial rotational lightcurve our major findings include 1 the satellites are red and similar in color comparable to d type asteroids some kbos iapetus and hyperion 2 none of the satellites including those from the phoebe group has any spectrophotometric relationship to phoebe and 3 s21 tarvos exhibits a rotational lightcurve although the data are not well constrained and more observations are required to fit a definitive period dust created by meteoritic impacts and ejected from these satellites and additional undiscovered ones may be the source of the exogenous material deposited on the low albedo side of iapetus recent work which states that the small irregular satellites of saturn have impacted phoebe at least 6 7 times in the age of the solar system nesvorny et al 2003 astron j 126 398 429 suggests that such collisions may have propelled additional material from both phoebe and the small irregular satellites toward iapetus the accretion of material from outer retrograde satellites may be a process that also occurs on callisto and the uranian satellites

expression of the bipolar see saw in antarctic climate records during the last deglaciation ice core records of climate from greenland and antarctica show asynchronous temperature variations on millennial timescales during the last glacial period 1 the warming during the transition from glacial to interglacial conditions was markedly different between the hemispheres a pattern attributed to the thermal bipolar see saw 2 however a record from the ross sea sector of east antarctica has been suggested to be synchronous with northern hemisphere climate change 3 here we present a temperature record from the talos dome ice core also located in the ross sea sector we compare our record with ice core analyses from greenland based on methane synchronization 4 and find clearly asynchronous temperature changes during the deglaciation we also find distinct differences in antarctic records pointing to differences in the climate evolution of the indo pacific and atlantic sectors of antarctica in the atlantic sector we find that the rate of warming slowed between 16 000 and 14 500 years ago parallel with the deceleration of the rise in atmospheric carbon dioxide concentrations and with a slight cooling over greenland in addition our chronology supports the hypothesis that the cooling of the antarctic cold reversal is synchronous with the bolling allerod warming in the northern hemisphere 14 700 years ago 5 the period from about 8 to 25kyr before present bp includes the climate transition from the last glacial to the holocene as documented from polar ice cores and other climate archives the pattern of climate changes throughout this transition is different between antarctica and the surrounding southern ocean and the northern hemisphere the steady antarctic deglacial warming reaches a first maximum antarctic isotopic maximum aim1 ref 1 followedbyaninterruptiontowardscoolerconditionsduring the antarctic cold reversal acr conversely greenland records show two rapid warming phases at the onset of the dansgaard oeschger 1 do1 event bllingaller dinterstadial b a andthe

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes