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Review Recent Advances in Understanding Antarctic Climate

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Review Recent Advances in Understanding Antarctic Climate Antarctic Science page 1 of 13 (2008) & Antarctic Science Ltd 2008 DOI: 10.1017/S0954102008000941 1 Review 56 2 57 3 Recent advances in understanding Antarctic climate evolution 58 4 MARTIN J. SIEGERT1*, PETER BARRETT2, ROBERT DECONTO3, ROBERT DUNBAR4,COLMO´ COFAIGH5, 59 5 SANDRA PASSCHIER6 and TIM NAISH2,7 60 6 61 1School of GeoSciences, University of Edinburgh, Grant Institute, Edinburgh EH9 3JW, UK 7 62 2Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington, New Zealand 8 3Department of Geosciences, 611 North Pleasant Street, 233 Morrill Science Center, University of Massachusetts, Amherst, 63 9 MA 01003-9297, USA 64 10 4Department of Geological and Environmental Sciences, 325 Braun Hall (bldg. 320), Stanford University, Stanford, CA 94305-2115, USA 65 5 11 Department of Geography, Durham University, Science Site, South Road, Durham DH1 3LE, UK 66 6 12 Department of Earth and Environmental Studies, Mallory Hall 252, Montclair State University, Montclair, NJ 07043, USA 67 7 13 Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand 68 *[email protected] 14 69 15 70 16 Abstract: Geological evidence shows that the ice sheet and climate in Antarctica has changed considerably 71 17 since the onset of glaciation around 34 million years ago. By analysing this evidence, important information 72 18 concerning processes responsible for ice sheet growth and decay can be determined, which is vital for 73 19 appreciating future changes in Antarctica. Geological records are diverse and their analyses require a variety 74 20 of techniques. They are, however, essential for the establishment of hypotheses regarding past Antarctic 75 21 changes. Numerical models of ice and climate are useful for testing such hypotheses, and in recent years 76 22 there have been several advances in our knowledge relating to ice sheet history gained from these tests. This 77 23 paper documents five case studies, employing a full range of techniques, to exemplify recent insights into 78 24 Antarctic climate evolution from modelling ice sheet inception in the earliest Oligocene to quantifying 79 25 Neogene ice sheet fluctuations and process-led investigations of recent (last glacial) changes. 80 26 81 27 Received 15 January 2007, accepted 29 August 2007 82 28 83 Key words: Cenozoic, environment, glacial history, ice sheet 29 84 30 85 31 86 Introduction 32 volume is a major driver of Earth’s climate and sea level, 87 33 Large ice sheets have existed on Antarctica since at least much effort has been expended in deriving numerical models 88 34 earliest Oligocene times (Wise et al. 1991, Barrett 1996), of its behaviour. Some of these models have been 89 35 approximately 34 million years ago. Since then it has successfully evaluated against modern conditions (Le Broqc 90 36 fluctuated considerably and has been one of the major 2007). Employing numerical models to evaluate past ice sheet 91 37 driving forces for changes in global sea level and climate. behaviour, using the record of changes in climate (inferred 92 38 The size and timing of these fluctuations has been the from ice cores, sedimentary facies, and seismic data), 93 39 subject of considerable debate. Knowing how large ice palaeoceanographic conditions (inferred from palaeoecology 94 40 masses and associated sea ice respond to external forcing is and climate proxies in ocean sediments) and palaeogeography 95 41 of vital importance, because ice volume variations change 1) (as recorded in landscape evolution), provides a powerful 96 42 global sea level on a scale of tens of metres or more, and 2) means by which quantitative process-led assessments of the 97 43 the capacity of ice sheets and sea ice as major heat sinks, cryosphere’s involvement in a variety of climate change 98 44 insulators and reflectors. It is thus important to assess the episodes can be established. Such assessment is critical to 99 45 stability of the cryosphere under a warming climate and predicting how the Antarctic Ice Sheet will respond to, and 100 46 higher atmospheric CO2 levels (IPCC 2001) when ice force, future environmental changes. 101 47 volumes may reduce, particularly as ice core records have Recognizing the importance of understanding past 102 48 yielded evidence of a strong correlation between CO2 in the changes in Antarctica to comprehending future changes, 103 49 atmosphere and palaeotemperatures (EPICA 2004). This the Scientific Committee on Antarctic Research has developed 104 50 concern is justified when CO2 levels are compared with a programme, entitled ACE (Antarctic Climate evolution), 105 51 temperature changes in the more distant past (Crowley & aimed at facilitating research in the broad area of Antarctic 106 52 Kim 1995, Pagani et al. 2005). For example, IPCC (2001) glacial and climate history. In this review we assess five 107 53 estimated global mean temperatures in 2300, from ‘best case’ areas of activity in which the ACE programme has been 108 54 projections of atmospheric CO2, that have not occurred on focused. These five examples are not meant to be an 109 55 Earth for over 50 Ma. Since variation in Antarctic ice exhaustive account of research undertaken on Antarctic 110 1 2 MARTIN J. SIEGERT et al. 111 166 112 167 113 168 114 169 115 170 116 171 117 172 118 173 119 174 120 175 121 176 122 177 123 178 124 179 125 180 126 181 127 182 128 183 129 184 130 185 131 186 132 187 133 188 134 189 135 Fig. 1. Ice volume (left) and corresponding ice sheet geometries (right) simulated by a coupled GCM ice sheet model in response to a slow 190 136 191 decline in atmospheric CO2 and idealized orbital cyclicity across the Eocene–Oligocene boundary. The sudden, two-step jump in ice volume 137 (left panel) corresponds to the Oi-1 event. The left panel shows simulated ice volume, extrapolated to an equivalent change in sea level and 192 138 the mean isotopic composition of the ocean (top). Arbitrary model years (left axis) and corresponding, prescribed atmospheric CO2 (right 193 139 axis) are also labelled. Atmospheric CO2 is shown as the multiplicative of pre-industrial (280 ppmv) levels. Ice sheet geometries (right 194 140 panels) show ice sheet thickness in metres. Black arrows correlate simulated ice volumes with the geometric evolution of the ice sheet 195 141 through the Oi-1 event (modified from DeConto & Pollard 2003b). 196 142 197 143 climate evolution, but they do provide a means of gauging (Tasmanian Passage) widened and deepened during the late 198 144 the variety of activities needed to gain a fuller appreciation Palaeogene and early Neogene (Lawver & Gahagan 1998), 199 145 of Antarctic glacial history. The review begins with a strengthening of the Antarctic Circumpolar Current and 200 146 discussion concerning the onset of glaciation in Antarctica Polar Frontal Zone were thought to have cooled the Southern 201 147 in the earliest Oligocene, with subsequent assessment of Ocean by limiting the advection of warm subtropical surface 202 148 ice sheet fluctuations during the Neogene. The review waters into high latitudes (Kennett 1977). A number of 203 149 continues with an account of ice sheet changes following ocean modelling studies have indeed shown that the opening 204 150 the last glacial maximum, and ends with an example for of both the Drake and Tasmanian gateways reduces poleward 205 151 how geological evidence can be used to characterise heat convergence in the Southern Ocean, cooling sea surface 206 152 modern ice sheet processes. temperatures by several degrees (Mikolajewicz et al. 1993, 207 153 Nong et al. 2000, Toggweiler & Bjornsson 2000). 208 154 Whereas the opening of the Tasmanian gateway broadly 209 Connection of CO2 and ice sheet inception at the 155 coincides with the earliest Oligocene glaciation event 210 Eocene–Oligocene boundary 156 (Oi-1) (Stickley et al. 2004), the tectonic history of the 211 157 Whereas the onset of major, continental-scale glaciation in Scotia Sea remains equivocal. Estimates for the opening of 212 158 the earliest Oligocene (Oi-1 event) has long been attributed Drake Passage range between 40 and 20 Ma (Barker & 213 159 to the opening of Southern Ocean gateways (Kennett & Burrell 1977, Livermore et al. 2004, Scher & Martin 214 160 Shackleton 1976, Kennett 1977, Exon et al. 2002), recent 2006), blurring the direct ‘cause and effect’ relationship 215 161 numerical modelling studies suggest declining atmospheric between the gateways and glaciation. Furthermore, recent 216 162 CO2 was the most important factor in Antarctic cooling atmosphere-ocean modelling (Huber et al. 2004) has shown 217 163 and glaciation. that the Tasmanian Gateway probably had a minimal effect on 218 164 As the passages between South America and the Antarctic oceanic heat convergence and sea surface temperatures around 219 165 Peninsula (Drake Passage), and Australia and East Antarctica the continent, because the warm East Australia Current does 220 ANTARCTIC CLIMATE EVOLUTION 3 221 not travel any further south if the gateway is open or closed. The geothermal heat flux (Pollard et al. 2005), Antarctic vegetation 276 222 gateway’s effect on East Antarctic climate and snowfall was also dynamics (Thorn & DeConto 2006), and Southern Ocean sea 277 223 showntobeminimal,pointingtosomeotherforcing(perhaps ice (DeConto et al. 2007) in the Eocene–Oligocene climatic 278 224 decreasing atmospheric CO2 concentrations) as the primary transition. 279 225 cause of Antarctic cooling and glaciation. The results of these studies can be summarized as follows. 280 226 One aspect of recent modelling has focused on the The timing of glaciation on East Antarctica was shown to be 281 227 development of coupled climate-ice sheet models capable sensitive to orbital forcing, mountain uplift, and continental 282 228 of running long (.106 yr), time-continuous simulations of vegetation, but only within a very narrow range of 283 229 specific climate events and transitions (DeConto & Pollard atmospheric CO2 concentrations around 2.8 times pre- 284 230 2003a).
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