http://www.clim-past.net/ Climate of the Past Latest Articlesenhttp://www.clim-past.net/9/1141/2013/<b>An assessment of particle filtering methods and nudging for climate state reconstructions</b><br /><br />Climate of the Past, 9, 1141-1152, 2013<br /><br />Author(s): S. Dubinkina and H. Goosse<br /><br />Using the climate model of intermediate complexity LOVECLIM in an idealised framework, we assess three data-assimilation methods for reconstructing the climate state. The methods are a nudging, a particle filter with sequential importance resampling, and a nudging proposal particle filter and the test case corresponds to the climate of the high latitudes of the Southern Hemisphere during the past 150 yr. The data-assimilation methods constrain the model by pseudo-observations of surface air temperature anomalies obtained from the same model, but different initial conditions. All three data-assimilation methods provide with good estimations of surface air temperature and of sea ice concentration, with the nudging proposal particle filter obtaining the highest correlations with the pseudo-observations. When reconstructing variables that are not directly linked to the pseudo-observations such as atmospheric circulation and sea surface salinity, the particle filters have equivalent performance and their correlations are smaller than for surface air temperature reconstructions but still satisfactory for many applications. The nudging, on the contrary, obtains sea surface salinity patterns that are opposite to the pseudo-observations, which is due to a spurious impact of the nudging on vertical exchanges in the ocean.Fri, 17 May 2013 00:00:00 +0200http://www.clim-past.net/9/1111/2013/<b>Historical and idealized climate model experiments: an intercomparison of Earth system models of intermediate complexity</b><br /><br />Climate of the Past, 9, 1111-1140, 2013<br /><br />Author(s): M. Eby, A. J. Weaver, K. Alexander, K. Zickfeld, A. Abe-Ouchi, A. A. Cimatoribus, E. Crespin, S. S. Drijfhout, N. R. Edwards, A. V. Eliseev, G. Feulner, T. Fichefet, C. E. Forest, H. Goosse, P. B. Holden, F. Joos, M. Kawamiya, D. Kicklighter, H. Kienert, K. Matsumoto, I. I. Mokhov, E. Monier, S. M. Olsen, J. O. P. Pedersen, M. Perrette, G. Philippon-Berthier, A. Ridgwell, A. Schlosser, T. Schneider von Deimling, G. Shaffer, R. S. Smith, R. Spahni, A. P. Sokolov, M. Steinacher, K. Tachiiri, K. Tokos, M. Yoshimori, N. Zeng, and F. Zhao<br /><br />Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO₂, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO₂ and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO₂ experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO₂ forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO₂ between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO₂, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions.Thu, 16 May 2013 00:00:00 +0200http://www.clim-past.net/9/1089/2013/<b>Climate of the last millennium: ensemble consistency of simulations and reconstructions</b><br /><br />Climate of the Past, 9, 1089-1110, 2013<br /><br />Author(s): O. Bothe, J. H. Jungclaus, D. Zanchettin, and E. Zorita<br /><br />Are simulations and reconstructions of past climate and its variability consistent with each other? We assess the consistency of simulations and reconstructions for the climate of the last millennium under the paradigm of a statistically indistinguishable ensemble. In this type of analysis, the null hypothesis is that reconstructions and simulations are statistically indistinguishable and, therefore, are exchangeable with each other. Ensemble consistency is assessed for Northern Hemisphere mean temperature, Central European mean temperature and for global temperature fields. Reconstructions available for these regions serve as verification data for a set of simulations of the climate of the last millennium performed at the Max Planck Institute for Meteorology. <br><br> Consistency is generally limited to some sub-domains and some sub-periods. Only the ensemble simulated and reconstructed annual Central European mean temperatures for the second half of the last millennium demonstrates unambiguous consistency. Furthermore, we cannot exclude consistency of an ensemble of reconstructions of Northern Hemisphere temperature with the simulation ensemble mean. <br><br> If we treat simulations and reconstructions as equitable hypotheses about past climate variability, the found general lack of their consistency weakens our confidence in inferences about past climate evolutions on the considered spatial and temporal scales. That is, our available estimates of past climate evolutions are on an equal footing but, as shown here, inconsistent with each other.Thu, 16 May 2013 00:00:00 +0200http://www.clim-past.net/9/1065/2013/<b>Tracking atmospheric and riverine terrigenous supplies variability during the last glacial and the Holocene in central Mediterranean</b><br /><br />Climate of the Past, 9, 1065-1087, 2013<br /><br />Author(s): V. Bout-Roumazeilles, N. Combourieu-Nebout, S. Desprat, G. Siani, J.-L. Turon, and L. Essallami<br /><br />A multiproxy study – coupling mineralogical, grain size and geochemical approaches – was used to tentatively retrace eolian and fluvial contributions to sedimentation in the Sicilian–Tunisian Strait since the last glacial. The eolian supply is dominant over the whole interval, excepted during the sapropel S1 when riverine contribution apparently became significant. Saharan contribution increased during the Bølling–Allerød, evidencing the persistence of aridity over North Africa although the northern Mediterranean already experienced moister and warmer conditions. The Younger Dryas is marked by proximal dust inputs, highlighting intense regional eolian activity. A southward migration of dust provenance toward Sahel occurred at the onset of the Holocene, likely resulting from a southward position of the Inter Tropical Convergence Zone that was probably associated with a large-scale atmospheric reorganization. Finally, a peculiar high terrigenous flux associated with drastic modifications of the mineralogical and geochemical sediment signature occurred during the sapropel S1, suggesting the propagation of fine particles derived from major floodings of the Nile River – resulting from enhanced rainfall on northeastern Africa – and their transportation across the Sicilian–Tunisian Strait by intermediate water masses.Wed, 15 May 2013 00:00:00 +0200http://www.clim-past.net/9/1053/2013/<b>Heinrich event 4 characterized by terrestrial proxies in southwestern Europe</b><br /><br />Climate of the Past, 9, 1053-1064, 2013<br /><br />Author(s): J. M. López-García, H.-A. Blain, M. Bennàsar, M. Sanz, and J. Daura<br /><br />Heinrich event 4 (H4) is well documented in the North Atlantic Ocean as a cooling event that occurred between 39 and 40 Ka. Deep-sea cores around the Iberian Peninsula coastline have been analysed to characterize the H4 event, but there are no data on the terrestrial response to this event. Here we present for the first time an analysis of terrestrial proxies for characterizing the H4 event, using the small-vertebrate assemblage (comprising small mammals, squamates and amphibians) from Terrassa Riera dels Canyars, an archaeo-palaeontological deposit located on the seaboard of the northeastern Iberian Peninsula. This assemblage shows that the H4 event is characterized in northeastern Iberia by harsher and drier terrestrial conditions than today. Our results were compared with other proxies such as pollen, charcoal, phytolith, avifauna and large-mammal data available for this site, as well as with the general H4 event fluctuations and with other sites where H4 and the previous and subsequent Heinrich events (H5 and H3) have been detected in the Mediterranean and Atlantic regions of the Iberian Peninsula. We conclude that the terrestrial proxies follow the same patterns as the climatic and environmental conditions detected by the deep-sea cores at the Iberian margins.Tue, 07 May 2013 00:00:00 +0200http://www.clim-past.net/9/1029/2013/<b>Spatial gradients of temperature, accumulation and &delta;¹⁸O-ice in Greenland over a series of Dansgaard&ndash;Oeschger events</b><br /><br />Climate of the Past, 9, 1029-1051, 2013<br /><br />Author(s): M. Guillevic, L. Bazin, A. Landais, P. Kindler, A. Orsi, V. Masson-Delmotte, T. Blunier, S. L. Buchardt, E. Capron, M. Leuenberger, P. Martinerie, F. Prié, and B. M. Vinther<br /><br />Air and water stable isotope measurements from four Greenland deep ice cores (GRIP, GISP2, NGRIP and NEEM) are investigated over a series of Dansgaard–Oeschger events (DO 8, 9 and 10), which are representative of glacial millennial scale variability. Combined with firn modeling, air isotope data allow us to quantify abrupt temperature increases for each drill site (1σ = 0.6 °C for NEEM, GRIP and GISP2, 1.5 °C for NGRIP). Our data show that the magnitude of stadial–interstadial temperature increase is up to 2 °C larger in central and North Greenland than in northwest Greenland: i.e., for DO 8, a magnitude of +8.8 °C is inferred, which is significantly smaller than the +11.1 °C inferred at GISP2. The same spatial pattern is seen for accumulation increases. This pattern is coherent with climate simulations in response to reduced sea-ice extent in the Nordic seas. The temporal water isotope (&delta;¹⁸O)–temperature relationship varies between 0.3 and 0.6 (±0.08) &permil; °C⁻¹ and is systematically larger at NEEM, possibly due to limited changes in precipitation seasonality compared to GISP2, GRIP or NGRIP. The gas age−ice age difference of warming events represented in water and air isotopes can only be modeled when assuming a 26% (NGRIP) to 40% (GRIP) lower accumulation than that derived from a Dansgaard–Johnsen ice flow model.Tue, 07 May 2013 00:00:00 +0200http://www.clim-past.net/9/1015/2013/<b>Large spatial variations in coastal ¹⁴C reservoir age &ndash; a case study from the Baltic Sea</b><br /><br />Climate of the Past, 9, 1015-1028, 2013<br /><br />Author(s): B. C. Lougheed, H. L. Filipsson, and I. Snowball<br /><br />Coastal locations are highly influenced by input from freshwater river runoff, including sources of terrestrial carbon, which can be expected to modify the ¹⁴C reservoir age, or <i>R (t)</i>, associated with marine water. In this Baltic Sea case study, pre-bomb museum collection mollusc shells of known calendar age, from 30 locations across a strategic salinity transect of the Baltic Sea, were analysed for ¹⁴C, &delta;¹³C and &delta;¹⁸O. <i>R (t)</i> was calculated for all 30 locations. Seven locations, of which six are within close proximity of the coast, were found to have relatively higher <i>R (t)</i> values, indicative of hard-water effects. Whenever possible, the <i>Macoma</i> genus of mollusc was selected from the museum collections, in order to exclude species specific reservoir age effects as much as possible. When the <i>Macoma</i> samples are exclusively considered, and samples from hard-water locations excluded, a statistically significant correlation between <i>Macoma</i> <i>R (t)</i> and average salinity is found, indicating a two end-member linear mixing model between ¹⁴C_marine and ¹⁴C_runoff. A map of Baltic Sea <i>Macoma</i> aragonite <i>R (t)</i> for the late 19th and early 20th centuries is produced. Such a map can provide an estimate for contemporary Baltic Sea <i>Macoma</i> <i>R (t)</i>, although one must exercise caution when applying such estimates back in time or to ¹⁴C dates obtained from different sample material. A statistically significant correlation is found between &delta;¹⁸O_aragonite and <i>Macoma</i> <i>R (t)</i>, suggesting that &delta;¹⁸O_aragonite can be used to estimate <i>Macoma</i> palaeo-<i>R (t)</i>, due to the &delta;¹⁸O_aragonite signal being dominated by the salinity gradient of the Baltic Sea. A slightly increased correlation can be expected when &delta;¹⁸O_aragonite is corrected for temperature fractionation effects. The results of this Baltic Sea case study, which show that <i>R (t)</i> is affected by hydrographic conditions and local carbon inputs, have important consequences for other coastal and estuarine locations, where <i>R (t)</i> is also likely to significantly vary on spatial and temporal bases.Tue, 07 May 2013 00:00:00 +0200http://www.clim-past.net/9/1001/2013/<b>Excursions to C₄ vegetation recorded in the Upper Pleistocene loess of Surduk (Northern Serbia): an organic isotope geochemistry study</b><br /><br />Climate of the Past, 9, 1001-1014, 2013<br /><br />Author(s): C. Hatté, C. Gauthier, D.-D. Rousseau, P. Antoine, M. Fuchs, F. Lagroix, S. B. Markovi&cacute;, O. Moine, and A. Sima<br /><br />Loess sequences have been intensively studied to characterize past glacial climates of the 40–50° north and south latitude zones. Combining different approaches of sedimentology, magnetism, geochemistry, geochronology and malacology allows the general pattern of the climate and environment of the last interglacial–glacial cycle in Eurasia and America to be characterized. Previous studies performed in Europe have highlighted the predominance (if not the sole occurrence) of C₃ vegetation. The presence of C₃ plants suggests a regular distribution of precipitation along the year. Therefore, even if the mean annual precipitation remained very low during the most extensive glacial times, free water was available for more than 2 months per year. Contrarily, the &delta;¹³C record of Surduk (Serbia) clearly shows the occurrence and dominance of C₄ plants during at least 4 episodes of the last glacial times at 28.0–26.0 kyr cal BP, 31.4–30.0 kyr cal BP, 53.4–44.5 kyr cal BP and 86.8–66.1 kyr. The C₄ plant development is interpreted as a specific atmospheric circulation pattern that induces short and dry summer conditions. As possible explanation, we propose that during "C₄ episodes", the Mediterranean Sea would have been under the combined influence of the following: (i) a strong meridional circulation unfavorable to water evaporation that reduced the Mediterranean precipitation on the Balkans; and (ii) a high positive North Atlantic Western Russian (NA/WR)-like atmospheric pattern that favored northerlies over westerlies and reduced Atlantic precipitation over the Balkans. This configuration would imply very dry summers that did not allow C₃ plants to grow, thus supporting C₄ development. The intra-"C₄ episode" periods would have occurred under less drastic oceanic and atmospheric patterns that made the influence of westerlies on the Balkans possible.Fri, 03 May 2013 00:00:00 +0200http://www.clim-past.net/9/983/2013/<b>Glacial–interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-&delta;¹⁵N measurements</b><br /><br />Climate of the Past, 9, 983-999, 2013<br /><br />Author(s): E. Capron, A. Landais, D. Buiron, A. Cauquoin, J. Chappellaz, M. Debret, J. Jouzel, M. Leuenberger, P. Martinerie, V. Masson-Delmotte, R. Mulvaney, F. Parrenin, and F. Prié<br /><br />Correct estimation of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice core studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: outputs of a firn densification model, and measurements of &delta;¹⁵N of N₂ in air trapped in ice core, assuming that &delta;¹⁵N is only affected by gravitational fractionation in the firn column. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi-coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available ice core air-&delta;¹⁵N measurements from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rates and temperature conditions. <br><br> Our &delta;¹⁵N profiles reveal a heterogeneous response of the firn structure to glacial–interglacial climatic changes. While firn densification simulations correctly predict TALDICE &delta;¹⁵N variations, they systematically fail to capture the large millennial-scale &delta;¹⁵N variations measured at BI and the &delta;¹⁵N glacial levels measured at JRI and EDML – a mismatch previously reported for central East Antarctic ice cores. <br><br> New constraints of the EDML gas–ice depth offset during the Laschamp event (~41 ka) and the last deglaciation do not favour the hypothesis of a large convective zone within the firn as the explanation of the glacial firn model–&delta;¹⁵N data mismatch for this site. While we could not conduct an in-depth study of the influence of impurities in snow for firnification from the existing datasets, our detailed comparison between the &delta;¹⁵N profiles and firn model simulations under different temperature and accumulation rate scenarios suggests that the role of accumulation rate may have been underestimated in the current description of firnification models.Thu, 02 May 2013 00:00:00 +0200http://www.clim-past.net/9/969/2013/<b>The sensitivity of the Arctic sea ice to orbitally induced insolation changes: a study of the mid-Holocene Paleoclimate Modelling Intercomparison Project 2 and 3 simulations</b><br /><br />Climate of the Past, 9, 969-982, 2013<br /><br />Author(s): M. Berger, J. Brandefelt, and J. Nilsson<br /><br />In the present work the Arctic sea ice in the mid-Holocene and the pre-industrial climates are analysed and compared on the basis of climate-model results from the Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) and phase 3 (PMIP3). The PMIP3 models generally simulate smaller and thinner sea-ice extents than the PMIP2 models both for the pre-industrial and the mid-Holocene climate. Further, the PMIP2 and PMIP3 models all simulate a smaller and thinner Arctic summer sea-ice cover in the mid-Holocene than in the pre-industrial control climate. The PMIP3 models also simulate thinner winter sea ice than the PMIP2 models. The winter sea-ice extent response, i.e. the difference between the mid-Holocene and the pre-industrial climate, varies among both PMIP2 and PMIP3 models. Approximately one half of the models simulate a decrease in winter sea-ice extent and one half simulates an increase. The model-mean summer sea-ice extent is 11 % (21 %) smaller in the mid-Holocene than in the pre-industrial climate simulations in the PMIP2 (PMIP3). In accordance with the simple model of Thorndike (1992), the sea-ice thickness response to the insolation change from the pre-industrial to the mid-Holocene is stronger in models with thicker ice in the pre-industrial climate simulation. Further, the analyses show that climate models for which the Arctic sea-ice responses to increasing atmospheric CO₂ concentrations are similar may simulate rather different sea-ice responses to the change in solar forcing between the mid-Holocene and the pre-industrial. For two specific models, which are analysed in detail, this difference is found to be associated with differences in the simulated cloud fractions in the summer Arctic; in the model with a larger cloud fraction the effect of insolation change is muted. A sub-set of the mid-Holocene simulations in the PMIP ensemble exhibit open water off the north-eastern coast of Greenland in summer, which can provide a fetch for surface waves. This is in broad agreement with recent analyses of sea-ice proxies, indicating that beach-ridges formed on the north-eastern coast of Greenland during the early- to mid-Holocene.Mon, 15 Apr 2013 00:00:00 +0200http://www.clim-past.net/9/955/2013/<b>Model sensitivity to North Atlantic freshwater forcing at 8.2 ka</b><br /><br />Climate of the Past, 9, 955-968, 2013<br /><br />Author(s): C. Morrill, A. N. LeGrande, H. Renssen, P. Bakker, and B. L. Otto-Bliesner<br /><br />We compared four simulations of the 8.2 ka event to assess climate model sensitivity and skill in responding to North Atlantic freshwater perturbations. All of the simulations used the same freshwater forcing, 2.5 Sv for one year, applied to either the Hudson Bay (northeastern Canada) or Labrador Sea (between Canada's Labrador coast and Greenland). This freshwater pulse induced a decadal-mean slowdown of 10–25% in the Atlantic Meridional Overturning Circulation (AMOC) of the models and caused a large-scale pattern of climate anomalies that matched proxy evidence for cooling in the Northern Hemisphere and a southward shift of the Intertropical Convergence Zone. The multi-model ensemble generated temperature anomalies that were just half as large as those from quantitative proxy reconstructions, however. Also, the duration of AMOC and climate anomalies in three of the simulations was only several decades, significantly shorter than the duration of ~150 yr in the paleoclimate record. Possible reasons for these discrepancies include incorrect representation of the early Holocene climate and ocean state in the North Atlantic and uncertainties in the freshwater forcing estimates.Wed, 10 Apr 2013 00:00:00 +0200http://www.clim-past.net/9/935/2013/<b>Climatic impacts of fresh water hosing under Last Glacial Maximum conditions: a multi-model study</b><br /><br />Climate of the Past, 9, 935-953, 2013<br /><br />Author(s): M. Kageyama, U. Merkel, B. Otto-Bliesner, M. Prange, A. Abe-Ouchi, G. Lohmann, R. Ohgaito, D. M. Roche, J. Singarayer, D. Swingedouw, and X Zhang<br /><br />Fresh water hosing simulations, in which a fresh water flux is imposed in the North Atlantic to force fluctuations of the Atlantic Meridional Overturning Circulation, have been routinely performed, first to study the climatic signature of different states of this circulation, then, under present or future conditions, to investigate the potential impact of a partial melting of the Greenland ice sheet. The most compelling examples of climatic changes potentially related to AMOC abrupt variations, however, are found in high resolution palaeo-records from around the globe for the last glacial period. To study those more specifically, more and more fresh water hosing experiments have been performed under glacial conditions in the recent years. Here we compare an ensemble constituted by 11 such simulations run with 6 different climate models. All simulations follow a slightly different design, but are sufficiently close in their design to be compared. They all study the impact of a fresh water hosing imposed in the extra-tropical North Atlantic. Common features in the model responses to hosing are the cooling over the North Atlantic, extending along the sub-tropical gyre in the tropical North Atlantic, the southward shift of the Atlantic ITCZ and the weakening of the African and Indian monsoons. On the other hand, the expression of the bipolar see-saw, i.e., warming in the Southern Hemisphere, differs from model to model, with some restricting it to the South Atlantic and specific regions of the southern ocean while others simulate a widespread southern ocean warming. The relationships between the features common to most models, i.e., climate changes over the north and tropical Atlantic, African and Asian monsoon regions, are further quantified. These suggest a tight correlation between the temperature and precipitation changes over the extra-tropical North Atlantic, but different pathways for the teleconnections between the AMOC/North Atlantic region and the African and Indian monsoon regions.Tue, 09 Apr 2013 00:00:00 +0200http://www.clim-past.net/9/913/2013/<b>Climate and vegetation changes during the Lateglacial and early–middle Holocene at Lake Ledro (southern Alps, Italy)</b><br /><br />Climate of the Past, 9, 913-933, 2013<br /><br />Author(s): S. Joannin, B. Vannière, D. Galop, O. Peyron, J. N. Haas, A. Gilli, E. Chapron, S. B. Wirth, F. Anselmetti, M. Desmet, and M. Magny<br /><br />Adding to the on-going debate regarding vegetation recolonisation (more particularly the timing) in Europe and climate change since the Lateglacial, this study investigates a long sediment core (LL081) from Lake Ledro (652 m a.s.l., southern Alps, Italy). Environmental changes were reconstructed using multiproxy analysis (pollen-based vegetation and climate reconstruction, lake levels, magnetic susceptibility and X-ray fluorescence (XRF) measurements) recorded climate and land-use changes during the Lateglacial and early–middle Holocene. The well-dated and high-resolution pollen record of Lake Ledro is compared with vegetation records from the southern and northern Alps to trace the history of tree species distribution. An altitude-dependent progressive time delay of the first continuous occurrence of <i>Abies</i> (fir) and of the <i>Larix</i> (larch) development has been observed since the Lateglacial in the southern Alps. This pattern suggests that the mid-altitude Lake Ledro area was not a refuge and that trees originated from lowlands or hilly areas (e.g. Euganean Hills) in northern Italy. Preboreal oscillations (ca. 11 000 cal BP), Boreal oscillations (ca. 10 200, 9300 cal BP) and the 8.2 kyr cold event suggest a centennial-scale climate forcing in the studied area. <i>Picea</i> (spruce) expansion occurred preferentially around 10 200 and 8200 cal BP in the south-eastern Alps, and therefore reflects the long-lasting cumulative effects of successive boreal and the 8.2 kyr cold event. The extension of <i>Abies</i> is contemporaneous with the 8.2 kyr event, but its development in the southern Alps benefits from the wettest interval 8200–7300 cal BP evidenced in high lake levels, flood activity and pollen-based climate reconstructions. Since ca. 7500 cal BP, a weak signal of pollen-based anthropogenic activities suggest weak human impact. The period between ca. 5700 and ca. 4100 cal BP is considered as a transition period to colder and wetter conditions (particularly during summers) that favoured a dense beech (<i>Fagus</i>) forest development which in return caused a distinctive yew (<i>Taxus</i>) decline. We conclude that climate was the dominant factor controlling vegetation changes and erosion processes during the early and middle Holocene (up to ca. 4100 cal BP).Tue, 09 Apr 2013 00:00:00 +0200http://www.clim-past.net/9/903/2013/<b>Stable isotopic evidence of El Niño-like atmospheric circulation in the Pliocene western United States</b><br /><br />Climate of the Past, 9, 903-912, 2013<br /><br />Author(s): M. J. Winnick, J. M. Welker, and C. P. Chamberlain<br /><br />Understanding how the hydrologic cycle has responded to warmer global temperatures in the past is especially important today as concentrations of CO₂ in the atmosphere continue to increase due to human activities. The Pliocene offers an ideal window into a climate system that has equilibrated with current atmospheric <i>p</i>CO₂. During the Pliocene the western United States was wetter than modern, an observation at odds with our current understanding of future warming scenarios, which involve the expansion and poleward migration of the subtropical dry zone. Here we compare Pliocene oxygen isotope profiles of pedogenic carbonates across the western US to modern isotopic anomalies in precipitation between phases of the El Niño–Southern Oscillation (ENSO). We find that when accounting for seasonality of carbonate formation, isotopic changes through the late Pliocene match modern precipitation isotopic anomalies in El Niño years. Furthermore, isotopic shifts through the late Pliocene mirror changes through the early Pleistocene, which likely represents the southward migration of the westerly storm track caused by growth of the Laurentide ice sheet. We propose that the westerly storm track migrated northward through the late Pliocene with the development of the modern cold tongue in the east equatorial Pacific, then returned southward with widespread glaciation in the Northern Hemisphere – a scenario supported by terrestrial climate proxies across the US. Together these data support the proposed existence of background El Niño-like conditions in western North America during the warm Pliocene. If the earth behaves similarly with future warming, this observation has important implications with regard to the amount and distribution of precipitation in western North America.Mon, 08 Apr 2013 00:00:00 +0200http://www.clim-past.net/9/887/2013/<b>Using data assimilation to investigate the causes of Southern Hemisphere high latitude cooling from 10 to 8 ka BP</b><br /><br />Climate of the Past, 9, 887-901, 2013<br /><br />Author(s): P. Mathiot, H. Goosse, X. Crosta, B. Stenni, M. Braida, H. Renssen, C. J. Van Meerbeeck, V. Masson-Delmotte, A. Mairesse, and S. Dubinkina<br /><br />From 10 to 8 ka BP (thousand years before present), paleoclimate records show an atmospheric and oceanic cooling in the high latitudes of the Southern Hemisphere. During this interval, temperatures estimated from proxy data decrease by 0.8 °C over Antarctica and 1.2 °C over the Southern Ocean. In order to study the causes of this cooling, simulations covering the early Holocene have been performed with the climate model of intermediate complexity LOVECLIM constrained to follow the signal recorded in climate proxies using a data assimilation method based on a particle filtering approach. The selected proxies represent oceanic and atmospheric surface temperature in the Southern Hemisphere derived from terrestrial, marine and glaciological records. Two mechanisms previously suggested to explain the 10–8 ka BP cooling pattern are investigated using the data assimilation approach in our model. The first hypothesis is a change in atmospheric circulation, and the second one is a cooling of the sea surface temperature in the Southern Ocean, driven in our experimental setup by the impact of an increased West Antarctic melting rate on ocean circulation. For the atmosphere hypothesis, the climate state obtained by data assimilation produces a modification of the meridional atmospheric circulation leading to a 0.5 °C Antarctic cooling from 10 to 8 ka BP compared to the simulation without data assimilation, without congruent cooling of the atmospheric and sea surface temperature in the Southern Ocean. For the ocean hypothesis, the increased West Antarctic freshwater flux constrainted by data assimilation (+100 mSv from 10 to 8 ka BP) leads to an oceanic cooling of 0.7 °C and a strengthening of Southern Hemisphere westerlies (+6%). Thus, according to our experiments, the observed cooling in Antarctic and the Southern Ocean proxy records can only be reconciled with the reconstructions by the combination of a modified atmospheric circulation and an enhanced freshwater flux.Wed, 03 Apr 2013 00:00:00 +0200http://www.clim-past.net/9/871/2013/<b>Impact of precipitation intermittency on NAO-temperature signals in proxy records</b><br /><br />Climate of the Past, 9, 871-886, 2013<br /><br />Author(s): M. Casado, P. Ortega, V. Masson-Delmotte, C. Risi, D. Swingedouw, V. Daux, D. Genty, F. Maignan, O. Solomina, B. Vinther, N. Viovy, and P. Yiou<br /><br />In mid and high latitudes, the stable isotope ratio in precipitation is driven by changes in temperature, which control atmospheric distillation. This relationship forms the basis for many continental paleoclimatic reconstructions using direct (e.g. ice cores) or indirect (e.g. tree ring cellulose, speleothem calcite) archives of past precipitation. However, the archiving process is inherently biased by intermittency of precipitation. Here, we use two sets of atmospheric reanalyses (NCEP (National Centers for Environmental Prediction) and ERA-interim) to quantify this precipitation intermittency bias, by comparing seasonal (winter and summer) temperatures estimated with and without precipitation weighting. We show that this bias reaches up to 10 °C and has large interannual variability. We then assess the impact of precipitation intermittency on the strength and stability of temporal correlations between seasonal temperatures and the North Atlantic Oscillation (NAO). Precipitation weighting reduces the correlation between winter NAO and temperature in some areas (e.g. Québec, South-East USA, East Greenland, East Siberia, Mediterranean sector) but does not alter the main patterns of correlation. The correlations between NAO, &delta;¹⁸O in precipitation, temperature and precipitation weighted temperature are investigated using outputs of an atmospheric general circulation model enabled with stable isotopes and nudged using reanalyses (LMDZiso (Laboratoire de Météorologie Dynamique Zoom)). In winter, LMDZiso shows similar correlation values between the NAO and both the precipitation weighted temperature and &delta;¹⁸O in precipitation, thus suggesting limited impacts of moisture origin. Correlations of comparable magnitude are obtained for the available observational evidence (GNIP (Global Network of Isotopes in Precipitation) and Greenland ice core data). Our findings support the use of archives of past &delta;¹⁸O for NAO reconstructions.Wed, 27 Mar 2013 00:00:00 +0100http://www.clim-past.net/9/859/2013/<b>Mismatch between the depth habitat of planktonic foraminifera and the calibration depth of SST transfer functions may bias reconstructions</b><br /><br />Climate of the Past, 9, 859-870, 2013<br /><br />Author(s): R. J. Telford, C. Li, and M. Kucera<br /><br />We demonstrate that the temperature signal in the planktonic foraminifera assemblage data from the North Atlantic typically does not originate from near-surface waters and argue that this has the potential to bias sea surface temperature reconstructions using transfer functions calibrated against near-surface temperatures if the thermal structure of the upper few hundred metres of ocean changes over time. CMIP5 climate models indicate that ocean thermal structure in the North Atlantic changed between the Last Glacial Maximum (LGM) and the pre-industrial (PI), with some regions, mainly in the tropics, of the LGM ocean lacking good thermal analogues in the PI. <br><br> Transfer functions calibrated against different depths reconstruct a marked subsurface cooling in parts of the tropical North Atlantic during the last glacial, in contrast to previous studies that reconstruct only a modest cooling. These possible biases in temperature reconstructions may affect estimates of climate sensitivity based on the difference between LGM and pre-industrial climate. Quantifying these biases has the potential to alter our understanding of LGM climate and improve estimates of climate sensitivity.Fri, 22 Mar 2013 00:00:00 +0100http://www.clim-past.net/9/841/2013/<b>Controls of Caribbean surface hydrology during the mid- to late Holocene: insights from monthly resolved coral records</b><br /><br />Climate of the Past, 9, 841-858, 2013<br /><br />Author(s): C. Giry, T. Felis, M. Kölling, W. Wei, G. Lohmann, and S. Scheffers<br /><br />Several proxy-based and modeling studies have investigated long-term changes in Caribbean climate during the Holocene, however, very little is known on its variability on short timescales. Here we reconstruct seasonality and interannual to multidecadal variability of sea surface hydrology of the southern Caribbean Sea by applying paired coral Sr/Ca and &delta;¹⁸O measurements on fossil annually banded <i>Diploria strigosa</i> corals from Bonaire. This allows for better understanding of seasonal to multidecadal variability of the Caribbean hydrological cycle during the mid- to late Holocene. The monthly resolved coral &Delta;&delta;¹⁸O records are used as a proxy for the oxygen isotopic composition of seawater (&delta;¹⁸O_sw) of the southern Caribbean Sea. Consistent with modern day conditions, annual &delta;¹⁸O_sw cycles reconstructed from three modern corals reveal that freshwater budget at the study site is influenced by both net precipitation and advection of tropical freshwater brought by wind-driven surface currents. In contrast, the annual &delta;¹⁸O_sw cycle reconstructed from a mid-Holocene coral indicates a sharp peak towards more negative values in summer, suggesting intense summer precipitation at 6 ka BP (before present). In line with this, our model simulations indicate that increased seasonality of the hydrological cycle at 6 ka BP results from enhanced precipitation in summertime. On interannual to multidecadal timescales, the systematic positive correlation observed between reconstructed sea surface temperature and salinity suggests that freshwater discharged from the Orinoco and Amazon rivers and transported into the Caribbean by wind-driven surface currents is a critical component influencing sea surface hydrology on these timescales.Fri, 22 Mar 2013 00:00:00 +0100http://www.clim-past.net/9/825/2013/<b>Mass-movement and flood-induced deposits in Lake Ledro, southern Alps, Italy: implications for Holocene palaeohydrology and natural hazards</b><br /><br />Climate of the Past, 9, 825-840, 2013<br /><br />Author(s): A. Simonneau, E. Chapron, B. Vannière, S. B. Wirth, A. Gilli, C. Di Giovanni, F. S. Anselmetti, M. Desmet, and M. Magny<br /><br />High-resolution seismic profiles and sediment cores from Lake Ledro combined with soil and riverbed samples from the lake's catchment area are used to assess the recurrence of natural hazards (earthquakes and flood events) in the southern Italian Alps during the Holocene. Two well-developed deltas and a flat central basin are identified on seismic profiles in Lake Ledro. Lake sediments have been finely laminated in the basin since 9000 cal. yr BP and frequently interrupted by two types of sedimentary events (SEs): light-coloured massive layers and dark-coloured graded beds. Optical analysis (quantitative organic petrography) of the organic matter present in soil, riverbed and lacustrine samples together with lake sediment bulk density and grain-size analysis illustrate that light-coloured layers consist of a mixture of lacustrine sediments and mainly contain algal particles similar to the ones observed in background sediments. Light-coloured layers thicker than 1.5 cm in the main basin of Lake Ledro are synchronous to numerous coeval mass-wasting deposits remoulding the slopes of the basin. They are interpreted as subaquatic mass-movements triggered by historical and pre-historical regional earthquakes dated to AD 2005, AD 1891, AD 1045 and 1260, 2545, 2595, 3350, 3815, 4740, 7190, 9185 and 11 495 cal. yr BP. Dark-coloured SEs develop high-amplitude reflections in front of the deltas and in the deep central basin. These beds are mainly made of terrestrial organic matter (soils and lignocellulosic debris) and are interpreted as resulting from intense hyperpycnal flood event. Mapping and quantifying the amount of soil material accumulated in the Holocene hyperpycnal flood deposits of the sequence allow estimating that the equivalent soil thickness eroded over the catchment area reached up to 5 mm during the largest Holocene flood events. Such significant soil erosion is interpreted as resulting from the combination of heavy rainfall and snowmelt. The recurrence of flash flood events during the Holocene was, however, not high enough to affect pedogenesis processes and highlight several wet regional periods during the Holocene. The Holocene period is divided into four phases of environmental evolution. Over the first half of the Holocene, a progressive stabilization of the soils present through the catchment of Lake Ledro was associated with a progressive reforestation of the area and only interrupted during the wet 8.2 event when the soil destabilization was particularly important. Lower soil erosion was recorded during the mid-Holocene climatic optimum (8000–4200 cal. yr BP) and associated with higher algal production. Between 4200 and 3100 cal. yr BP, both wetter climate and human activities within the drainage basin drastically increased soil erosion rates. Finally, from 3100 cal. yr BP to the present-day, data suggest increasing and changing human land use.Thu, 21 Mar 2013 00:00:00 +0100http://www.clim-past.net/9/811/2013/<b>Skill and reliability of climate model ensembles at the Last Glacial Maximum and mid-Holocene</b><br /><br />Climate of the Past, 9, 811-823, 2013<br /><br />Author(s): J. C. Hargreaves, J. D. Annan, R. Ohgaito, A. Paul, and A. Abe-Ouchi<br /><br />Paleoclimate simulations provide us with an opportunity to critically confront and evaluate the performance of climate models in simulating the response of the climate system to changes in radiative forcing and other boundary conditions. Hargreaves et al. (2011) analysed the reliability of the Paleoclimate Modelling Intercomparison Project, PMIP2 model ensemble with respect to the MARGO sea surface temperature data synthesis (MARGO Project Members, 2009) for the Last Glacial Maximum (LGM, 21 ka BP). Here we extend that work to include a new comprehensive collection of land surface data (Bartlein et al., 2011), and introduce a novel analysis of the predictive skill of the models. We include output from the PMIP3 experiments, from the two models for which suitable data are currently available. We also perform the same analyses for the PMIP2 mid-Holocene (6 ka BP) ensembles and available proxy data sets. <br><br> Our results are predominantly positive for the LGM, suggesting that as well as the global mean change, the models can reproduce the observed pattern of change on the broadest scales, such as the overall land–sea contrast and polar amplification, although the more detailed sub-continental scale patterns of change remains elusive. In contrast, our results for the mid-Holocene are substantially negative, with the models failing to reproduce the observed changes with any degree of skill. One cause of this problem could be that the globally- and annually-averaged forcing anomaly is very weak at the mid-Holocene, and so the results are dominated by the more localised regional patterns in the parts of globe for which data are available. The root cause of the model-data mismatch at these scales is unclear. If the proxy calibration is itself reliable, then representativity error in the data-model comparison, and missing climate feedbacks in the models are other possible sources of error.Thu, 21 Mar 2013 00:00:00 +0100