W. Richard Peltier

Global changes in postglacial sea level: A numerical calculation☆

The sea-level rise due to ice-sheet melting since the last glacial maximum was not uniform everywhere because of the deformation of the Earths surface and its geoid by changing ice and water loads. A numerical model is employed to calculate global changes in relative sea level on a spherical viscoelastic Earth as northern hemisphere ice sheets melt and fill the ocean basins with meltwater. Predictions for the past 16,000 years explain a large proportion of the global variance in the sea-level record, particularly during the Holocene. Results indicate that the oceans can be divided into six zones, each of which is characterized by a specific form of the relative sea-level curve. In four of these zones emerged beaches are predicted, and these may form even at considerable distance from the ice sheets themselves. In the remaining zones submergence is dominant, and no emerged beaches are expected. The close agreement of these predictions with the data suggests that, contrary to the beliefs of many, no net change in ocean volume has occurred during the past 5000 years. Predictions for localities close to the ice sheets are the most in error, suggesting that slight modifications of the assumed melting history and/or the rheological model of the Earths interior are necessary.

Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model

Ice sheets may have reached the Equator in the late Proterozoic era (600–800 Myr ago), according to geological and palaeomagnetic studies, possibly resulting in a ‘snowball Earth’. But this period was a critical time in the evolution of multicellular animals, posing the question of how early life survived under such environmental stress. Here we present computer simulations of this unusual climate stage with a coupled climate/ice-sheet model. To simulate a snowball Earth, we use only a reduction in the solar constant compared to present-day conditions and we keep atmospheric CO2 concentrations near present levels. We find rapid transitions into and out of full glaciation that are consistent with the geological evidence. When we combine these results with a general circulation model, some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals.

Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic coast of the United States

Accurate estimates of global sea-level rise in the pre-satellite era provide a context for 21 st century sea-level predictions, but the use of tide-gauge records is complicated by the contributions from changes in land level due to glacial isostatic adjustment (GIA). We have constructed a rigorous quality-controlled database of late Holocene sea-level indices from the U.S. Atlantic coast, exhibiting subsidence rates of <0.8 mm a –1 in Maine, increasing to rates of 1.7 mm a –1 in Delaware, and a return to rates <0.9 mm a –1 in the Carolinas. This pattern can be attributed to ongoing GIA due to the demise of the Laurentide Ice Sheet. Our data allow us to defi ne the geometry of the associated collapsing proglacial forebulge with a level of resolution unmatched by any other currently available method. The corresponding rates of relative sea-level rise serve as background rates on which future sea-level rise must be superimposed. We further employ the geological data to remove the GIA component from tide-gauge records to estimate a mean 20 th century sea-level rise rate for the U.S. Atlantic coast of 1.8 ± 0.2 mm a –1 , similar to the global average. However, we fia distinct spatial trend in the rate of 20 th century sea-level rise, increasing from Maine to South Carolina. This is the fi rst evidence of this phenomenon from observational data alone. We suggest this may be related to the melting of the Greenland ice sheet and/or ocean steric effects.

The Puzzle of Global Sea-Level Rise

Measuring the rate of sea level rise over the past century requires modeling the behavior of Earth’s crust over the past 20 000 years.

Terminating the 100 kyr ice age cycle

We report a simulation of the most recent 100,000-year glaciationdeglaciation cycle of the late Pleistocene ice age, a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record. Our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one-level energy balance model (EBM) and vertically integrated ice sheet model (ISM). The theory fully incorporates the influences of orbital insolation forcing, glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases. It correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21,000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently. The latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model, a limitation of all previous attempts to understand this global scale climate cycle. Our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of CO2 Is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence. With the incorporation of this influence, model-predicted ice thickness distributions at last glacial maximum (LGM) are very similar to those of the recently described ICE-4G reconstruction that was based upon the inversion of postglacial relative sea level histories.

Impact of thermomechanical ice sheet coupling on a model of the 100 kyr ice age cycle

Simulations of the most recent 100 kyr glaciation-deglaciation cycle of the late Pleistocene Ice Age are presented which feature a newly constructed thermomechanically coupled three-dimensional ice sheet model which is itself coupled to a previously employed global energy balance climate model with realistic geography. The model incorporates both orbital insolation forcing due to the slow time evolution of orbital geometry (arising from many body effects in the solar system) as well as the forcing due to varying atmospheric concentrations of greenhouse gases. Simulations of the Greenland ice sheet are presented with which we are able to investigate the extent to which the ice flow law employed in the ice dynamics component of the model is constrained. The good agreement that we are able to achieve between the model-generated ice sheet topography and the observed Greenland topography provides a clear demonstration of the quality of the ice dynamical model we have developed. The incorporation of full thermomechanical coupling using the standard Glen flow law in the ice dynamics component of the model is shown to increase the difficulty of achieving complete termination of the 100 kyr cycle that the model delivers. Furthermore, a 20 fold flow parameter enhancement relative to that used for Greenland is required to match the aspect ratio of the ICE-4G reconstruction [Peltier, 1994]. Analyses presented herein suggest that basal processes are unlikely to account for this need for flow parameter retuning. However, the new thermomechanically coupled model now provides a clear separation of the Cordilleran and Hudson Bay domes at Last Glacial Maximum in contradistinction to analyses previously performed with isothermal ice sheet models. This innovation therefore leads to a major improvement of the generated ice sheet topography in relation to geological inferences. Aside from this important difference, the overall results regarding the ability of the model to fully account for the most recent 100 kyr cycle of glaciation and deglaciation without the necessity of introducing additional ad hoc feedbacks confirms the validity of this conclusion, reached previously on the basis of isothermal model integrations.

Glacial isostatic adjustment observed using very long baseline interferometry and satellite laser ranging geodesy

In global space geodetic solutions, radial site motions are usually estimated relative to the geocenter (the center of figure of the solid Earth). Most geodesists estimate the motion of the geocenter assuming both that sites do not move radially and that sites move laterally as predicted by plate motion model NUVEL-1A [DeMets et al., 1990, 1994]. Here we estimate the motion of the geocenter assuming that the plate interiors deform radially and laterally as predicted by the postglacial rebound model of Peltier [1994] or that of Peltier [1996] without assuming a priori knowledge about relative plate motion. Radial site motions estimated relative to this rebound-adjusted geocenter are in the same reference frame as the rebound model predictions, whereas site motions estimated without adjusting for rebound are not. We further constrain the motion of the rebound-adjusted geocenter using satellite laser rangings sensitivity to the center of mass (of the solid Earth, the oceans, and the atmosphere) by assuming that the mean velocity between the rebound-adjusted geocenter and the center of mass is negligible over the time period of geodetic measurement. Twenty years of observation with satellite laser ranging and very long baseline interferometry record the isostatic response of the solid Earth to the unloading of the late Pleistocene ice sheets. The misfits of the postglacial rebound model of Peltier [1994] and that of Peltier [1996] are 34% and 16% less, respectively, than the misfit of the rigid plate model. Sites at Onsala (Sweden) and Algonquin Park (Ontario) are observed to be rising at 3 mm/yr and 2 mm/yr, respectively, reflecting unloading of the Fennoscandian and Laurentide ice sheets. Sites along the east coast of the United States are subsiding at <2 mm/yr, indicating that the forebulge produced by the Laurentide ice sheet is currently collapsing very slowly. Sites beneath the margins of the ice sheets during the last glacial maximum are currently moving laterally away from the ice sheet centers at <1.5 mm/yr, in disagreement with the moderately fast outward motion predicted by the model of Peltier [1996].

Snowball Earth prevention by dissolved organic carbon remineralization

The ‘snowball Earth’ hypothesis posits the occurrence of a sequence of glaciations in the Earth’s history sufficiently deep that photosynthetic activity was essentially arrested. Because the time interval during which these events are believed to have occurred immediately preceded the Cambrian explosion of life, the issue as to whether such snowball states actually developed has important implications for our understanding of evolutionary biology. Here we couple an explicit model of the Neoproterozoic carbon cycle to a model of the physical climate system. We show that the drawdown of atmospheric oxygen into the ocean, as surface temperatures decline, operates so as to increase the rate of remineralization of a massive pool of dissolved organic carbon. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of the surface of the Earth, and the prevention of a snowball state.

Coupled energy‐balance/ice‐sheet model simulations of the glacial cycle: A possible connection between terminations and terrigenous dust

We apply a coupled energy-balance/ice-sheet climate model in an investigation of northern hemisphere ice-sheet advance and retreat over the last glacial cycle. When driven only by orbital insolation variations, the model predicts ice-sheet advances over the continents of North America and Eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses. The orbital forcing alone, however, is unable to induce the observed rapid ice-sheet retreat, and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting. In the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination. These include marine instability, thermohaline circulation effects, carbon dioxide variations, and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration. For the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records. Of these various influences, our analyses suggest that the albedo variations in the ice-sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism. Using our parameterization of “dirty” snow in the ablation zone we find glacial retreat to be strongly accelerated, such that complete collapse of the otherwise stable Laurentide ice sheet ensues. The last glacial maximum configurations of the Laurentide and Fennoscandian complexes are also brought into much closer accord with the ICE-3G reconstruction of Tushingham and Peltier (1991,1992) and the ICE-4G reconstruction of Peltier (1994) when this effect is reasonably introduced.

CO2 levels required for deglaciation of a “near‐snowball” Earth

Geologic evidence suggests that in the Late Neoproterozoic (-600 Ma) almost all land masses were glaciated, with sea-level glaciation existing even at the equator. A recent modeling study has shown that it is possible to simulate an ice-covered Earth glaciation with a coupled climate/ice-sheet model. However, separate general circulation model experiments suggest that a second solution may exist with a substantial area of ice free ocean in the tropics. Although 0.1 to 0.3 of an atmosphere of CO 2 (-300 to 1000 X) is required for deglaciation of a Snowball Earth, the exit CO 2 levels for an open water solution could be significantly less. In this paper we utilize a coupled climate/ice sheet model to demonstrate four points: (1) the open water solution can be simulated in the coupled model if the sea ice parameter is adjusted slightly; (2) a major reduction in ice volume from the open water/equatorial ice solution occurs at a CO, level of about 4X present values -about two orders of magnitude less than required for exit from the hard snowball initial state; (3) additional CO 2 increases are required to get fuller meltback of the ice; and (4) the open water solution exhibits hysteresis properties, such that climates with the same level of CO 2 may evolve into either the snowball, open water, or a warmer world solution, with the trajectory depending on initial conditions. These results set useful targets for geochemical calculations of CO 2 changes associated with the open-water solution.