Abstract
Abstract. The geometry of Antarctic ice sheets during warm periods of the geological past is difficult to determine from geological evidence, but is important to know because such reconstructions enable a more complete understanding of how the ice-sheet system responds to changes in climate. Here we investigate how Antarctica evolved under orbital and greenhouse gas conditions representative of an interglacial in the early Pliocene at 4.23 Ma, when Southern Hemisphere insolation reached a maximum. Using offline-coupled climate and ice-sheet models, together with a new synthesis of high-latitude palaeoenvironmental proxy data to define a likely climate envelope, we simulate a range of ice-sheet geometries and calculate their likely contribution to sea level. In addition, we use these simulations to investigate the processes by which the West and East Antarctic ice sheets respond to environmental forcings and the timescales over which these behaviours manifest. We conclude that the Antarctic ice sheet contributed 8.6 ± 2.8 m to global sea level at this time, under an atmospheric CO2 concentration identical to present (400 ppm). Warmer-than-present ocean temperatures led to the collapse of West Antarctica over centuries, whereas higher air temperatures initiated surface melting in parts of East Antarctica that over one to two millennia led to lowering of the ice-sheet surface, flotation of grounded margins in some areas, and retreat of the ice sheet into the Wilkes Subglacial Basin. The results show that regional variations in climate, ice-sheet geometry, and topography produce long-term sea-level contributions that are non-linear with respect to the applied forcings, and which under certain conditions exhibit threshold behaviour associated with behavioural tipping points.
Highlights
The response of the Antarctic ice sheets (AIS) to predicted future oceanic and atmospheric warming will dictate the magnitude of global sea level changes for millennia, yet the sensitivity of the AIS system is unclear, leading to a wide range of sea-level predictions for coming centuries (Ritz et al, 2015; Golledge et al, 2015; DeConto and Pollard, 2016)
Novel isotope-enabled climate and ice-sheet modelling has allowed the likely GMSL contribution from Antarctica to be constrained to 3–12 m during the warmest parts of the mid-Pliocene, with an absolute maximum of 13 m (Gasson et al, 2016), which requires at least some loss of ice from East Antarctica
Recent studies have alluded to the idea that the Antarctic ice sheets may be susceptible to “tipping points” in their stability that could give rise to non-linear behaviour and rapid contributions to sea level
Summary
The response of the Antarctic ice sheets (AIS) to predicted future oceanic and atmospheric warming will dictate the magnitude of global sea level changes for millennia, yet the sensitivity of the AIS system is unclear, leading to a wide range of sea-level predictions for coming centuries (Ritz et al, 2015; Golledge et al, 2015; DeConto and Pollard, 2016). Warm periods of the Pliocene (2.58–5.33 Ma) are considered some of the most appropriate analogues for future environmental conditions (Haywood et al, 2016), especially the early to midPliocene (4–5 Ma), when atmospheric CO2 concentrations were in the range 365–415 ppm (Pagani et al, 2010), similar to present day. Golledge et al.: Early Pliocene Antarctic ice sheet present (Haywood et al, 2013), comparable to warmings anticipated by 2100 under mid-range emissions scenarios of the Intergovernmental Panel on Climate Change Fifth Assessment Report (Collins et al, 2013). Geological records from close to the East Antarctic coast suggest that a portion of this sea-level contribution may have originated from the Wilkes Subglacial Basin (WSB), when the ice sheet margin is thought to have migrated hundreds of kilometres inland (Cook et al, 2013; Patterson et al, 2014)
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