http://www.clim-past-discuss.net/ Climate of the Past Discussions Latest Articlesenhttp://www.clim-past-discuss.net/8/481/2012/<b>Identification of climatic state with limited proxy data</b><br /><br />Climate of the Past Discussions, 8, 481-503, 2012<br /><br />Author(s): J. D. Annan and J. C. Hargreaves<br /><br />We investigate the identifiability of the climate by limited proxy data. We test a data assimilation approach through perfect model pseudoproxy experiments, using a simple likelihood-based weighting based on the particle filtering process. Our experimental set-up enables us to create a massive 10 000-member ensemble at modest computational cost, thus enabling us to generate statistically robust results. We find that the method works well when data are sparse and imprecise, but in this case the reconstruction has a rather low accuracy as indicated by residual RMS errors. Conversely, when data are relatively plentiful and accurate, the estimate tracks the target closely, at least when considering the hemispheric mean. However, in this case, our prior ensemble size of 10 000 appears to be inadequate to correctly represent the true posterior, and the regional performance is poor. Using correlations to assess performance gives a more encouraging picture, with significant correlations ranging from about 0.3 when data are sparse to values over 0.7 when data are plentiful, but the residual RMS errors are substantial in all cases. Our results imply that caution is required in interpreting climate reconstructions, especially when considering the regional scale, as skill on this basis is markedly lower than on the large scale of hemispheric mean temperature.Tue, 31 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/435/2012/<b>Planetary fertility during the past 400 ka based on the triple isotope composition of O₂ in trapped gases from the Vostok ice core</b><br /><br />Climate of the Past Discussions, 8, 435-479, 2012<br /><br />Author(s): T. Blunier, M. L. Bender, B. Barnett, and J. C. von Fisher<br /><br />The productivity of the biosphere leaves its imprint on the isotopic composition of atmospheric oxygen. Ultimately atmospheric oxygen, through photosynthesis, originates from seawater. Fractionations during the passage from seawater to atmospheric O₂ and during respiration are mass dependent, affecting &delta;¹⁷O about half as much as &delta;¹⁸O. An "anomalous" (also termed mass independent) fractionation process changes &delta;¹⁷O about 1.7 times as much as &delta;¹⁸O during isotope exchange between O₂ and CO₂ in the stratosphere. The relative rates of biological O₂ production and stratospheric processing determine the relationship between &delta;¹⁷O and &delta;¹⁸O of O₂ in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of mass dependent O₂ production by photosynthesis vs. the rate of mass independent O₂-CO₂ exchange in the stratosphere. However, the analysis of the ¹⁷O anomaly is complicated because each hydrological and biological process influencing &delta;¹⁷O and &delta;¹⁸O fractionates ¹⁷O and ¹⁸O in slightly different proportions. In this study we present oxygen data covering the last 400 kyr from the Vostok ice core. We reconstruct oxygen productivities from the triple isotope composition of atmospheric oxygen with a box model. Our steady state model for the oxygen cycle takes into account fractionation during photosynthesis and respiration of the land and ocean biosphere as well as fractionation when oxygen passes through the stratosphere. We consider changes of fractionation factors linked to climate variations taking into account the span of estimates of the main factors affecting our calculations. We find that ocean oxygen productivity was likely elevated relative to modern during glacials. However, this increase probably did not fully compensate for a reduction in land ocean productivity resulting in a slight reduction in total oxygen production during glacials.Mon, 30 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/391/2012/<b>Ranges of moisture-source temperatures estimated from Antarctic ice core stable isotope records over the glacial-interglacial cycles</b><br /><br />Climate of the Past Discussions, 8, 391-434, 2012<br /><br />Author(s): R. Uemura, V. Masson-Delmotte, J. Jouzel, A. Landais, H. Motoyama, and B. Stenni<br /><br />A single isotope ratio (&delta;D or &delta;¹⁸O) of water is widely used as an air-temperature proxy in Antarctic ice cores. These isotope ratios, however, do not solely depend on air-temperature but also on the extent of distillation of heavy isotopes out of atmospheric water vapor from an oceanic moisture source to a precipitation site. The temperature changes at the oceanic moisture source (&Delta;<i>T</i>_source) and at the precipitation site (&Delta;<i>T</i>_site) can be retrieved by using deuterium-excess (<i>d</i>) data. A new <i>d</i> record from Dome Fuji, Antarctica is produced spanning the past 360 000 yr and compared with records from Vostok and EPICA Dome C ice cores. To retrieve &Delta;<i>T</i>_source and &Delta;<i>T</i>_site information, different linear regression equations have been proposed using theoretical isotope distillation models. A major source of uncertainty lies in the coefficient of regression, <i>&beta;</i>_site which is related to the sensitivity of <i>d</i> to &Delta;<i>T</i>_site. We show that different ranges of temperature and selections of isotopic model outputs may increase the value of <i>&beta;</i>_site by a factor of two. To explore the impacts of this coefficient on the reconstructed temperatures, we apply for the first time the exact same methodology to the isotope records from the three Antarctica ice cores. We show that uncertainties in the <i>&beta;</i>_site coefficient strongly affect (i) the glacial-interglacial magnitude of &Delta;<i>T</i>_source; (ii) the imprint of obliquity in &Delta;<i>T</i>_source and in the site-source temperature gradient. By contrast, we highlight the robustness of &Delta;<i>T</i>_site reconstruction using water isotopes records.Mon, 30 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/349/2012/<b>Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?</b><br /><br />Climate of the Past Discussions, 8, 349-389, 2012<br /><br />Author(s): B. Meyssignac, D. Salas y Melia, M. Becker, W. Llovel, and A. Cazenave<br /><br />We investigate the spatio-temporal variability of sea level trend patterns observed by satellite altimetry since 1993, focusing on the Tropical Pacific. The objective of this study is two fold. On the basis of a 2-D past sea level reconstruction (over 1950–2009) – based on a combination of observations and ocean modelling – and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we investigate how these sea level trend patterns evolved during the last decades and centuries, and what their characteristic time scales are. Using 20th century coupled climate model runs, we also examine whether observed trend patterns are driven by external forcing factors (i.e. solar plus volcanic variability and changes in anthropogenic forcing) or if they essentially result from natural climate variability. For this analysis, we computed sea level trend patterns over successive 17 yr windows (i.e. the length of the altimetry record) both for the reconstructed sea level and model runs. We compared them to altimetry-based observed trends. The 2-D sea level reconstruction shows similar spatial trend patterns to those observed during the altimetry era. The patterns appear to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. Similar behaviour is found in multi-centennial control runs of the coupled climate models. The same analysis, performed on 20th century model runs does not display significant differences. This suggests that Tropical Pacific sea level trend fluctuations are still dominated by the internal natural variability of the ocean-atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effects on the regional sea level patterns are still hardly detectable.Fri, 13 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/321/2012/<b>Climate bifurcation during the last deglaciation</b><br /><br />Climate of the Past Discussions, 8, 321-348, 2012<br /><br />Author(s): T. M. Lenton, V. N. Livina, V. Dakos, and M. Scheffer<br /><br />The last deglaciation was characterised by two abrupt warming events, at the start of the Bølling-Allerød and at the end of the Younger Dryas, but their underlying causes are unclear. Some abrupt climate changes may involve gradual forcing past a bifurcation point, in which a prevailing climate state loses its stability and the climate tips into an alternative state, providing an early warning signal in the form of slowing responses to perturbations. However, the abrupt Dansgaard-Oeschger (DO) events during the last ice age were probably triggered by stochastic fluctuations without bifurcation or early warning, and whether the onset of the Bølling-Allerød (DO event 1) was preceded by slowing down or not is debated. Here we show that the interval from the Last Glacial Maximum to the end of the Younger Dryas, as recorded in three Greenland ice cores with two different climate proxies, was accompanied by a robust slowing down in climate dynamics and an increase in climate variability, consistent with approaching bifurcation. Prior to the Bølling warming there was a robust increase in climate variability but no consistent slowing down signal, suggesting this abrupt change was probably triggered by a stochastic fluctuation. The Bølling warming marked a distinct destabilisation of the climate system, which excited an internal mode of variability in Atlantic meridional overturning circulation strength, causing multi-centennial climate fluctuations. There is some evidence for slowing down in the transition to and during the Younger Dryas. We infer that a bifurcation point was finally approached at the end of the Younger Dryas, in which the cold climate state, with weak Atlantic overturning circulation, lost its stability, and the climate tipped irreversibly into a warm interglacial state. The lack of a large triggering perturbation at the end of the Younger Dryas, and the fact that subsequent meltwater perturbations did not cause sustained cooling, support the bifurcation hypothesis.Thu, 12 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/263/2012/<b>Statistical framework for evaluation of climate model simulations by use of climate proxy data from the last millennium</b><br /><br />Climate of the Past Discussions, 8, 263-320, 2012<br /><br />Author(s): A. Hind, A. Moberg, and R. Sundberg<br /><br />A statistical framework for comparing the output of ensemble simulations from global climate models with networks of climate proxy and instrumental records is developed, focusing on near-surface temperatures for the last millennium. This framework includes the formulation of a joint statistical model for proxy data, instrumental data and simulation data, which is used to optimize a quadratic distance measure for ranking climate model simulations. An essential underlying assumption is that the simulations and the proxy/instrumental series have a shared component of variability that is due to temporal changes in external forcing, such as volcanic aerosol load, solar irradiance changes and greenhouse gas concentrations. Two statistical tests are formulated. Firstly, a preliminary test to establish whether a significant temporal correlation exists between instrumental/proxy and simulation data. Secondly, the distance measure is expressed in the form of a test statistic of whether a forced simulation is closer to the instrumental/proxy series than unforced simulations. The proposed framework allows any number of proxy locations to be used jointly, with different seasons, record lengths and statistical precision. The new methods are applied in a pseudo-proxy experiment. Here, a set of previously published millennial forced model simulations, including both "low" and "high" solar radiative forcing histories together with other common forcings, were used to define "true" target temperatures as well as pseudo-proxy and pseudo-instrumental series. The pseudo-proxies were created to reflect current proxy locations and noise levels, where it was found that the low and high solar full-forcing simulations could be distinguished when the latter were used as targets. When the former were used as targets, a greater number of proxy locations were needed to make this distinction. It was also found that to improve detectability of the low solar simulations, increasing the signal-to-noise ratio was more efficient than increasing the spatial coverage of the proxy network. In the next phase of the work, we will apply these methods to real proxy and instrumental data, with the aim to distinguish which of the two solar forcing histories is most compatible with the observed/reconstructed climate.Thu, 12 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/215/2012/<b>Little Ice Age climate and oceanic conditions of the Ross Sea, Antarctica from a coastal ice core record</b><br /><br />Climate of the Past Discussions, 8, 215-262, 2012<br /><br />Author(s): R. H. Rhodes, N. A. N. Bertler, J. A. Baker, H. C. Steen-Larsen, S. B. Sneed, U. Morgenstern, and S. J. Johnsen<br /><br />The Little Ice Age (LIA) is the most recent abrupt climate change event. Understanding its forcings and associated climate system feedbacks is made difficult by a scarcity of Southern Hemisphere paleoclimate records. In this paper we utilise ice core glaciochemical records to reconstruct atmospheric and oceanic conditions in the Ross Sea sector of Antarctic, a region influenced by two contrasting meteorological regimes: katabatic winds and cyclones. Stable isotope (&delta;<i>D</i>) and lithophile element concentration (e.g., Al) records indicate that the region experienced ~1.75 °C cooler temperatures and strong (>57 m s⁻¹) prevailing katabatic winds during the LIA. We observe that the 1590–1875 record is characterised by high d-excess values and marine element (e.g., Na) concentrations, which are linked to the intrusion of cyclonic systems. The strongest katabatic wind events of the LIA, marked by Al, Ti and Pb concentration increases of an order of magnitude (>120 ppb Al), also occur during this interval. Furthermore, concentrations of the biogenic sulphur species MS^&minus; suggest that biological productivity in the Ross Sea Polynya was ~80% higher prior to 1875 than in the subsequent time. We propose that colder temperatures and intensified cyclonic activity in the Ross Sea promoted stronger katabatic winds across the Ross Ice Shelf, resulting in an enlarged polynya with increased sea ice and bottom water production. It is therefore hypothesised that increased bottom water formation during the LIA occurred in response to atmospheric circulation change.Tue, 10 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/169/2012/<b>Modelling large-scale ice-sheet–climate interactions following glacial inception</b><br /><br />Climate of the Past Discussions, 8, 169-213, 2012<br /><br />Author(s): J. M. Gregory, O. J. H. Browne, A. J. Payne, J. K. Ridley, and I. C. Rutt<br /><br />We have coupled the FAMOUS global AOGCM (atmosphere–ocean general circulation model) to the Glimmer thermomechanical ice-sheet model in order to study the development of ice-sheets in North-East America (Laurentia) and North-West Europe (Fennoscandia) following glacial inception. This first use of a coupled AOGCM-ice-sheet model for a study of change on long palæoclimate timescales is made possible by the low computational cost of FAMOUS, despite its inclusion of physical parameterisations of a similar complexity to those of higher-resolution AOGCMs. With the orbital forcing of 115 ka BP, FAMOUS-Glimmer produces ice-caps on the Canadian Arctic islands, on the north-west coast of Hudson Bay, and in Southern Scandinavia, which over 50 ka grow to occupy the Keewatin region of the Canadian mainland and all of Fennoscandia. Their growth is eventually halted by increasing coastal ice discharge. The expansion of the ice-sheets influences the regional climate, which becomes cooler, reducing the ablation, while precipitation increases. Ice accumulates in places that initially do not have positive surface mass balance. The results suggest the possibility that the Laurentide glaciation could have begun on the Canadian Arctic islands, producing a regional climate change that caused or enhanced the growth of ice on the mainland. The increase in albedo due to snow and ice cover is the dominant feedback on the area of the ice-sheets, and acts rapidly, whereas the feedback of topography on SMB does not become significant for several centuries, but eventually has a large effect on the thickening of the ice-sheets. These two positive feedbacks are mutually reinforcing. In addition the change in topography perturbs the tropospheric circulation, producing some reduction of cloud and mitigating the local cooling along the margin of the Laurentide ice-sheet. Our experiments demonstrate the importance and complexity of the interactions between ice-sheets and local climate.Mon, 09 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/145/2012/<b>Interpreting last glacial to Holocene dust changes at Talos Dome (East Antarctica): implications for atmospheric variations from regional to hemispheric scales</b><br /><br />Climate of the Past Discussions, 8, 145-168, 2012<br /><br />Author(s): S. Albani, B. Delmonte, V. Maggi, C. Baroni, J.-R. Petit, B. Stenni, C. Mazzola, and M. Frezzotti<br /><br />Central East Antarctica ice cores preserve stratigraphic records of mineral dust originating from remote sources in the Southern Hemisphere, and represent useful indicators of climatic variations on glacial-interglacial time scales. The peripheries of the East Antarctic Ice Sheet, where ice-free areas with the potential to emit dust exist, have been less explored from this point of view. Here we present a new profile of dust deposition flux and grain size distributions from an ice core drilled at Talos Dome (Northern Victoria Land, East Antarctica), where there is a significant input of dust from proximal Antarctic ice-free areas. We analyze dust and stable water isotopes variations from the Last Glacial Maximum to the Late Holocene, and compare them to the EPICA Dome C profiles from Central East Antarctica. The smaller glacial-interglacial variations at Talos Dome compared to Dome C, and a distinctive decreasing trend during the Holocene, characterize the TALDICE dust profile. By deciphering the composite dust signal from both remote and local sources, we show the potential of this combined proxy of source activity and atmospheric transport to give information on both regional and larger spatial scales. In particular, we show how a regional signal, which we related to the deglaciation history of the Ross Sea embayment, can be superimposed to the broader scale glacial-interglacial variability that characterizes other Antarctic sites.Thu, 05 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/121/2012/<b>The Aptian evaporites of the South Atlantic: a climatic paradox?</b><br /><br />Climate of the Past Discussions, 8, 121-144, 2012<br /><br />Author(s): A.-C. Chaboureau, Y. Donnadieu, P. Sepulchre, C. Robin, F. Guillocheau, and S. Rohais<br /><br />For a long time, evaporitic sequences have been interpreted as indicative of an arid climate. Such systematic interpretations led to the suggestion that the central segment of the South Atlantic (20&deg;–0&deg;) was characterized by an arid climate during the upper Aptian. Indeed, synchronous to this period that corresponds to the rifting and to the opening of this part of the South Atlantic, a large evaporitic sequence spreads out from the equator to 20&deg; S. Using the fully ocean atmosphere coupled model FOAM, we test the potential for the Aptian geography to produce an arid area over the central segment. Sensitivity to the altitude of the rift shoulders separating the Africa and the South America cratons, to the water depth of the central segment and to the drainage pattern have been performed. Using seawater salinity as a diagnostic, our simulations show that the southern part of the central segment is characterized by very high salinity in the case of catchment areas draining the water out of the central segment. Conversely, whatever the boundary conditions used, the northern part of the central segment remains humid and salinities are very low. Hence, we conclude that the evaporites deposited in the southern part of the central segment may have been controlled by the climate favouring aridity and high saline waters. In contrast, the evaporites of the northern part can hardly be reconciled with the climatic conditions occurring there and may be due to hydrothermal sources. Our interpretations are in agreement with the gradient found in the mineralogical compositions of the evaporites from the north to the south, i.e. the northern evaporites are at least 4 times more concentrated than the southern one.Thu, 05 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/103/2012/<b>Winter temperature variations over middle and lower reaches of the Yangtze River during the past three centuries</b><br /><br />Climate of the Past Discussions, 8, 103-119, 2012<br /><br />Author(s): Z.-X. Hao, J.-Y. Zheng, Q.-S. Ge, and W.-C. Wang<br /><br />We present statistically reconstructed annual winter (December–February) mean temperature in the Middle and Lower Reaches of the Yangtze River (24–34° N, east of 108° E) back to 1736. The reconstructions are based on information from snowfall days from <i>Yu</i>-<i>Xue</i>-<i>Fen</i>-<i>Cun</i> archive (one of historical documents proxies) in Qing Dynasty (1644–1911). Those information are calibrated with regional winter temperature series spanning the period 1951 to 2007 period. The gap from 1912 to 1950 is filled using early instrumental observation. With respect to the 1951–2007 climatology, the 18th century was 0.6 °C colder, and the 19th century was 1.0 °C colder. But since the 20th century, climate entered into the warming phase, particular in the last 30 yr, the mean temperature from 1981 to 2007 is 0.25 °C higher than that of climatology, a highest level of the past 300 yr. The uncertainty is existed for the period prior of 1900, and possible causes have been discussed here.Wed, 04 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/63/2012/<b>The impact of different glacial boundary conditions on atmospheric dynamics and precipitation in the North Atlantic region</b><br /><br />Climate of the Past Discussions, 8, 63-101, 2012<br /><br />Author(s): D. Hofer, C. C. Raible, A. Dehnert, and J. Kuhlemann<br /><br />Using a highly resolved atmospheric general circulation model the impact of different glacial boundary conditions on precipitation and atmospheric dynamics in the North Atlantic region is investigated. Seven 30-yr time slice experiments of the Last Glacial Maximum (21 ka ago) and of a less pronounced glacial state – the Middle Weichselian (65 ka ago) – are compared to analyse the sensitivity to changes in the ice sheet distribution, in the radiative forcing, and in the prescribed time-varying lower boundary conditions, which are taken from a lower-resolved but fully-coupled atmosphere-ocean general circulation model. <br><br> The strongest differences are found for simulations with different heights of the Laurentide ice sheet. A large altitude of this ice sheet leads to a southward displacement of the jet stream and the storm track in the North Atlantic region. These changes in the atmospheric dynamics generate a band of increased precipitation in the mid-latitudes across the Atlantic to southern Europe in winter, while the precipitation pattern in summer is only marginally affected. The impact of the radiative forcing differences between the two glacial periods and of the prescribed time-varying lower boundary conditions – evaluated using two simulations of the Last Glacial Maximum with a global mean temperature difference of 1.1 &deg;C – are of second order compared to the one of the Laurentide ice sheet. They affect the atmospheric dynamics and precipitation in a similar but less pronounced manner as the topographic changes.Wed, 04 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/31/2012/<b>A modelling approach to assessing the timescale uncertainties in proxy series with chronological errors</b><br /><br />Climate of the Past Discussions, 8, 31-61, 2012<br /><br />Author(s): D. V. Divine, F. Godtliebsen, and H. Rue<br /><br />The paper proposes an approach to assessment of timescale errors in proxy-based series with chronological uncertainties. The method relies on approximation of the physical process(es) forming a proxy archive by a random Gamma process. Parameters of the process are partly data-driven and partly determined from prior assumptions. For a particular case of a linear accumulation model and absolutely dated tie points an analytical solution is found suggesting the Beta-distributed probability density on age estimates along the length of a proxy archive. In a general situation of uncertainties in the ages of the tie points the proposed method employs MCMC simulations of age-depth profiles yielding empirical confidence intervals on the constructed piecewise linear best guess timescale. It is suggested that the approach can be further extended to a more general case of a time-varying expected accumulation between the tie points. The approach is illustrated by using two ice and two lake/marine sediment cores representing the typical examples of paleoproxy archives with age models based on tie points of mixed origin.Tue, 03 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/8/1/2012/<b>The initiation of modern soft and hard Snowball Earth climates in CCSM4</b><br /><br />Climate of the Past Discussions, 8, 1-29, 2012<br /><br />Author(s): J. Yang and W. R. Peltier<br /><br />Geochemical and geological evidence suggested that several global-scale glaciation events occurred during the Neoproterozoic era at 750–580 million years ago. The initiation of these glaciations is thought to have been a consequence of the combined influence of a result of low-level carbon dioxide and an approximately 6% weakening of solar luminosity. The latest version of the Community Climate System Model (CCSM4) is employed herein to explore the detailed combination of forcings required to trigger such extreme glaciation under present-day geography and topography conditions. It is found that runaway glaciation occurs in the model under the following conditions: (1) a 8–9% reduction in solar radiation with 286 ppmv CO₂ or (2) a 6% reduction in solar radiation with 70–100 ppmv CO₂. These thresholds are only moderately different from those found to be characteristic of the previous CCSM3 model reported recently in Yang et al. (2011a,b) for which the respective critical points corresponded to a 10–10.5% reduction in solar radiation with 286 ppmv CO₂ or a 6% reduction in solar radiation with 17.5–20 ppmv CO₂. The most important reason for these differences is that the sea-ice/snow albedo in CCSM4 is somewhat higher than in CCSM3. Differences in cloud radiative forcings and oceanic and atmospheric heat transports between CCSM3 and CCSM4 also influence the bifurcation points. <br><br> The forcings required to trigger a "hard Snowball" Earth in either CCSM3 or CCSM4 may be not met by the conditions expected to be characteristic of the Neoproterozoic. Furthermore, there exist "soft Snowball" Earth states, in which the sea-ice coverage reaches approximately 60–65%, land masses in low latitudes are covered by perennial snow, and runaway glaciation does not develop. This is also qualitatively consistent with our previous results of the CCSM3 model. These results suggest that a "soft Snowball" solution for the Neoproterozoic is entirely plausible and may in fact be preferred.Mon, 02 Jan 2012 00:00:00 +0100http://www.clim-past-discuss.net/7/4401/2011/<b>A 500 kyr record of global sea level oscillations in the Gulf of Lion, Mediterranean Sea: new insights into MIS 3 sea level variability</b><br /><br />Climate of the Past Discussions, 7, 4401-4428, 2011<br /><br />Author(s): J. Frigola, M. Canals, I. Cacho, A. Moreno, F. J. Sierro, J. A. Flores, S. Berné, G. Jouet, B. Dennielou, G. Herrera, C. Pasqual, J. O. Grimalt, M. Galavazi, and R. Schneider<br /><br />Borehole PRGL1-4 drilled in the upper slope of the Gulf of Lion provides an exceptional record to investigate the impact of Late Pleistocene orbitally-driven glacio-eustatic sea level oscillations on the sedimentary outbuilding of a river fed continental margin. High-resolution grain-size and geochemical records supported by oxygen isotope chronostratigraphy allow reinterpreting the last 500 ka upper slope seismostratigraphy of the Gulf of Lion which consists of five main sequences stacked during the sea level lowering phases of the last five glacial-interglacial 100-kyr cycles. The high sensitivity to sea level oscillations of the grain-size record along the borehole, favoured by the large width of the Gulf of Lion continental shelf, demonstrates that sea level driven changes in accommodation space over the shelf are able to cyclically modify the depositional mode of the entire margin. PRGL1-4 data also illustrate the imprint of sea level oscillations at millennial scale, as shown for Marine Isotopic Stage 3, and provide unambiguous evidence of relative high sea levels at the onset of each Dansgaard-Oeschger Greenland warm interstadial. The PRGL1-4 grain-size record represents the first evidence ever for a one-to-one coupling of millennial-scale sea level oscillations associated with each Dansgaard-Oeschger cycle.Tue, 20 Dec 2011 00:00:00 +0100http://www.clim-past-discuss.net/7/4375/2011/<b>Precipitation variability in the winter rainfall zone of South Africa during the last 1400 yr linked to the austral westerlies</b><br /><br />Climate of the Past Discussions, 7, 4375-4399, 2011<br /><br />Author(s): J. C. Stager, P. A. Mayewski, J. White, B. M. Chase, F. H. Neumann, M. E. Meadows, C. D. King, and D. A. Dixon<br /><br />The austral westerlies strongly influence precipitation and ocean circulation in the southern temperate zone, with important consequences for cultures and ecosystems. Global climate models anticipate poleward contraction of the austral westerlies with future warming, but the available paleoclimate records that might test these models have been largely limited to South America, are not fully consistent with each other, and may be complicated by influences from other climatic factors. Here we present the first fine-interval diatom and sedimentological records from the winter rainfall region of South Africa, representing precipitation during the last 1400 yr. Inferred rainfall increased ~1400–1200 cal yr BP and most notably during the Little Ice Age with pulses centered on ~600, 530, 470, 330, 200, and 90 cal yr BP. Synchronous fluctuations in Antarctic ice core chemistry strongly suggest that these variations are linked to changes in the westerlies. Partial inconsistencies among South African and South American records warn against the simplistic application of local-scale histories to the Southern Hemisphere as a whole. Nonetheless, these findings in general do support model projections of increasing aridity in austral winter rainfall zones with future warming.Tue, 20 Dec 2011 00:00:00 +0100http://www.clim-past-discuss.net/7/4335/2011/<b>Correlation of Greenland ice-core isotope profiles and the terrestrial record of the Alpine Rhine glacier for the period 32–15 ka</b><br /><br />Climate of the Past Discussions, 7, 4335-4373, 2011<br /><br />Author(s): M. G. G. De Jong, L. W. S. de Graaff, A. C. Seijmonsbergen, and A. R. Böhm<br /><br />We present a newly extended stratigraphic subdivision of the Greenland NGRIP, GRIP and GISP2 ice cores for the period 32–15 ka. Our classification emphasizes the multiscale nature of the climatic oscillations. Spectral trend analysis of isotopic data supports this interpretation. We compare this time series with the stratigraphy of the last major Pleistocene (Weichselian, Würmian) glaciation in the area of the Rhine glacier (Germany and Austria) as chronicled by a detailed inventory of landforms and deposits, dated in part with ¹⁴C analyses, and their interpretation in terms of climate change. Both time series show a major climatic oscillation, consisting of a colder period (glaciation) followed by a warmer period (deglaciation). The available dates allow the time of change, at 23.4 ka, to be correlated between the two. Pattern analysis clearly indicates that higher-order oscillations were superimposed on the major oscillation in both areas, emphasizing the multiscale nature of the underlying pattern of climate change. The correlation between the two areas is sufficiently good to propose that the pattern of climate change was synchronous (within the available time resolution) between Greenland and the Rhine glacier area. Comparison of our results with other high resolution climate proxies is expected to further improve the understanding of the climate changes during the Late Weichselian.Fri, 16 Dec 2011 00:00:00 +0100http://www.clim-past-discuss.net/7/4297/2011/<b>Precipitation changes in the South American Altiplano since 1300 AD reconstructed by tree-rings</b><br /><br />Climate of the Past Discussions, 7, 4297-4334, 2011<br /><br />Author(s): M. S. Morales, D. A. Christie, R. Villalba, J. Argollo, J. Pacajes, J. S. Silva, C. A. Alvarez, J. C. Llancabure, and C. Solíz<br /><br />During the second half of the 20th century, the Central Andes has experienced significant climatic and environmental changes characterized by a persistent warming trend, an increase in elevation of the 0 °C isotherm, and a sustained shrinkage of glaciers. These changes have occurred in conjunction with a steady growing demand for water resources. Given the short span of instrumental hidroclimatic records in this region, longer records are needed to understand the nature of climate variability and improve the predictability of precipitation, a key factor modulating the socio-economic development in the South American Altiplano and the adjacent arid lowlands. In this study we present the first quasi-millennial, tree-ring based precipitation reconstruction for the South American Altiplano. This annual (November–October) precipitation reconstruction is based on <i>Polylepis tarapacana</i> tree-ring series and represents the closest dendroclimatological record to the Equator in South America. This high-resolution reconstruction covers the past 707 yr and provides a unique record to characterize the occurrence of extreme events and consistent oscillations in precipitation, as well as to check the spatial and temporal stabilities of the teleconnections between rainfall in the Altiplano and hemispheric forcings such as El Niño-Southern Oscillation. Since the 1930s up to present a persistent negative trend in precipitation is recorded in the reconstruction, with the three driest years since 1300 AD occurring in the last 70 yr. The reconstruction contains a clear ENSO-like pattern at interannual to multicentennial time scales which determines inter-hemispheric linkages between our reconstruction and other precipitation-sensitive records modulated by ENSO in North America. Our reconstruction points out that century-scale dry periods are a recurrent feature in the Altiplano climate, and that the potential coupling of natural and anthropogenic-induced droughts in the future would have a severe impact on current socio-economical activities in the region. Water resource managers must anticipate these changes to adapt for future climate change, reduce vulnerability and provide water equitably to all users.Mon, 12 Dec 2011 00:00:00 +0100http://www.clim-past-discuss.net/7/4261/2011/<b>Hydrometeorological extremes and their impacts, as derived from taxation records for south-eastern Moravia, Czech Republic, AD 1751–1900</b><br /><br />Climate of the Past Discussions, 7, 4261-4296, 2011<br /><br />Author(s): R. Brázdil, K. Chromá, H. Valášek, and L. Dolák<br /><br />Historical written records associated with tax relief at ten estates located in south-eastern Moravia (Czech Republic) are used for the study of hydrometeorological extremes and their impacts during the period AD 1751–1900. At the time, the taxation system in Moravia allowed farmers to request tax relief if their crop yields had been negatively affected by hydrological and meteorological extremes. The documentation involved contains information about the type of extreme event and the date of its occurrence, while the impact on crops may often be derived. A total of 175 extreme events resulting in some kind of damage is documented for 1751–1900, with the highest concentration between 1811 and 1860 (74.9% of all events analysed). The nature of events leading to damage (of a possible 272 types) include hailstorm (25.7%), torrential rain (21.7%), and flood (21.0%), followed by thunderstorm, flash flood, late frost and windstorm. The four most outstanding events, affecting the highest number of settlements, were thunderstorms with hailstorms (25 June 1825, 20 May 1847 and 29 June 1890) and flooding of the River Morava (mid-June 1847). Hydrometeorological extremes in the 1816–1855 period are compared with those occurring during the recent 1961–2000 period. The results obtained are inevitably influenced by uncertainties related to taxation records, such as their temporal and spatial incompleteness, the limits of the period of outside agricultural work (i.e. mainly May–August) and the purpose for which they were originally collected (primarily tax alleviation, i.e. information about hydrometeorological extremes was of secondary importance). Taxation records constitute an important source of data for historical climatology and historical hydrology and have a great potential for use in many European countries.Fri, 09 Dec 2011 00:00:00 +0100http://www.clim-past-discuss.net/7/4223/2011/<b>Extreme pointer years in tree-ring records of Central Spain as evidence of volcanic eruptions (Huaynaputina, Peru, 1600 AC) and other climatic events</b><br /><br />Climate of the Past Discussions, 7, 4223-4259, 2011<br /><br />Author(s): M. Génova<br /><br />The study of pointer years based on the numerous tree-ring chronologies of the central Iberian Peninsula (Sierra de Guadarrama) could provide complementary information about climate variability over the last 405 years. In total, 64 pointer years have been identified: 30 negative (representing minimum growths) and 34 positive (representing maximum growths), the most significant of these being 1601, 1963 and 1996 for the negative ones, and 1734 and 1737 for the positive ones. Given that summer precipitation has been the most incident factor in the general variability of growth of <i>Pinus</i> in the Sierra de Guadarrama in the second half of the 20th century, it is also an explanatory factor in almost 50% of the extreme growths. Furthermore, the data show that there has been variability over the centuries in the distribution of the frequencies of pointer years and intervals. The first half of the 17th century, together with the second half of the 20th century, constitute the two most notable periods for the frequency of negative pointer years in Central Spain. This variability was sufficiently notable to affirm that, both in the 17th and 20th centuries, the macroclimatic anomalies that affected growth were more frequent and more extreme than in the other two centuries analysed. <br><br> The period 1600–1602 is of special significance, being one of the most unfavourable for tree growth in the centre of Spain, with 1601 representing the minimum index in the regional chronology. It is possible to infer that these phenomena are the effect of the eruption of Huaynaputina, which occurred in Peru at the beginning of 1600 AD. This is the first time that the effects of this eruption in the tree-ring records of central and southern Europe have been demonstrated.Thu, 08 Dec 2011 00:00:00 +0100