Sea Level Contribution Research Articles

Tracer transport in the Greenland Ice Sheet: constraints on ice cores and glacial history

The climate history and dynamics of the Greenland Ice Sheet are studied using a coupled model of the depositional provenance and transport of glacier ice, allowing simultaneous prediction of the detailed isotopic stratigraphy of ice cores at all the major Greenland sites. Adopting a novel method for reconstructing the age–depth relationship, we greatly improve the accuracy of semi-Lagrangian tracer tracking schemes and can readily incorporate an age-dependent ice rheology. The larger aim of our study is to impose new constraints on the glacial history of the Greenland Ice Sheet. Leading sources of uncertainty in the climate and dynamic history are encapsulated in a small number of parameters: the temperature and elevation isotopic sensitivities, the glacial–interglacial precipitation contrast and the effective viscosity of ice in the flow law. Comparing predicted and observed ice layering at ice core sites, we establish plausible ranges for the key model parameters, identify climate and dynamic histories that are mutually consistent and recover the past depositional elevation of ice cores to ease interpretation of their climatic records. With the coupled three-dimensional model of ice dynamics and provenance transport we propose a method to place all the ice core records on a common time scale and use discrepancies to adjust the reconstructed climate history. Analysis of simulated GRIP ice layering and borehole temperature profiles confirms that the GRIP record is sensitive to the dynamic as well as to the climatic history, but not enough to strongly limit speculation on the state of the Greenland Ice Sheet during the Eemian. In contrast, our study indicates that the Dye 3 and Camp Century ice cores are extremely sensitive to ice dynamics and greatly constrain Eemian ice sheet reconstructions. We suggest that the maximum Eemian sea-level contribution of the ice sheet was in the range of 3.5–4.5 m.

Comparisons of ice-sheet surface mass budgets from Paleoclimate Modeling Intercomparison Project (PMIP) simulations

As a contribution to the Paleoclimate Modeling Intercomparison Project (PMIP), ice-sheet surface mass budgets are computed using the meteorologic forcing from 17 GCMs for three time periods: present, 6000 and 21,000 calendar years before present. The ice-budget calculations are performed on a common and relatively fine grid (0.5°×1°), by interpolating the monthly GCM fields, making straightforward corrections for the GCMs' topographic errors, and adding synthetic diurnal cycles of air temperature and solar radiation. These fields drive a common snow-ice vertical column model (LSX) through annual cycles to predict net precipitation, melt, and ablation at each fine-grid point over Greenland, Antarctica, and the 21 k Laurentide and Eurasian ice sheets. Considerable scatter among GCMs is found in the computed budgets for all ice sheets except Antarctica, even among the higher-resolution GCMs. This is mainly due to scatter in snow and ice melt around the ice-sheet margins, which is caused by differences of several degrees in summer surface–air temperatures in these regions. Wind speed and cloudiness also vary widely among GCMs, but contribute little to the scatter in melt. Although the temperature differences are not large by GCM standards, they have a large effect on melt rate. By driving a 2-D dynamic ice-sheet model with the predicted mass balances, it is shown that the scatter would cause drastic uncertainties in coupled ice-sheet/GCM simulations of long-term ice-sheet evolution. Much the same scatter occurs with a commonly used perturbative method (using present observed temperatures and precipitation, modified by the GCMs' predicted changes from the present). Although some of the scatter may be due to local differences in ice albedos between GCMs, it is argued that most of it are due to larger-scale climatic differences. Thus, the stringent requirements of consistency in ice-sheet forcing are not met by this set of GCMs, and consistent results from ice-sheet/GCM simulations of future sea-level contributions and past ice-age evolution will require improvements in GCM performance.

Postglacial rebound and sea level contributions to changes in the geoid and the Earth's rotation axis

SUMMARY The rate of change of the spherical harmonic degree 2 component of the Earth’s gravitational potential (CoA 20) and polar wander velocity are two signals which are sensitive to Late Pleistocene deglaciation, current changes in sea level and deep mantle viscosity. Diierent load and earth models have been used in earlier papers to predict the component of these geophysical signals caused by the collapse of the last great ice sheets and recent melting of polar ice caps. In this paper, we present a systematic analysis of the dependence of the predictions on parameters of the ice and earth model. We show that the key parameters of the ice model which govern the predictions are the mass, the location of the centre of mass and the midpoint of the deglaciation phase. Of secondary importance is the length of the deglaciation phase and the mean ice load prior to the Last Glacial Maximum.These conclusions enable us to make a more robust inference of mantle viscosity than has been made before, allowing for the uncertainties in the model of Late Pleistocene and present deglaciation. As previous authors have shown, the lower-mantle viscosity is the most important rheological parameter and therefore theCoA 20 observation complements sea level observations, which are primarily sensitive to lithospheric thickness and the viscosity of the upper part of the mantle. Using realistic constraints on the sizes, locations and timing of deglaciation of the Late Pleistocene ice sheets and current changes in polar ice caps, the observation of CoA 20 is used to infer lower-mantle viscosity as a function of the present rate of sea level change. If the present rate of non-steric sea level change is 1 mm yr {1 and that change has been occurring for less than 1000 years, then the lower-mantle viscosity satis¢es log10 glm~21:82+0:15, which is consistent with inferences drawn from recent sea level analyses and con¢rms other analyses of theCoA 20 observation. If the polar wander signal is produced entirely by postglacial rebound and current sea level change, no more than 20 per cent of the present contribution to global sea level change comes from Greenland. The above conclusions also hold if the density discontinuities at 420 and 670 km are modelled as phase boundaries rather than material (chemical) boundaries.

Late Quaternary stratigraphy of western Yamal Peninsula, Russia: New constraints on the configuration of the Eurasian ice sheet

Ice-sheet reconstructions for the last glacial maximum in northern Eurasia range from nearly complete coverage by a contiguous marine-based ice sheet to large unglaciated areas. Stratigraphic records from Yamal Peninsula, Russia, provide new insight on the eastern limit of the Eurasian ice sheet in the Kara Sea. Radiocarbon and infrared stimulated luminescence ages from coastal cliff sections date the emplacement of the Kara diamicton as older than ca. 40 ka, reflecting regional glaciation. Ice-wedge growth, peat accumulation, and eolian and fluvial deposition characterized the past ~40 k.y. and do not support coverage of Yamal Peninsula by an ice sheet or proximity to a glacier margin. Thus, the late Weichselian Eurasian ice sheet was largely confined to Barents Sea and its global sea-level contribution is reduced to ~7 m of sea-level equivalent.

A review of the Milankovitch climatic beat: template for Plio–Pleistocene sea-level changes and sequence stratigraphy

The Milankovitch theory of climate change predicts that global ice volume, and hence sea-level changes, were controlled by long-term quasi-periodic variations in the earth's orbital parameters (obliquity, precession and eccentricity). δ 18O records from deep-sea cores are a proxy for sea-level changes and have an orbitally tuned chronology covering the last 5 Ma. The sea-level signal in δ 18O data from east equatorial Pacific core V19-30 is well calibrated with sea-level data from coral terraces on Huon Peninsula, New Guinea, over the last 140 ka, and with less certainty back to 340 ka. Over the last 140 ka, the sea-level contribution to benthic glacial–interglacial isotopic variation is about 1.2–1.3‰ or 0.011‰ m −1, and for core V19-30 the glacial-age temperature contribution from deep ocean cooling of 1.7°C is 0.4‰. Independent constraints on Late Pliocene sea-level changes interpreted from shallow marine continental margin records indicate that the sea-level δ 18O calibration may not have been the same over the last 2.6 Ma, and the temperature correction is unlikely to have been the same in all glacial periods and all ocean settings. Nevertheless, the astronomically tuned isotopic records from deep-sea cores provide an accurate chronology and approximate the magnitudes of sea-level changes over the last 2.6 Ma, against which the facies architecture of stratigraphic sequences can be analysed and the concepts of sequence stratigraphy properly evaluated.

MASS BALANCE OF POLAR ICE FROM LONG WAVELENGTH FEATURES OF THE EARTH'S GRAVITATIONAL FIELD

We have used satellite solutions to the low degree zonal harmonics of the Earth's gravitational potential, and rates of surface accumulation to partially constrain, by means of repeated forward solution, the time rates of thickness change over the Antarctic and Greenland Ice Sheets (dTA and dTG respectively). In addition to the observed zonal coefficients j2 through j5 we impose only one other constraint: That dTA and dTG are proportional to surface accumulation. The lagged response of the Earth to secular changes in ice thickness spanning recent time periods (up to 2000 years before present) and the late Pleistocene is accounted for by means of two viscoelastic rebound models. The sea level contributions from the ice sheets, calculated from dTA and dTG, lower mantle viscosity, and the start time of present-day thickness change are all variables subject to the constraints. For a given set of post glacial rebound inputs, a family of solutions that have similar characteristics and that agree well with observation are obtained from the large number of forward solutions. The off axis position of the Greenland ice sheet makes its contribution to the low degree zonal coefficients less sensitive to the spatial details of the mass balance than to the overall sea level contribution. dTG is therefore modeled as surface mass balance offset by a uniform and constant mass loss. Though dTA varies widely with choices of input parameters, the combined sea level contribution from both ice sheets is reasonably well constrained by the gravity coefficients, and is predicted to range from -0.9 to +1.6 mm yr-1. The sign of the slope of the low degree zonal coefficients versus sea level contribution for Greenland is positive, but for Antarctica, the sign of the slope is positive for even degree and negative for odd degree harmonics. By using this property of the zonal coefficients, it is possible to determine the individual sea level contributions for Greenland and Antarctica. They vary from -0.6 to +0.3 mm yr-1 for the Greenland Ice Sheet, and from -0.3 to +1.3 mm yr-1 for the Antarctic Ice Sheet.

Projections of global mean sea level rise calculated with a 2D energy-balance climate model and dynamic ice sheet models

Projections of changes in surface air temperature and global mean sea level over the next century are presented for all IS92 radiative forcing scenarios. A zonal mean energy-balance climate model is used to estimate temperature changes and thermal expansion, precipitation-dependent sensitivity values are used to estimate the sea-level contribution of glaciers and small ice caps and dynamic ice-sheet models coupled to surface mass balance models are employed with regard to the Greenland and Antarctic ice sheets. A few of the sea-level projections have been included in the IPCC96-report for comparison with the revised IPCC96 projections. Here it is demonstrated that the observed inter-model differences are similar for all IS92 radiative forcing scenarios: the projections of global surface air temperature change resemble the revised IPCC96 projections, but the projections of global sea-level rise are 30%—50% smaller than the revised IPCC96 projections. In this paper, the reasons for the inter-model differences in sea-level results are considered. The largest inter-model differences in individual sea-level contributions are found for thermal expansion and for the Antarctic ice-sheet. Sensitivity experiments are presented that show the importance of different assumptions about the temperature forcing of the glacier and ice-sheet models and about weakening of the ocean circulation. Furthermore, uncertainties in thermal expansion caused by uncertainties in ocean heat mixing are considered. It is concluded that the inter-model differences in sea-level projections are caused by the use of essentially different models in this paper and in the revised IPCC96 projections.

Impact of transient increases in atmospheric CO2 on the accumulation and mass balance of the Antarctic ice sheet

The response of the Antarctic ice sheet to climate change over the next 500 years is calculated using the output of a transient-coupled ocean-atmosphere simulation assuming the atmospheric CO2value increases up to three times present levels. The main effects on the ice sheet on this time-scale include increasing rates of accumulation, minimal surface melting, and basal melting of ice shelves. A semi-Lagrangian transport scheme for moisture was used to improve the model’s ability to represent realistic rates of accumulation under present-day conditions, and thereby increase confidence in the anomalies calculated under a warmer climate. The response of the Antarctic ice sheet to the warming is increased accumulation inland, offset by loss from basal melting from the floating ice, and increased ice flow near the grounding line. The preliminary results of this study show that the change to the ice-sheet balance for the transient-coupled model forcing amounted to a minimal sea-level contribution in the next century, but a net positive sea-level rise of 0.21 m by 500 years. This new result supercedes earlier results that showed the Antarctic ice sheet made a net negative contribution to sea-level rise over the next century. However, the amplitude of the sea-level rise is still dominated In the much larger contributions expected from thermal expansion of the ocean of 0.25 m for 100 years and 1.00 m for 500 years.

Vertical motion and ice thickness variation in western North America

Maps of the ice covered area of western North America and records of mass balance of 23 mountain glaciers in this region, were used to construct a gridded data set of average mass balance and to calculate a detailed description of time averaged radial displacement (uplift) in the region for the period, 1961–1990. While the region studied is tectonically active, there is some agreement between the rates of vertical uplift at several tide gauge sites along the Inside Passage of Alaska, and the measured sea level drop at these stations. The average sea level contribution from the melting of western North American glaciers, calculated from these gridded data is 0.4±0.2 mm/yr, and is greater than prior estimates for the same time period.