Antarctic Temperature at Orbital Timescales Controlled by Local Summer Duration

During the late Pleistocene epoch, proxies for Southern Hemisphere climate from the Antarctic ice cores vary nearly in phase with Northern Hemisphere insolation intensity at the precession and obliquity timescales. This coherence has led to the suggestion that Northern Hemisphere insolation controls Antarctic climate. However, it is unclear what physical mechanisms would tie southern climate to northern insolation. Here we call on radiative equilibrium estimates to show that Antarctic climate could instead respond to changes in the duration of local summer. Simple radiative equilibrium dictates that warmer annual average atmospheric temperatures occur as a result of a longer summer, as opposed to a more intense one, because temperature is more sensitive to insolation when the atmosphere is cooler. Furthermore, we show that a single-column atmospheric model reproduces this radiative equilibrium effect when forced exclusively by local Antarctic insolation, generating temperature variations that are coherent and in phase with proxies of Antarctic atmospheric temperature and surface conditions. We conclude that the duration of Southern Hemisphere summer is more likely to control Antarctic climate than the intensity of Northern Hemisphere summer with which it (often misleadingly) covaries. In our view, near interhemispheric climate symmetry at the obliquity and precession timescales arises from a northern response to local summer intensity and a southern response to local summer duration.Earth and Planetary Science

Climate and atmospheric history of the past 420,000 years from the Vostok ice core,

Antarctica has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial-interglacial cycles. The succession of changes through each climate cycle and termination was similar, and atmospheric and climate properties oscillated between stable bounds. Interglacial periods differed in temporal evolution and duration. Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years. The late Quaternary period (the past one million years) is punctuated by a series of large glacial-interglacial changes with cycles that last about 100,000 years (ref. 1). Glacial-interglacial climate changes are documented by complementary climate records 1,2 largely derived from deep sea sediments, continental deposits of flora, fauna and loess, and ice cores. These studies have documented the wide range of climate variability on Earth. They have shown that much of the variability occurs with periodicities corresponding to that of the precession, obliquity and eccentricity of the Earth's orbit 1,3 . But understanding how the climate system responds to this initial orbital forcing is still an important issue in palaeoclimatology, in particular for the generally strong ϳ100,000-year (100-kyr) cycle. Ice cores give access to palaeoclimate series that includes local temperature and precipitation rate, moisture source conditions, wind strength and aerosol fluxes of marine, volcanic, terrestrial, cosmogenic and anthropogenic origin. They are also unique with their entrapped air inclusions in providing direct records of past changes in atmospheric trace-gas composition. The ice-drilling project undertaken in the framework of a long-term collaboration between Russia, the United States and France at the Russian Vostok station in East Antarctica (78Њ S, 106Њ E, elevation 3,488 m, mean temperature −55 ЊC) has already provided a wealth of such information for the past two glacial-interglacial cycles [4][5][6][7][8][9] Here we present a series of detailed Vostok records covering this ϳ400-kyr period. We show that the main features of the more recent Vostok climate cycle resemble those observed in earlier cycles. In particular, we confirm the strong correlation between atmospheric greenhouse-gas concentrations and Antarctic temperature, as well as the strong imprint of obliquity and precession in most of the climate time series. Our records reveal both similarities and differences between the successive interglacial periods. They suggest the lead of Antarctic air temperature, and of atmospheric greenhousegas concentrations, with respect to global ice volume and Greenland air-temperature changes during glacial terminations. The ice record The data are shown in Figs 1, 2 and 3 (see Supplementary Information for the numerical data). They include the deuterium content of the ice (dD ice , a proxy of local temperature change), the dust content (desert aerosols), the concentration of sodium (marine aerosol), and from the entrapped air the greenhouse gases CO 2 and CH 4 , and the d 18 O are defined in the legends to Figs 1 and 2, respectively.) All these measurements have been performed using methods previously described except for slight modifications (see The detailed record of dD ic

State of balance of the cryosphere

This is the published version, also available here: http://dx.doi.org/10.1029/91RG00784.The current state of balance of the terrestrial ice sheets and glaciers is poorly known. What little data are available suggest that, worldwide, mountain glaciers have receded since about the mid-nineteenth century, with occasional interruptions of the retreat. The interior part of the Greenland ice sheet appears to be thickening or in near equilibrium, but this ice sheet may be thinning in the coastal areas. Estimates of the mass balance of the Antarctic ice sheet suggest that it is positive, although the error limits allow for a slightly negative balance. There is an urgent need to greatly improve the current estimates and to monitor the ice sheets continuously for changes in volume and extent. A program based on satellite observation techniques, in cooperation with ground-based surveys repeated over long time periods (many years or decades), appears to be most opportune to achieve this

Beyond equilibrium climate sensitivity

ISSN:1752-0908ISSN:1752-089

Rapport I.6. Informations climatologiques stockées dans les calottes polaires

Different methods enable the dating of samples obtained by drilling in Antarctica and Greenland : periodic changes in certain properties (in particular the stable isotope content), radioisotopes, ice flow models. The accuracy of these methods decreases with the age and the depth of the samples. There has been no systematic change in the precipitation rate in the course of the last century and, as it seems, even further back in time. Radioisotope contents used as air mass tracers have made it possible to obtain information concerning residence times and atmospheric exchanges. Isotopic profiles (deuterium, oxygen 18) constitute at present a detailed climatic record covering the last one hundred thousand years and may make a tendency prediction possible. Important variations that correspond to the end of the last glaciation occurred at approximately the same time in the ice-sheets of both hemispheres ; these variations could also be due in part to significant changes in the thickness of the ice-sheets. Trace element contents are more particularly connected with particulate exchange processes within the “continents-atmosphere-oceans” system. Whether it be the cause or the effet of climatic changes, an increase in trace elements has been observed in ice formed during the last glaciation. In recently deposited layers there is a significant increase in the concentrations for certain elements. This increase is attributed to industrial activity.Différentes méthodes permettent de dater les échantillons obtenus par carottage en Antarctique et au Groenland : variations périodiques de certaines propriétés (en particulier teneur en isotopes stables), radioisotopes, modèles d’écoulement de la glace, ceci avec une précision qui décroît pour les échantillons anciens prélevés en profondeur. Il n’apparaît pas de variation systématique du taux de précipitation au cours du dernier siècle et même, semble-t-il, au-delà. Les teneurs en radioéléments, utilisés comme traceurs des masses d’air, ont permis d’obtenir des informations sur les temps de résidence et les échanges atmosphériques. Les profils de teneur isotopique (deuterium, oxygène 18) constituent un enregistrement climatique détaillé portant actuellement sur les derniers 100 000 ans et rendront peut-être possible une prévision des tendances. Les variations importantes qui traduisent la fin de la dernière glaciation se sont produites approximativement à la même époque dans les calottes des deux hémisphères. Elles peuvent être aussi en partie causées par des changements notables des épaisseurs de glace. Les teneurs en éléments traces sont notamment liées aux échanges de matière dans le système continents-atmosphère-océans ; cause ou effet des variations climatiques, on observe une augmentation des teneurs dans la glace formée durant la dernière glaciation. Dans les couches récemment déposées on note pour certains éléments un accroissement sensible des concentrations attribué à l’activité industrielle.Lorius Claude. Rapport I.6. Informations climatologiques stockées dans les calottes polaires. In: Influence des activités de l'homme sur le cycle hydrométéorologique. Compte rendu des treizièmes journées de l'hydraulique. Paris, 16-18 septembre 1974. Tome 1, 1975