Climate of the Past

Published by European Geosciences Union
Online ISSN: 1814-9332
Publications
Article
[Abridged] This paper describes the creation of HadISD: an automatically quality-controlled synoptic resolution dataset of temperature, dewpoint temperature, sea-level pressure, wind speed, wind direction and cloud cover from global weather stations for 1973--2011. The full dataset consists of over 6000 stations, with 3427 long-term stations deemed to have sufficient sampling and quality for climate applications requiring sub-daily resolution. As with other surface datasets, coverage is heavily skewed towards Northern Hemisphere mid-latitudes. The dataset is constructed from a large pre-existing ASCII flatfile data bank that represents over a decade of substantial effort at data retrieval, reformatting and provision. These raw data have had varying levels of quality control applied to them by individual data providers. The work proceeded in several steps: merging stations with multiple reporting identifiers; reformatting to netCDF; quality control; and then filtering to form a final dataset. Particular attention has been paid to maintaining true extreme values where possible within an automated, objective process. Detailed validation has been performed on a subset of global stations and also on UK data using known extreme events to help finalise the QC tests. Further validation was performed on a selection of extreme events world-wide (Hurricane Katrina in 2005, the cold snap in Alaska in 1989 and heat waves in SE Australia in 2009). Although the filtering has removed the poorest station records, no attempt has been made to homogenise the data thus far. Hence non-climatic, time-varying errors may still exist in many of the individual station records and care is needed in inferring long-term trends from these data. A version-control system has been constructed for this dataset to allow for the clear documentation of any updates and corrections in the future.
 
Overlap with CO 2 and CH 4 . Reduction in radiative forcing due to overlapping absorption. Trace gas concentrations are held at 
Trace gas overlap. Reduction in radiative forcing due to overlapping absorption. Gas concentrations are held at concentrations which give a 10 W m − 2 radiative forcing for an atmosphere with 1 bar of N 2 . 
Figure A1. Sensitivity Study. Columns from left to right: Temperature structure, H 2 O structure, Net flux of radiation at tropopause, radiative 
Article
Despite reduced insolation in the late Archean, evidence suggests a warm climate which was likely sustained by a stronger greenhouse effect, the so-called Faint Young Sun Problem (FYSP). CO2 and CH4 are generally thought to be the mainstays of this enhanced greenhouse, though many other gases have been proposed. We present high accuracy radiative forcings for CO2, CH4 and 26 other gases, performing the radiative transfer calculations at line-by-line resolution and using HITRAN 2012 line data for background pressures of 0.5, 1, and 2 bar of atmospheric N2. For CO2 to resolve the FYSP alone at 2.8 Gyr BP (80% of present solar luminosity), 0.32 bar is needed with 0.5 bar of atmospheric N2, 0.20 bar with 1 bar of atmospheric N2, or 0.11 bar with 2 bar of atmospheric N2. For CH4, we find that near-infrared absorption is much stronger than previously thought, arising from updates to the HITRAN database. CH4 radiative forcing peaks at 10.3, 9, or 8.3 Wm-2 for background pressures of 0.5, 1 or 2 bar, likely limiting the utility of CH4 for warming the Archean. For the other 26 HITRAN gases, radiative forcings of up to a few to 10 Wm-2 are obtained from concentrations of 0.1-1 ppmv for many gases. For the 20 strongest gases, we calculate the reduction in radiative forcing due to overlap. We also tabulate the modern sources, sinks, concentrations and lifetimes of these gases and summaries the literature on Archean sources and concentrations. We recommend the forcings provided here be used both as a first reference for which gases are likely good greenhouse gases, and as a standard set of calculations for validation of radiative forcing calculations for the Archean.
 
Article
The climate record preserved in polar glaciers, mountain glaciers, and widespread cave deposits shows repeated occurrence of abrupt global transitions between cold/dry stadial and warm/wet interstadial states during glacial periods. These abrupt transitions occur on millennial time scale and in the absence of any known global-scale forcing. Here a theory is advanced implicating a feedback between atmospheric dust and the hydrological cycle in producing these abrupt transitions. Calculations are performed using a radiative-convective model that includes the interaction of aerosols with radiation to reveal the mechanism of the dust/precipitation interaction feedback process and a Langevin equation is used to model glacial climate destabilization by this mechanism. This theory explains the observed bimodal, stochastic, and abrupt nature of the transitions as well as their intrinsic connection with the hydrological cycle.
 
Article
A synthesis of paleoclimate responses from Lake Baikal during the MIS 11 interglacial is presented based on proxy records from two drill sites 245 km apart. BDP-99 is located in vicinity of the delta of the major Baikal tributary, whereas the BDP-96 site represents hemipelagic setting distant from riverine influence. The comparison of thicknesses of interglacial intervals in these contrasting depositional settings confirms the extended ca. 33-kyr duration of the MIS 11 interglacial. The new BDP-99 diatom biostratigraphic record matches that of the BDP-96-2 holostratotype and thus allows establishing establishes robust correlation between the records on the same orbitally-tuned timescale. The first detailed MIS 11 palynological record from the BDP-99 drill core indicates the dominance of boreal conifer (taiga) forest vegetation in the Baikal region throughout the MIS 11 interglacial, since at least 424 ka till ca. 396 ka. The interval ca. 420–405 ka stands out as a "conifer optimum" with abundant Abies sibirica , indicative of climate significantly warmer and less continental than today. The closest Baikal analog to this type of vegetation in the history of the current Holocene interglacial is at ca. 9–7 ka. The warm conifer phase lasted for ca. 15 kyr during MIS 11 interrupted by two millennial-scale cooling episodes at ca. 411–410 and 405–404 ka. Reconstructed annual precipitation of 450–550 mm/yr during the MIS 11 interglacial is by ca. 100 mm higher than during the Holocene; regional climate was less continental with warmer mean temperatures both in summer and in winter. At both drill sites, the two-peak structure of the MIS 11 diatom abundance profiles reflects the orbital signature of precession in the interglacial paleoclimate record of continental Eurasia. MIS 11 interglacial was characterized by the sustained high level of primary production and accumulation of autochthonous organic matter at both study sites. The responses of paleoclimate-sensitive indices in the mineralogy of the MIS 11 sediments in BDP-96-2 are consistent with those during the Holocene. Illitization of secondary clay minerals in the Baikal watershed was an important process, but it appears to have been subdued during the first half of the MIS 11, apparently due to elevated humidity and muted seasonality of regional climate.
 
Depth-age relationship for the EDML ice core (A), and corresponding thickness of annual layers (B) shown as uncorrected true depth for EDML (black, bold line) and for EDC (blue, fine line). Note, that variations in the EDML layer thickness may also include possible spatial upstream variations in snow accumulation in the past while upstream effects at the dome position of EDC do not exist.  
Article
A chronology called EDML1 has been developed for the EPICA ice core from Dronning Maud Land (EDML). EDML1 is closely interlinked with EDC3, the new chronology for the EPICA ice core from Dome-C (EDC) through a stratigraphic match between EDML and EDC that consists of 322 volcanic match points over the last 128 ka. The EDC3 chronology comprises a glaciological model at EDC, which is constrained and later selectively tuned using primary dating information from EDC as well as from EDML, the latter being transferred using the tight stratigraphic link between the two cores. Finally, EDML1 was built by exporting EDC3 to EDML. For ages younger than 41 ka BP the new synchronized time scale EDML1/EDC3 is based on dated volcanic events and on a match to the Greenlandic ice core chronology GICC05 via <sup>10</sup>Be and methane. The internal consistency between EDML1 and EDC3 is estimated to be typically ~6 years and always less than 450 years over the last 128 ka (always less than 130 years over the last 60 ka), which reflects an unprecedented synchrony of time scales. EDML1 ends at 150 ka BP (2417 m depth) because the match between EDML and EDC becomes ambiguous further down. This hints at a complex ice flow history for the deepest 350 m of the EDML ice core.
 
Article
This paper presents quantitative reconstructions of vegetation and climate along the pollen sequence of Lake Barombi Mbo, southwestern Cameroon (4°39'45.75" N, 9°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.
 
Article
We examine several aspects of the ocean-atmosphere system over the last 30 000 years, by carrying out simulations with prescribed ice sheets, atmospheric CO<sub>2</sub> concentration, and orbital parameters. We use the GENIE-1 model with a frictional geostrophic ocean, dynamic sea ice, an energy balance atmosphere, and a land-surface scheme with fixed vegetation. A transient simulation, with boundary conditions derived from ice-core records and ice sheet reconstructions, is compared with equilibrium snapshot simulations, including the Last Glacial Maximum (21 000 years before present; 21 kyrBP), mid-Holocene (6 kyrBP) and pre-industrial. The equilibrium snapshot simulations are all very similar to their corresponding time period in the transient simulation, indicating that over the last 30 000 years, the model&apos;s ocean-atmosphere system is close to equilibrium with its boundary conditions. However, our simulations neglect the transfer of fresh water from and to the ocean, resulting from the growth and decay of ice sheets, which would, in reality, lead to greater disequilibrium. Additionally, the GENIE-1 model exhibits a rather limited response in terms of its Atlantic Meridional Overturning Circulation (AMOC) over the 30 000 years; a more sensitive AMOC would also be likely to lead to greater disequilibrium. We investigate the method of accelerating the boundary conditions of a transient simulation and find that the Southern Ocean is the region most affected by the acceleration. The Northern Hemisphere, even with a factor of 10 acceleration, is relatively unaffected. The results are robust to changes to several tunable parameters in the model. They also hold when a higher vertical resolution is used in the ocean.
 
Article
High-temporal resolution pollen record from the Alboran Sea ODP Site 976, pollen-based quantitative climate reconstruction and biomisation show that changes of Mediterranean vegetation have been clearly modulated by short and long term variability during the last 25 000 years. The reliability of the quantitative climate reconstruction from marine pollen spectra has been tested using 22 marine core-top samples from the Mediterranean. The ODP Site 976 pollen record and climatic reconstruction confirm that Mediterranean environments have a rapid response to the climatic fluctuations during the last Termination. The western Mediterranean vegetation response appears nearly synchronous with North Atlantic variability during the last deglaciation as well as during the Holocene. High-resolution analyses of the ODP Site 976 pollen record show a cooling trend during the Bölling/Allerød period. In addition, this period is marked by two warm episodes bracketing a cooling event that represent the Bölling-Older Dryas-Allerød succession. During the Holocene, recurrent declines of the forest cover over the Alboran Sea borderlands indicate climate events that correlate well with several events of increased Mediterranean dryness observed on the continent and with Mediterranean Sea cooling episodes detected by alkenone-based sea surface temperature reconstructions. These events clearly reflect the response of the Mediterranean vegetation to the North Atlantic Holocene cold events.
 
Article
Atmospheric CO<sub>2</sub> measured in Antarctic ice cores shows a natural variability of 80 to 100 ppmv during the last four glacial cycles and variations of approximately 60 ppmv in the two cycles between 410 and 650 kyr BP. We here use various paleo-climatic records from the EPICA Dome C Antarctic ice core and from oceanic sediment cores covering the last 740 kyr to force the ocean/atmosphere/biosphere box model of the global carbon cycle BICYCLE in a forward mode over this time in order to interpret the natural variability of CO<sub>2</sub>. Our approach is based on the previous interpretation of carbon cycle variations during Termination I (Köhler et al., 2005a). In the absense of a process-based sediment module one main simplification of BICYCLE is that carbonate compensation is approximated by the temporally delayed restoration of deep ocean [CO<sub>3</sub><sup>2?</sup>]. Our results match the low frequency changes in CO<sub>2</sub> measured in the Vostok and the EPICA Dome C ice core for the last 650 kyr BP ( r <sup>2</sup>?0.75). During these transient simulations the carbon cycle reaches never a steady state due to the ongoing variability of the overall carbon budget caused by the time delayed response of the carbonate compensation to other processes. The average contributions of different processes to the rise in CO<sub>2</sub> during Terminations I to V and during earlier terminations are: the rise in Southern Ocean vertical mixing: 36/22 ppmv, the rise in ocean temperature: 26/11 ppmv, iron limitation of the marine biota in the Southern Ocean: 20/14 ppmv, carbonate compensation: 15/7 ppmv, the rise in North Atlantic deep water formation: 13/0 ppmv, the rise in gas exchange due to a decreasing sea ice cover: ?8/?7 ppmv, sea level rise: ?12/?4 ppmv, and rising terrestrial carbon storage: ?13/?6 ppmv. According to our model the smaller interglacial CO<sub>2</sub> values in the pre-Vostok period prior to Termination V are mainly caused by smaller interglacial Southern Ocean SST and an Atlantic THC which stayed before MIS 11 (before 420 kyr BP) in its weaker glacial circulation mode.
 
Article
Chironomids preserved in a sediment core from Lago di Origlio (416 m a.s.l.), a lake in the foreland of the Southern Swiss Alps, allowed quantitative reconstruction of Late Glacial and Early Holocene summer temperatures using a combined Swiss-Norwegian temperature inference model based on chironomid assemblages from 274 lakes. We reconstruct July air temperatures of ca. 10 °C between 17 300 and 16 000 cal yr BP, a rather abrupt warming to ca. 12.0 °C at ca. 16 500-16 000 cal yr BP, and a strong temperature increase at the transition to the Bølling/Allerød interstadial with average temperatures of about 14 °C. During the Younger Dryas and earliest Holocene similar temperatures are reconstructed as for the interstadial. The rather abrupt warming at 16 500-16 000 cal yr BP is consistent with sea-surface temperature as well as speleothem records, which indicate a warming after the end of Heinrich event 1 (sensu stricto) and before the Bølling/Allerød interstadial in southern Europe and the Mediterranean Sea. Pollen records from Origlio and other sites in southern Switzerland and northern Italy indicate an early reforestation of the lowlands 2000-1500 yr prior to the large-scale afforestation of Central Europe at the onset of the Bølling/Allerød period at ca. 14 700-14 600 cal yr BP. Our results suggest that these early afforestation processes in the formerly glaciated areas of northern Italy and southern Switzerland have been promoted by increasing temperatures.
 
July precipitation difference between with and without ice sheets experiments where the 506 ka BP ice sheets volumes are assumed to be the same as they were at the Last Glacial Maximum and the centre of the NA ice sheet is moved one grid point to the south of its LGM position. Color shading and contour lines of confidence levels are like in Fig. 2.  
Exp. 5 (506 ka BP with ice sheets) minus Exp. 1 (Pre-Industrial) for (a) July precipitation (cm/year) and (b) July geopotential (m 2 s −2 ) and wind (m/s) at 800 hPa level. In (a) color shading and contour lines of confidence levels are like in Fig. 2.
Article
Deep-sea and ice-core records show that interglacial periods were overall less "warm" before about 420 000 years ago than after, with relatively higher ice volume and lower greenhouse gases concentration. This is particularly the case for the interglacial Marine Isotope Stage 13 which occurred about 500 000 years ago. However, by contrast, the loess and other proxy records from China suggest an exceptionally active East Asian summer monsoon during this interglacial. A three-dimension Earth system Model of Intermediate complexity was used to understand this seeming paradox. The astronomical forcing and the remnant ice sheets present in Eurasia and North America were taken into account in a series of sensitivity experiments. Expectedly, the seasonal contrast is larger and the East Asian summer monsoon is reinforced compared to Pre-Industrial time when Northern Hemisphere summer is at perihelion. Surprisingly, the presence of the Eurasian ice sheet was found to reinforce monsoon, too, through a south-eastwards perturbation planetary wave. The trajectory of this wave is influenced by the Tibetan plateau.
 
Left: comparison of the EDC3 age scale (red) (Parrenin et al., 2007a) and the modeled age scale using the simple 1-D model with m = 0.5 (black), m = 0.3 (blue) and m = 0.7 (purple). Right: comparison of the measured Dome C temperature profile (red) and the modeled temperature profile using the 1-D model for m = 0.5 with constant thermal conductivity K and heat capacity c at the bottom temperature T b = 270.4 K (black), at the measured mean temperature over the entire ice column T m = 240.9 K (blue) and using temperature dependent K and c (green). Also shown are versions with constant K and c using the bottom temperature, but for m = 0.3 (blue) and m = 0.7 (purple), which are very similar to the one for m = 0.5. 
Article
The recovery of a 1.5 Myr long ice core from Antarctica represents a keystone to our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we show that such old ice is most likely to exist in the plateau area of the East Antarctic Ice Sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle positions on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, we strongly argue for significantly reduced ice thickness to avoid bottom melting, while at the same time maximizing the resolution and the distance of such old ice to the bedrock. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than 2500 m would be required to find 1.5 Myr old ice. However, the final choice is strongly dependent on the local geothermal heat flux, which is largely unknown for the EAIS and has to be determined beforehand. In addition, the detailed bedrock topography and ice flow history for candidates of an Oldest Ice ice coring site has to be reconstructed. Finally, we argue strongly for rapid access drilling before any full deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.
 
(a) Surface temperature anomalies relative to 1000-1850 AD reference period averaged over the Northern Hemisphere in SGI, on annual mean (grey line). The thick black line is the ten-year running mean applied on the annual averages, sampled every ten years to keep the number of degrees of freedom. The same treatment is done on the CTRL simulation, drawn on grey blue. (b) Fingerprints of solar (red lines) and CO 2 blue lines) forcings, estimated with univariate (dashed lines) and bivariate linear regressions and anomalies of TSI and CO 2 concentrations relative to the 1000-1850 AD period. The fingerprint of both solar and CO 2 forcings together is represented by the dark red line. The black thin line shows the 10 yr-running mean of the NH temperature of SGI presented on panel (a). (c) Fingerprint of both solar and CO 2 forcings together (dark red line). The residuals corresponding to SGI minus total forced fingerprint are represented by the green line. The light shaded area shows the interval corresponding to 1 standard deviation (SD) of the residuals, and the dark shaded area depicts the ±2SD of the residuals. The black line plotted over is the ten-year NH temperature of SGI presented on panel (a).
Signal-Noise Ratio (SNR) calculated as a function of the spatial scale with the ratio of variance 2
Percentage of temperature variance explained at first order by the terms in Eq. (4) corresponding to the fingerprints of solar (d, e, f), CO 2 (g, h, i), and orbital (j, k, l) forcings, and of all the forcings together (m, n, o). The white areas depicts the grid points where the fingerprints explain less than 5% of the total variance of the temperature at the given grid points, or where the regression coefficients shown on Fig. 5 are not significant at 99.5%. The upper panels (a, b, c) show the temperature variance of the ten-year smoothed time series used in this study. All those diagnostics are done on annual (a, d, g, j, m), summer (b, e, h, k, n) and winter averages (c, f, i, l, o).
comparison between the Arctic summer temperature reconstruction provided by (Kaufman et al., 2  
Percentage of Earth surface (on x-axis) where the variances of the signatures of the forcings used in SGI explain at least the percentage of temperature variance represented on y-axis. This diagnostic is represented for solar (red line), CO 2 (blue line) and orbital (orange line) forcing and for all the forcings together (black line). The horizontal dashed lines underline 25 and 50% of temperature variance.  
Article
Studying the climate of the last millennium gives the possibility to deal with a relatively well-documented climate essentially driven by natural forcings. We have performed two simulations with the IPSLCM4 climate model to evaluate the impact of Total Solar Irradiance (TSI), CO 2 and orbital forcing on secular temperature variability during the preindustrial part of the last millennium. The Northern Hemisphere (NH) temperature of the simulation reproduces the amplitude of the NH temperature reconstructions over the last millennium. Using a linear statistical decomposition we evaluated that TSI and CO 2 have similar contributions to secular temperature variability between 1425 and 1850 AD. They generate a temperature minimum comparable to the Little Ice Age shown by the temperature reconstructions. Solar forcing explains ̃80% of the NH temperature variability during the first part of the millennium (1000-1425 AD) including the Medieval Climate Anomaly (MCA). It is responsible for a warm period which occurs two centuries later than in the reconstructions. This mismatch implies that the secular variability during the MCA is not fully explained by the response of the model to the TSI reconstruction. With a signal-noise ratio (SNR) estimate we found that the temperature signal of the forced simulation is significantly different from internal variability over area wider than ̃5.106 km2, i.e. approximately the extent of Europe. Orbital forcing plays a significant role in latitudes higher than 65° N in summer and supports the conclusions of a recent studyon an Arctic temperature reconstruction over past two millennia. The forced variability represents at least half of the temperature signal on only ̃30% of the surface of the globe. This study suggests that regional reconstructions of the temperature between 1000 and 1850 AD are likely to show weak signatures of solar, CO 2 and orbital forcings compared to internal variability.
 
Article
ODP Site 1078 situated under the coast of Angola provides the first record of the vegetation history for Angola. The upper 11 m of the core covers the past 30 thousand years, which has been analysed palynologically in decadal to centennial resolution. Alkenone sea surface temperature estimates were analysed in centennial resolution. We studied sea surface temperatures and vegetation development during full glacial, deglacial, and interglacial conditions. During the glacial the vegetation in Angola was very open consisting of grass and heath lands, deserts and semi-deserts, which suggests a cool and dry climate. A change to warmer and more humid conditions is indicated by forest expansion starting in step with the earliest temperature rise in Antarctica, 22 thousand years ago. We infer that around the period of Heinrich Event 1 a northward excursion of the Angola Benguela Front and the Congolian Air Boundary resulted in cool sea surface temperatures and a northward extension of desert vegetation along the coast. Rain forest and dry forest returned 15 thousand years ago. During the Holocene, dry forests and Miombo woodlands expanded. Also in Angola globally recognised climate changes at 8 thousand and 4 thousand years ago had an impact on the vegetation. During the past 2 thousand years, savannah vegetation became dominant.
 
Article
Nine thousand years ago (9 ka BP), the Northern Hemisphere experienced enhanced seasonality caused by an orbital configuration close to the minimum of the precession index. To assess the impact of this "Holocene Insolation Maximum" (HIM) on the Mediterranean Sea, we use a regional ocean general circulation model forced by atmospheric input derived from global simulations. A stronger seasonal cycle is simulated by the model, which shows a relatively homogeneous winter cooling and a summer warming with well-defined spatial patterns, in particular, a subsurface warming in the Cretan and western Levantine areas. The comparison between the SST simulated for the HIM and a reconstruction from planktonic foraminifera transfer functions shows a poor agreement, especially for summer, when the vertical temperature gradient is strong. As a novel approach, we propose a reinterpretation of the reconstruction, to consider the conditions throughout the upper water column rather than at a single depth. We claim that such a depth-integrated approach is more adequate for surface temperature comparison purposes in a situation where the upper ocean structure in the past was different from the present-day. In this case, the depth-integrated interpretation of the proxy data strongly improves the agreement between modelled and reconstructed temperature signal with the subsurface summer warming being recorded by both model and proxies, with a small shift to the south in the model results. The mechanisms responsible for the peculiar subsurface pattern are found to be a combination of enhanced downwelling and wind mixing due to strengthened Etesian winds, and enhanced thermal forcing due to the stronger summer insolation in the Northern Hemisphere. Together, these processes induce a stronger heat transfer from the surface to the subsurface during late summer in the western Levantine; this leads to an enhanced heat piracy in this region, a process never identified before, but potentially characteristic of time slices with enhanced insolation.
 
Maps showing the modern environmental pattern and Loess Plateau in China. (a) Modern environmental pattern and the prevailing atmospheric circulation. Dotted arrows indicate the southwest and southeast Asian summer monsoons and solid arrows indicate the Asian winter monsoon. (b) Loess Plateau in China and the studied sites.  
Correlation of the China loess proxies of Asian dust intensity and winter monsoon with relevant ice and marine records. (a) Marine δ 18 O record (Lisiecki and Raymo, 2005) (blue) with the oxygen isotope stages (MIS) labelled at the top part, and loess magnetic susceptibility at Xifeng (red) with the major soil and loess units labelled; (b) Dust ( blue) and Fe 2 O 3 flux (red) at Xifeng plotted on log scales; (c) Antarctic (Vostok and EDC) CO 2 records (Luthi et al., 2008; Petit et al., 1999; Siegenthaler et al., 2005); (d) EDC (Lambert et al., 2008) (red, log scale) and equatorial Pacific (Winckler et al., 2008) (blue) dust fluxes; (e) Loess grain-size changes shown by the content of the >32 µm fraction at Xifeng (red) and EDC dust size normalized (Lambert et al., 2008) (blue). (f) Summer daily insolation (Berger, 1978) at 65 • N (red) and at 65 • S (blue). Marine data are plotted versus their own timescales. Antarctic data are plotted versus EDC3 chronology (Parrenin et al., 2007).
Correlation of the China loess proxies of Asian summer monsoon with relevant ice and marine records. (a) Chemical weathering index (Guo et al., 2000) (Fed/Fet) at Xifeng (blue), Changwu (green) and the redness at Xifeng (red); (b) EDC δD (blue) (Jouzel et al., 2007) and CH 4 record (red) (Loulergue et al., 2008); (c) Benthic δ 13 C records from the North Atlantic site 552 (black) (Raymo et al., 1997), tropical Atlantic site 664 (blue) (Raymo et al., 1997), North Pacific site 849 (red) (Raymo et al., 1997) and Southern Ocean site RC 13-229 (green) (Oppo et al., 1990) versus their own timescales. (d) Summer daily insolation (Berger, 1978) at 30 • N.
Article
We correlate the China loess and Antarctica ice records to address the inter-hemispheric climate link over the past 800 ka. The results show a broad coupling between Asian and Antarctic climates at the glacial-interglacial scale. However, a number of decoupled aspects are revealed, among which marine isotope stage (MIS) 13 exhibits a strong anomaly compared with the other interglacials. It is characterized by unusually positive benthic oxygen (δ18O) and carbon isotope (δ13C) values in the world oceans, cooler Antarctic temperature, lower summer sea surface temperature in the South Atlantic, lower CO2 and CH4 concentrations, but by extremely strong Asian, Indian and African summer monsoons, weakest Asian winter monsoon, and lowest Asian dust and iron fluxes. Pervasive warm conditions were also evidenced by the records from northern high-latitude regions. These consistently indicate a warmer Northern Hemisphere and a cooler Southern Hemisphere, and hence a strong asymmetry of hemispheric climates during MIS-13. Similar anomalies of lesser extents also occurred during MIS-11 and MIS-5e. Thus, MIS-13 provides a case that the Northern Hemisphere experienced a substantial warming under relatively low concentrations of greenhouse gases. It suggests that the global climate system possesses a natural variability that is not predictable from the simple response of northern summer insolation and atmospheric CO2 changes. During MIS-13, both hemispheres responded in different ways leading to anomalous continental, marine and atmospheric conditions at the global scale. The correlations also suggest that the marine δ18O record is not always a reliable indicator of the northern ice-volume changes, and that the asymmetry of hemispheric climates is one of the prominent factors controlling the strength of Asian, Indian and African monsoon circulations, most likely through modulating the position of the inter-tropical convergence zone (ITCZ) and land-sea thermal contrasts.
 
Difference in the latitudinal and seasonal insolation (Wm −2 ) distribution between 506 and 495 ka BP. Labels on the X-axis indicate the true longitude of the Sun from the beginning to the end of the year. Insolation is calculated from the long-term variations of eccentricity, precession and obliquity (Berger, 1978; Berger et al., 1993).
Orbital parameters and ice volumes used for the exper- iments in this paper. The same greenhouse gases concentrations (CO 2 =240 ppmv, CH 4 =510 ppbv and N 2 O=280 ppbv) are used in all these experiments.
Difference between Exp. 5 and Exp. 1 in July 800 hPa geopotential (m 2 s −2 ) and wind (m/s) over East Asia.
Article
Simulations with an Earth System Model of Intermediate Complexity are made to investigate the role of insolation and the size of ice sheets on the regional and global climate for marine isotope stage (MIS) 13. The astronomical forcing is selected at two dates with opposite precession, one when Northern Hemisphere summer (NHS) occurs at perihelion (at 506 ka BP) and the other when it occurs at aphelion (at 495 ka BP). Experiments with five different volumes of the Eurasian and North American ice sheets (ranging from 0 to the Last Glacial Maximum one) are done under these two astronomical conditions. When NHS is at perihelion, the Earth is warmer, the seasonal contrast in Northern (Southern) Hemisphere is larger (smaller) and summer precipitation in Northern Hemisphere monsoon regions is more abundant than when it is at aphelion. The global cooling due to the ice sheets is mainly related to the ice sheet area, little to their height. The regional cooling and warming anomalies caused by the ice sheets get intensified with increasing ice sheet size. The cooling is different whether the NHS occurs at aphelion or at perihelion. Precipitation over different monsoon regions responds differently to the size of the ice sheets. Over North Africa, the ice sheets always reduce precipitation, larger the size less the precipitation. Over East China, when NHS is at perihelion, the ice sheets reinforce the summer precipitation whatever their sizes. But when NHS is at aphelion, there is a threshold in the ice volume beyond which the ice sheets start to reduce the precipitation over East China. This underlies the importance of insolation in shaping the ice sheet impact on the precipitation over the East Asian Summer Monsoon (EASM) region.
 
Location of Lake Mbalang in the Adamawa plateau and morphometric features of the lake and its area. The location of the lake is shown with a black star in (b).  
Sketch of atmospheric features (clouds cover and air movement) and relative modifications of Lake Mbalang level, in the dry season (January) and rainy season (August) before 3600 cal yrs BP (a) and afterwards (b). Before 3600 cal yrs BP, stratiform cloud cover were abundant, convective cloud are dominant after 3600 cal yrs BP.
Article
Past limnological conditions of Lake Mbalang (7°19´ N, 13°44´ E, alt: 1130 m) and vegetation type were reconstructed from diatoms and sedimentary stable carbon isotope records (δ13C) since 7200 cal yrs BP. The data showed that before 3600 yrs cal BP the water column was preferentially cold and stable except around 5000–5300 cal yrs BP where diatom evidenced mixed upper water layer, δ13C data suggest more forested vegetation in the landscape. These stable conditions can be explained by a strong monsoonal flux and correlatively northern position of the ITCZ that entailed high/low rainfall well distributed over the year to allow the development mountainous forest taxa. The decreasing trend of the monsoonal flux towards mid-Holocene was however affected by several centennial to millennial time scale abrupt weakening at 6700, 5800–6000, 5000–5300, 4500 and 3600 cal yrs BP although their impact on vegetation is not visible probably because rainfall distribution was favourable to forest maintenance or extension. After 3600 cal yrs BP, water column became very mixed as a result of more intense NE trade winds (Harmattan) that led at ~3000 cal yrs BP to the instalment of savana in the vegetation landscape. At that time, rainfall was probably reduced following the southwards shift of the ITCZ and the distribution of yearly rainfall was no more favourable to forest development. Thus a strong seasonality with a well marked dry season was established, conditions that maintained the savana vegetation till today. Diatom data suggest the lake did not dried during the last 7200 cal yrs BP, however, a low lake level observed at 2400–2100 cal yrs BP is contemporaneous to a climatic event evidenced in several areas of tropical Africa and could correspond to the southernmost position of the ITCZ. Other low lake levels are observed at 1800 and 1400 cal yrs BP, after which lake rose to its present level.
 
Article
Using an intermediate-complexity UVic Earth System Climate Model (UVic Model), the geographical and seasonal implications and indicative senses of the Guliya temperature proxy found in the Guliya δ18O ice core record (hereinafter, the Guliya δ18O-temperature proxy record) are investigated under time-dependent orbital and CO2 forcings with an acceleration factor of 50 over the past 130 ka. The results reveal that the simulated August-September Guliya surface air temperature (SAT) reproduces the 21-ka precession and 43-ka obliquity cycles of the Guliya δ18O-temperature proxy record, showing an in-phase variation with the latter. Moreover, the Guliya δ18O-temperature proxy record may be also an indicator of the August-September Northern Hemispheric (NH) SAT. Corresponding to the difference between the extreme warm and cold phases of the precession cycle in the Guliya August-September SAT, there are two anomalous patterns in SAT and sea surface temperature (SST). The first anomalous pattern shows increases of SAT and SST toward the Arctic, which is possibly associated with an increase of the NH incoming solar radiation that is caused by the in-phase superposition between the precession and obliquity cycles. The second anomalous pattern shows increases of SAT and SST toward the equator, which is possibly due to a decrease of incoming solar radiation over the NH polar that results from the anti-phase counteraction between the precession and obliquity cycles. The summer (winter) Guliya and NH temperatures are higher (lower) in the warm phases of the August-September Guliya than in their cold phases. Moreover, in August-September, the Guliya SAT is closely related to the North Atlantic SST, in which the Guliya precipitation might act as a "bridge" linking the Guliya SAT and the North Atlantic SST.
 
Comparisons of the Law Dome volcanic event start dates with other independently dated records. Volcanic Event Law Dome ice date NGRIP 1 DML 2 South Pole 3 NH tree ring signature 4
Comparison of Law Dome to other ice core volcanic records. Figure 2 illustrates the number of years of difference between the dating of volcanic events common to the NGRIP, DML and South Pole (SP045C) ice cores relative to Law Dome. Volcanic event dates have been matched on eruption signature. Squares indicate volcanic event dates. DML and South Pole cores are from relatively low accumulation sites (0.073 and 0.075 m yr −1 ice equivalent respectively), which may be a factor in the variability and drifting of those timescales relative to Law Dome (0.70 m yr −1 ice equivalent). NGRIP at (0.19 m yr −1 ice equivalent) still agrees well with Law Dome.
Volcanic sulphate deposition at Law Dome. For events identified as inter-hemispheric, NGRIP depositional data are provided also.
Article
Volcanic eruptions are an important cause of natural climate variability. In order to improve the accuracy of climate models, precise dating and magnitude of the climatic effects of past volcanism are necessary. Here we present a 2000-yr record of Southern Hemisphere volcanism recorded in ice cores from the high accumulation Law Dome site, East Antarctica. The ice cores were analysed for a suite of chemistry signals and are independently dated via annual layer counting, with 11 ambiguous years at 23 BCE, which has presently the lowest error of all published long Antarctic ice cores. Independently dated records are important to avoid circular dating where volcanic signatures are assigned a date from some external information rather than using the date it is found in the ice core. Forty-five volcanic events have been identified using the sulphate chemistry of the Law Dome record. The low dating error and comparison with the NGRIP (North Greenland Ice Core Project) volcanic records (on the GICC05 timescale) suggest Law Dome is the most accurately dated Antarctic volcanic dataset, which will improve the dating of individual volcanic events and potentially allow better correlation between ice core records, leading to improvements in global volcanic forcing datasets. One of the most important volcanic events of the last two millennia is the large 1450s CE event, usually assigned to the eruption of Kuwae, Vanuatu. In this study, we review the evidence surrounding the presently accepted date for this event, and make the case that two separate eruptions have caused confusion in the assignment of this event. Volcanic sulphate deposition estimates are important for modelling the climatic response to eruptions. The largest volcanic sulphate events in our record are dated at 1458 CE (Kuwae?, Vanuatu), 1257 and 422 CE (unidentified).
 
Article
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 R (t), 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, δ13C and δ18O. R (t) was calculated for all 30 locations. Seven locations, of which six are within close proximity of the coast, were found to have relatively higher R (t) values, indicative of hard-water effects. Whenever possible, the Macoma genus of mollusc was selected from the museum collections, in order to exclude species specific reservoir age effects as much as possible. When the Macoma samples are exclusively considered, and samples from hard-water locations excluded, a statistically significant correlation between Macoma R (t) and average salinity is found, indicating a two end-member linear mixing model between 14Cmarine and 14Crunoff. A map of Baltic Sea Macoma aragonite R (t) for the late 19th and early 20th centuries is produced. Such a map can provide an estimate for contemporary Baltic Sea Macoma R (t), 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 δ18Oaragonite and Macoma R (t), suggesting that δ18Oaragonite can be used to estimate Macoma palaeo-R (t), due to the δ18Oaragonite signal being dominated by the salinity gradient of the Baltic Sea. A slightly increased correlation can be expected when δ18Oaragonite is corrected for temperature fractionation effects. The results of this Baltic Sea case study, which show that R (t) is affected by hydrographic conditions and local carbon inputs, have important consequences for other coastal and estuarine locations, where R (t) is also likely to significantly vary on spatial and temporal bases.
 
Article
We present and analyse a high-resolution regional climate palaeosimulation encompassing the European region for the period 1500-1990. We use the regional model MM5 coupled to the global model ECHO-G. Both models were driven by reconstructions of three external factors: greenhouse gas concentrations, Total Solar Irradiance and volcanic activity. The simulation has been assessed in a recent period by comparing the model results with the Climate Research Unit (CRU) database. The results show that although the regional model is tightly driven by the boundary conditions, it is able to improve the reliability of the simulations, narrowing the differences to the observations, especially in areas of complex topography. Additionally, the evolution of the spatial distributions of temperature and precipitation through the last five centuries has been analysed. The mean values of temperature reflects the influence of the external forcings but, contrary to the results obtained under climate change scenario conditions, we found that higher-order momenta of the probability distribution of seasonal temperature and precipitation are hardly affected by changes in the external forcings
 
Article
In this paper, we develop a new methodology to estimate past changes of growing season temperature at Fontainebleau (northern France). Northern France temperature fluctuations have been documented by homogenised instrumental temperature records (at most 140 year long) and by grape harvest dates (GHD) series, incorporated in some of the European-scale temperature reconstructions. We have produced here three new proxy records: δ<sup>18</sup>O and δ<sup>13</sup>C of latewood cellulose of living trees and timbers from Fontainebleau Forest and Castle, together with ring widths of the same samples. δ<sup>13</sup>C data appear to be influenced by tree and age effects; ring widths are not controlled by a single climate parameter. By contrast, δ<sup>18</sup>O and Burgundy GHD series exhibit strong links with Fontainebleau growing season maximum temperature. Each of these records can also be influenced by other factors such as vine growing practices, local insolation, or moisture availability. In order to reduce the influence of these potential biases, we have used a linear combination of the two records to reconstruct inter-annual fluctuations of Fontainebleau growing season temperature from 1596 to 2000. Over the instrumental period, the reconstruction is well correlated with the temperature data ( R <sup>2</sup>=0.60). This reconstruction is associated with an uncertainty of ~1.1°C (1.5 standard deviation), and is expected to provide a reference series for the variability of growing season maximum temperature in Western Europe. Spectral analyses conducted on the reconstruction clearly evidence (i) the interest of combining the two proxy records in order to improve the power spectrum of the reconstructed versus observed temperature, (ii) changes in the spectral properties over the time, with varying weights of periodicities ranging between ~6 and ~25 years. Available reconstructions of regional growing season temperature fluctuations get increasingly divergent at the interannual or decadal scale prior to 1800. Our reconstruction suggests a warm interval in the late 17th century, with the 1680s as warm as the 1940s, followed by a prolonged cool period from the 1690s to the 1850s culminating in the 1770s. The persistency of the late 20th century warming trend appears unprecedented.
 
Article
An ensemble of simulations of the climate of the past millennium conducted with a three-dimensional climate model of intermediate complexity are constrained to follow temperature histories obtained from a recent compilation of well-calibrated surface temperature proxies using a simple data assimilation technique. Those simulations provide a reconstruction of the climate of the Arctic that is compatible with the model physics, the forcing applied and the proxy records. Available observational data, proxy-based reconstructions and our model results suggest that the Arctic climate is characterized by substantial variations in surface temperature over the past millennium. Though the most recent decades are likely to be the warmest of the past millennium, we find evidence for substantial past warming episodes in the Arctic. In particular, our model reconstructions show a prominent warm event during the period 1470–1520. This warm period is likely related to the internal variability of the climate system, that is the variability present in the absence of any change in external forcing. We examine the roles of competing mechanisms that could potentially produce this anomaly. This study leads us to conclude that changes in atmospheric circulation, through enhanced southwesterly winds towards northern Europe, Siberia and Canada, are likely the main cause of the late 15th/early 16th century Arctic warming.
 
Article
The study of pointer years of numerous tree-ring chronologies of the central Iberian Peninsula (Sierra de Guadarrama) could provide complementary information about climate variability over the last 405 yr. 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 was found to be the most limiting factor for the growth of Pinus 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, these pointer years and intervals are not evenly distributed throughout time. Both in the first half of the 17th and in the second half of 20th, they were more frequent and more extreme and these periods are the most notable for the frequency of negative pointer years in Central Spain. The interval 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. We infer that this special minimum annual increase was 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 Southern Europe have been demonstrated.
 
Distribution of the ice coring sites on the Tibetan Plateau. Dots indicate meteoro-stations around the Plateau; numbers are the sites of the ice cores discussed in the paper.
(a) Cramer's test verification, t indicates relationship between 31-year running mean and the whole serial mean; (b) Puruogangri ice core yearly accumulation record since AD 1600.  
Decadal changes of precipitation based on glacial accumulation reconstructed from Guliya ice core (a), Dunde ice core (b), and Puruogangri ice core (c). The decadal temperature reconstructed by the composition of δ 18 O from Puruogangri, Guliya and Dunde ice core (d). The thick lines are linear trends of precipitation and temperature reconstructions.  
Article
Lack of reliable long-term precipitation record from the northern Tibetan Plateau has constrained our understanding of precipitation variations in this region. We drilled an ice core on the Puruogangri Ice Field in the central Tibetan Plateau in 2000 to reveal the precipitation variations. The well dated part of the core extends back to AD 1600, allowing us to construct a 400-year annual accumulation record. This record shows that the central Tibetan plateau experienced a drier period with an average annual precipitation of ~300 mm in the 19th century, compared to ~450 mm in the wetter periods during 1700–1780 and the 20th century. This pattern agrees with precipitation reconstructions from the Dunde and Guliya ice cores on the northern Plateau but differs from that found in the Dasuopu ice cores from the southern Plateau The north-south contrasts in precipitation reconstruction reveals difference in moisture origin between the south Tibetan Plateau dominated by the Asian monsoon and the north Tibetan Plateau dominated by the continental recycling and the westerlies.
 
Article
We explore the climatic information contained in the record of length fluctuations of Glaciar Frías, in the north Patagonian Andes of Argentina. This record is one of the longest and most detailed glacier records in southern South America, starting in 1639. In order to interpret the length variations of Glaciar Frías since the maximum Little Ice Age extent, we use a combination of a simplified surface energy-balance model to calculate the glacier mass balance, and a flowline model to account for the dynamical response of the glacier to changes in the climatic forcing. The overall retreat of the glacier observed over 1639-2009 is best explained by an annual mean temperature increase of 1.2 °C or a decrease in annual precipitation of 34%, most of which would have occurred during the 20th century. The glacier model is also forced with two independent tree-ring and multi-proxy reconstructions of precipitation and temperature. The uncertainties in these reconstructions are rather large, leading to a wide range in the modelled glacier length that includes most of the observations. However, in both reconstructions, the mid-17th century seems to be too cold and the early 19th century too warm to explain the observed glacier lengths with the glacier model forced with the reconstructions. Forcing with reconstructed precipitation and temperature separately shows that the influence of historical variations in precipitation on the glacier fluctuations of Glaciar Frías is smaller than that of the temperature fluctuations. This suggests that the observed 1639-2009 retreat could be best explained by a warming close to 1.2 °C.
 
Article
In this manuscript we have attempted to consolidate the common signal in previously defined proxy reconstructions of the El Niño-Southern Oscillation into one individual proxy titled the Unified ENSO Proxy (UEP). While correlating well with the majority of input reconstructions, the UEP provides better representation of observed indices of ENSO, discrete ENSO events and documented historical chronologies of ENSO than any of these input ENSO reconstructions. Further to this, the UEP also provides a means to reconstruct the PDO/IPO multi-decadal variability of the Pacific Ocean as the low-pass filtered UEP displays multi-decadal variability that is consistent with the 20th century variability of the PDO and IPO. The UEP is then used to describe changes in ENSO variability which have occurred since 1650 focusing on changes in ENSOs variance, multi-year ENSO events, PDO-like multi-decadal variability and the effects of volcanic and solar forcing on ENSO. We find that multi-year El Niño events similar to the 1990–1995 event have occurred several times over the last 3 1/2 centuries. Consistent with earlier studies we find that volcanic forcing can induce a statistically significant change in the mean state of ENSO in the year of the eruption and a doubling of the probability of an El Niño (La Niña) event occurring in the year of (three years after) the eruption.
 
Article
This paper examines the processes underlying changes to the once-extensive Bermejo Wetland, east of the city of Mendoza, Argentina (32°55' S, 68°51' W). Historical documents and maps from the 16th to 20th century are used to reconstruct environmental shifts. Historical documents indicate periods of increased snowfall in the adjacent Andes mountains, as well as high flow volumes in the Mendoza River. Data from georeferenced maps, the first from 1802 and the last from 1903, reflect the changes in the surface area of the wetland. The combined data sets show pulses of growth and retraction, in which major expansions coincided with more intense snowstorms and increased flow in the Mendoza River, which in turn influenced socio-economic activities. The wetland became progressively drier during the 19th century, before drying up completely around 1930, due in part to the construction of drainages and channels.
 
map showing the location of the Undarsmosse and Store Mosse bogs on the south-west coast of Sweden. The locations of two other sites mentioned in the text (Vejers dunefield, lake Igelsjön) are also shown.  
Stratigraphy and age models for the Undarsmosse and Store Mosse bog. The upper 30 cm in the Store Mosse bog was omitted from the stratigraphy since this layer is though to have been cut from a deeper adjacent peat section, and placed on the surface to dry. This is indicated by the erroneously old ages, as wel as in the pollen assemblage, organic bulk density values and ASI record.  
Radiocarbon dates from the Undarsmosse and Store Mosse bogs, calibrated ages and error margins. The radiocarbon dates were calibrated with the IntCal04 calibration curve (Reimer et al., 2004).
The most common testate amoebae at Undarsmosse shown as the percentage of total testate amoebae plotted on common timescale (right y-axis) and cal. yrs BP (left y-axis). The testate amoebae are grouped according to their ecological wetness preferences.  
Comparison between aeolian sediment influx data (ASI) and pollen types indicating land use. Shown are the total pollen percentage for pollen types indicating (b) crop cultivation (Secale, Cerealea) and (c) grassland areas (Poaceae, Taraxacum, Plantago lanceolata, Potentilla, Rumex acetosa/acetosella).  
Article
Analyses of testate amoebae and aeolian sediment influx (ASI) were used to reconstruct effective humidity changes and aeolian activity in the coastal zone of south-west Sweden. Cores were taken from an ombrotrophic peat sequence from the Undarsmosse bog. Since both types of analysis were carried out on the same core, a direct comparison between humidity fluctuations in the bog and aeolian activity was possible, potentially providing detailed information on atmospheric circulation changes in this region. Relatively wet bog surface conditions occurred from around 1500 to 1230 and 770 to 380 cal. yrs BP, whereas dry conditions dominated from ca. 1630 to 1530, 1160 to 830 and 300 to 50 cal. yrs BP. The transitions between these phases occurred within 60?100 years and are characterised by a major change in the testate amoebae assemblages. A watertable reconstruction was used to study the hydrological changes at the bog surface in more detail. ASI peak events were reconstructed around 1450, 1150, 850 and after 370 cal. yrs BP. Most interestingly, these aeolian activity peaks started during the recorded hydrological transitions, regardless of the direction of these shifts. Our results therefore suggest that humidity shifts in this region were associated with temporary intensifications of atmospheric circulation during the past 1700 years. Several ASI peaks apparently coincide with reduced solar activity, possibly suggesting a solar related cause for some of the observed events.
 
Article
We present statistically reconstructed mean annual winter (December-February) temperatures from the middle and lower reaches of the Yangtze River (24° N-34° N, 108° E-123° E within mainland China) extending back to 1736. The reconstructions are based on information regarding snowfall days from historical documents of the Yu-Xue-Fen-Cun archive recorded during the Qing Dynasty (1644-1911). This information is calibrated with regional winter temperature series spanning the period from 1951 to 2007. The gap from 1912 to 1950 is filled using early instrumental observations. With the reference period of 1951-2007, the 18th century was 0.76 °C colder, and the 19th century was 1.18 °C colder. However, since the 20th century, the climate has been in a warming phase, particularly in the last 30 yr, and the mean temperature from 1981 to 2007 was 0.25 °C higher than that of the reference period of 1951-2007, representing the highest temperatures of the past 300 yr. Uncertainty existed for the period prior to 1900, and possible causes of this uncertainty, such as physical processes involved in the interaction between temperature and snowfall days and changing of observers, are discussed herein.
 
Article
Among the different meteorological hazards, droughts are those with the highest socio-economical impact on the Iberian Peninsula. Drought events have been largely studied in the instrumental period, but very little is known about the characteristics of droughts in the preinstrumental period. In this work, several series of rogation ceremonies are used to identify severe droughts within the period 1750-1850. The overlapping of the rogation series with some instrumental series served to identify some climatic characteristics of rogation ceremonies: (a) during spring, rainfall deficits needed to celebrate rogation ceremonies are smaller than in any other season; (b) the hydrological deficit in a particular region increases with the number of locations celebrating rogations simultaneously. On the other hand, it was found that between 1750-1754 and 1779-1783 are probably the driest periods of the 101 analyzed years. Both show an important number of rogations all over Iberia and during all the seasons. The most extended drought of this period occurred during the spring of 1817, affecting 15 of the 16 locations studied. This drought was influenced by the Tambora eruption (1815). The study of the climate footprint of this eruption and its comparison with similar situations in the series suggest that the spring drought of 1824 may be associated with the eruptions of the Galunggung and Usu volcanoes (1822). Further studies are required to confirm this fact and understand the atmospheric mechanisms involved.
 
Article
The paper is oriented on social vulnerability to climate in Switzerland and in the Czech Lands during the early 1770s. Documentary sources of climate related to man-made archives are discussed. Methods of temperature and precipitation reconstruction based on this evidence as well as climate impact analyses are presented. Modelling of Little Ice Age-type Impacts (LIATIMP) is applied to highlight climate impacts during the period 1750?1800 in the Swiss Plateau and in the Czech Lands. LIATIMP are defined as adverse climate situations affecting grain production, mainly in terms of rainy autumns, cold springs and rainy harvest-periods. The most adverse weather patterns according to this model occurred from 1769 to 1771 causing two, in the case of the Czech Lands even three successive harvest failures. The paper addresses the social and economic consequences of this accumulation of climatic stress and explores how the authorities and the victims dealt with this situation.
 
Article
In this paper, series of drought occurrence and drought extension in the Iberian Peninsula are constructed for the 1600–1750 period from seven rogation series. These rogation ceremony records come from Bilbao, Catalonia, Zamora, Zaragoza, Toledo, Murcia and Seville. They are distributed across the Peninsula and include the areas with the most characteristic Iberian climate types, influenced by the Atlantic and the Mediterranean conditions, described from modern data. A seasonal division of the series shows that spring is a critical season for rogation series in most of Iberia, being Bilbao the only site were the highest number of rogations is detected for a different season. The annual analysis of the series shows a dramatic difference between the period 1600–1652, when droughts are characterized by its local character; and the period 1653–1749, when they affect to broader regions or even to the whole Peninsula. The analysis of spring series confirms the existence of the two periods detected in the annual analysis. Finally, secondary documentary sources are used to further characterise the two most extended droughts in the period, 1664 and 1680, and to verify the extension of the areas affected by droughts recorded through rogation series.
 
Article
A 33 m ice core was retrieved from the Tanggula Mts, central Tibetan Plateau at 5743 m a.s.l. in August 2005. Annual average δ18O values were determined for the upper 17 m depth (14.6 m w.eq.), representing the time series since the mid-1930s. Data are compared to previous results of an ice core from Mt. Geladaindong, 100 km to the northwest, for the period 1935–2003. During the time 1935–1960, δ18O values differed by 2–3‰ between the two ice cores, with generally lower ratios preserved in the Tanggula 2005 core. Differences in interannual variability and overall average ratios between the two study locations highlight the spatially variable climate controls on ice core isotope ratios within the boundary of monsoon- and westerly-impacted regions of the central Tibetan Plateau. Average annual net accumulation was 261 mm w.eq. for the period 1935–2004. The overall average δ18O value was −13.2‰ and exhibited a statistically significant increase from the 1935–1969 average (−13.7‰) to the 1970–2004 average (−12.6‰). Despite the observed increase in isotope ratios, isotopic temperature dependence was not evident, based on comparison with long-term data from meteorological stations to the north and southwest of the study location. Lack of correlation between average δ18O values and temperature is likely due to monsoon influence, which results in relatively greater isotopic depletion of moisture during the warm season. Evidence of monsoon impacts on precipitation in the central Tibetan Plateau has been previously documented, and statistically significant negative correlation (r=−0.37, p
 
(a) Climatology (× 10 10 kg s −1 ) and (b) trend (× 10 10 kg s −1 per decade) of the mass stream function of the Hadley Circulation for 1871-2008.
Spectrum and red-noise confidence intervals for the time series of the annual mean width of the Hadley Circulation.
Scatter plots of the strength and width of the Hadley Circulation versus the averaged 20CR2 surface air temperature in the tropics and subtropics (30 • S-30 • N) during the cold (1871-1925, blue) and warm (1926-2008, red) periods: (a) the strength of the northern component, (b) the strength of the southern component, and (c) the width.
Article
Recent studies demonstrate that the Hadley Circulation has intensified and expanded for the past three decades, which has important implications for subtropical societies and may lead to profound changes in global climate. However, the robustness of this intensification and expansion that should be considered when interpreting long-term changes of the Hadley Circulation is still a matter of debate. It also remains largely unknown how the Hadley Circulation has evolved over longer periods. Here, we present long-term variability of the Hadley Circulation using the 20th Century Reanalysis. It shows a slight strengthening and widening of the Hadley Circulation since the late 1970s, which is not inconsistent with recent assessments. However, over centennial timescales (1871-2008), the Hadley Circulation shows a tendency towards a more intense and narrower state. More importantly, the width of the Hadley Circulation might have not yet completed a life-cycle since 1871. The strength and width of the Hadley Circulation during the late 19th to early 20th century show strong natural variability, exceeding variability that coincides with global warming in recent decades. These findings raise the question of whether the recent change in the Hadley Circulation is primarily attributed to greenhouse warming or to a long-period oscillation of the Hadley Circulation - substantially longer than that observed in previous studies.
 
Spatial distribution of the cited temperature proxy series (a) and temperature changes at a centennial timescale (5-point FFT filter) in five regions of China (Northeast, Central East, Southeast, Northwest, and the Tibet Plateau) (b). The temperature anomaly in the right-hand plot is the temperature departure from the 1851–1950 average. The decadal series are from Ge et al. (2010) and are updated for the Tibet Plateau and Northwest China. Dashed lines indicate the overall mean value for each temperature series.  
(a) Ensemble temperature reconstructions based on PLS (red lines) and PCR (blue lines) methods at decadal (thin lines) and centennial timescales (solid lines; smoothed by a 5-point FFT filter), along with the 95 % confidence level (shading). The reference value is the mean temperature from 1851 to 1950. The green line indicates the observed average air temperature. (b) Numbers of samples used.  
Comparison of temperature reconstructions for China (a) and the Northern Hemisphere (b). The reference period is from 1851 to 1950. The right-hand axis (marked " Sigma Unit " ) is for the Yang et al. (2002) series. All color lines have been smoothed using a 100 yr FFT filter.  
Selected number of components from the PLS regression and the first two components from the PCR between proxies and observations. Verifications are based on the leave-one-out cross- validation method.
Article
We use principal component regression and partial least squares regression to separately reconstruct a composite series of temperature variations in China, and associated uncertainties, at a decadal resolution over the past 2000 yr. The reconstruction is developed using proxy temperature data with relatively high confidence levels from five regions across China, and using a temperature series from observations by the Chinese Meteorological Administration, covering the period from 1871 to 2000. Relative to the 1851-1950 climatology, our two reconstructions show four warm intervals during AD 1-AD 200, AD 551-AD 760, AD 951-AD 1320, and after AD 1921, and four cold intervals during AD 201-AD 350, AD 441-AD 530, AD 781-AD 950, and AD 1321-AD 1920. The temperatures during AD 981-AD 1100 and AD 1201-AD 1270 are comparable to those of the Present Warm Period, but have an uncertainty of ±0.28 °C to ±0.42 °C at the 95% confidence interval. Temperature variations over China are typically in phase with those of the Northern Hemisphere (NH) after 1000, a period which covers the Medieval Climate Anomaly, the Little Ice Age, and the Present Warm Period. In contrast, a warm period in China during AD 541-AD 740 is not obviously seen in the NH.
 
Pdf for climate sensitivity (in degrees Celsius) from AH06 (blue triangles), pdf obtained by ignoring the volcanic cooling constraint (green solid line) and pdf obtained by ignoring the volcanic cooling constraint and adding 0.5 degrees to the upper and lower bound of the 95% confidence level in the pdf corresponding to the LGM (red solid line). 
Article
In their article from 2006, Annan and Hargreaves present a probabilistic estimate of climate sensitivity obtained by using Bayes' theorem to combine information from different sources. In this comment article we critisize two aspects of their reasoning, namely using probability density functions and likelihood functions interchangeably and the assumed independence of evidence from the different sources. The derivation of their result rests on key assumptions, some stated explicitly and some left implicit, which could be unrealistic. Thus their study does not convincingly reduce the large uncertainty of climate sensitivity remaining in previous observationally-based studies.
 
Article
Henriksson et al. (2010), hereafter HALTL10, criticize Annan and Hargreaves (2006a) (AH06) primarily on the grounds that we assumed that different sources of data were conditionally independent given the climate sensitivity. While we consider this approximation to have been a reasonable one under the circumstances (and provided arguments to justify this approach), we also acknowledged its importance in our original paper and performed several sensitivity analyses. The alternative calculations presented by HALTL10 appear to strengthen rather than contradict our conclusion. HALTL10 additionally criticize Annan and Hargreaves (2009) (AH09) for proposing a Cauchy type prior (as an alternative to the use of a uniform prior, which was widespread up to that time) "without sufficient support", and further claim that anticipated economic damages were used as a means of selecting the prior. We are surprised by these claims, especially considering that the proposed prior was justified at some length both on the basis of both the "Charney report" (National Research Council, 1979) and basic physical arguments, and also in light of our elementary demonstration of the pathological failings of the most commonly-used alternative. Thus, these claims are factually incorrect.
 
Article
In a recent paper, Chylek and Lohmann (2008) used data from the Vostok ice core together with simple energy balance arguments to simultaneously estimate both the dust radiative forcing effect and the climate sensitivity, generating surprisingly high and low values for these respective parameters. However, their results depend critically on their selection of single unrepresentative data points from time series which exhibit a large amount of short-term variability, and are highly unstable with respect to other arbitrarily selected data points. When temporal averages are used in accordance with accepted norms within the paleoclimate community, the results obtained are entirely unremarkable and in line with previous analyses.
 
Article
Visual observations of clouds have been performed since the establishment of meteorological observatories during the early instrumental period, and have become more systematic and reliable after the mid-19th century due to the establishment of the first national weather services. During the last decades a large number of studies have documented the trends of the total cloud cover (TCC) and cloudy types; most of these studies focus on the trends since the second half of the 20th century. Due to the lower reliability of former observations, and the fact that most of this data is not accessible in digital format, there is a lack of studies focusing on the trends of cloudiness since the mid-19th century. In the first part, this work attempts to review previous studies analyzing TCC changes with information covering at least the first half of the 20th century. Then, the study analyses a database of cloudiness observations in Southern Europe (Spain) since the second half of the 19th century. Specifically, monthly TCC series were reconstructed since 1866 by means of a so-called parameter of cloudiness, calculated from the number of cloudless and overcast days. These estimated TCC series show a high interannual and decadal correlation with the observed TCC series originally measured in oktas. After assessing the temporal homogeneity of the estimated TCC series, the mean annual and seasonal series for the whole of Spain and several subregions were calculated. The mean annual TCC shows a general tendency to increase from the beginning of the series until the 1960s; at this point, the trend becomes negative. The linear trend for the annual mean series, estimated over the 1866-2010 period, is a highly remarkable (and statistically significant) increase of +0.44% per decade, which implies an overall increase of more than +6% during the analyzed period. These results are in line with the majority of the trends observed in many areas of the world in previous studies, especially for the records before the 1950s when a widespread increase of TCC can been considered as a common feature.
 
Article
In this study, we assess how the anthropogenically induced increase in greenhouse gas concentrations affects the climate of central and southern South America. We utilise two regional climate simulations for present day (PD) and pre-industrial (PI) times. These simulations are compared to historical reconstructions in order to investigate the driving processes responsible for climatic changes between the different periods. The regional climate model is validated against observations for both re-analysis data and GCM-driven regional simulations for the second half of the 20th century. Model biases are also taken into account for the interpretation of the model results. The added value of the regional simulation over global-scale modelling relates to a better representation of hydrological processes that are particularly evident in the proximity of the Andes Mountains. Climatic differences between the simulated PD minus PI period agree qualitatively well with proxy-based temperature reconstructions, albeit the regional model overestimates the amplitude of the temperature increase. For precipitation the most important changes between the PD and PI simulation relate to a dipole pattern along the Andes Mountains with increased precipitation over the southern parts and reduced precipitation over the central parts. Here only a few regions show robust similarity with studies based on empirical evidence. However, from a dynamical point-of-view, atmospheric circulation changes related to an increase in high-latitude zonal wind speed simulated by the regional climate model are consistent with numerical modelling studies addressing changes in greenhouse gas concentrations. Our results indicate that besides the direct effect of greenhouse gas changes, large-scale changes in atmospheric circulation and sea surface temperatures also exert an influence on temperature and precipitation changes in southern South America. These combined changes in turn affect the relationship between climate and atmospheric circulation between PD and PI times and should be considered for the statistical reconstruction of climate indices calibrated within present-day climate data.
 
Article
The development of northern high-latitude peatlands played an important role in the carbon (C) balance of the land biosphere since the Last Glacial Maximum (LGM). At present, carbon storage in northern peatlands is substantial and estimated to be 500 ± 100 Pg C (1 Pg C = 1015 g C). Here, we develop and apply a peatland module embedded in a dynamic global vegetation model (LPX). The peatland module features a dynamic nitrogen cycle, a dynamic C transfer between peatland acrotelm (upper oxic layer) and catotelm (deep anoxic layer), hydrology- and temperature-dependent respiration rates, and peatland specific plant functional types. Nitrogen limitation down-regulates average modern net primary productivity over peatlands by almost a factor of two. Decadal acrotelm-to-catotelm C fluxes vary between -20 and +50 g C m-2 yr-1 over the Holocene. Key model parameters are calibrated with reconstructed peat accumulation rates from peat-core data. The model reproduces the major features of the peat core data and of the observation-based modern circumpolar soil carbon distribution. Results from a set of simulations for possible evolutions of northern peat development and areal extent show that soil C stocks in modern peatlands increased by 365-550 Pg C since the LGM, of which 175-272 Pg C accumulated between 11 and 5 kyr BP. Furthermore, our simulations suggest a persistent C sequestration rate of 35-50 Pg C per 1000 yr in peatlands under current climate conditions, and that this C sink could either vanish or turn into a small source by 2100 AD depending on climate trajectories as projected for different representative greenhouse gas concentration pathways.
 
Article
The summer sea ice extent strongly decreased in the Arctic over the last decades. This decline is very likely to continue in the future but uncertainty on projections is very large. An ensemble of experiments with the climate model LOVECLIM using 5 different parameter sets has been performed to show that summer sea ice changes for the early Holocene and for the 21st century are strongly linked, allowing to reduce this uncertainty. Using the limited information presently available for the early Holocene, simulations presenting very large changes for the 21st century could reasonably be rejected. On the other hand, simulations displaying low to moderate changes during the second half of the 20th century are not consistent with recent observations. Using this evidence based on observations during both the early Holocene and the last decades, the most realistic projection indicates a nearly disappearance of the sea ice at the end of the 21st century for a moderate increase in atmospheric greenhouse gas concentrations. For a faster increase in those concentrations, the Arctic Ocean would become almost ice-free in summer as early as 2060 AD.
 
Article
The Levant is a key region in terms of both long-term hydroclimate dynamics and human cultural evolution. Our understanding of the regional response to glacial-interglacial boundary conditions is limited by uncertainties in proxy-data interpretation and the lack of long-term records from different geographical settings. The present paper provides a 250 ka paleoenvironmental reconstruction based on a multi-proxy approach from northern Levant, derived from a 36 m lacustrine-palustrine sequence cored in the small intra-mountainous karstic Yammoûneh basin from northern Lebanon. We combined time series of sediment properties, paleovegetation, and carbonate oxygen isotopes (δ<sub>c</sub>), to yield a comprehensive view of paleohydrologic-paleoclimatic fluctuations in the basin over the two last glacial-interglacial cycles. Integration of all available proxies shows that Interglacial maxima (early-mid MIS 7, MIS 5.5 and early MIS 1) experienced relatively high effective moisture, evidenced by the dominance of forested landscapes (although with different forest types) associated with authigenic carbonate sedimentation in a productive waterbody. Synchronous and steep δ<sub>c</sub> increases can be reconciled with enhanced mean annual moisture when changes in seasonality are taken into account. During Glacials periods (MIS 2 and MIS 6), open vegetation tends to replace the forests, favouring local erosion and detrital sedimentation. However, all proxy data reveal an overall wetting during MIS 6, while a drying trend took place during MIS4-2, leading to extremely harsh LGM conditions possibly linked to water storage as ice in the surrounding highlands. Over the past 250 ka, the Yammoûneh record shows an overall decrease in local effective water, coincident with a weakening of seasonal insolation contrasts linked to the decreasing amplitude of the eccentricity cycle. The Yammoûneh record is roughly consistent with long-term climatic fluctuations in the northeastern Mediterranean region (except during MIS 6). It suggests that the role of seasonality on effective moisture, already highlighted for MIS 1, also explains older interglacial climate. The Yammoûneh record shares some features with speleothem isotope records of western Israel, while the Dead Sea basin generally evolved in opposite directions. Changes in atmospheric circulation, regional topographic patterns and site-specific hydrological factors are invoked as potential causes of spatial heterogeneities. Further work is needed to refine the Yammoûneh chronology, better understand its functioning through hydrological and climate modelling, and acquire other long records from northern Levant to disentangle the relative effects of local versus regional factors.
 
Article
We present a new high-resolution marine pollen record from NW Iberian margin sediments (core MD03-2697) covering the interval between 340 000 and 270 000 years ago, a time period centred on Marine Isotope Stage (MIS) 9 and characterized by particular baseline climate states. This study enables the documentation of vegetation changes in the north-western Iberian Peninsula and therefore the terrestrial climatic variability at orbital and in particular at millennial scales during MIS 9, directly on a marine stratigraphy. Suborbital vegetation changes in NW Iberia in response to cool/cold events are detected throughout the studied interval even during MIS 9e ice volume minimum. However, they appear more frequent and of higher amplitude during the 30 000 years following the MIS 9e interglacial period and during the MIS 9a-8 transition, which correspond to intervals of an intermediate to high ice volume and mainly periods of ice growth. Each suborbital cold event detected in NW Iberia has a counterpart in the Southern Iberian margin SST record. High to moderate amplitude cold episodes detected on land and in the ocean appear to be related to changes in deep water circulation and probably to iceberg discharges at least during MIS 9d, the mid-MIS 9c cold event and MIS 9b. This work provides therefore additional evidence of pervasive millennial-scale climatic variability in the North Atlantic borderlands throughout past climatic cycles of the Late Pleistocene, regardless of glacial state. However, ice volume might have an indirect influence on the amplitude of the millennial climatic changes in Southern Europe.
 
Article
Tree rings, ice cores and glacial geologic histories for the past several centuries offer an opportunity to characterize climate variability and to identify the key climate parameters forcing glacier expansions. A newly developed larch ring-width chronology is presented for Kamchatka that is sensitive to past summer temperature variability. This record provides the basis to compare with other proxy records of inferred temperature and precipitation change from ice core and glacier records, and to characterize climate for the region over the past 400 years. Individual low growth years in the larch record are associated with several known and proposed volcanic events that have been observed in other proxy records from the Northern Hemisphere. Comparison of the tree-rings with an ice core record of melt feature index for Kamchatka's Ushkovsky volcano confirms a 1?3 year dating accuracy for this ice core series over the late 18th to 20th centuries. Decadal variations of low summer temperatures (tree-ring record) and high annual precipitation (ice core record) are broadly consistent with intervals of positive mass balance measured and estimated at several glaciers, and with moraine building, provides a basis to interpret geologic glacier records.
 
Top-cited authors
Ayako Abe-Ouchi
  • The University of Tokyo
Paul J Valdes
  • University of Bristol
Masa Kageyama
  • French National Centre for Scientific Research
Sandy Harrison
  • University of Reading
Michel Crucifix
  • Université Catholique de Louvain - UCLouvain