Vivien Gornitz, Vanishing Ice: Glaciers, Ice Sheets, and Rising Seas. New York: Columbia University Press, 400 pp. ISBN 978-02-3116-824-3.

Many Arctic marine‐terminating glaciers have undergone rapid retreats in recent decades. Seasonal and year‐to‐year variations in terminus position act on all tidewater glaciers, but the key controls on those changes vary from region to region. Here, we examined seasonal and decadal changes in termini positions of seven tidewater glaciers in the inner part of Hornsund, the southernmost fjord of Spitsbergen (Svalbard Archipelago), based on a variety of data from 1992 to 2020. Combining satellite imagery, basic meteorological data (air temperature, positive degree day index (PDD), liquid precipitation), sea surface temperature (SST), mean temperature in the glacier forefield bays, fast sea ice cover, and bathymetry near the glacier front, we examined the influence of potential controlling parameters on interannual and seasonal variability of the glacier termini. We found regional synchrony between terminus advance/retreat and climate variables. At a regional scale, annual fluctuation changes are related to PDD and SST, while summer fluctuations are linked to PDD, although individual glaciers are shown to have differing sensitivities to potential climate drivers. We also found that the retreat period in Hornsund generally lasts from June to October‐December. Onset of the retreat is related to sea and air temperature, and in some cases follows the disappearance of the ice cover. These results indicate that the expected increase in meltwater runoff in Svalbard, the input of relatively warm Atlantic water to the fjord, and the increasing trend of longer summer and warmer winter periods will have implications for glacier velocity and frontal ablation.

Sea level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate for the deep ocean and ice sheets. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientists and practitioners builds on a framework of discussing physical evidence to quantify high-end global SLR for practitioners. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2°C in 2100 (RCP2.6/SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for a (RCP8.5/SSP5-8.5), we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2°C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while here we emphasize the importance of the timing of ice shelf collapse around Antarctica. This is highly uncertain due to low understanding of the driving processes. Hence both process understanding and emission scenario control high-end SLR.

In recent decades, meltwater runoff has accelerated to become the dominant mechanism for mass loss in the Greenland ice sheet1–3. In Greenland’s high-elevation interior, porous snow and firn accumulate; these can absorb surface meltwater and inhibit runoff4, but this buffering effect is limited if enough water refreezes near the surface to restrict percolation5,6. However, the influence of refreezing on runoff from Greenland remains largely unquantified. Here we use firn cores, radar observations and regional climate models to show that recent increases in meltwater have resulted in the formation of metres-thick, low-permeability ‘ice slabs’ that have expanded the Greenland ice sheet’s total runoff area by 26 ± 3 per cent since 2001. Although runoff from the top of ice slabs has added less than one millimetre to global sea-level rise so far, this contribution will grow substantially as ice slabs expand inland in a warming climate. Runoff over ice slabs is set to contribute 7 to 33 millimetres and 17 to 74 millimetres to global sea-level rise by 2100 under moderate- and high-emissions scenarios, respectively—approximately double the estimated runoff from Greenland’s high-elevation interior, as predicted by surface mass balance models without ice slabs. Ice slabs will have an important role in enhancing surface meltwater feedback processes, fundamentally altering the ice sheet’s present and future hydrology. Observations and regional climate models show that the increasing coverage of ice slabs on the Greenland ice sheet could lead to a global sea-level rise of up to 74 millimetres by 2100.

ABSTRACTWhile the terms ‘glacier’ and ‘ice cap’ have distinct morphological meanings, no easily defined boundary or transition distinguishes one from the other. Despite this, the exponent of the power law function relating volume to surface area differs sharply for glaciers and ice caps, suggesting a fundamental distinction beyond a smoothly transitioning morphology. A standard percolation technique from statistical physics is used to show that valley glaciers are in fact differentiated from ice caps by an abrupt geometric transition. The crossover is a function of increasing glacier thickness, but it owes its existence more to the nature of the underlying bedrock topography than to specifics of glacier mechanics: the crossover is caused by a switch from directed flow that is constrained by surrounding bedrock topography to unconstrained radial flow of thicker ice that has subsumed the topography. The crossover phenomenon is nonlinear and rapid so that few if any glaciers will have geometries or dynamics that blend the two extremes. The exponents of scaling relationships change abruptly at the crossover from one regime to another; in particular, the volume/area scaling exponent will switch from γ = 1.375 for glaciers to γ = 1.25 for ice caps, with few, if any, ice bodies having exponents that fall between these values.

Appendix A lists key practitioners from the Dhaka Water Supply and Sewerage Authority, Institute of Water Modelling, Bangladesh Water Development Board, and others who were consulted in Dhaka to estimate the costs of implementing the structural measures recommended in this study. Appendix B discusses the basin and region modeling as well as the rainfall analyses of the Brahmaputra (Jamuna) River basin. Appendix C reviews the findings of previous research on Dhaka City Rainfall Analyses. Appendix D lists the DAP-proposed (Detailed Area Plan) water retention areas and water bodies. Appendix E provides the status of 26 Khals under Dhaka Water Supply and Sewerage Authority (DWASA) jurisdiction.

Volume‐area power law scaling, one of a set of analytical scaling techniques based on principals of dimensional analysis, has become an increasingly important and widely used method for estimating the future response of the world's glaciers and ice caps to environmental change. Over 60 papers since 1988 have been published in the glaciological and environmental change literature containing applications of volume‐area scaling, mostly for the purpose of estimating total global glacier and ice cap volume and modeling future contributions to sea level rise from glaciers and ice caps. The application of the theory is not entirely straightforward, however, and many of the recently published results contain analyses that are in conflict with the theory as originally described by Bahr et al. (1997). In this review we describe the general theory of scaling for glaciers in full three‐dimensional detail without simplifications, including an improved derivation of both the volume‐area scaling exponent γ and a new derivation of the multiplicative scaling parameter c. We discuss some common misconceptions of the theory, presenting examples of both appropriate and inappropriate applications. We also discuss potential future developments in power law scaling beyond its present uses, the relationship between power law scaling and other modeling approaches, and some of the advantages and limitations of scaling techniques.

Estimating a glacier's volume by inferring properties at depth (e.g. bed topography or basal slip) from properties observed at the surface (e.g. area and slope) creates a calculation instability that grows exponentially with the size of the glacier. Random errors from this inversion instability can overwhelm all other sources of error and can corrupt thickness and volume calculations, unless problematic short spatial wavelengths are specifically excluded. Volume/area scaling inherently filters these short wavelengths and automatically eliminates the instability, while numerical inversions can also give stable solutions by filtering the correct wavelengths explicitly, as is frequently done when 'regularizing' a model. Each of the scaling and numerical techniques has applications to which it is better suited, and there are trade-offs in resolution and accuracy; but when calculating volume, neither the modeling nor the scaling approach offers a fundamental advantage over the other. Both are significantly limited by the inherently 'ill-posed' inversion, and even though both provide stable volume solutions, neither can give unique solutions.

The Randolph Glacier Inventory (RGI) is a globally complete collection of digital outlines of glaciers, excluding the ice sheets, developed to meet the needs of the Fifth Assessment of the Intergovernmental Panel on Climate Change for estimates of past and future mass balance. The RGI was created with limited resources in a short period. Priority was given to completeness of coverage, but a limited, uniform set of attributes is attached to each of the similar to 198 000 glaciers in its latest version, 3.2. Satellite imagery from 1999-2010 provided most of the outlines. Their total extent is estimated as 726 800 +/- 34 000 km(2). The uncertainty, about +/- 5%, is derived from careful single-glacier and basin-scale uncertainty estimates and comparisons with inventories that were not sources for the RGI. The main contributors to uncertainty are probably misinterpretation of seasonal snow cover and debris cover. These errors appear not to be normally distributed, and quantifying them reliably is an unsolved problem. Combined with digital elevation models, the RGI glacier outlines yield hypsometries that can be combined with atmospheric data or model outputs for analysis of the impacts of climatic change on glaciers. The RGI has already proved its value in the generation of significantly improved aggregate estimates of glacier mass changes and total volume, and thus actual and potential contributions to sea-level rise.

Sea-level changes on a wide range of time and space scales. This article considers changes in mean sea level, that is, sea level averaged over a sufficient period of time to remove fluctuations associated with surface waves, tides, and individual storm surge events. Changes in sea level changes over the last hundred years or so and on how it might change over the next one hundred years are or particular interest. However, to understand these changes requires consideration of sea level since the last glacial maximum 20,000 years ago. The longer-term implications of changes in the Earth's climate arising from changes in atmospheric greenhouse gas concentrations are also considered.

We discuss GPR reflection profiles that we recorded on glacial till and a colluvial diamict at several locations in New Hampshire, and from which we interpret water contents, depths and rates of signal loss. We used pulses centered from 150–200 MHz and 300–360 MHz. The boulder-rich sediments reside over granitic and metavolcanics, the horizons of which we recognize from the relative strengths and phase of their waveforms, underlying fractures, and well-developed diffraction asymptotes. The till produced an apparent dense distribution of diffractions with limited asymptotes and dispersion, and occasional minor stratification. We use these diffractions and moveout profiles to calculate relative dielectric permittivities between 17 and 27, values which suggest up to 30% volumetric water, and likely saturation within these over-consolidated sediments. The evidence for transitions from till to bedrock ranges from a simple horizon to complex horizon segments, all characterized by diffractions and amenable to single-layer migration. A gradational loss in diffraction strength with depth suggests gradational weathering or changes in grain size as the cause. Maximum profiled depths range from 4 m to at least 10 m, with estimated scattering attenuation rates of about 3.3 dB m−1. In contrast, one and possible two colluvial diamicts, which likely contained 3-m-size boulders, show short segments of stratification, rare diffraction asymptotes, allow more than 20-m penetration and provide scattering losses of about 0.5 dB m−1. We measured extremely low conductivity and calculated permittivities ranging from 9–12, which suggest high densities and volumetric water content of 4–12%. Low, single scattering loss and deep penetration in the till are consistent with evidence of ground waves traveling up to 40 m one way. The phase polarity of waveforms within till and colluvial events show they may originate from either high or low dielectric contrasts, likely related to water or large boulders, respectively.

Mark Meier, wearing a national Georgian cap at the Grossaletschgletscher, Switzerland, in 1978, following a conference in Tbilisi. Photo by Vladimir Kotlyakov.In 1981 I was a new graduate student a...