Abstract

Antarctic Climate Evolution has been, and will be, hugely influential in the development of Earth's environment. This book has detailed how Antarctica changed during several key stages in the Cenozoic. Here we take stock of past changes and consider how they may be helpful in evaluating future changes in Antarctica.

Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice in Greenland and in the Antarctic Peninsula, and rising global sea levels. Eleven of the 12 years between 1995 and 2006 are among the 12 warmest years in the instrumental record since 1850 (IPCC, 2007). According to the latest IPCC worst-case scenario projections (i.e. continued greenhouse gas emissions at or above current rates), global annual mean temperatures by 2100 are likely to exceed those that have been experienced by the Earth in the last 40myr, that is before the Antarctic Ice Sheet first developed (IPCC, 2007). If warming continues, the implication is that ice loss to the ocean may far outweigh gain from ice accumulation, with huge implications for global sea level (and associated

"Corresponding author. Tel.: +39 0651860 383; Fax: +39 0651860 397; E-mail: [email protected] (F. Florindo).

Figure 13.1: Ice-sheet surface elevation changes, recorded by satellite altimetric measurements, between 1992 and 2003. Note how ice loss is concentrated at the margins of ice streams in the Amundsen Bay region of East Antarctica, and the Totten and Cook glaciers in East Antarctica. These regions of ice loss are characterised by ice resting on a bed suppressed several hundred metres below sea-level, implying an ice-ocean connection to the current retreat (from Shepherd and Wingham (2007), with permission of the authors).

Figure 13.1: Ice-sheet surface elevation changes, recorded by satellite altimetric measurements, between 1992 and 2003. Note how ice loss is concentrated at the margins of ice streams in the Amundsen Bay region of East Antarctica, and the Totten and Cook glaciers in East Antarctica. These regions of ice loss are characterised by ice resting on a bed suppressed several hundred metres below sea-level, implying an ice-ocean connection to the current retreat (from Shepherd and Wingham (2007), with permission of the authors).

major changes in coastlines and the inundation of low-lying areas), atmospheric composition and dynamics, and ocean circulation.

Recent comprehensive, and near continuous, satellite altimetric observations indicate that the Antarctic Ice Sheet is losing mass, and that the rate of loss has increased steadily during the last few decades (Fig. 13.1, e.g. Davis et al., 2005; Shepherd and Wingham, 2007; Rignot et al., 2008). Most loss is from the West Antarctic Ice Sheet (WAIS), especially along the Bellingshausen and Amundsen seas (132760 Gtyr-1 in 2006). During the past ca. 40 years many glaciers, and a series of ice-shelves, have retreated in the Antarctic Peninsula (e.g. Cook et al., 2005) and, since the new millennium, two large ice shelves, located either side of the Antarctic Peninsula (the Larsen B and Wilkins ice shelves, with extents of 3,250 km2 and over 570 km2, respectively; Fig. 13.2), have suddenly broken up in dramatic fashion and separated from the continent. The decay of these ice shelves will not have affected sea level because ice shelves are afloat and thus already displacing their weight of water. However, ice-shelf decay has the potential to greatly reduce buttressing forces on grounded ice that resist the flow from the continent to the ocean. In other words, ice-shelf decay may encourage further loss of grounded ice, which will affect global sea level. A number of factors contribute to ice-shelf collapse, including atmospheric and oceanic

Figure 13.2: High-resolution, enhanced-colour image of the Wilkins Ice Shelf in Antarctica, taken on 8 March 2008 by Taiwan's Formosat-2 satellite (Credit: left, National Snow and Ice Data Center; right, National Snow and Ice Data Center/Courtesy Cheng-Chien Liu, National Cheng Kung University (NCKU), Taiwan and Taiwan's National Space Organization (NSPO); processed at Earth Dynamic System Research Center at NCKU, Taiwan).

Figure 13.2: High-resolution, enhanced-colour image of the Wilkins Ice Shelf in Antarctica, taken on 8 March 2008 by Taiwan's Formosat-2 satellite (Credit: left, National Snow and Ice Data Center; right, National Snow and Ice Data Center/Courtesy Cheng-Chien Liu, National Cheng Kung University (NCKU), Taiwan and Taiwan's National Space Organization (NSPO); processed at Earth Dynamic System Research Center at NCKU, Taiwan).

temperatures. If these continue to rise, we may see more ice shelves decaying across the Antarctic Peninsula and elsewhere in Antarctica (e.g. Morris and Vaughan, 2003; MacAyeal and others, 2006; Glasser and Scambos, 2008). It is worth noting that, over the past 50 years, the western part of the Antarctic Peninsula has experienced the greatest temperature increase on Earth, rising by nearly 3°C (e.g. King et al., 2003; Turner et al., 2005). This is approximately 10 times the mean rate of global warming, as reported by the IPCC.

The temperature increase is not confined to the Antarctic Peninsula; it is also occurring in and over the oceans surrounding Antarctica. For example, it is now well-established that the waters of the Antarctic Circumpolar Current (ACC) are warming more rapidly than the global ocean as a whole. A comparison of temperature measurements from the 1990s with data from earlier decades shows a large-scale warming of around 0.2°C in the ACC waters at around 700-1,100 m depth (Gille, 2002).

Across the East Antarctica Ice Sheet (EAIS) the mass balance is near a steady state with the flow of ice. Its interior is increasing in overall thickness, which is probably the result of increased ice accumulation due to warmer (and moister) atmospheric conditions over the continental interior (Church et al., 2001). This mass gain is balanced by ice thinning/loss across the potentially dynamic marine sectors in Wilkes Land (Davis et al., 2005). Further atmospheric and ocean warming may be incompatible with sustaining mass balance in this region, implying it too is susceptible to further changes in a warming world. A greater understanding of past changes in this region of East Antarctica is clearly warranted.

Comprehending glacial history since the Last Glacial Maximum (LGM), which culminated around 20,000 years ago, is also critical to knowing whether recent changes are a result of human activities and/or a result of icesheet relaxation since the LGM. The geological record tells us that the WAIS retreat occurred during the Holocene, far later than Northern Hemisphere ice sheets. It is certainly possible that changes observed in some locations today can be at least part explained by post-LGM ice retreat to a more stable interglacial condition. Geological evidence on the rate of ice loss since the LGM is therefore needed at several sites across the Antarctic continent.

Understanding Antarctic climate evolution requires continent-wide as well as continent-to-deep-sea studies of past climate records, to decipher their separate but related histories. Some of these activities are long-term undertakings but commitment within the frame of IPY is essential to laying the groundwork for long-term success. Over the next few years ACE aims to integrate geoscience data provided by sediment cores (ANDRILL, andrill.org; SHALDRILL, www.shaldril.rice.edu; IODP, www.iodp.org) and ice cores (EPICA, www.esf.org; ITASE, www2.umaine.edu/itase/; TAL-DICE; WAIS Divide, www.waisdivide.unh.edu/) with the new generation of climate models that couple atmosphere, ocean, ice sheet and sediment modelling on key time periods from the distant past (i.e. tens of millions of years ago when global temperature was several degrees warmer than today) to the recent past (i.e. during the Holocene, prior to anthropogenic impacts as well as at the LGM). Acquisition of sedimentary records from the floors of subglacial lakes is also anticipated in the next few years. Such information may inform us about ice-sheet histories, and when West Antarctica was last deglaciated. Drilling, sampling and studying subglacial lakes remotely and without causing their contamination represent a considerable technological challenge. In this regard the exploration of sub-ice lakes represents a good analogue for the exploration of planets and satellites such as Europa. A number of subglacial lakes have been identified in Antarctica and, of these, one subglacial lake in West Antarctica, named Lake Ellsworth (from the American explorer Lincoln Ellsworth), is well suited to exploratory research (www.geos.ed.ac.uk/ellsworth).

This book has demonstrated that the Antarctic Ice Sheet has undergone many changes, and has varied in size considerably over the Cenozoic. The nature of these changes has been shown to be associated with global climate conditions, forced externally or through interaction with ice and climate conditions in Antarctica and the Southern Ocean. IPCC predictions of atmospheric greenhouse gases are pertinent to future ice volumes in Antarctica. If such gases continue to rise, in a few centuries time the value of atmospheric carbon dioxide may be greater than at any time in the Cenozoic. The obvious danger is that, in a warming world, the Antarctic Ice Sheet may respond to climate and ocean changes as it has done in the past. In other words, the palaeo-ice-sheet reconstructions highlighted in this book may be relevant to assessments of future changes in Antarctica.

Over the next decade, ACE will be pursuing a broad range of objectives to better comprehend past Antarctic changes, through the organisation of workshops, where interdisciplinary research can be discussed, and through publications (e.g. Florindo et al., 2003, 2005, 2008; Barrett et al., 2006), allowing dissemination of results to a wide audience. It is only through such integration of geological data and numerical modelling that quantitative assessments of past changes, and possible future scenarios, can be achieved.

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