Astronomical frequencies for climate research at the decadal to century time scale

Short-term variations of the elements representing the Earth's motion around the Sun and its rotation have been analyzed over the last 6000 years using 1-year steps. Their low-frequency part is compared first to the values obtained from a secular theory of the planetary long-term motion showing that they can be considered reliable enough to represent adequately the motion of the Earth over the last 5000 years. Spectral analysis of these values shows that the main periodicities are 2.67, 3.98, 5.26, 5.93, 7.9, 9.8, 11.9, 14.7, 15.8, 29, 42, 61, 122, 165 and 250 years for the eccentricity as well as for the climatic precession, with an additional component at around 930 years for the eccentricity and around 840 years for the climatic precession. Periodicities at 2.67, 3.8, 5.9, 8.0, 9.3, 11.9, 14.7, 18.6, 29, 135, 250 and 840 yr are also shown for the obliquity. Spectral analyses of the daily July mid-month insolation at 65°N show essentially the same periodicities as the climatic precession and the obliquity, i.e. 2.67, 3.98, 5.92, 8.1, 11.9, 15.7, 18.6, 29, 40, 61 and around 900 years. Finally a wider analysis of the insolation pattern was performed related to the large periodicity band of the insolation time series for the solstices and the equinoxes for 7 different latitudes. In equatorial latitudes the insolation variance is largely explained by precession. But precession dominates everywhere with the obliquity signal being stronger at polar latitudes at the solstices. The amplitudes of the insolation change at these frequencies is of the order of 0.2 Wm^–2 at the maximum. Offprint requests to: A Berger     

The role of forcing and internal dynamics in explaining the “Medieval Climate Anomaly”

Proxy reconstructions suggest that peak global temperature during the past warm interval known as the Medieval Climate Anomaly (MCA, roughly 950–1250 AD) has been exceeded only during the most recent decades. To better understand the origin of this warm period, we use model simulations constrained by data assimilation establishing the spatial pattern of temperature changes that is most consistent with forcing estimates, model physics and the empirical information contained in paleoclimate proxy records. These numerical experiments demonstrate that the reconstructed spatial temperature pattern of the MCA can be explained by a simple thermodynamical response of the climate system to relatively weak changes in radiative forcing combined with a modification of the atmospheric circulation, displaying some similarities with the positive phase of the so-called Arctic Oscillation, and with northward shifts in the position of the Gulf Stream and Kuroshio currents. The mechanisms underlying the MCA are thus quite different from anthropogenic mechanisms responsible for modern global warming.     

The Earth's Climate in the Next Hundred Thousand years (100 kyr)

One of the most striking features of the Quaternary paleoclimate records remains the so-called 100-kyr cycle which is undoubtedly linked to the future of our climate. Such a 100-kyr cycle is indeed characterised by long glacial periods followed by a short-interglacial (10–15 kyr long). As we are now in an interglacial, the Holocene, the previous one (the Eemian, which corresponds quite well to Marine Isotope Stage 5e, peaking at 125 kyr before present, BP) was assumed to be a good analogue for our present-day climate. In addition, as the Holocene is 10 kyr long, paleoclimatologists were naturally inclined to predict that we are quite close to the next ice age. Simulations using the 2-D climate model of Louvain-la-Neuve show, however, that the current interglacial will most probably last much longer than any previous ones. It is suggested here that this is related to the shape of the Earth's orbit around the Sun, which will be almost circular over the next tens of thousands of years. As this is primarily related to the 400-kyr cycle of eccentricity, the best and closest analogue for such a forcing is definitely Marine Isotopic Stage 11 (MIS-11), some 400 kyr ago, not MIS-5e. Because the CO₂ concentration in the atmosphere also plays an important role in shaping long-term climatic variations – especially its phase with respect to insolation – a detailed reconstruction of this previous interglacial from deep sea and ice records is urgently needed. Such a study is particularly important in the context of the already exceptional present-day CO₂ concentrations (unprecedented over the past million years) and, even more so, because of even larger values predicted to occur during the 21st century due to human activities.     

西藏丁青弧前蛇绿岩的地球化学特征

丁青蛇绿岩位于班化湖－丁青－怒江蛇绿岩带的东段，其地幔岩出露规模是该带中最大的。本文报道的丁青蛇绿岩主要由地幔橄榄岩、堆晶岩、辉长岩和斜长花岗岩组成。蛇绿岩剖面上覆硅岩中的放射虫化石是早株罗世和晚三叠世诺利克期的，中侏罗统砂岩和砾岩不整合覆盖在蛇绿岩之上，由此确定丁青蛇绿岩是晚三叠－早侏罗世的，在中株罗世之前侵位，丁青蛇绿岩属于玻安岩系，玻安岩的特点是富Si、Mg和大离子亲石元素（LILE），贫高场强元素（Ti、P、Zr、Y、Yb和Nb）。丁青蛇绿岩的堆晶岩、辉长岩和辉绿岩均具“U”型REE分布，暗示丁青玻安岩是由于亏损的地幔源岩和来自消减带的水和流体两组分的混合形成的。丁青玻安岩的地球化学特征类似西太平洋第三纪玻安岩，而明显不同于MORB的地球化学性质， 表明丁青玻安岩应当形成于洋内岛弧的弧前环境，属于弧前蛇绿岩。     

Sensitivity of the LLN climate model to the astronomical and CO2 forcings over the last 200 ky

 The Louvain-la-Neuve climate model (here referred to as the LLN 2-D model has been used extensively to simulate the Northern Hemisphere ice volume under both the insolation and CO₂ forcings. The period analysed here covers the last 200 ky. First, sensitivity analyses to constant CO₂ concentration were performed. The model was accordingly forced by insolation changes only, the CO₂ concentration being kept constant to respectively 210, 250 and 290 ppmv. Results show that the simulated ice volume variations are comparable to the geological reconstructions only when the CO₂ concentration is low (210 ppmv) and that the sensitivity of the simulated Northern Hemisphere ice volume to CO₂ is not constant through time. Second, three CO₂ reconstructions were used to force the LLN 2-D model in addition to insolation. Results show (1) a better agreement with the SPECMAP oxygen isotope time series, in particular as far as the amplitude of the signal is concerned, and (2) that the simulated Northern Hemisphere ice volume is not very sensitive to the slight differences between these three reconstructions.     

CO2 and Northern Hemisphere ice volume variations over the middle and late Quaternary

 The atmospheric CO₂ concentrations have been reconstructed over the past 600 ka based on regression between the Vostok CO₂ data and the SPECMAP oxygen isotope values. A lag of 4.5 ka (CO₂ preceding δ¹⁸O) gives the best results. A polynomial of order 5 explains 66% of the Vostok CO₂ variance over the last 220 ka. The Northern Hemisphere ice-sheet volume was simulated over the past 575 ka using the LLN 2-D model, forced by insolation and these statistically reconstructed atmospheric CO₂ concentrations. The simulated ice volume fluctuations resemble the deep-sea oxygen isotope variations. CO₂ of interglacial level is necessary for explaining both the interglacial at oxygen isotopic stage 11 and our present-day interglacial.     

Climate oscillations evidenced by spectral analysis of Southern Chilean lacustrine sediments: the assessment of ENSO over the last 600 years

The Chilean Lake District (38–42°S) is strongly influenced by Southern westerlies-driven precipitations. At 40°S Lago Puyehue provides high resolution sedimentation rates (∼1–2 mm/yr) suitable for annual climate reconstruction. Several short and long sediment cores were collected in this lake. Their analysis aim at a better understanding of climate mechanisms related to ENSO in this part of the world. The recognition of ENSO related periodicities and their stability is studied through the analysis of two short varved cores collected from underflow and interflow key sites. According to varve chronology controlled by ¹³⁷Cs and ²¹⁰Pb profiles and chronostratigraphical markers, the short core from underflow site (PU-I) spans 294 ± 18 years and the core in the interflow site (PU-II) covers 592 ± 9 years. Several methods of spectral analysis were applied on the total varve thickness to identify potential periodicities in the signal. Blackman–Tuckey, Maximum Entropy, Multi-Taper Methods (MTM) and singular spectrum analysis were applied on the whole record. In addition, evolutive MTM and wavelet analyses allow to identify temporal influence of some periodicities. In the PU-I studied interval (AD 1700–2000), a period at ∼3.0 years appears in a large part of the interval, mostly in the recent part. Periods at ∼5.2 and ∼23 years also show up. PU-II record (AD 1400–2000) displays the most robust periodicities at around 15, 9, 4.4, 3.2 and 2.4 years. These periodicities are in good agreement with the sub-decadal periods identified by Dean and Kemp (2004) and linked to the El Nino Southern Oscillation and the Pacific Decadal Oscillation. Differences in the recorded periodicities between PU-I and PU-II sites are consistent with different sedimentation processes in the lake. According to climate instrumental data for the last 20 years, varves in PU-I site are mostly related to fluvial dynamics and regional climate factors, i.e., precipitation, temperature and wind. In PU-II site, varves increment is related to both regional and global climate forcing factors, i.e., El Nino Southern Oscillation. The evolutive MTM analysis and the wavelet analysis suggest a striking break in the periodicities at around AD 1820. Finally relationships between El Nino and longer term climate phase like the Little Ice Age (LIA) are also assessed. This is the seventh in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.     

A 17,900-year multi-proxy lacustrine record of Lago Puyehue (Chilean Lake District): introduction

This paper introduces the background and main results of a research project aimed at unravelling the paleolimnological and paleoclimatological history of Lago Puyehue (40° S, Lake District, Chile) since the Last Glacial Maximum (LGM), based on the study of several sediment cores from the lake and on extensive fieldwork in the lake catchment. The longest record was obtained in an 11-m-long piston core. An age-depth model was established by AMS ¹⁴C dating, ²¹⁰Pb and ²³⁷Cs measurements, identification of event-deposits, and varve-counting for the past 600 years. The core extends back to 17,915 cal. yr. BP, and the seismic data indicate that an open-lake sedimentary environment already existed several thousands of years before that. The core was submitted to a multi-proxy analysis, including sedimentology, mineralogy, grain-size, major geochemistry and organic geochemistry (C/N ratio, δ¹³C), loss-on-ignition, magnetic susceptibility, diatom analysis and palynology. Along-core variations in sediment composition reveal that the area of Lago Puyehue was characterized since the LGM by a series of rapid climate fluctuations superimposed on a long-term warming trend. Identified climate fluctuations confirm a.o. the existence of a Late-Glacial cold reversal predating the northern-hemisphere Younger Dryas cold period by 500–1,000 years, as well as the existence of an early southern-hemisphere Holocene climatic optimum. Varve-thickness analyses over the past 600 years reveal periodicities similar to those associated with the El Niño Southern Oscillation and the Pacific Decadal Oscillation, as well as intervals with increased precipitation, related to an intensification of the El Niño impact during the southern-hemisphere equivalent of the Little Ice Age.     

Impact of Greenland and Antarctic ice sheet interactions on climate sensitivity

We use the Earth system model of intermediate complexity LOVECLIM to show the effect of coupling interactive ice sheets on the climate sensitivity of the model on a millennial time scale. We compare the response to a 2×CO₂ warming scenario between fully coupled model versions including interactive Greenland and Antarctic ice sheet models and model versions with fixed ice sheets. For this purpose an ensemble of different parameter sets have been defined for LOVECLIM, covering a wide range of the model??s sensitivity to greenhouse warming, while still simulating the present-day climate and the climate evolution over the last millennium within observational uncertainties. Additional freshwater fluxes from the melting ice sheets have a mitigating effect on the model??s temperature response, leading to generally lower climate sensitivities of the fully coupled model versions. The mitigation is effectuated by changes in heat exchange within the ocean and at the sea?Cair interface, driven by freshening of the surface ocean and amplified by sea?Cice-related feedbacks. The strength of the effect depends on the response of the ice sheets to the warming and on the model??s climate sensitivity itself. The effect is relatively strong in model versions with higher climate sensitivity due to the relatively large polar amplification of LOVECLIM. With the ensemble approach in this study we cover a wide range of possible model responses.