PLIOMAX - PLIOcene MAXimum sea level
Scientists interested in how modern climate change will impact the stability of polar ice sheets may take several approaches in their research. The first is to use climate model simulations of the response of ice sheets and glaciers under elevated greenhouse gas levels. The second is to look to the past and infer from the geological record the fate of the Earth’s polar ice sheets when levels of warming were similar to what is expected in the next few decades to centuries. The interdisciplinary PLIOMAX project took the latter approach, exploring the geophysical processes that obscure the signal of ancient ice volumes embedded within the geological record of sea level change.
The aim of the PLIOMAX project, a five-year research project funded by the US National Science Foundation, was to deliver more accurate sea level and ice volume data for the Pliocene, a period 3 million years ago when atmospheric CO2 was at ~400 ppmv, a benchmark that was recently surpassed. The mid-Pliocene warm period thus provides a natural analogue for a warmer, higher CO2 world that can be used as a testing ground for the climate and ice sheet models that are being used to predict the future response of Earth’s climate to increasing levels of greenhouse gases. We have shown that existing Pliocene sea level estimates have large errors (±15 m) due to previously unaccounted for solid earth effects (such as glacial isostatic adjustment and mantle dynamic topography), precluding firm estimates of sea level at that time. More accurate estimates of Pliocene peak sea level will require improved modeling of solid earth deformation effects, including better constraints on mantle viscosity, as well as additional field studies of Pliocene and Pleistocene sedimentary sequences around the globe. We are continuing these efforts with our WarmCoasts/EEMAX initiative that focuses on understanding sea level rise in the Eemian.
Through a collaboration between marine geologists, geophysicists, and ice and climate modelers, we have published numerous papers on different aspects of Pliocene and Pleistocene SL change and climate. Major accomplishments of the PLIOMAX project include:
(1) The first paper showing how to correct ancient shorelines for glacial isostatic adjustment (GIA; Raymo et al., 2011) and identifying this process as one of the reasons for the Pliocene sea level paradox—namely the different elevations of Pliocene shorelines around the globe. The techniques we outline in this paper can be used to correct all shorelines from deep time for GIA and are not restricted to Pliocene shorelines. We also conclude that the remaining disagreement between Pliocene shorelines around the world, after correcting for GIA, is likely due to dynamic topography effects due to mantle convection (see #4 below).
(2) A paper showing that the unusually long duration of the MIS11 interglacial must be taken into account when making GIA shoreline corrections (Raymo & Mitrovica, 2012). We show that the extremely high estimated sea level for this time is actually much lower, closer to 10 m when a comparative time reference frame to today is applied. This paper also demonstrated how knowing when in an interglacial a shoreline feature formed is essential for making accurate GIA corrections. A follow-up paper by Chen et al. 2014 further developed these ideas.
(3) O’Leary et al., 2013 published a detailed MIS 5e (Eemian) sea level history from a large database of new and existing field data in Western Australia supporting the hypothesis that a late rapid rise in eustatic sea level occurred at the end of the Eemian. This idea remains controversial and is driving current research both within our group as well as in other research groups, focusing on both data and modeling. Understanding the timing of sea level events within MIS5e will lead to a better understanding of Antarctic ice sheet dynamics and its response to local insolation with direct implications for future sea level change.
(4) A series of papers based on our field work on four continents show that Pliocene shorelines can be used to set constraints on the magnitude of dynamic topography (DT) over millions of years (Rowley et al., 2013; Rovere et al., 2014, 2015, 2016; Raymo et al., in prep.; Hearty et al., in prep). Post-doc Alessio Rovere showed that dynamic topography uplift rates of 10-20 m/Ma are supported by field evidence from around the globe. This further implies that even much younger Eemian shoreline features have the possibility of being biased by DT by 1-2 meters (e.g., Austermann et al., 2017) and thus are likely overestimated. Lastly, building on the insight of Gomez et al. (2012, 2013), graduate student Jacky Austermann (Austermann et al., 2015) demonstrated that modeling of mid-Pliocene ice sheets must take into account long-term changes in sub-ice bedrock topography due to dynamic topography. In particular, the Wilkes Sub-Glacial Basin was several hundred meters lower during mid-Pliocene than had been recognized in previous work--incorporating this insight into an ice sheet model led to a significant retreat of ice cover over this region, reconciling model results with geological observations that suggest a dynamically evolving ice sheet in this area during the mid-Pliocene.
(5) During the Eemian, giant boulders weighing many tons appear to have been emplaced onto coastal clifftops in the Bahamas. Some studies suggest they were put there by “super storms” far more powerful than anything seen in recorded human time–generated by a climate warmer than that of today. Rovere et al. (2017) shows that storms typical of today, combined with a few meters increase in sea level during this warm time, would have been sufficient to move boulders of this size. Current projections say that sea level will rise about a meter by the end of this century, but some models put it much higher. This study shows that even if the intensity of storms does not increase, sea-level rise alone will dramatically increase the power of waves against hard barriers such as cliffs, buildings, and walls.
(6) Natalya Gomez, while a graduate student, developed the first ice sheet model coupled to a gravitationally self-consistent ice age sea level model (Gomez et al., 2012, 2013). The new model demonstrated that the drop in sea level at the margin of a rapidly melting ice sheet acted as a stabilizing influence on the grounding line of marine-based ice sheets such as the West Antarctic Ice Sheet. Prior to this work it had been assumed that melting of an ice sheet led to a globally uniform sea level rise plus a simplified viscous uplift of the crust in the zone of melting. These earlier analyses supported the so-called Marine Ice Sheet Instability Hypothesis, which holds that a grounded, marine-based ice sheet lying on a bed that tilts down toward the center of the ice sheet will experience runaway collapse as the ice sheet melts. The simulations in Gomez et al. (2012, 2013) showed that this hypothesis is not universally correct. The sea-level feedback will act to slow the collapse of a marine-based ice sheet and, in cases where the bedrock tilt is moderate, may even lead to the cessation of the collapse. This work – and the Gomez effect – is now universally understood as a fundamentally important ingredient in ice sheet modeling in response to global warming.
(7) The inference of Earth’s viscosity based on ice age data sets is a classic problem in geophysics, but despite a half-century of study there remain significant (order of magnitude) differences in published estimates. Graduate student Harriet Lau, using a Bayesian inversion methodology, resolved this contentious issue by demonstrating that these differences originate from varying treatments of ice age effects on the Earth’s gravity field (Lau et al., 2016). By accounting for contamination of the gravity signal associated with modern global change processes (i.e., glacier melting), the paper demonstrated that mantle viscosity increases by two orders of magnitude from the base of tectonic plates to the core-mantle-boundary. In future work, our group will build on this study to infer the lateral variation on mantle viscosity in addition to the depth variation – i.e., build a 3-D map of mantle viscosity.
(8) Pliomax graduate student Ed Gasson demonstrated, using isotope-enable ice sheet models, how changes in ice-sheet geometry can impact sea level estimates based on the marine oxygen isotope record (Gasson et al., 2016). Work by Raymo et al. (2017) and Evans et al. (2016) further explore the numerous factors that contribute to uncertainty in Pliocene sea level estimates derived from marine geochemical proxies such as d18O and Mg/Ca ratios.
(9) Pliocene atmospheric modeling that considers the impacts of a smaller Antarctic ice sheet on regional winds supports the notion that marine diatoms found in the Pliocene Sirius Formation (Antarctica) were wind blown, and sourced from deglaciated sub-glacial basins (exposed to weathering after rebound). This paper provides a fresh perspective on a long-standing geological debate (Scherer et al., 2016).
(10) For the first time, paleo sea-level estimates (Pliocene and Last Interglacial) were used to tune ice-sheet model physics using large ensemble techniques. The calibrated models were then used to simulate the future response of the Antarctic ice sheet to greenhouse-gas forcing, following the RCP scenarios. The simulations implied higher and faster possible rates of ice sheet retreat than previously published estimates (DeConto and Pollard, 2016) though the results are highly dependent on the paleo sea-level estimates which still have large uncertainties.
(11) New ice-sheet model physics representing processes not previously considered at the ice-sheet scale (hydrofracturing of ice shelves, and ice cliff collapse), produce much higher model-derived estimates of Pliocene sea-level than in any of our previous modeling (Pollard et al., 2015). Model advances included: a novel inverse method to deduce basal sliding coefficients under currently grounded ice by fitting to modern observed ice elevations and iterating in forward runs (Pollard and DeConto, 2012a), more sophisticated parameterizations of calving, including simple damage advection (Pollard and DeConto, 2012b; Pollard et al., 2015), and more physical and robust parameterization of oceanic melting under floating ice shelves (Pollard and DeConto, 2012b).
(12) Large ensembles of Antarctic runs were conducted, both for the past deglacial period over the last 40 kyrs, and for the future using RCP climate scenarios (as in DeConto and Pollard, 2016), with the coupled ice-Earth model (Gomez et al., 2013) and looking in detail at the effects of different Earth rheological profiles. In particular, the potential negative feedbacks on ice retreat with a weak upper mantle and thin lithosphere, appropriate for West Antarctica, were highlighted. A full-length paper was produced describing the results (Pollard et al., 2017), following on from Gomez et al. (2015). The main findings were that most Earth profiles gave similar results to the simpler ELRA (Elastic Lithosphere Rebounding Asthenosphere) bedrock model, but with the weak-upper-mantle thin-lithosphere profile representative of areas of West Antarctica, significant negative feedbacks occur in the future scenarios that limit ice retreat.
(13) The Penn State ice sheet model was coupled with LOVECLIM, the Earth Model of Intermediate Complexity (EMIC) maintained by the U. Hawaii group. This EMIC requires much less CPU time than the global and regional climate models (GCM, RCM) used in the work above, and offers the capability of fully coupled and time-continuous ice-climate simulations through glacial cycles, and orbital cycles during the Pliocene. D. Pollard also significantly improved the surface ice-snow mass balance scheme in the ice model (which was based on Positive Degree Days). It was converted into a surface energy balance scheme, including solar insolation variations on diurnal, seasonal and orbital scales, with more physical treatment of snow water content and refreezing. Extensive modern calibration for modern Greenland led to a new reference model that continues to be refined.
(14) A pair of companion papers (data and modeling) show that the Antarctic Ice Sheet had the potential to contribute >25m of sea-level variability in the Miocene. This work has substantially reduced the longstanding “hysteresis problem”, because this much orbitally-paced sea-level variability had been difficult to explain without invoking pre-Pliocene Northern Hemispheric ice sheets (Gasson et al., 2016; Levy et al., 2016). The model advance comes from the addition of new ice sheet dynamical processes and coupling between atmospheric and ice-sheet models. A combination of ice sheet and GIA modeling, integrated with marine isotope records hints at a causal relationship between late Miocene Antarctic Ice-Sheet variability and the Messinian Salinity Crisis (desiccation of the Mediterranean Basin) (Ohnheiser et al., 2015).
(15) Finally, we have made numerous contributions to the wider sea level research community including participation in the publication of three review papers, including on in Science (Dutton et al., 2015, more than 250 citations; Kemp et al., 2015; and Miller et al., 2019).
References Cited Above
Austermann, J., Mitrovica, J. X., Huybers, P., & Rovere, A. (2017). Detection of a dynamic topography signal in last interglacial sea-level records. Science Advances, 3(7), e1700457.
Austermann, J., Pollard, D., Mitrovica, J. X., Moucha, R., Forte, A. M., DeConto, R. M., ... & Raymo, M. E. (2015). The impact of dynamic topography change on Antarctic ice sheet stability during the mid-Pliocene warm period. Geology, 43(10), 927-930.
Chen, F., Friedman, S., Gertler, C. G., Looney, J., O’Connell, N., Sierks, K., & Mitrovica, J. X. (2014). Refining estimates of polar ice volumes during the MIS11 Interglacial using sea level records from South Africa. Journal of Climate, 27(23), 8740-8746.
DeConto, R., and Pollard, D. (2016). Antarctica’s contribution to past and future sea-level rise. Nature, v. 531, pp. 591-597.
Dutton, A., Carlson, A. E., Long, A., Milne, G. A., Clark, P. U., DeConto, R., ... & Raymo, M. E. (2015). Sea-level rise due to polar ice-sheet mass loss during past warm periods. Science, 349(6244), aaa4019.
Gasson, E., DeConto, R. M., & Pollard, D. (2016). Modeling the oxygen isotope composition of the Antarctic ice sheet and its significance to Pliocene sea level. Geology, 44(10), 827-830.
Gasson, E., DeConto, R., Pollard, D., Levy, R. (2016). Dynamic Antarctic ice sheet during the early to mid-Miocene. PNAS, v. 113, pp. 3459–3464.
Gomez, N., Pollard, D., & Holland, D. (2015). Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss. Nature Communications, 6, 8798.
Gomez, N., Pollard, D., & Mitrovica, J. X. (2013). A 3-D coupled ice sheet–sea level model applied to Antarctica through the last 40 ky. Earth and Planetary Science Letters, 384, 88-99.
Gomez, N., Pollard, D., Mitrovica, J. X., Huybers, P., & Clark, P. U. (2012). Evolution of a coupled marine ice sheet–sea level model. Journal of Geophysical Research: Earth Surface, 117(F1).
Hearty et al., in prep., Plio-Pleistocene stratigraphy and sea level estimates, Republic of South Africa.
Kemp, A. C., Dutton, A., & Raymo, M. E. (2015). Paleo constraints on future sea-level rise. Current Climate Change Reports, 1(3), 205-215.
Lau, H. C., Mitrovica, J. X., Austermann, J., Crawford, O., Al‐Attar, D., & Latychev, K. (2016). Inferences of mantle viscosity based on ice age data sets: Radial structure. Journal of Geophysical Research: Solid Earth, 121(10), 6991-7012.
Levy, R., et al., incl. DeConto, R. (2016). Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene. Proceedings of the National Academy of Sciences, 113(13), 3453-3458.
Miller, K.E., Raymo, M.E., Browning, J.V., Rosenthal, Y., Wright, J.D., in press. Peak sea level during the warm Pliocene: Errors, limitations, and constraints. Past Global Changes.
O’Leary, M. J., Hearty, P. J., Thompson, W. G., Raymo, M. E., Mitrovica, J. X., & Webster, J. M. (2013). Ice sheet collapse following a prolonged period of stable sea level during the last interglacial. Nature Geoscience, 6(9), 796.
Ohneiser, C., Florindo, F., Stocchi, P., Roberts, A. P., DeConto, R. M., & Pollard, D. (2015). Antarctic glacio-eustatic contributions to late Miocene Mediterranean desiccation and reflooding. Nature Communications, 6, 8765.
Pollard, D., & DeConto, R. M. (2012a). A simple inverse method for the distribution of basal sliding coefficients under ice sheets, applied to Antarctica. The Cryosphere, 6(5), 953.
Pollard, D., & DeConto, R. M. (2012b). Description of a hybrid ice sheet-shelf model, and application to Antarctica. Geoscientific Model Development, 5(5), 1273.
Pollard, D., Chang, W., Haran, M., Applegate, P., & DeConto, R. (2015). Large ensemble modeling of last deglacial retreat of the West Antarctic Ice Sheet: Comparison of simple and advanced statistical techniques. Geoscientific Model Development Discussions, 8, 9925-9963.
Pollard, D., DeConto, R. M., & Alley, R. B. (2015). Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure. Earth and Planetary Science Letters, 412, 112-121.
Pollard, D., et al., incl. DeConto, R. (2016). Large ensemble modeling of the last deglacial West Antarctic Ice Sheet: comparison of simple and advanced statistical techniques. GMD v. 9, pp. 697-1723.
Pollard, D., Gomez, N., & Deconto, R. M. (2017). Variations of the Antarctic Ice Sheet in a Coupled Ice Sheet‐Earth‐Sea Level Model: Sensitivity to Viscoelastic Earth Properties. Journal of Geophysical Research: Earth Surface, 122(11), 2124-2138.
Raymo, M. E., & Mitrovica, J. X. (2012). Collapse of polar ice sheets during the stage 11 interglacial. Nature, 483(7390), 453.
Raymo, M. E., Mitrovica, J. X., O’Leary, M. J., DeConto, R. M., & Hearty, P. J. (2011). Departures from eustasy in Pliocene sea-level records. Nature Geoscience, 4(5), 328.
Rovere, A., Raymo, M. E., Mitrovica, J. X., Hearty, P. J., OʼLeary, M. J., & Inglis, J. D. (2014). The Mid-Pliocene sea-level conundrum: Glacial isostasy, eustasy and dynamic topography. Earth and Planetary Science Letters, 387, 27-33.
Rovere, A., Hearty, P. J., Austermann, J., Mitrovica, J. X., Gale, J., Moucha, R., & Raymo, M. E. (2015). Mid-Pliocene shorelines of the US Atlantic Coastal Plain—An improved elevation database with comparison to Earth model predictions. Earth-science reviews, 145, 117-131.
Rovere, A., Raymo, M. E., Vacchi, M., Lorscheid, T., Stocchi, P., Gomez-Pujol, L., & Hearty, P. J. (2016). The analysis of Last Interglacial (MIS 5e) relative sea-level indicators: Reconstructing sea-level in a warmer world. Earth-Science Reviews, 159, 404-427.
Rovere, A., Casella, E., Harris, D. L., Lorscheid, T., Nandasena, N. A., Dyer, B., & Raymo, M. E. (2017). Giant boulders and Last Interglacial storm intensity in the North Atlantic. Proceedings of the National Academy of Sciences, 114(46), 12144-12149.
Rowley, D. B., Forte, A. M., Moucha, R., Mitrovica, J. X., Simmons, N. A., & Grand, S. P. (2013). Dynamic topography change of the eastern United States since 3 million years ago. Science, 340(6140), 1560-1563.
Scherer, R. P., DeConto, R. M., Pollard, D., & Alley, R. B. (2016). Windblown Pliocene diatoms and East antarctic ice sheet retreat. Nature Communications, 7, 12957.
The aim of the PLIOMAX project, a five-year research project funded by the US National Science Foundation, was to deliver more accurate sea level and ice volume data for the Pliocene, a period 3 million years ago when atmospheric CO2 was at ~400 ppmv, a benchmark that was recently surpassed. The mid-Pliocene warm period thus provides a natural analogue for a warmer, higher CO2 world that can be used as a testing ground for the climate and ice sheet models that are being used to predict the future response of Earth’s climate to increasing levels of greenhouse gases. We have shown that existing Pliocene sea level estimates have large errors (±15 m) due to previously unaccounted for solid earth effects (such as glacial isostatic adjustment and mantle dynamic topography), precluding firm estimates of sea level at that time. More accurate estimates of Pliocene peak sea level will require improved modeling of solid earth deformation effects, including better constraints on mantle viscosity, as well as additional field studies of Pliocene and Pleistocene sedimentary sequences around the globe. We are continuing these efforts with our WarmCoasts/EEMAX initiative that focuses on understanding sea level rise in the Eemian.
Through a collaboration between marine geologists, geophysicists, and ice and climate modelers, we have published numerous papers on different aspects of Pliocene and Pleistocene SL change and climate. Major accomplishments of the PLIOMAX project include:
(1) The first paper showing how to correct ancient shorelines for glacial isostatic adjustment (GIA; Raymo et al., 2011) and identifying this process as one of the reasons for the Pliocene sea level paradox—namely the different elevations of Pliocene shorelines around the globe. The techniques we outline in this paper can be used to correct all shorelines from deep time for GIA and are not restricted to Pliocene shorelines. We also conclude that the remaining disagreement between Pliocene shorelines around the world, after correcting for GIA, is likely due to dynamic topography effects due to mantle convection (see #4 below).
(2) A paper showing that the unusually long duration of the MIS11 interglacial must be taken into account when making GIA shoreline corrections (Raymo & Mitrovica, 2012). We show that the extremely high estimated sea level for this time is actually much lower, closer to 10 m when a comparative time reference frame to today is applied. This paper also demonstrated how knowing when in an interglacial a shoreline feature formed is essential for making accurate GIA corrections. A follow-up paper by Chen et al. 2014 further developed these ideas.
(3) O’Leary et al., 2013 published a detailed MIS 5e (Eemian) sea level history from a large database of new and existing field data in Western Australia supporting the hypothesis that a late rapid rise in eustatic sea level occurred at the end of the Eemian. This idea remains controversial and is driving current research both within our group as well as in other research groups, focusing on both data and modeling. Understanding the timing of sea level events within MIS5e will lead to a better understanding of Antarctic ice sheet dynamics and its response to local insolation with direct implications for future sea level change.
(4) A series of papers based on our field work on four continents show that Pliocene shorelines can be used to set constraints on the magnitude of dynamic topography (DT) over millions of years (Rowley et al., 2013; Rovere et al., 2014, 2015, 2016; Raymo et al., in prep.; Hearty et al., in prep). Post-doc Alessio Rovere showed that dynamic topography uplift rates of 10-20 m/Ma are supported by field evidence from around the globe. This further implies that even much younger Eemian shoreline features have the possibility of being biased by DT by 1-2 meters (e.g., Austermann et al., 2017) and thus are likely overestimated. Lastly, building on the insight of Gomez et al. (2012, 2013), graduate student Jacky Austermann (Austermann et al., 2015) demonstrated that modeling of mid-Pliocene ice sheets must take into account long-term changes in sub-ice bedrock topography due to dynamic topography. In particular, the Wilkes Sub-Glacial Basin was several hundred meters lower during mid-Pliocene than had been recognized in previous work--incorporating this insight into an ice sheet model led to a significant retreat of ice cover over this region, reconciling model results with geological observations that suggest a dynamically evolving ice sheet in this area during the mid-Pliocene.
(5) During the Eemian, giant boulders weighing many tons appear to have been emplaced onto coastal clifftops in the Bahamas. Some studies suggest they were put there by “super storms” far more powerful than anything seen in recorded human time–generated by a climate warmer than that of today. Rovere et al. (2017) shows that storms typical of today, combined with a few meters increase in sea level during this warm time, would have been sufficient to move boulders of this size. Current projections say that sea level will rise about a meter by the end of this century, but some models put it much higher. This study shows that even if the intensity of storms does not increase, sea-level rise alone will dramatically increase the power of waves against hard barriers such as cliffs, buildings, and walls.
(6) Natalya Gomez, while a graduate student, developed the first ice sheet model coupled to a gravitationally self-consistent ice age sea level model (Gomez et al., 2012, 2013). The new model demonstrated that the drop in sea level at the margin of a rapidly melting ice sheet acted as a stabilizing influence on the grounding line of marine-based ice sheets such as the West Antarctic Ice Sheet. Prior to this work it had been assumed that melting of an ice sheet led to a globally uniform sea level rise plus a simplified viscous uplift of the crust in the zone of melting. These earlier analyses supported the so-called Marine Ice Sheet Instability Hypothesis, which holds that a grounded, marine-based ice sheet lying on a bed that tilts down toward the center of the ice sheet will experience runaway collapse as the ice sheet melts. The simulations in Gomez et al. (2012, 2013) showed that this hypothesis is not universally correct. The sea-level feedback will act to slow the collapse of a marine-based ice sheet and, in cases where the bedrock tilt is moderate, may even lead to the cessation of the collapse. This work – and the Gomez effect – is now universally understood as a fundamentally important ingredient in ice sheet modeling in response to global warming.
(7) The inference of Earth’s viscosity based on ice age data sets is a classic problem in geophysics, but despite a half-century of study there remain significant (order of magnitude) differences in published estimates. Graduate student Harriet Lau, using a Bayesian inversion methodology, resolved this contentious issue by demonstrating that these differences originate from varying treatments of ice age effects on the Earth’s gravity field (Lau et al., 2016). By accounting for contamination of the gravity signal associated with modern global change processes (i.e., glacier melting), the paper demonstrated that mantle viscosity increases by two orders of magnitude from the base of tectonic plates to the core-mantle-boundary. In future work, our group will build on this study to infer the lateral variation on mantle viscosity in addition to the depth variation – i.e., build a 3-D map of mantle viscosity.
(8) Pliomax graduate student Ed Gasson demonstrated, using isotope-enable ice sheet models, how changes in ice-sheet geometry can impact sea level estimates based on the marine oxygen isotope record (Gasson et al., 2016). Work by Raymo et al. (2017) and Evans et al. (2016) further explore the numerous factors that contribute to uncertainty in Pliocene sea level estimates derived from marine geochemical proxies such as d18O and Mg/Ca ratios.
(9) Pliocene atmospheric modeling that considers the impacts of a smaller Antarctic ice sheet on regional winds supports the notion that marine diatoms found in the Pliocene Sirius Formation (Antarctica) were wind blown, and sourced from deglaciated sub-glacial basins (exposed to weathering after rebound). This paper provides a fresh perspective on a long-standing geological debate (Scherer et al., 2016).
(10) For the first time, paleo sea-level estimates (Pliocene and Last Interglacial) were used to tune ice-sheet model physics using large ensemble techniques. The calibrated models were then used to simulate the future response of the Antarctic ice sheet to greenhouse-gas forcing, following the RCP scenarios. The simulations implied higher and faster possible rates of ice sheet retreat than previously published estimates (DeConto and Pollard, 2016) though the results are highly dependent on the paleo sea-level estimates which still have large uncertainties.
(11) New ice-sheet model physics representing processes not previously considered at the ice-sheet scale (hydrofracturing of ice shelves, and ice cliff collapse), produce much higher model-derived estimates of Pliocene sea-level than in any of our previous modeling (Pollard et al., 2015). Model advances included: a novel inverse method to deduce basal sliding coefficients under currently grounded ice by fitting to modern observed ice elevations and iterating in forward runs (Pollard and DeConto, 2012a), more sophisticated parameterizations of calving, including simple damage advection (Pollard and DeConto, 2012b; Pollard et al., 2015), and more physical and robust parameterization of oceanic melting under floating ice shelves (Pollard and DeConto, 2012b).
(12) Large ensembles of Antarctic runs were conducted, both for the past deglacial period over the last 40 kyrs, and for the future using RCP climate scenarios (as in DeConto and Pollard, 2016), with the coupled ice-Earth model (Gomez et al., 2013) and looking in detail at the effects of different Earth rheological profiles. In particular, the potential negative feedbacks on ice retreat with a weak upper mantle and thin lithosphere, appropriate for West Antarctica, were highlighted. A full-length paper was produced describing the results (Pollard et al., 2017), following on from Gomez et al. (2015). The main findings were that most Earth profiles gave similar results to the simpler ELRA (Elastic Lithosphere Rebounding Asthenosphere) bedrock model, but with the weak-upper-mantle thin-lithosphere profile representative of areas of West Antarctica, significant negative feedbacks occur in the future scenarios that limit ice retreat.
(13) The Penn State ice sheet model was coupled with LOVECLIM, the Earth Model of Intermediate Complexity (EMIC) maintained by the U. Hawaii group. This EMIC requires much less CPU time than the global and regional climate models (GCM, RCM) used in the work above, and offers the capability of fully coupled and time-continuous ice-climate simulations through glacial cycles, and orbital cycles during the Pliocene. D. Pollard also significantly improved the surface ice-snow mass balance scheme in the ice model (which was based on Positive Degree Days). It was converted into a surface energy balance scheme, including solar insolation variations on diurnal, seasonal and orbital scales, with more physical treatment of snow water content and refreezing. Extensive modern calibration for modern Greenland led to a new reference model that continues to be refined.
(14) A pair of companion papers (data and modeling) show that the Antarctic Ice Sheet had the potential to contribute >25m of sea-level variability in the Miocene. This work has substantially reduced the longstanding “hysteresis problem”, because this much orbitally-paced sea-level variability had been difficult to explain without invoking pre-Pliocene Northern Hemispheric ice sheets (Gasson et al., 2016; Levy et al., 2016). The model advance comes from the addition of new ice sheet dynamical processes and coupling between atmospheric and ice-sheet models. A combination of ice sheet and GIA modeling, integrated with marine isotope records hints at a causal relationship between late Miocene Antarctic Ice-Sheet variability and the Messinian Salinity Crisis (desiccation of the Mediterranean Basin) (Ohnheiser et al., 2015).
(15) Finally, we have made numerous contributions to the wider sea level research community including participation in the publication of three review papers, including on in Science (Dutton et al., 2015, more than 250 citations; Kemp et al., 2015; and Miller et al., 2019).
References Cited Above
Austermann, J., Mitrovica, J. X., Huybers, P., & Rovere, A. (2017). Detection of a dynamic topography signal in last interglacial sea-level records. Science Advances, 3(7), e1700457.
Austermann, J., Pollard, D., Mitrovica, J. X., Moucha, R., Forte, A. M., DeConto, R. M., ... & Raymo, M. E. (2015). The impact of dynamic topography change on Antarctic ice sheet stability during the mid-Pliocene warm period. Geology, 43(10), 927-930.
Chen, F., Friedman, S., Gertler, C. G., Looney, J., O’Connell, N., Sierks, K., & Mitrovica, J. X. (2014). Refining estimates of polar ice volumes during the MIS11 Interglacial using sea level records from South Africa. Journal of Climate, 27(23), 8740-8746.
DeConto, R., and Pollard, D. (2016). Antarctica’s contribution to past and future sea-level rise. Nature, v. 531, pp. 591-597.
Dutton, A., Carlson, A. E., Long, A., Milne, G. A., Clark, P. U., DeConto, R., ... & Raymo, M. E. (2015). Sea-level rise due to polar ice-sheet mass loss during past warm periods. Science, 349(6244), aaa4019.
Gasson, E., DeConto, R. M., & Pollard, D. (2016). Modeling the oxygen isotope composition of the Antarctic ice sheet and its significance to Pliocene sea level. Geology, 44(10), 827-830.
Gasson, E., DeConto, R., Pollard, D., Levy, R. (2016). Dynamic Antarctic ice sheet during the early to mid-Miocene. PNAS, v. 113, pp. 3459–3464.
Gomez, N., Pollard, D., & Holland, D. (2015). Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss. Nature Communications, 6, 8798.
Gomez, N., Pollard, D., & Mitrovica, J. X. (2013). A 3-D coupled ice sheet–sea level model applied to Antarctica through the last 40 ky. Earth and Planetary Science Letters, 384, 88-99.
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