Could climate engineering save the Greenland Ice Sheet?

Goelzer et al (2012) paint a portentous picture of what is likely to happen to the global sea-level over the next 1000 years. This worrying assessment is based on our current best understanding of how the world's giant ice sheets of Greenland and Antarctica, as well as a quarter of a million smaller glacial ice masses, and the ocean collectively respond to ongoing climate change. Theirs is a state of the science study that integrates these key contributors of sea-level change based on the latest models and current understanding, and an integrated Earth systems modelling approach termed LOVECLIM. As they point out in their study, only a handful of global climate models to date—i.e. models that are used to make predictions of future climate change—incorporate dynamically (fully) coupled ice-sheet models.

Solar radiation management (SRM) geoengineering has been proposed as one means ofhelping avoid the occurrence of dangerous climate change and undesirable state transitions(‘tipping points’) in the Earth system. The irreversible melting of the GreenlandIce Sheet is a case in point—a state transition that could occur as a result ofCO2-driven elevated global temperatures, and one leading to potentially catastrophic sea-levelrise. SRM schemes such as the creation of a ‘sunshade’ or injection of sulfate aerosols intothe stratosphere could reduce incoming solar radiation, and in theory balance, in aglobal mean, the greenhouse warming resulting from elevated concentrations ofCO2 in the atmosphere. Previous work has highlighted that a geoengineered world would have:warming towards the poles, cooling in the tropics, and a reduction in the globalhydrological cycle, which may have important implications for the Greenland IceSheet. Using a fully coupled global climate model in conjunction with an icesheet model, we assess the consequences for the mass balance of the GreenlandIce Sheet of the reorganization of climate patterns by the combination of highCO2 and geoengineering. We find that Greenland surface temperature and precipitation anomalies,compared to the pre-industrial situation, decrease almost linearly with increasing levels of SRMgeoengineering, but that these combine to create a highly non-linear response of the ice sheet.The substantial melting of the Greenland Ice Sheet predicted for four times pre-industrialCO2 levels is prevented in our model with only a partial application of SRM, and hence withouthaving to fully restore the global average temperature back to pre-industrial levels. Thissuggests that the degree of SRM geoengineering required to mitigate the worst impacts ofgreenhouse warming, such as sea-level rise, need not be as extensive as generally assumed.

<div><span>Model-based projections of ice-sheet thresholds and global sea-level rise are severely constrained by </span><span>instrumental observations being only decadal to century-long. As we improve our understanding of these processes, projections just a few years old are now considered conservative, raising concerns about our ability to successfully plan for abrupt future change. </span><span>Past periods of abrupt and extreme warming offer ‘process analogues’ that can provide new insights into the future rate of response of polar ice sheets to warming of the Earth system. The Last Termination </span><span>(20,000-10,000 years ago or 20-10 ka BP) </span>in the North Atlantic region was characterised by a series of abrupt climatic changes including rapid warming at 14.7 ka BP (the start of the “Bølling”, or GI-1 in the Greenland ice-core isotope stratigraphy) which was accompanied by an Antarctic Cold Reversal (ACR) in the south. Potentially important, during the onset of GI-1, warming persisted in the south for some 256±133 calendar years before the ACR, providing a period of time during which both polar regions experienced increasing temperatures. Sometime around the onset of GI-1 and the ACR, Meltwater Pulse 1A (MWP-1A) formed an abrupt sea level rise of ~15 metres, and was coincident with a period of enhanced iceberg flux in the Southern Ocean. It seems likely the majority of the sea level rise came from the Northern Hemisphere – up to 5-6 metres from the Laurentide Ice Sheet – though the timing remains uncertain. The contribution of Antarctic Ice Sheets (AIS) to global mean sea level (GMSL) rise during MWP-1A range from ‘high-end’ scenarios (>10 m contributing over half of the total GMSL rise), to ‘low-end’ (scenarios with little to no contribution). Here we report the results of a multidisciplinary study, with refined age and Antarctic ice-sheet modelling of the MWP-1A sea-level rise. With the recently released international radiocarbon calibration curve (IntCal20), our Bayesian age modelling of terrestrial ages from flooded mangrove swamps suggests global <span>sea level rose across a mean age range of 14.58 ka BP to 14.42 ka BP, with a mean rate of sea-level rise of 0.94 metres per decade (14.97 metres over 160 years). Because the calibrated age range at 95% confidence overlaps in this age model, it is possible the 15 metre rise during MWP1A could have taken place essentially instantaneously. Even the most conservative age modelling we have undertaken indicates an extraordinary rapid rate of sea-level rise; two orders of magnitude larger than the mean rate of global sea level rise since 1993 (0.03±0.003 metres per decade). Our ice-sheet modelling suggests a substantial and rapid loss of Antarctic ice mass (mostly from the Weddell Sea Embayment and the Antarctic Peninsula), synchronous with warming and ice loss in the North Atlantic. The drivers and mechanisms of the observed near-synchronous interhemispheric changes will be discussed, with implications for the future.</span></div>

Abstract. As the most recent warm period in Earth's history with a sea-level stand higher than present, the Last Interglacial (LIG, ∼ 130 to 115 kyr BP) is often considered a prime example to study the impact of a warmer climate on the two polar ice sheets remaining today. Here we simulate the Last Interglacial climate, ice sheet, and sea-level evolution with the Earth system model of intermediate complexity LOVECLIM v.1.3, which includes dynamic and fully coupled components representing the atmosphere, the ocean and sea ice, the terrestrial biosphere, and the Greenland and Antarctic ice sheets. In this setup, sea-level evolution and climate–ice sheet interactions are modelled in a consistent framework.Surface mass balance change governed by changes in surface meltwater runoff is the dominant forcing for the Greenland ice sheet, which shows a peak sea-level contribution of 1.4 m at 123 kyr BP in the reference experiment. Our results indicate that ice sheet–climate feedbacks play an important role to amplify climate and sea-level changes in the Northern Hemisphere. The sensitivity of the Greenland ice sheet to surface temperature changes considerably increases when interactive albedo changes are considered. Southern Hemisphere polar and sub-polar ocean warming is limited throughout the Last Interglacial, and surface and sub-shelf melting exerts only a minor control on the Antarctic sea-level contribution with a peak of 4.4 m at 125 kyr BP. Retreat of the Antarctic ice sheet at the onset of the LIG is mainly forced by rising sea level and to a lesser extent by reduced ice shelf viscosity as the surface temperature increases. Global sea level shows a peak of 5.3 m at 124.5 kyr BP, which includes a minor contribution of 0.35 m from oceanic thermal expansion. Neither the individual contributions nor the total modelled sea-level stand show fast multi-millennial timescale variations as indicated by some reconstructions.

The present-day perturbations of the Earth's rotation are sensitive to the glacial isostatic adjustment (GIA) arising from the Late Pleistocene glacial cycles and also to the recent mass balance of polar ice caps. In this study, we evaluate the polar wander and the change of degree-two harmonic of the Earth's geopotential , proportional to the rotation rate, for four Late Pleistocene ice models. We examine these perturbations as a function of lower- and upper-mantle viscosities and lithospheric thickness and rheology (elastic or viscoelastic), in which a compressible Earth model with elasticity and density given by the seismological model PREM is used. By considering the observations and predictions including the GIA process arising from the Late Pleistocene ice and recent mass balance of polar ice caps, we discuss the recent mass balance of the Antarctic and Greenland ice sheets. We also examine the effects of internal processes and the melting of mountain glaciers, although this work is only preliminary. The results shown below seem to be supported even if these effects are included. Two solutions are obtained for source areas of the recent Antarctic melting. We denote an equivalent sea level (ESL) rise (mm yr−1)) from the Greenland and Antarctic ice sheets as and , respectively, being positive for melting and negative for growth. One, solution s1, is a solution satisfying the relationship , and the other, solution s2, generally satisfies the relationship and . In most cases, the magnitude of for solution s2 is larger than that for solution s1. The melting area of the Antarctic ice sheet for solution s1 roughly corresponds to the Weddell Sea region, approximately located on the symmetric part of Greenland to the Equator. The area for solution s2 is located on the symmetric part of Greenland to the centre of the Earth. In both solutions, therefore, the polar wander direction caused by the mass imbalance of each ice sheet is in an opposite direction. The reason for this is that the observed polar wander direction is nearly identical to the prediction from the GIA process for the Late Pleistocene ice models. However, it is difficult to independently examine which solution is better. If we consider a recent ESL rise of ∼0.6 mm yr−1 from the Greenland ice sheet, then a similar ESL rise of 0.5–1.0 mm yr−1 is also suggested for the Antarctic ice sheet around the Weddell Sea region. This solution also suggests the lower-mantle viscosity to be ∼1022 Pa s.

Twentieth century sea-level rise inferred from tide gauge, geologically derived and thermosteric sea-level changes

West Antarctica’s contribution to sea level rise as Earth’s climate warms Dr Dan Lubin, Researcher at the Scripps Institution of Oceanography, explores the contribution of West Antarctica to sea level rise as Earth’s climate warms. Sea level rise (SLR) is one of the most pressing concerns with greenhouse gas-induced climate warming. Over one-third of the human population, including nearly two-thirds of the global urban population, lives within 100 km of the sea. SLR increases flooding from storm surges and high tides, deterioration of infrastructure, shoreline erosion, contamination of agriculture, and numerous other hardships. Climate warming causes SLR in two primary ways: the melting of polar ice sheets and glaciers and the thermal expansion of the ocean as it warms. The two largest contributors to SLR from the steadily warming polar regions are the Greenland Ice Sheet (GIS) and the West Antarctic Ice Sheet (WAIS), at approximately 280 and 160 gigatons of ice per year, respectively. The GIS and WAIS have contrasts in the details of their ice mass loss.

(TYPE=abstract)Present-day sea-level change around Greenland is examined by assessing the role played by glacial-isostatic adjustment (GIA). We consider the contributions from: (1) the ongoing GIA due to changes in the extent and thickness of the Greenland Ice Sheet (GIS) following the Last Glacial Maximum (LGM), (2) the equivalent signal associated with the continental ice masses located outside of Greenland, and (3) present-day changes in the GIS. Changes in the GIS arising from the last glacial-interglacial transition generally result in falling sea level today. The contribution from ice-load changes outside of Greenland causes rising sea level, owing to Greenland's location on the collapsing forebulge that surrounds the former North American ice sheets. Combining predictions of these contributions gives results showing rising sea level in the southwest and falling sea level in the north and east. However, this is strongly dependent upon the neoglacial part of the GIS's history. The present-day behaviour of the GIS is predicted to cause falling sea level with rates of several mm yr$^{-1}$ around areas experiencing the larger ice-load changes. The available tide-gauge data are considered unusable by the standards of many workers. Nevertheless, we compare rates of local sea-level change inferred from this type of data with our predictions. In Southern Greenland, where the tide-gauge stations are located, sea level is predicted to be rising at a rate of 4 to 5 mm yr$^{-1}$. Our predictions match most of the rates obtained from the tide-gauge time series, with the exception of Qaqortoq where the inferred rates may also reflect additional oceanic and meteorological effects. Similarly, our predictions are consistent with GPS observations, with again the exception of Qaqortoq.

The last interglacial (LIG; ~130 to ~118 thousand years ago, ka) was the last time global sea level rose well above the present level. Greenland Ice Sheet (GrIS) contributions were insufficient to explain the highstand, so that substantial Antarctic Ice Sheet (AIS) reduction is implied. However, the nature and drivers of GrIS and AIS reductions remain enigmatic, even though they may be critical for understanding future sea-level rise. Here we complement existing records with new data, and reveal that the LIG contained an AIS-derived highstand from ~129.5 to ~125 ka, a lowstand centred on 125–124 ka, and joint AIS + GrIS contributions from ~123.5 to ~118 ka. Moreover, a dual substructure within the first highstand suggests temporal variability in the AIS contributions. Implied rates of sea-level rise are high (up to several meters per century; m c−1), and lend credibility to high rates inferred by ice modelling under certain ice-shelf instability parameterisations.

During the last interglacial period (the Eemian), global sea level was at least three metres, and probably more than five metres, higher than at present. Complete melting of either the West Antarctic ice sheet or the Greenland ice sheet would today raise sea levels by 6-7 metres. But the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the West Antarctic ice sheet, with model studies attributing only 1-2 m of sea-level rise to meltwater from Greenland. This result was considered consistent with ice core evidence, although earlier work had suggested a much reduced Greenland ice sheet during the last interglacial period. Here we reconsider the Eemian evolution of the Greenland ice sheet by combining numerical modelling with insights obtained from recent central Greenland ice-core analyses. Our results suggest that the Greenland ice sheet was considerably smaller and steeper during the Eemian, and plausibly contributed 4-5.5 m to the sea-level highstand during that period. We conclude that the high sea level during the last interglacial period most probably included a large contribution from Greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the West Antarctic ice sheet.

Estimates of future Greenland ice sheet (GrIS) melt are mostly based on regional climate modelling for a fixed GrIS topography or on ice sheet modelling with forcing from climate models. This prevents the modelling of climate and GrIS feedbacks and other types of interaction. Here we examine a set of multi-century simulations with the Community Earth System Model featuring an interactive GrIS to explore future relationship between global climate change and ice sheet change. To this end, we compare a set of coupled CESM-CISM 1% CO2 increase simulations until stabilization at two, two and a half, three and four times pre-industrial CO2 levels to examine the sensitivity of the GrIS to emission mitigation. Here we find a large role of ocean circulation weakening and associated regional climate changes on GrIS melt for moderate emission scenarios and large melt differences between the three times and four times CO2 stabilization scenarios. In addition, we examine the role of feedbacks on ice sheet evolution by comparing a 1% to 4xCO2 coupled simulation with a simulation where the GrIS topography and meltwater fluxes to the ocean are prescribed as pre-industrial. Finally, we explore the effects on GrIS melt rates of a fast 5% CO2 reduction from four times to pre-industrial levels, with a focus on restoration of high latitude climate, GrIS albedo, surface energy fluxes and refreezing capacity.  

Projections of global sea-level rise into the future have become more pessimistic over the past five years or so. A global rise by more than one metre by the year 2100 is now widely accepted as a serious possibility if greenhouse gas emissions continue unabated. That is witnessed by the scientific assessments that were made since the last IPCC report was published in 2007. The Delta Commission of the Dutch government projected up to 1.10 m as a 'high-end' scenario (Vellinga et al 2009). The Scientific Committee on Antarctic Research (SCAR) projected up to 1.40 m (Scientific Committee on Antarctic Research 2009), and the Arctic Monitoring and Assessment Programme (AMAP) gives a range of 0.90–1.60 m in its 2011 report (Arctic Monitoring and Assessment Programme 2011). And recently the US Army Corps of Engineers recommends using a 'low', an 'intermediate' and a 'high' scenario for global sea-level rise when planning civil works programmes, with the high one corresponding to a 1.50 m rise by 2100 (US Army Corps of Engineers 2011).

Abstract. The Greenland Ice Sheet (GrIS) plays a crucial role in sea level rise (SLR). We investigate its response to warming thresholds over two centuries using a coupled regional-atmospheric ice sheet model (MAR-PISM, respectively run at 25 and 4.5 km resolutions). We explore responses to global atmospheric temperature increases from +0.6 to +5.8 °C since the pre-industrial period and assess GrIS recovery if the climate reverts to present conditions, while prescribing unchanged ocean conditions. Our study focuses exclusively on evaluating the effect of atmospheric changes on the Greenland Ice Sheet without considering oceanic warming. Moderate atmospheric warmings (+0.6 to +1.4 °C) yield steady and similar SLR contributions (from +8.35 to +9.55 cm in 2200), close to levels already committed under the present climate. Global temperature increases beyond +1.4 °C mark a critical threshold, triggering non-linear mass loss due to feedback mechanisms like the melt–albedo effect and firn saturation. The SLR increase between the +1.4 and +2.3 °C experiments is larger (+7.56 cm), highlighting an accelerating mass loss. This trend is further reinforced by the even greater increase of 15.51 cm between +4.4 and +5.2 °C, underscoring the amplified impact of higher warming levels. Reversing the climate after surpassing +2.3 °C demonstrates the potential for a substantial slowdown in GrIS mass loss, indicating a trend toward stabilization at a reduced state (approximately 4 % smaller). These findings underscore the impact of thresholds and time spent above them, highlighting the importance of limiting anthropogenic warming to mitigate GrIS mass loss and the long-term SLR associated with it.

Humboldt Glacier, North Greenland, has retreated and accelerated through the 21st century, raising concerns that it could be a significant contributor to future sea-level rise. We use a data-constrained ensemble of three-dimensional higher-order ice sheet model simulations to estimate the likely range of sea-level rise from the continued retreat of Humboldt Glacier. We first solve for basal traction using observed ice thickness, bed topography, and ice surface velocity from the year 2007 in a partial differential equation constrained optimization. Next, we impose calving rates to match mean observed 2007–2017 retreat rates in a transient calibration of the exponent in the power-law basal friction relationship. We find that power law exponents in the range of 1/7–1/5 — rather than the commonly used 1/3–1 — are necessary to reproduce the observed speedup over this period. We then tune an iceberg calving parameterization based on the von Mises stress yield criterion in another transient calibration step from 2007–2017 to approximate both observed ice velocities and terminus position in 2017. Finally, we use the range of basal friction relationship exponents and calving parameter values to generate the ensemble of model simulations from 2007–2100 under three climate forcing scenarios from CMIP5 (two RCP 8.5 forcings) and CMIP6 (one SSP5-8.5 forcing). Our simulations predict 5.5–9.2 mm of sea-level rise from Humboldt Glacier, significantly higher than a previous estimate (~3.5 mm) and equivalent to a substantial fraction of the 40–140 mm predicted by ISMIP6 from the whole Greenland Ice Sheet. Our larger future sea-level rise prediction results from the transient calibration of our basal friction law to match the observed 2007–2017 speedup, which requires a semi-plastic bed rheology. In many simulations, our model predicts the growth of a sizable ice shelf in the middle of the 21st century. Thus, atmospheric warming could lead to more retreat than predicted here if increased surface melt promotes hydrofracture of the ice shelf. Our data-constrained simulations of Humboldt Glacier underscore the sensitivity of model predictions of Greenland outlet glacier response to warming to choices of basal shear stress and iceberg calving parameterizations. Further, transient calibration of these parameterizations, which has not typically been performed, is necessary to reproduce observed behavior. Current estimates of future sea-level rise from the Greenland Ice Sheet could, therefore, contain significant biases.

Thermal expansion of the oceans, as well as melting of glaciers, ice sheets and ice caps have been the main contributors to global sea level rise over the past century. The greatest uncertainty in predicting future sea level changes lies with our estimates of the mass balance of the ice sheets in Greenland and Antarctica. Satellite measurements have been used to determine changes in these ice sheets on short timescales, demonstrating that surface-elevation changes on timescales of decades or less result mainly from variations in snow accumulation. Here we present direct measurements of the changes in surface elevation between 1954 and 1995 on a traverse across the north Greenland ice sheet. Measurements over a time interval of this length should reflect changes in ice flow-the important quantity for predicting changes in sea level-relatively unperturbed by short-term fluctuations in snow accumulation. We find only small changes in the eastern part of the transect, except for some thickening of the north ice stream. On the west side, however, the thinning rates of the ice sheet are significantly higher and thinning extends to higher elevations than had been anticipated from previous studies.