EPICA Dome C record of glacial and interglacial intensities
Introduction
Glacial–interglacial climate change is expected to result from the orbital forcing, changes in atmospheric composition and internal feedbacks, such as changes in planetary albedo (Paillard, 1998). Along the last million years, changes in land–sea geographical distribution can be considered as negligible in driving past changes in climate and atmospheric composition, in contrast with deeper times. Over this time period, the orbital forcing is well known from astronomical calculations (Berger and Loutre, 1991, Laskar et al., 1993). It is therefore critical to document the relative intensities of past glacial and interglacial periods, in terms of changes in atmospheric composition and global temperatures. Each climate archive provides a complementary view of the global climate system. Globally relevant climate data can be extracted from marine records, where proxies document past changes in tropical sea surface temperature (Lea, 2004), deep ocean temperature (Zachos et al., 2001), or past changes in sea level (Bintanja et al., 2005, Lisiecki and Raymo, 2005), or from continental records documenting past changes in vegetation cover (Cheddadi et al., 2005, Tzedakis et al., 2006) or monsoon intensity (Sun et al., 2006, Wang et al., 2008).
There are specific areas, such as the tropics or Antarctica (Fig. 1), where climate models show a strong link between local temperature changes and global temperature changes (Lea, 2004, Masson-Delmotte et al., 2006a, Schneider von Deimling et al., 2006, Hargreaves et al., 2007, IPCC, 2007). Here, we focus on deep Antarctic ice cores. They offer a variety of new climate and environmental records including the documentation of past atmospheric composition over several glacial–interglacial cycles, at Vostok (Petit et al., 1999), Dome Fuji (Watanabe et al., 2003, Kawamura et al., 2007) or EPICA Dome C (Jouzel et al., 2007, Loulergue et al., 2008, Lüthi et al., 2008). Recently, multiple parameters have been measured on the EPICA Dome C (hereafter EDC) ice core spanning the past 800 000 years (Section 2).
These parameters track climate changes from a variety of latitudes. Past temperatures are relevant for central Antarctica. Past changes in sea salt sodium have been suggested to be related to sea ice surface area, modulated by uplift and transport from the sea ice surface towards central Antarctica. Past changes in methane concentrations are relevant for continental wetland temperature and moisture, mainly in the tropics and the Northern Hemisphere. Past changes in carbon dioxide concentration are relevant for changes in terrestrial and marine carbon sources and sinks, and past changes in dust or non-sea-salt calcium are expected to document dust production on southern hemisphere mid-latitude continents and its transport to Antarctica. Recent studies have been conducted on the sequence of events during terminations (Röthlisberger et al., 2008), but this approach remains difficult to extend to the phase relationships between atmospheric composition and climate due to relative dating uncertainties for the gas and the ice records caused by firnification processes (Caillon et al., 2003, Loulergue et al., 2007).
Section 3 is focused on the relative intensity of glacial and interglacial extrema as identified in various ice core proxies. First, we discuss the uncertainties on past Antarctic temperatures which are estimated from measurements of water stable isotopes along the EPICA EDC ice core over the past 800 000 years. We then discuss the consistency between stable isotope-derived temperature and other environmental and climatic proxies measured in the EDC ice core regarding the relative intensity of glacial and interglacial extrema.
Section 4 is dedicated to the links between radiative perturbations and climate and the new results from the EDC ice core. We discuss the weights of orbital frequencies in EDC temperature. We analyse the links between EDC temperature and greenhouse gas radiative forcing. We use climate models to discuss the relationship between Antarctic and global temperature, highlighting the problems of changes in local ice sheet elevation. A reconstruction of past global temperature variations is compared to the simulations of global temperature response to global radiative forcings estimated from past changes in greenhouse gas concentrations and in northern hemisphere glaciation.
As we show that EDC interglacial temperature variations cannot be explained by a simple response to radiative forcing perturbations, we finally focus our discussion (Section 5) on the warmest interglacial period identified in the EDC ice cores (MIS 5.5). We compare the response of climate models forced by changes in orbital configurations with observational constraints from Greenland and Antarctica. We show that MIS 5.5 Antarctic warmth is not captured by the response of ocean and atmosphere dynamics to orbital forcing and that other feedbacks involving the coupling with the cryosphere must be at play.
Section snippets
Data
In this analysis, we use the available datasets measured and previously published, on the EPICA EDC3 age scale (Parrenin et al., 2007a). This age scale is generated by combining an accumulation and an ice flow model optimised using absolute age markers (Parrenin et al., 2007b). For the time period from 300 to 800 kyr (thousands of years before present), the age scale is obtained from a glaciological interpolation of precession age markers identified in EDC δ18O of O2 (Dreyfus et al., 2007). The
EDC temperature reconstructions
Fig. 1 shows different temperature reconstructions based on EDC stable isotope data. The detailed basis of temperature reconstructions using ice core isotopic composition has been previously published (Jouzel et al., 2003, Masson-Delmotte et al., 2006b, Masson-Delmotte et al., 2008). Due to isotopic distillation processes, changes in central Antarctic snow δD are primarily driven by changes in condensation temperature. Past temperature reconstructions rely on the explicit assumptions of a
Links between changes in radiative forcing and climate
In this section, we now discuss the links between EDC temperature and climate forcings using different approaches. At the local scale, radiative forcings involve changes in obliquity (which affects annual mean insolation) (Section 4.1) and greenhouse gas concentrations (which affects the infra-red loss) (Section 4.2.1). We analyse in Section 4.2.2 the links between EDC temperature and local radiative forcings caused by obliquity and greenhouse gas concentrations.
In a second step, we then use
Discussion of Antarctic interglacial optima
We focus here on MIS 5.5, which appears as an exceptional interglacial period in EDC temperature with the highest temperature of the past 800 ka. It is also exceptional with respect to the residual between EDC temperature and the greenhouse gas radiative forcing (Fig. 6b). Here we use climate models to explore climate feedbacks involved in the simulated response to MIS 5.5 orbital forcing and compare the simulated polar responses in the northern and southern hemispheres (Greenland and
Conclusions and perspectives
The EDC ice core offers a wealth of information on past climate glacial–interglacial dynamics. We have discussed the uncertainties linked with the intensity of glacial and interglacial periods as derived from water stable isotope records, and shown limited effects of changes in local elevation. A deeper understanding of the uncertainties linked with changes in moisture origin will soon be provided by the full record of EDC deuterium excess data.
A solid knowledge of past Antarctic temperature
Acknowledgments
We thank J.R. Petit, G. Raisbeck and M. Werner, A. Timmermann and two anonymous reviewers for constructive comments on the manuscript. This work is a contribution to the European Project for Ice Coring in Antarctica (EPICA), a joint European Science Foundation/European Commission scientific programme, funded by the EU (EPICA-MIS) and by national contributions from Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland and the United Kingdom. The main logistic
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