http://www.clim-past.net/ Climate of the Past Latest Articlesenhttp://www.clim-past.net/8/227/2012/<b>Northern Hemisphere temperature patterns in the last 12 centuries</b><br /><br />Climate of the Past, 8, 227-249, 2012<br /><br />Author(s): F. C. Ljungqvist, P. J. Krusic, G. Brattström, and H. S. Sundqvist<br /><br />We analyse the spatio-temporal patterns of temperature variability over Northern Hemisphere land areas, on centennial time-scales, for the last 12 centuries using an unprecedentedly large network of temperature-sensitive proxy records. Geographically widespread positive temperature anomalies are observed from the 9th to 11th centuries, similar in extent and magnitude to the 20th century mean. A dominance of widespread negative anomalies is observed from the 16th to 18th centuries. Though we find the amplitude and spatial extent of the 20th century warming is within the range of natural variability over the last 12 centuries, we also find that the rate of warming from the 19th to the 20th century is unprecedented in the context of the last 1200 yr. The positive Northern Hemisphere temperature change from the 19th to the 20th century is clearly the largest between any two consecutive centuries in the past 12 centuries. These results remain robust even after removing a significant number of proxies in various tests of robustness showing that the choice of proxies has no particular influence on the overall conclusions of this study.Fri, 03 Feb 2012 00:00:00 +0100http://www.clim-past.net/8/215/2012/<b>Warm Middle Jurassic–Early Cretaceous high-latitude sea-surface temperatures from the Southern Ocean</b><br /><br />Climate of the Past, 8, 215-226, 2012<br /><br />Author(s): H. C. Jenkyns, L. Schouten-Huibers, S. Schouten, and J. S. Sinninghe Damsté<br /><br />Although a division of the Phanerozoic climatic modes of the Earth into "greenhouse" and "icehouse" phases is widely accepted, whether or not polar ice developed during the relatively warm Jurassic and Cretaceous Periods is still under debate. In particular, there is a range of isotopic and biotic evidence that favours the concept of discrete "cold snaps", marked particularly by migration of certain biota towards lower latitudes. Extension of the use of the palaeotemperature proxy TEX₈₆ back to the Middle Jurassic indicates that relatively warm sea-surface conditions (26–30 °C) existed from this interval (&sim;160 Ma) to the Early Cretaceous (&sim;115 Ma) in the Southern Ocean, with a general warming trend through the Late Jurassic followed by a general cooling trend through the Early Cretaceous. The lowest sea-surface temperatures are recorded from around the Callovian–Oxfordian boundary, an interval identified in Europe as relatively cool, but do not fall below 25 °C. The early Aptian Oceanic Anoxic Event, identified on the basis of published biostratigraphy, total organic carbon and carbon-isotope stratigraphy, records an interval with the lowest, albeit fluctuating Early Cretaceous palaeotemperatures (&sim;26 °C), recalling similar phenomena recorded from Europe and the tropical Pacific Ocean. Extant belemnite &delta;¹⁸O data, assuming an isotopic composition of waters inhabited by these fossils of −1&permil; SMOW, give palaeotemperatures throughout the Upper Jurassic–Lower Cretaceous interval that are consistently lower by &sim;14 °C than does TEX₈₆ and the molluscs likely record conditions below the thermocline. The long-term, warm climatic conditions indicated by the TEX₈₆ data would only be compatible with the existence of continental ice if appreciable areas of high altitude existed on Antarctica, and/or in other polar regions, during the Mesozoic Era.Thu, 02 Feb 2012 00:00:00 +0100http://www.clim-past.net/8/205/2012/<b>Reconstruction of southeast Tibetan Plateau summer climate using tree ring &delta;¹⁸O: moisture variability over the past two centuries</b><br /><br />Climate of the Past, 8, 205-213, 2012<br /><br />Author(s): C. Shi, V. Daux, Q.-B. Zhang, C. Risi, S.-G. Hou, M. Stievenard, M. Pierre, Z. Li, and V. Masson-Delmotte<br /><br />A tree-ring &delta;¹⁸O chronology of Linzhi spruce, spanning from AD 1781 to 2005, was developed in Bomi, Southeast Tibetan Plateau (TP). During the period with instrumental data (AD 1961–2005), this record is strongly correlated with regional CRU (Climate Research Unit) summer cloud data, which is supported by a precipitation &delta;¹⁸O simulation conducted with the isotope-enabled atmospheric general circulation model LMDZiso. A reconstruction of a regional summer cloud index, based upon the empirical relationship between cloud and diurnal temperature range, was therefore achieved. This index reflects regional moisture variability in the past 225 yr. The climate appears drier and more stable in the 20th century than previously. The drying trend in late 19th century of our reconstruction is consistent with a decrease in the TP glacier accumulation recorded in ice cores. An exceptional dry decade is documented in the 1810s, possibly related to the impact of repeated volcanic eruptions on monsoon flow.Thu, 02 Feb 2012 00:00:00 +0100http://www.clim-past.net/8/191/2012/<b>Towards orbital dating of the EPICA Dome C ice core using &delta;O₂/N₂</b><br /><br />Climate of the Past, 8, 191-203, 2012<br /><br />Author(s): A. Landais, G. Dreyfus, E. Capron, K. Pol, M. F. Loutre, D. Raynaud, V. Y. Lipenkov, L. Arnaud, V. Masson-Delmotte, D. Paillard, J. Jouzel, and M. Leuenberger<br /><br />Based on a composite of several measurement series performed on ice samples stored at −25 °C or −50 °C, we present and discuss the first δO₂/N₂ record of trapped air from the EPICA Dome C (EDC) ice core covering the period between 300 and 800 ka (thousands of years before present). The samples stored at −25 °C show clear gas loss affecting the precision and mean level of the δO₂/N₂ record. Two different gas loss corrections are proposed to account for this effect, without altering the spectral properties of the original datasets. Although processes at play remain to be fully understood, previous studies have proposed a link between surface insolation, ice grain properties at close-off, and δO₂/N₂ in air bubbles, from which orbitally tuned chronologies of the Vostok and Dome Fuji ice core records have been derived over the last four climatic cycles. Here, we show that limitations caused by data quality and resolution, data filtering, and uncertainties in the orbital tuning target limit the precision of this tuning method for EDC. Moreover, our extended record includes two periods of low eccentricity. During these intervals (around 400 ka and 750 ka), the matching between δO₂/N₂ and the different insolation curves is ambiguous because some local insolation maxima cannot be identified in the δO₂/N₂ record (and vice versa). Recognizing these limitations, we restrict the use of our δO₂/N₂ record to show that the EDC3 age scale is generally correct within its published uncertainty (6 kyr) over the 300–800 ka period.Tue, 31 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/171/2012/<b>Bridging the Faraoni and Selli oceanic anoxic events: late Hauterivian to early Aptian dysaerobic to anaerobic phases in the Tethys</b><br /><br />Climate of the Past, 8, 171-189, 2012<br /><br />Author(s): K. B. Föllmi, M. Bôle, N. Jammet, P. Froidevaux, A. Godet, S. Bodin, T. Adatte, V. Matera, D. Fleitmann, and J. E. Spangenberg<br /><br />A detailed geochemical analysis was performed on the upper part of the Maiolica Formation in the Breggia (southern Switzerland) and Capriolo sections (northern Italy). The analysed sediments consist of well-bedded, partly siliceous, pelagic carbonate, which lodges numerous thin, dark and organic-rich layers. Stable-isotope, phosphorus, organic-carbon and a suite of redox-sensitive trace-element contents (RSTE: Mo, U, Co, V and As) were measured. The RSTE pattern and C_org:P_tot ratios indicate that most organic-rich layers were deposited under dysaerobic rather than anaerobic conditions and that latter conditions were likely restricted to short intervals in the latest Hauterivian, the early Barremian and the pre-Selli early Aptian. <br><br> Correlations are both possible with organic-rich intervals in central Italy (the Gorgo a Cerbara section) and the Boreal Lower Saxony Basin, as well as with the facies and drowning pattern in the Helvetic segment of the northern Tethyan carbonate platform. Our data and correlations suggest that the latest Hauterivian witnessed the progressive installation of dysaerobic conditions in the Tethys, which went along with the onset in sediment condensation, phosphogenesis and platform drowning on the northern Tethyan margin, and which culminated in the Faraoni anoxic episode. This episode is followed by further episodes of dysaerobic conditions in the Tethys and the Lower Saxony Basin, which became more frequent and progressively stronger in the late early Barremian. Platform drowning persisted and did not halt before the latest early Barremian. The late Barremian witnessed diminishing frequencies and intensities in dysaerobic conditions, which went along with the progressive installation of the Urgonian carbonate platform. Near the Barremian-Aptian boundary, the increasing density in dysaerobic episodes in the Tethyan and Lower Saxony Basins is paralleled by a change towards heterozoan carbonate production on the northern Tethyan shelf. The following return to more oxygenated conditions is correlated with the second phase of Urgonian platform growth and the period immediately preceding and corresponding to the Selli anoxic episode is characterised by renewed platform drowning and the change to heterozoan carbonate production. Changes towards more humid climate conditions were the likely cause for the repetitive installation of dys- to anaerobic conditions in the Tethyan and Boreal basins and the accompanying changes in the evolution of the carbonate platform towards heterozoan carbonate-producing ecosystems and platform drowning.Mon, 30 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/149/2012/<b>Impact of oceanic processes on the carbon cycle during the last termination</b><br /><br />Climate of the Past, 8, 149-170, 2012<br /><br />Author(s): N. Bouttes, D. Paillard, D. M. Roche, C. Waelbroeck, M. Kageyama, A. Lourantou, E. Michel, and L. Bopp<br /><br />During the last termination (from ~18 000 years ago to ~9000 years ago), the climate significantly warmed and the ice sheets melted. Simultaneously, atmospheric CO₂ increased from ~190 ppm to ~260 ppm. Although this CO₂ rise plays an important role in the deglacial warming, the reasons for its evolution are difficult to explain. Only box models have been used to run transient simulations of this carbon cycle transition, but by forcing the model with data constrained scenarios of the evolution of temperature, sea level, sea ice, NADW formation, Southern Ocean vertical mixing and biological carbon pump. More complex models (including GCMs) have investigated some of these mechanisms but they have only been used to try and explain LGM versus present day steady-state climates. <br><br> In this study we use a coupled climate-carbon model of intermediate complexity to explore the role of three oceanic processes in transient simulations: the sinking of brines, stratification-dependent diffusion and iron fertilization. Carbonate compensation is accounted for in these simulations. We show that neither iron fertilization nor the sinking of brines alone can account for the evolution of CO₂, and that only the combination of the sinking of brines and interactive diffusion can simultaneously simulate the increase in deep Southern Ocean &delta;¹³C. The scenario that agrees best with the data takes into account all mechanisms and favours a rapid cessation of the sinking of brines around 18 000 years ago, when the Antarctic ice sheet extent was at its maximum. In this scenario, we make the hypothesis that sea ice formation was then shifted to the open ocean where the salty water is quickly mixed with fresher water, which prevents deep sinking of salty water and therefore breaks down the deep stratification and releases carbon from the abyss. Based on this scenario, it is possible to simulate both the amplitude and timing of the long-term CO₂ increase during the last termination in agreement with ice core data. The atmospheric &delta;¹³C appears to be highly sensitive to changes in the terrestrial biosphere, underlining the need to better constrain the vegetation evolution during the termination.Fri, 20 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/135/2012/<b>Change in dust variability in the Atlantic sector of Antarctica at the end of the last deglaciation</b><br /><br />Climate of the Past, 8, 135-147, 2012<br /><br />Author(s): A. Wegner, P. Gabrielli, D. Wilhelms-Dick, U. Ruth, M. Kriews, P. De Deckker, C. Barbante, G. Cozzi, B. Delmonte, and H. Fischer<br /><br />We present a Rare Earth Elements (REE) record determined on the EPICA ice core drilled at Dronning Maud Land (EDML) in the Atlantic sector of the East Antarctic Plateau. The record covers the transition from the last glacial stage (LGS) to the early Holocene (26 600–7500 yr BP) at decadal to centennial resolution. Additionally, samples from potential source areas (PSAs) for Antarctic dust were analyzed for their REE characteristics. The dust provenance is discussed by comparing the REE fingerprints in the ice core and the PSA samples. We find a shift in variability in REE composition at ~15 000 yr BP in the ice core samples. Before 15 000 yr BP, the dust composition is very uniform and its provenance was most certainly dominated by a South American source. After 15 000 yr BP, multiple sources such as Australia and New Zealand become relatively more important, although South America remains the major dust source. A similar change in the dust characteristics was observed in the EPICA Dome C ice core at around ~15 000 yr BP, accompanied by a shift in the REE composition, thus suggesting a change of atmospheric circulation in the Southern Hemisphere.Thu, 19 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/117/2012/<b>Climate variability in Andalusia (southern Spain) during the period 1701&ndash;1850 based on documentary sources: evaluation and comparison with climate model simulations</b><br /><br />Climate of the Past, 8, 117-133, 2012<br /><br />Author(s): F. S. Rodrigo, J. J. Gómez-Navarro, and J. P. Montávez Gómez<br /><br />In this work, a reconstruction of climatic conditions in Andalusia (southern Iberian Peninsula) during the period 1701–1850, as well as an evaluation of its associated uncertainties, is presented. This period is interesting because it is characterized by a minimum in solar irradiance (Dalton Minimum, around 1800), as well as intense volcanic activity (for instance, the eruption of Tambora in 1815), at a time when any increase in atmospheric CO₂ concentrations was of minor importance. The reconstruction is based on the analysis of a wide variety of documentary data. The reconstruction methodology is based on counting the number of extreme events in the past, and inferring mean value and standard deviation using the assumption of normal distribution for the seasonal means of climate variables. This reconstruction methodology is tested within the pseudoreality of a high-resolution paleoclimate simulation performed with the regional climate model MM5 coupled to the global model ECHO-G. The results show that the reconstructions are influenced by the reference period chosen and the threshold values used to define extreme values. This creates uncertainties which are assessed within the context of climate simulation. An ensemble of reconstructions was obtained using two different reference periods (1885–1915 and 1960–1990) and two pairs of percentiles as threshold values (10–90 and 25–75). The results correspond to winter temperature, and winter, spring and autumn rainfall, and they are compared with simulations of the climate model for the considered period. The mean value of winter temperature for the period 1781–1850 was 10.6 &plusmn; 0.1 &deg;C (11.0 &deg;C for the reference period 1960–1990). The mean value of winter rainfall for the period 1701–1850 was 267 &plusmn; 18 mm (224 mm for 1960–1990). The mean values of spring and autumn rainfall were 164 &plusmn; 11 and 194 &plusmn; 16 mm (129 and 162 mm for 1960–1990, respectively). Comparison of the distribution functions corresponding to 1790–1820 and 1960–1990 indicates that during the Dalton Minimum the frequency of dry and warm (wet and cold) winters was lower (higher) than during the reference period: temperatures were up to 0.5 &deg;C lower than the 1960–1990 value, and rainfall was 4% higher.Tue, 10 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/89/2012/<b>Benchmarking homogenization algorithms for monthly data</b><br /><br />Climate of the Past, 8, 89-115, 2012<br /><br />Author(s): V. K. C. Venema, O. Mestre, E. Aguilar, I. Auer, J. A. Guijarro, P. Domonkos, G. Vertacnik, T. Szentimrey, P. Stepanek, P. Zahradnicek, J. Viarre, G. Müller-Westermeier, M. Lakatos, C. N. Williams, M. J. Menne, R. Lindau, D. Rasol, E. Rustemeier, K. Kolokythas, T. Marinova, L. Andresen, F. Acquaotta, S. Fratianni, S. Cheval, M. Klancar, M. Brunetti, C. Gruber, M. Prohom Duran, T. Likso, P. Esteban, and T. Brandsma<br /><br />The COST (European Cooperation in Science and Technology) Action ES0601: advances in homogenization methods of climate series: an integrated approach (HOME) has executed a blind intercomparison and validation study for monthly homogenization algorithms. Time series of monthly temperature and precipitation were evaluated because of their importance for climate studies and because they represent two important types of statistics (additive and multiplicative). The algorithms were validated against a realistic benchmark dataset. The benchmark contains real inhomogeneous data as well as simulated data with inserted inhomogeneities. Random independent break-type inhomogeneities with normally distributed breakpoint sizes were added to the simulated datasets. To approximate real world conditions, breaks were introduced that occur simultaneously in multiple station series within a simulated network of station data. The simulated time series also contained outliers, missing data periods and local station trends. Further, a stochastic nonlinear global (network-wide) trend was added. <br><br> Participants provided 25 separate homogenized contributions as part of the blind study. After the deadline at which details of the imposed inhomogeneities were revealed, 22 additional solutions were submitted. These homogenized datasets were assessed by a number of performance metrics including (i) the centered root mean square error relative to the true homogeneous value at various averaging scales, (ii) the error in linear trend estimates and (iii) traditional contingency skill scores. The metrics were computed both using the individual station series as well as the network average regional series. The performance of the contributions depends significantly on the error metric considered. Contingency scores by themselves are not very informative. Although relative homogenization algorithms typically improve the homogeneity of temperature data, only the best ones improve precipitation data. Training the users on homogenization software was found to be very important. Moreover, state-of-the-art relative homogenization algorithms developed to work with an inhomogeneous reference are shown to perform best. The study showed that automatic algorithms can perform as well as manual ones.Tue, 10 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/79/2012/<b>Technical note: Late Pliocene age control and composite depths at ODP Site 982, revisited</b><br /><br />Climate of the Past, 8, 79-87, 2012<br /><br />Author(s): N. Khélifi, M. Sarnthein, and B. D. A. Naafs<br /><br />Ocean Drilling Program (ODP) Site 982 provided a key sediment section at Rockall Plateau for reconstructing northeast Atlantic paleoceanography and monitoring benthic &delta;¹⁸O stratigraphy over the late Pliocene to Quaternary onset of major Northern Hemisphere glaciation. A renewed hole-specific inspection of magnetostratigraphic reversals and the addition of epibenthic &delta;¹⁸O records for short Pliocene sections in holes 982A, B, and C, crossing core breaks in the &delta;¹⁸O record published for Hole 982B, now imply a major revision of composite core depths. After tuning to the orbitally tuned reference record LR04, the new composite &delta;¹⁸O record results in a hiatus, where the Kaena magnetic subchron might have been lost, and in a significant age reduction for all proxy records by 130 to 20 ky over the time span 3.2–2.7 million years ago (Ma). Our study demonstrates the general significance of reliable composite-depth scales and &delta;¹⁸O stratigraphies in ODP sediment records for generating ocean-wide correlations in paleoceanography. The new concept of age control makes the late Pliocene trends in SST (sea surface temperature) and atmospheric <i>p</i>CO₂ at Site 982 more consistent with various paleoclimate trends published from elsewhere in the North Atlantic.Fri, 06 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/59/2012/<b>Pollen, vegetation change and climate at Lake Barombi Mbo (Cameroon) during the last ca. 33 000 cal yr BP: a numerical approach</b><br /><br />Climate of the Past, 8, 59-78, 2012<br /><br />Author(s): J. Lebamba, A. Vincens, and J. Maley<br /><br />This paper presents quantitative reconstructions of vegetation and climate along the pollen sequence of Lake Barombi Mbo, southwestern Cameroon (4&deg;39'45.75" N, 9&deg;23'51.63" E, 303 m a.s.l.) during the last 33 000 cal yr BP, improving previous empirical interpretations. The biomisation method was applied to reconstruct potential biomes and forest successional stages. Mean annual precipitation, mean annual potential evapotranspiration and an index of moisture availability were reconstructed using modern analogues and an artificial neural network technique. The results show a dense forested environment around Lake Barombi Mbo of mixed evergreen/semi-deciduous type during the most humid phases (highest precipitation and lowest evapotranspiration), but with a more pronounced semi-deciduous type from ca. 6500 cal yr BP to the present day, related to increased seasonality. This forest displays a mature character until ca. 2800 cal yr BP, then becomes of secondary type during the last millennium, probably due to increased human activity. Two episodes of forest fragmentation are shown, which are synchronous with the lowest reconstructed precipitation and highest potential evapotranspiration values. The first of these occurs during the LGM, and the second one from ca. 3000 to ca. 1200 cal yr BP, mainly linked to high precipitation seasonality. Savanna were, however, never extensive within the Barombi Mbo basin, existing instead inside the forest in form of savanna patches. The climate reconstructions at Lake Barombi Mbo suggest that the artificial neural networks technique would be more reliable in this region, although the annual precipitation values are likely under-estimated through the whole sequence.Thu, 05 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/37/2012/<b>Tropical climate and vegetation changes during Heinrich Event 1: a model-data comparison</b><br /><br />Climate of the Past, 8, 37-57, 2012<br /><br />Author(s): D. Handiani, A. Paul, and L. Dupont<br /><br />Abrupt climate changes from 18 to 15 thousand years before present (kyr BP) associated with Heinrich Event 1 (HE1) had a strong impact on vegetation patterns not only at high latitudes of the Northern Hemisphere, but also in the tropical regions around the Atlantic Ocean. To gain a better understanding of the linkage between high and low latitudes, we used the University of Victoria (UVic) Earth System-Climate Model (ESCM) with dynamical vegetation and land surface components to simulate four scenarios of climate-vegetation interaction: the pre-industrial era, the Last Glacial Maximum (LGM), and a Heinrich-like event with two different climate backgrounds (interglacial and glacial). We calculated mega-biomes from the plant-functional types (PFTs) generated by the model to allow for a direct comparison between model results and palynological vegetation reconstructions. <br><br> Our calculated mega-biomes for the pre-industrial period and the LGM corresponded well with biome reconstructions of the modern and LGM time slices, respectively, except that our pre-industrial simulation predicted the dominance of grassland in southern Europe and our LGM simulation resulted in more forest cover in tropical and sub-tropical South America. <br><br> The HE1-like simulation with a glacial climate background produced sea-surface temperature patterns and enhanced inter-hemispheric thermal gradients in accordance with the "bipolar seesaw" hypothesis. We found that the cooling of the Northern Hemisphere caused a southward shift of those PFTs that are indicative of an increased desertification and a retreat of broadleaf forests in West Africa and northern South America. The mega-biomes from our HE1 simulation agreed well with paleovegetation data from tropical Africa and northern South America. Thus, according to our model-data comparison, the reconstructed vegetation changes for the tropical regions around the Atlantic Ocean were physically consistent with the remote effects of a Heinrich event under a glacial climate background.Wed, 04 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/25/2012/<b>Internal and external variability in regional simulations of the Iberian Peninsula climate over the last millennium</b><br /><br />Climate of the Past, 8, 25-36, 2012<br /><br />Author(s): J. J. Gómez-Navarro, J. P. Montávez, P. Jiménez-Guerrero, S. Jerez, R. Lorente-Plazas, J. F. González-Rouco, and E. Zorita<br /><br />In this study we analyse the role of internal variability in regional climate simulations through a comparison of two regional paleoclimate simulations for the last millennium. They share the same external forcings and model configuration, differing only in the initial condition used to run the driving global model simulation. A comparison of these simulations allows us to study the role of internal variability in climate models at regional scales, and how it affects the long-term evolution of climate variables such as temperature and precipitation. The results indicate that, although temperature is homogeneously sensitive to the effect of external forcings, the evolution of precipitation is more strongly governed by random unpredictable internal dynamics. There are, however, some areas where the role of internal variability is lower than expected, allowing precipitation to respond to the external forcings. In this respect, we explore the underlying physical mechanisms responsible for it. This study identifies areas, depending on the season, in which a direct comparison between model simulations of precipitation and climate reconstructions would be meaningful, but also other areas where good agreement between them should not be expected even if both are perfect.Wed, 04 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/17/2012/<b>Ventilation changes in the western North Pacific since the last glacial period</b><br /><br />Climate of the Past, 8, 17-24, 2012<br /><br />Author(s): Y. Okazaki, T. Sagawa, H. Asahi, K. Horikawa, and J. Onodera<br /><br />We reconstructed the ventilation record of deep water at 2100 m depth in the mid-latitude western North Pacific over the past 25 kyr from radiocarbon measurements of coexisting planktic and benthic foraminiferal shells in sediment with a high sedimentation rate. The ¹⁴C data on fragile and robust planktic foraminiferal shells were concordant with each other, ensuring high quality of the reconstructed ventilation record. The radiocarbon activity changes were consistent with the atmospheric record, suggesting that no massive mixing of old carbon from the abyssal reservoir occurred throughout the glacial to deglacial periods.Tue, 03 Jan 2012 00:00:00 +0100http://www.clim-past.net/8/1/2012/<b>Deglaciation records of ¹⁷O-excess in East Antarctica: reliable reconstruction of oceanic normalized relative humidity from coastal sites</b><br /><br />Climate of the Past, 8, 1-16, 2012<br /><br />Author(s): R. Winkler, A. Landais, H. Sodemann, L. Dümbgen, F. Prié, V. Masson-Delmotte, B. Stenni, and J. Jouzel<br /><br />We measured &delta;¹⁷O and &delta;¹⁸O in two Antarctic ice cores at EPICA Dome C (EDC) and TALDICE (TD), respectively, and computed ¹⁷O-excess with respect to VSMOW. The comparison of our ¹⁷O-excess data with the previous record obtained at Vostok (Landais et al., 2008a) revealed differences up to 35 ppm in ¹⁷O-excess mean level and evolution for the three sites. Our data show that the large increase depicted at Vostok (20 ppm) during the last deglaciation is a regional and not a general pattern in the temporal distribution of ¹⁷O-excess in East Antarctica. The EDC data display an increase of 12 ppm, whereas the TD data show no significant variation from the Last Glacial Maximum (LGM) to the Early Holocene (EH). A Lagrangian moisture source diagnostic revealed very different source regions for Vostok and EDC compared to TD. These findings combined with the results of a sensitivity analysis, using a Rayleigh-type isotopic model, suggest that normalized relative humidity (RH_n) at the oceanic source region (OSR) is a determining factor for the spatial differences of ¹⁷O-excess in East Antarctica. However, ¹⁷O-excess in remote sites of continental Antarctica (e.g. Vostok) may be highly sensitive to local effects. Hence, we consider ¹⁷O-excess in coastal East Antarctic ice cores (TD) to be more reliable as a proxy for RH_n at the OSR.Tue, 03 Jan 2012 00:00:00 +0100http://www.clim-past.net/7/1459/2011/<b>Reconstruction of a continuous high-resolution CO₂ record over the past 20 million years</b><br /><br />Climate of the Past, 7, 1459-1469, 2011<br /><br />Author(s): R. S. W. van de Wal, B. de Boer, L. J. Lourens, P. Köhler, and R. Bintanja<br /><br />The gradual cooling of the climate during the Cenozoic has generally been attributed to a decrease in CO₂ concentration in the atmosphere. The lack of transient climate models and, in particular, the lack of high-resolution proxy records of CO₂, beyond the ice-core record prohibit, however, a full understanding of, for example, the inception of the Northern Hemisphere glaciation and mid-Pleistocene transition. Here we elaborate on an inverse modelling technique to reconstruct a continuous CO₂ series over the past 20 million year (Myr), by decomposing the global deep-sea benthic &delta;¹⁸O record into a mutually consistent temperature and sea level record, using a set of 1-D models of the major Northern and Southern Hemisphere ice sheets. We subsequently compared the modelled temperature record with ice core and proxy-derived CO₂ data to create a continuous CO₂ reconstruction over the past 20 Myr. Results show a gradual decline from 450 ppmv around 15 Myr ago to 225 ppmv for mean conditions of the glacial-interglacial cycles of the last 1 Myr, coinciding with a gradual cooling of the global surface temperature of 10 K. Between 13 to 3 Myr ago there is no long-term sea level variation caused by ice-volume changes. We find no evidence of change in the long-term relation between temperature change and CO₂, other than the effect following the saturation of the absorption bands for CO₂. The reconstructed CO₂ record shows that the Northern Hemisphere glaciation starts once the long-term average CO₂ concentration drops below 265 ppmv after a period of strong decrease in CO₂. Finally, only a small long-term decline of 23 ppmv is found during the mid-Pleistocene transition, constraining theories on this major transition in the climate system. The approach is not accurate enough to revise current ideas about climate sensitivity.Wed, 21 Dec 2011 00:00:00 +0100http://www.clim-past.net/7/1457/2011/<b>Corrigendum to "Impact of CO₂ and climate on the Last Glacial Maximum vegetation: results from the ORCHIDEE/IPSL models" published in Clim. Past, 7, 557&ndash;577, 2011</b><br /><br />Climate of the Past, 7, 1457-1457, 2011<br /><br />Author(s): M.-N. Woillez, M. Kageyama, G. Krinner, N. de Noblet-Ducoudré, N. Viovy, and M. Mancip<br /><br />No abstract available.Wed, 21 Dec 2011 00:00:00 +0100http://www.clim-past.net/7/1439/2011/<b>Volcanic impact on the Atlantic Ocean over the last millennium</b><br /><br />Climate of the Past, 7, 1439-1455, 2011<br /><br />Author(s): J. Mignot, M. Khodri, C. Frankignoul, and J. Servonnat<br /><br />The oceanic response to volcanic eruptions over the last 1000 years is investigated with a focus on the North Atlantic Ocean, using a fully coupled AOGCM forced by a realistic time series of volcanic eruptions, total solar irradiance (TSI) and atmospheric greenhouse gases concentration. The model simulates little response to TSI variations but a strong and long-lasting thermal and dynamical oceanic adjustment to volcanic forcing, which is shown to be a function of the time period of the volcanic eruptions. The thermal response consists of a fast tropical cooling due to the radiative forcing by the volcanic eruptions, followed by a penetration of this cooling in the subtropical ocean interior one to five years after the eruption, and propagation of the anomalies toward the high latitudes. The oceanic circulation first adjusts rapidly to low latitude anomalous wind stress induced by the strong cooling. The Atlantic Meridional Overturning Circulation (AMOC) shows a significant intensification 5 to 10 years after the eruptions of the period post-1400 A.D., in response to anomalous atmospheric momentum forcing, and a slight weakening in the following decade. In response to the stronger eruptions occurring between 1100 and 1300, the AMOC shows no intensification and a stronger reduction after 10 years. This study thus stresses the diversity of AMOC response to volcanic eruptions in climate models and discusses possible explanations.Wed, 21 Dec 2011 00:00:00 +0100http://www.clim-past.net/7/1427/2011/<b>Annual layering in the NGRIP ice core during the Eemian</b><br /><br />Climate of the Past, 7, 1427-1437, 2011<br /><br />Author(s): A. Svensson, M. Bigler, E. Kettner, D. Dahl-Jensen, S. Johnsen, S. Kipfstuhl, M. Nielsen, and J. P. Steffensen<br /><br />The Greenland NGRIP ice core continuously covers the period from present day back to 123 ka before present, which includes several thousand years of ice from the previous interglacial period, MIS 5e or the Eemian. In the glacial part of the core, annual layers can be identified from impurity records and visual stratigraphy, and stratigraphic layer counting has been performed back to 60 ka. In the deepest part of the core, however, the ice is close to the pressure melting point, the visual stratigraphy is dominated by crystal boundaries, and annual layering is not visible to the naked eye. In this study, we apply a newly developed setup for high-resolution ice core impurity analysis to produce continuous records of dust, sodium and ammonium concentrations as well as conductivity of melt water. We analyzed three 2.2 m sections of ice from the Eemian and the glacial inception. In all of the analyzed ice, annual layers can clearly be recognized, most prominently in the dust and conductivity profiles. Part of the samples is, however, contaminated in dust, most likely from drill liquid. It is interesting that the annual layering is preserved despite a very active crystal growth and grain boundary migration in the deep and warm NGRIP ice. Based on annual layer counting of the new records, we determine a mean annual layer thickness close to 11 mm for all three sections, which, to first order, confirms the modeled NGRIP time scale (ss09sea). The counting does, however, suggest a longer duration of the climatically warmest part of the NGRIP record (MIS5e) of up to 1 ka as compared to the model estimate. Our results suggest that stratigraphic layer counting is possible basically throughout the entire NGRIP ice core, provided sufficiently highly-resolved profiles become available.Fri, 16 Dec 2011 00:00:00 +0100http://www.clim-past.net/7/1415/2011/<b>The role of orbital forcing, carbon dioxide and regolith in 100 kyr glacial cycles</b><br /><br />Climate of the Past, 7, 1415-1425, 2011<br /><br />Author(s): A. Ganopolski and R. Calov<br /><br />The origin of the 100 kyr cyclicity, which dominates ice volume variations and other climate records over the past million years, remains debatable. Here, using a comprehensive Earth system model of intermediate complexity, we demonstrate that both strong 100 kyr periodicity in the ice volume variations and the timing of glacial terminations during past 800 kyr can be successfully simulated as direct, strongly nonlinear responses of the climate-cryosphere system to orbital forcing alone, if the atmospheric CO₂ concentration stays below its typical interglacial value. The existence of long glacial cycles is primarily attributed to the North American ice sheet and requires the presence of a large continental area with exposed rocks. We show that the sharp, 100 kyr peak in the power spectrum of ice volume results from the long glacial cycles being synchronized with the Earth's orbital eccentricity. Although 100 kyr cyclicity can be simulated with a constant CO₂ concentration, temporal variability in the CO₂ concentration plays an important role in the amplification of the 100 kyr cycles.Wed, 14 Dec 2011 00:00:00 +0100