5 Reconstruction of past climates
The Antarctic ice sheet contains within its layers a rich archive of information on past climatic and environmental changes. As snow is successively buried it is compressed to form glacial ice. This ice carries traces of dissolved impurities, variations in water isotopes, and bubbles of air, sealed within as the snow is compacted, which can be analysed to study past atmospheric and climatic variations.
Photo: Rowan Butler
Australian contributions have focussed on ice cores from the relatively high snowfall areas in the coastal region of east Antarctica, mainly at Law Dome. In 1993 a major project there culminated in the recovery of a 1200 m long surface-to-bedrock ice core from near the Dome's summit. This core provides a record extending back some 90,000 years, with higher time-resolution for recent millennia than any other Antarctic ice core. It also provides information on the response of the active coastal zone of the ice sheet to the sea level and climate changes associated with the transition from the last Ice Age. The scientific output from this ice core research has made a significant contribution and improved our knowledge in a range of areas from climate forcing (with studies of relevance to volcanic, greenhouse gas and solar forcing), natural climate variability and recent change, with connections to Australian climate, and are included in the 3rd and 4th IPCC Assessment Reports.
The emphasis on detailed time resolution enabled by the Law Dome ice core has been the principal contribution of the Australian programme. The high resolution allows annual layers to be counted accurately, which gives very well dated records (Palmer et al., 2001) that can be compared, through the period of overlap, with instrumental meteorological data to calibrate ice core 'proxy' climate signals (van Ommen and Morgan, 1997; McMorrow et al., 2004).
Very few annually resolved records are available from Antarctica, and the Law Dome record has provided a fundamental resource in reconstructions of climate. This includes a recent two hundred year reconstruction of Antarctic temperatures (Schneider et al., 2006). This is the longest such record and shows overall warming in phase with that of the Southern Hemisphere, but interrupted in the late 20th century, likely by strengthening of high-latitude westerly winds: a manifestation of increasing strength of the atmospheric Southern Annular Mode (SAM), itself a product of changes in the concentration of atmospheric ozone.
A 50 year continent-wide snowfall reconstruction which made significant use of the LD record (Monaghan et al., 2006) showed, contrary to model expectations for a warming climate, that there has not been any significant change in Antarctic snowfall over 50 years, although it also reveals large spatial and temporal variability. The Law Dome climate record has also been used in hemispheric and global climate reconstructions of the last 2000 years (Mann and Jones, 2003; Jones and Mann, 2004). The Law Dome record was the only source of proxy data for Antarctica over this period in the reconstruction.
In addition to climate reconstructions, the Law Dome ice core data provide proxies for climate forcing. In particular, the record of volcanic eruptions (Palmer et al., 2001a, 2002) provides a reference forcing series for modelling climate changes over recent centuries. Solar variability is also an important factor in understanding and modelling past climate variations. Ice core beryllium-10 data are used to infer past variations, and studies at Law Dome are providing calibration constraints that will lead to better understanding of this proxy (Pedro et al., 2006). Solar variations are also playing a role in atmospheric chemistry and climate. Studies on the Law Dome chemistry and particularly on nitrate have been used to place limits on the size of the solar influence (Palmer et al., 2001b).
Photo: MacFarling Meure et al., 2006
Looking at more recent climate history, the detailed ice core records are being used to understand variability during the era of anthropogenic influence. Changes are seen in a range of proxies, and these can be compared with the longer term record.
Photo: Adapted from Curran et al., 2003
The strong maritime climate at Law Dome provides not only high resolution, but also climate information that links with mid-latitudes. Winter sea-salt levels in the ice core are connected with mid-latitude atmospheric pressure (Goodwin et al., 2004; Souney et al., 2002) and have been used as a proxy for the winter SAM strength. Recent analyses showing strong correlation between precipitation changes in south-western Australia and those in Antarctica are providing a context for the current drought in that part of Australia (van Ommen and Morgan, 2007).
Photo: Karen Holmes
Changes are evident in atmospheric circulation through the termination of the last ice age. Comparison of chemical concentrations with inland ice cores shows that the present maritime coastal climate at Law Dome only developed in the late stages of the deglaciation (Curran et al., 2007) and indeed the full modern-day high precipitation régime developed only well into the present interglacial period (van Ommen et al., 2004).
The high resolution of the Law Dome ice core allows for detection of relatively rapid climate events that are difficult to detect in other Antarctic records. One such event – the final in a series that punctuated the last ice age – occurred around 8200 years ago and is well known from Greenland ice cores. The event almost certainly has its origins in the northern hemisphere during an outburst from a lake system created during the demise of the Northern Hemisphere ice sheets of the last ice age. The changes left a large signature in global atmospheric methane, but it is not clear if it also triggered climate responses in the southern hemisphere. The Law Dome record shows the clearest and best-timed Antarctic signature of the methane marker and reveals that there is little evidence for an accompanying climate response in Antarctica (van Ommen et al., 2007).
As the mechanisms underlying the climate links and the proxies measured in the ice are better understood, there is considerable potential to shed light on observed climate changes in Australia as well as Antarctica and the southern hemisphere generally. Such work will be most fruitful as it draws together multiple proxy and multiple ice core records. With current international concern on climate change and in the context of increasing international collaboration directed toward ice core research, plans are in train to extend this work (Brook and Wolff, 2006). Under the banner of the International Partnership in Ice Core Sciences, plans are being developed to establish a denser network of Antarctic ice cores and also to recover an ice core extending back over 1 million years when the Earth's glacial-interglacial climate cycle changed from 41,000 years to its present 100,000 years. An ice core spanning this period will allow the relationship between atmospheric composition and environmental temperature to be determined. As it is predicted that some of Antarctica's oldest ice is to be found in Australia's Antarctic Territory it behoves Australia to take international leadership in such a fundamental study.
The keys to understanding some processes controlling climate and ice-sheet dynamics, and atmospheric CO2 lie in oceanic records. Palaeoceanography has contributed to the understanding of these climate dynamics on three main time scales: 1) the Cenozoic time frame (65.5 million years ago to present) over which the ice sheets, the ocean basins, and continental positions have reached their current arrangements; 2) the Plio-Pleistocene time frame (5.3 million years-11,000 years ago) during which the 100,000 year cycle of orbital forcing has dominated climate variability and in which the Southern Ocean has played a controlling role in determining atmospheric CO2; and 3) the Holocene (11,000 years ago to the present), in which the background climate variability can be measured, and against which the human climate impact can be detected.
For example, the change from 41,000 year cycles to 100,000 year climate cycles occurred during the Plio-Pleistocene time period – a period which has attracted considerable interest from Australian palaeoceanographers. Cores taken from the sea bed contain records of past flora and fauna whose shells and skeletons contain a record of sea temperature and water characteristics. From these 'proxies' Australian scientists have materially assisted our understanding of the evolution of the climate, from the start of the Cenozoic to historical times during which analyses of air trapped in ice cores can provide accurate data on climates.
The circumpolar Southern Ocean formed when Australia broke away from Antarctica and began drifting north about 83 million years ago, according to seismic reflection, magnetic data and drilling of the Antarctic and Australian margins. Drilling by the Ocean Drilling Program led by Australian scientists showed that a deep seaway opened between Australia and Antarctica about 35-33.5 million years ago (Exon et al. 2002; Stickley et al. 2004), concurrent with glacial ice first reaching sea level on the Antarctica coast (Barrett 1999; Cooper and O'Brien 2004). Oxygen isotope measurements on marine microfossils indicate a major increase in global ice volumes and cooling of the ocean at ~33.9 million years ago (Zachos et al. 2001).
Australian scientists developed a model for the inception of Antarctic glaciation based on the thermal isolation of Antarctica resulting from the development of the Antarctic Circumpolar Current. This allowed the East Antarctic Ice Sheet (EAIS) to develop (Kennett 1977; Kennett and Exon 2004). An alternative view from climate modelling suggests falling global atmospheric CO2 levels were the cause of Antarctic cooling (DeConto and Pollard 2003; Huber et al. 2004). Integrated Ocean Drilling Program sediment cores show cooling of the Arctic Ocean at the same time the EAIS developed supporting the role of CO2 in polar cooling and ice sheet formation, rather than the instigation of the Antarctic Circumpolar Current. Carbon dioxide levels, inferred from geochemical proxies, were 1000 to 2000 ppm, during the warm, ice-free "Greenhouse" world of the early Cenozoic (Pagani et al. 2005; Pearson and Palmer 2000). CO2 levels have been below 500 ppm since "Icehouse" climates began ~33 Ma (Pearson and Palmer 2000) during which time the Antarctic Ice Sheet has oscillated between ~50%-125% of its current size )Cooper and O'Brien 2004; Donda et al. 2007; Pekar and DeConto 2006; Wade and Palike 2004).
The Southern Ocean has played a key role in modulating Southern Hemisphere climate and atmospheric CO2 over the Plio-Pleistocene and Australian scientists have been closely involved in field and theoretical studies. Geochemical models suggest that Southern Ocean sea-ice, circulation and productivity could all play a role in controlling atmospheric CO2 (Howard and Prell 1994; Sigman and Boyle 2000), but insufficient data constrains our full understanding of the past behaviour of these physical and biogeochemical records. Nevertheless, Southern Ocean cycles of temperature (Brathauer and Abelmann 1999; Howard and Prell 1992), sea ice, (Armand and Leventer 2003; Crosta et al. 2004) carbon isotopic variability (Moy et al. 2006) and other variables are all nearly "in-phase" with global climate cycles. The possible mechanisms maintaining this near-synchronicity include the level of atmospheric CO2 , thus suggesting Southern Ocean processes are in themselves important global climate feedback mechanisms.
Additionally, Southern Ocean palaeoceanographic data have given us access to a range of environmental variability unattainable in the historical record and subsequently provides limits on the dynamic range of key physical and ecological components of the Southern Ocean. Microfossil data has shown that biome boundaries like the Antarctic Polar Front Zone (APFZ), one of the most extensive biological gradients on the planet (Tynan 1998), have moved as much as 5 degrees of latitude during the glacial-interglacial cycles of the past ~500,000 years (CLIMAP 1981; Morley 1989; Howard and Prell 1992; King and Howard 2000; Armand and Leventer 2003; Gersonde et al. 2005). The ecological, physical oceanographic, and biogeochemical implications of such large-scale movements of the APFZ are dramatic. These datasets carry lasting messages about the structure and biodiversity of Southern Ocean ecosystems: their persistence, ability to re-organise, and resilience in the face of climate perturbations.
Marine records equally provide evidence, unattainable by ice cores, on the behaviour of the ice-sheet itself in the face of large-scale climate forcing by changes in the Earth's orbit, CO2, and sea-level. The EAIS fluctuated mainly in a 41,000 year cycle associated with orbital tilt (Grützner et al. 2003; Naish et al. 2001), until ~900,000 years ago when global and Antarctic climate cycles changed to be dominated by 100,000 year cycles (Tziperman and Gildor 2003). Australian led ODP drilling off Prydz Bay, suggests that the highest ice volumes ever recorded were reached prior to 1.1 million years ago (Cooper and O'Brien, 2004), and the present Antarctic ice sheet state developed between 500,000 and 900,000 years ago (Fink et al. 2006; Whitehead et al. 2006).
Finally, marine sediments indicate warmer-than-present conditions during the progressive cooling of the Plio-Pleistocene age (McKelvey et al. 2001; Whitehead et al. 2006). The role of Antarctic ice-volume variations in Pleistocene sea-level cycles is still a matter of debate (Rohling et al. 2004) because the indicators of sea level (for example oxygen isotope ratios in marine microfossils or coral terraces) do not uniquely identify the locations of ice sheets responsible for the sea-level changes. Thus there is a need for direct observations on the maximum extent of the EAIS at glacial maxima; i.e. we need to know where the edge of the ice sheet has been (O'Brien et al. 1999). Since the EAIS edge at the glacial maxima is today located under water, obtaining well-dated records of ice-sheet grounding requires the recovery and analysis of further marine sediment samples from shelf basins.
The global view of Holocene climate variability, which spans the period from 11,000 years ago to the present, is one of relative stability and low variability, yet little is known of Southern Ocean Holocene variability. Some core records from the South Atlantic suggest mid-Holocene cooling (Hodell et al. 2001). However, there are few marine records of sufficient resolution in the Australian Antarctic sector to detect Holocene events such as the 8,200 year ago (cooling) event, an anomaly observed in Northern Hemisphere records (Morrill and Jacobsen 2005). Australian research reveals records capable of resolving Holocene variability do not show significant Holocene climate anomalies in sea ice extent or sea-surface temperature, suggesting that palaeoclimate variations are not uniform around the Antarctic and that records from the Australian Sector may have unique characteristics not seen in the South Atlantic or other basins (Howard et al. 2007). Sediment cores from continental shelf basins do, however, suggest variability in bottom-water production and these records may put modern changes in deep-water circulation into perspective (Harris et al. 2001). Some high-accumulation-rate deposits in continental shelf basins can resolve interannual-to-interdecadal-scale variability arising from such modes of variability as El Niño, the Southern Annular Mode, and solar variability (Crosta et al. 2005; Leventer et al. 1996; Stickley et al. 2005). Such records are important in being directly comparable to ice-core records like those recovered at Law Dome.