W. Tad Pfeffer

• PhD Geophysics, U. Washington
• Professor at University of Colorado Boulder

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...

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 sixty papers since 1988 have been published in the glaciological a...

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 w...

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 corru...

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 w...

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 w...

In his News and Analysis piece reporting on the newly released fifth assessment report (AR5) by Working Group I (WGI) of the Intergovernmental Panel on Climate Change (IPCC) (“A Stronger IPCC Report,” 4 October, p. [23][1]), R. A. Kerr highlights three fundamental conclusions about climate

ISI Document Delivery No.: 274ZI Times Cited: 0 Cited Reference Count: 0 Church, John A. Clark, Peter U. Cazenave, Anny Gregory, Jonathan M. Jevrejeva, Svetlana Levermann, Anders Merrifield, Mark A. Milne, Glenn A. Nerem, R. Steven Nunn, Patrick D. Payne, Antony J. Pfeffer, W. Tad Stammer, Detlef Unnikrishnan, Alakkat S. payne, antony/A-8916-2008;...

The recent retreat and speedup of outlet glaciers, as well as enhanced surface melting around the ice sheet margin, have increased Greenland's contribution to sea level rise to 0.6 ± 0.1 mm yr[superscript −1] and its discharge of freshwater into the North Atlantic. The widespread, near-synchronous glacier retreat, and its coincidence with a period...

Glaciers distinct from the Greenland and Antarctic Ice Sheets are losing large amounts of water to the world’s oceans. However, estimates of their contribution to sea level rise disagree. We provide a consensus estimate by standardizing existing, and creating new, mass-budget estimates from satellite gravimetry and altimetry and from local glaciolo...

One of the grand challenges in cryospheric science has been the creation of a detailed and globally complete glacier inventory. Such a dataset is required for a more precise calculation of the contribution of glaciers and ice caps (GIC) to global sea level rise, local to regional scale hydrology (run-off, hydropower), change assessment, future glac...

Consumer-grade digital cameras have become ubiquitous accessories of science. Particularly in glaciology, the recognized importance of short-term variability has motivated their deployment for increasingly time-critical observations. However, such devices were never intended for precise timekeeping, and their use as such needs to be accompanied by...

Published by the American Geophysical Union as part of the Special Publications Series. This book, beautifully illustrated with dozens of extraordinary photographs, not only tells the history of the expeditions to explore the Columbia Glacier, but also shows how warming over the last century in combination with internal physics of the glacier act t...

Information about glacier volume and ice thickness distribution is essential for many glaciological applications, but direct measurements of ice thickness can be difficult and costly. We present a new method that calculates ice thickness via an estimate of ice flux. We solve the familiar continuity equation between adjacent flowlines, which decreas...

Estimating a glacier's volume by inferring properties at depth from properties observed at the surface creates an instability that grows exponentially with the size of the glacier. Random errors from this instability can overwhelm the volume calculation unless problematic short spatial wavelengths are specifically excluded. Volume-area scaling inhe...

Surface melt on the Greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era. Records for melt were broken in 2005, 2007, 2010 and 2012. Much of the increased surface melt is occurring in the percolation zone, a region of the accumulation area that is perennially covered by snow and firn...

Due to the abundance of observational datasets collected since the onset of its retreat (c. 1983), Columbia Glacier, Alaska, provides an exciting modeling target. We perform Monte Carlo simulations of the form and flow of Columbia Glacier, using a 1-D (depth-integrated) flowline model, over a wide range of parameter values and forcings. An ensemble...

The glaciers on the summit plateau of Kibo, the main peak of the Kilimanjaro massif (3°S, 37°E, 5895 m a.s.l.) in Tanzania, are characterized by steep ice cliffs at their margins. These form-persistent cliffs continuously retreat and, consequently, govern the decrease in plateau glacier area. In order to quantify the ice cliff recession and study t...

We discuss 150 MHz GPR profiles of glacial till, reworked till, colluvium or decaying granite, which mainly contain random inclusions related to lithology, grain size, or water. The profiles contain numerous diffractions and limited stratification to possibly more than 40 m depth. The penetration, lack of dispersion, and lack of well developed asym...

Conventional mass loss estimates for large mountain glacier systems require the extrapolation of sparse mass balance measurements from a few individual glaciers that typically represent only a small fraction of the total glaciated area. The GRACE satellite gravity mission, which was launched in spring 2002 and continues to return high-quality data,...

Glaciers and ice caps (GIC) are important contributors to present-day global mean sea-level rise (SLR). Most previous global mass balance estimates for GIC rely on extrapolation of sparse mass balance measurements representing only a small fraction of the GIC area- leaving their overall contribution to SLR unclear. Here we show that GIC, excluding...

Due to the abundance of observational datasets collected since the onset of its retreat (c. 1983), Columbia Glacier, Alaska, provides an exciting modeling target. We perform Monte Carlo simulations of the form and flow of Columbia Glacier, using a 1-D (depth-integrated) flowline model, over a wide range of parameter values and forcings. An ensemble...

Glaciers and ice caps (GICs) are important contributors to present-day global mean sea level rise. Most previous global mass balance estimates for GICs rely on extrapolation of sparse mass balance measurements representing only a small fraction of the GIC area, leaving their overall contribution to sea level rise unclear. Here we show that GICs, ex...

Greater understanding of variations in firn densification is needed to distinguish between dynamic and melt-driven elevation changes on the Greenland ice sheet. This is especially true in Greenland’s percolation zone, where firn density profiles are poorly documented because few ice cores are extracted in regions with surface melt. We used georadar...

Poorly understood processes controlling retention of meltwater in snow and firn have important implications for Greenland Ice Sheet's mass balance and flow dynamics. Here we present results from a 3 year (2007-2009) field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west Greenland....

We present a comprehensive study of the ice cliffs on Kilimanjaro including a morphological description, results from four high-resolution photogrammetrical surveys, a physical analysis of their recession and the micro-climatological drivers behind it, and how the ice cliffs determine glacier area loss on Africa’s highest mountain. Since more than...

Within the percolation and soaked facies of the Greenland ice sheet, the relationship between radar-derived internal reflection horizons and the layered structure of the firn column is unclear. We conducted two small-scale ground-penetrating radar (GPR) surveys in conjunction with 10 m firn cores that we collected within the percolation and soaked...

Sea level rise is among the most tangible and potentially costly global changes facing society in the near future. Much of the uncertainty in future sea level rise lies in the determination of glacier and ice sheet contributions through melting of ice and through the discharge of icebergs directly into the ocean. As a consequence, many aspects of m...

Glaciers (including the Ice Caps) are major contributors to the observed Sea Level Rise both regionally and globally. We have reasonably good estimates of small scale contributions and contributions from selected regions, and also gross estimates of global contributions. Gaps in knowledge are known and not all of them can be closed. Yet, the number...

The present-day assessment of contributions to sea level rise from glaciers and ice sheets depends to a large degree on new technologies that allow efficient and precise detection of change in otherwise inaccessible polar regions. The creation of an overall research strategy, however, was set in early collaborative efforts nearly 30 years ago to as...

A numerical model reconstruction was made of the northeastern Laurentide Ice Sheet in the Baffin Island - Foxe Basin region using geophysical, terrestrial, and marine geologic evidence for initial and boundary conditions. The simulated ice sheet consists of a Foxe Dome with additional smaller Hall and Amadjuak domes and a Penny Ice Divide. A specif...

At the Columbia, a tidewater glacier in Alaska’s Prince William Sound, aerial stereophotography has been flown at least twice each year since 1976. After costly capture and careful processing, the images yield snapshots of the elevations and velocities at the ice surface, recounting the story of the glacier’s dynamic retreat in high resolution 3D....

Although accurate predictions of sea level rise are highly desired, uncertainties in predictions of near-future sea level rise are large, especially for the mountain glaciers. Regional- to global-scale glacier mass balance assessments typically neglect dynamic mass loss (iceberg calving), thus produce minimal estimates of potential sea level rise....

Since fulfilling Austin Post's prediction of impending retreat in the late 1970s, Columbia Glacier has repeatedly surprised both casual and careful observers with its ability for rapid change. Over the last three decades, Columbia Glacier has lost approximately 18 km of its original 66 km length, while thinning by approximately 50% at the present t...

Net mass change in the aggregate global Mountain Glacier and Ice Cap (MGIC) cryospheric component is presently a significant factor in changing land hydrology, regional/local alterations of ocean salinity, and as a contributor to sea level change. The accurate evaluation of this net mass change is complicated by the very large number (potentially a...

The terminus of Columbia Glacier, Alaska, unexpectedly became ungrounded in 2007 during its prolonged retreat. Visual observations showed that calving changed from a steady release of low-volume bergs, to episodic flow-perpendicular rifting, propagation, and release of very large icebergs - a style reminiscent of calving from ice shelves. Here, we...

Land-based ice cliffs are intriguing features at the margins of glaciers around the world, but little is known about mechanisms of their formation and maintenance. A review on those ice cliffs, which are mostly associated with dry calving events, is presented. The focus is on the persistent, rarely-calving ice cliffs on the plateau glaciers of Kibo...

Ice cliffs are intriguing features of glaciers around the world, but little is known about mechanisms of their formation and maintenance. Ice cliffs also characterize the plateau glaciers on Kilimanjaro, Tanzania (3° S, 37° E). Their heights range from 3 to more than 40 meters and they have at least persisted since the late 19th century when early...

Changes in Arapaho Glacier, Front Range, Colorado, are determined using historical maps, aerial photography, and field surveys using ground penetrating radar (GPR) and Global Positioning System data. Arapaho Glacier lost 52% of its area during the 20th century, decreasing from 0.34 to 0.16 km(2). Between 1900 and 1999 glacial area loss rates increa...

On Kilimanjaro’s summit plateau (3°04’S, 37°21’E) tabular-shaped plateau glaciers are a distinct feature on the flat ash ground. Their margins consist of near-vertical ice cliffs with heights ranging from 3 to more than 40 meters. They have been persistent at least since the late 19th century when first visitors documented them. Snow can accumulate...

Data from two local seismic networks deployed around Alaskan tidewater glaciers are compared side by side with an emphasis on signals associated with calving icebergs. Ten short-period and one broadband sensor logged data at Columbia Glacier, AK during 2004-2005. The broadband sensor was reinstalled during 2008, and continues to record today. A 15-...

We present results from a field campaign focused on meltwater infiltration and horizontal water transport processes in firn of western Greenland. Data were collected during 2007-2009 along a 90 km transect extending from 2000-1300 m elevation. Fifteen intensive study sites were spaced 5-10 km along the transect. Near-surface heat flow was measured...

The Extreme Ice Survey is the most wide-ranging glacier study ever conducted using ground-based, real-time photography. EIS uses time-lapse photography, conventional photography, and video to document the rapid changes now occurring to the Earth's glacial ice. The EIS team has installed 27 time-lapse cameras at 15 sites in Greenland, Iceland, Alask...

We present a test of the hypothesis that firn densification rates in the upper percolation zone have strongly increased in recent decades due to infiltrating meltwater. During 2007-2009 we collected detailed firn stratigraphy and mass measurements along a 90 km transect of the percolation zone in western Greenland, extending from 2000-1400 m elevat...

Calving poses the largest uncertainty in the prediction of sea-level rise in response to global climate changes. A physically-based calving law has yet to be successfully implemented into ice-sheet models in order to adequately describe the mass loss of tidewater glaciers and ice shelves. Observations from a variety of glacial environments are need...

Regularly spaced bumps that arise on ski slopes defy intuition by migrating uphill, even though skiers and snow move downhill.

Measurements of basal water pressure from 15 boreholes located at both local (tens of meters) and regional (kilometers) length scales were used to elucidate the pressure/sliding relationship during an autumn rapid motion event on Bench Glacier, Alaska, USA. The 8 day event had two distinct phases, each with a ten-fold speed-up with respect to winte...

The glaciers on the summit plateau of Mount Kilimanjaro (5700m a.s.l., 3°04’S, 37°21’E) are characterized by nearly vertical ice cliffs at their margins. Recent studies have shown that glacier retreat on Africa’s highest peak is closely linked to the recession of these cliffs. More than one year of distance measurements between an automatic weather...

Mountain glaciers are sensitive probes of the local climate, and, thus, they present an opportunity and a challenge to interpret climates of the past and to predict future changes. Furthermore, glaciers can constitute hazards, including: glacier outburst floods; changes in the magnitude and timing of runoff in the mountains and adjacent regions; an...

Surface meltwater transport on the Greenland Ice Sheet has important implications for mass balance. Melt runoff is the principal loss term in the surface mass balance, and any meltwater that reaches the englacial and subglacial hydraulic systems can contribute to sliding and dynamic losses from the ice sheet. In the accumulation zone of a glacier o...

We conducted ground based georadar surveys at thirteen locations along a 70 km long transect of the EGIG line in the percolation zone of the western part of the Greenland Ice Sheet. The purpose of these surveys is both to gain an understanding of the hydrologic pathways of surface generated meltwater as well as to measure densification of the upper...

After approximately seven years of slowed retreat rate, the terminus of Columbia Glacier (Prince William Sound) has backed up out of the major constriction in the mid-point of its fjord and has started retreating rapidly once again. In 2 months during the summer of 2006, the flow speed at the terminus dropped to approximately 1 m/day, and the lower...

A snow traverse from the percolation to saturation zone of the Greenland ice sheet has obtained year long temperature records from 10m boreholes. Each borehole records a temporally and spatially dense (delta time: 15mins, delta space: 0.25m) vertical temperature distribution over the summer melt season and winter freeze-up. Analysis of this thermal...

On the basis of climate modeling and analogies with past conditions, the potential for multimeter increases in sea level by the end of the 21st century has been proposed. We consider glaciological conditions required for large sea-level rise to occur by 2100 and conclude that increases in excess of 2 meters are physically untenable. We find that a...

Water levels were measured in boreholes spaced along the entire length of Bench Glacier, Alaska, USA, for a period in excess of 2 years. Instrumented boreholes were arranged as nine pairs along the center line of the glacier and an orthogonal grid of 16 boreholes in a 3600 m2 region at the center of the ablation area. Diurnal fluctuations of the wa...

Present-day contributions to sea level rise from Greenland, Antarctica, and all other glaciers and ice caps, are moderately well known, but prediction of future sea level rise is complicated by lack of knowledge of future ice- dynamic response from marine-based outlet glaciers and ice sheets. Much attention has been focused on Greenland's potential...

We present a simple numerical model that generates the water level changes observed during the summer- winter transition of a temperate valley glacier. The model allows us to approximate the basal area sampled by a borehole during this transition. For a period spanning three years, borehole water levels were recorded continuously in 43 different bo...

We conducted a variety of ice penetrating radar surveys along the EGIG line on the western side of the Greenland ice sheet. The purpose of these surveys is to1) gain an understanding of the hydrologic pathways of surface generated meltwater, and 2) document the rate of firn densification in the upper 80 m of the percolation zone. Our various data c...

We present results from a field campaign focused on meltwater infiltration and horizontal water transport processes in the percolation zone of the Greenland Ice Sheet. Field data were collected during a period of heavy melt in June/July 2007 along a ~50 km transect (from ~2000 m to ~1600 m elevation) of the EGIG line of west Greenland. Snow and fir...

Seismograms recorded during iceberg calving contain information pertaining to source processes during calving events. However, locally variable material properties may cause signal distortions, known as site and path effects, which must be eliminated prior to commenting on source mechanics. We applied the technique of horizontal/vertical spectral r...

Ice loss to the sea currently accounts for virtually all of the sea-level rise that is not attributable to ocean warming, and about 60% of the ice loss is from glaciers and ice caps rather than from the two ice sheets. The contribution of these smaller glaciers has accelerated over the past decade, in part due to marked thinning and retreat of mari...

Tidewater glacier retreat is abrupt and typically irreversible. The mechanics of abrupt onset are evaluated by considering the behavior of a sliding law that depends inversely upon effective pressure (pice - pwater); for such a law, flux can increase with decreasing thickness if the loss of bed traction exceeds the loss of driving stress. Character...

We present the first glacier-wide detailed measurement of basal effective pressure and related observations including bed separation to elucidate the role of water in sliding. The hard bedded glacier instrumented in our study exhibited two phases of accelerated sliding motion apparently driven by separate mechanisms. The first acceleration phase (u...

Contributions to sea level rise from rapidly retreating marine-terminating glaciers are large and increasing. Strong increases in iceberg calving occur during retreat, which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone. To study this process, we deployed an 11-sensor passive seismic network at Col...

Downslope transport of surface meltwater in the accumulation zone of the Greenland Ice Sheet is poorly understood, although the location and timing of meltwater production is observed by a variety of remote sensing instruments. Meltwater generated at the surface may be partitioned into a fraction which percolates vertically, altering the hydrologic...

Since 2002 we have been investigating the link between water and glacier sliding using Bench Glacier, Alaska as a field laboratory. Our approach is to test current theory with observations collected at local to glacier-wide length scales and second to year-long time scales. We are integrating diverse data sets including: (1) local and regional effe...

Water levels were measured in boreholes spaced along the entire length of Bench Glacier, Alaska for a period in excess of two years. Instrumented boreholes were arranged as 9 pairs along the centerline of the glacier and an orthogonal grid of 16 boreholes at the center of the ablation area. Diurnal fluctuations of the water levels were found to be...

During 2004-2005, an 11 station seismic array was deployed around the margins of south-central Alaska's rapidly retreating Columbia Glacier. Supplemental instrumentation and a detailed observer record have allowed us build and tune a frequency domain event detector that separates calving-generated seismicity from the total seismic record at the fir...

We critically review and integrate recent measurements of ice volume changes on the Greenland Ice Sheet, the Antarctic Ice Sheets, and small glaciers and ice caps around the world, with attention to disparities and discrepancies due to differing methodologies and data coverage. We find that ice loss to the sea currently accounts for virtually all o...

Columbia Glacier, which terminates in a grounded calving margin in Prince William Sound in central coastal Alaska, has retreated 15 km in the last 25 years, and with annually-averaged speeds in excess of 25 meters per day, it is among the world's fastest glaciers. Discharge flux of ice to the ocean has now exceeded 7 cubic kilometers of ice per yea...

One uncertainty in ice sheet mass balance is the destination of surface meltwater. The runoff limit is the elevation above which surface melt percolates into the underlying cold snow and refreezes, thus the local mass balance above this elevation remains unchanged due to melt. Below the runoff elevation, the annual quantity of melt is great enough...

An array of eleven seismometers including one broadband instrument was deployed around the lower Columbia Glacier margin during June 2004 for a period one year. This network recorded small local seismic events (icequakes) and four explosions detonated in boreholes on the glacier. Additionally, we documented changes in glacier geometry with a single...

Abstract Available from http://www.agu.org

Changes in driving and resistive stresses play an essential role in governing the buoyancy forces that are important controls on the speed and irreversibility of tidewater glacier retreats. We describe changes in geometry, velocity and strain rate, and present a topdown force balance analysis performed over the lower reach of Columbia Glacier. Our...

Observations from basal water-pressure sensors along the length of Bench Glacier, Alaska, USA, show that diurnal fluctuations of water pressure are seasonal and restricted to summer. Most notable about these fluctuations is their disappearance in the late summer and early autumn, long before the seasonal end of diurnal meltwater input. Here we pres...

Observations from along the length of Bench Glacier, Alaska, USA, show that the subglacial water-pressure field undergoes a multiphase transition from a winter mode to a summer mode. Data were collected at the glacier surface, the outlet stream, and in a network of 47 boreholes spanning the length of the 7 km long glacier. The winter pressure field...

Data on the sub-glacial hydrologic conditions under the Bench Glacier, coastal Alaska, have been obtained over the full length of the glacier and have been recorded continuously spanning 3 summer melt seasons and 2 winters. Data includes basal water system measurements of ionic conductivity, turbidity, water speed, and water pressure. This talk foc...

We present a suite of observations that elucidate the subglacial and englacial conditions leading to an eight fold speed-up of a valley glacier. Data were collected on the 7.5 km long temperate Bench Glacier, Chugach Mountains, Alaska. We documented the speed-up event, which lasted for approximately one week of June, with data that include: 1) subg...

We present a time/space analysis of the diurnal fluctuations in basal water pressure, surface velocity, stream discharge, and melt water input along the length of Bench Glacier, Alaska. Basal water pressure was measured in 43 different boreholes and surface velocities were recorded at four locations using differential GPS. The melt water input was...

Recent developments in digital photogrammetry allow reliable, accurate, stereometric models of detailed objects such as glaciers to be constructed at reduced cost and effort in comparison to conventional film-based aerial or terrestrial photogrammetric methods. These developments include: 1) Correction of lens distortion by digital remapping of ima...

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Borehole measurements are used to examine the long time/space evolution of the subglacial drainage system. Over the course of two field seasons more than 50 boreholes were drilled to the bed of Bench Glacier, a temperate valley glacier in Alaska. The holes are located at 15 sites that span the 8 km from the headwall to the terminus of the glacier....

Recent observations on the Bench Glacier and other temperate glaciers in the coastal mountains of Southeast Alaska have shown the existence and importance of crevasses that crack over the full glacier depth. Direct video and hydrologic observations show these pervasive crevasses to be rapidly evolving and laterally and vertically extensive. Hydrolo...