Plenary talk at the Earthscope National Meeting

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Tim will be attending the 2015 Earthscope National Meeting in Stowe, VT, to deliver a plenary talk on the use of seismology and GPS to learn about glacier dynamics.  This talk, on June 15th, will cover some of the projects Tim has been involved with in Alaska to understand subglacial hydrology, fast glacier flow, and iceberg calving, as well as future opportunities in Alaska and Greenland.

The meeting, from June 15-17, will broadly be discussing Earth’s deformation in North America and beyond, and the future of the Earthscope project.

Time series of calving icequakes can be used to track variations in iceberg calving rate, for example, how calving responds to rising and falling diurnal ocean tides.
Time series of calving icequakes can be used to track variations in iceberg calving rate, for example, how calving responds to rising and falling diurnal ocean tides.

The extended abstract for my presentation is below.

Understanding the Processes Driving Glacier Change with Alaskan Seismic and GPS Data

Timothy C. Bartholomaus, Christopher F. Larsen, Michael E. West, Shad O’Neel, Ginny Catania

Worldwide, glaciers and ice sheets are losing mass and increasing global sea level (Shepherd and others, 2012; Gardner and others, 2013). However, the processes controlling these changes are not well understood. Changes in glacier hydrology and iceberg calving can both increase rates of glacier flow, thereby hastening delivering of ice to the ocean and low elevation regions. The understanding of these two processes is not yet sufficient to reliably include them in ice flow models for the prediction of sea level rise.

The application of seismology and GPS techniques within glaciology allows insight into glacier hydrology and iceberg calving processes. At Yahtse Glacier, a tidewater glacier in Alaska, we seismically quantified calving at unprecedented tidal to seasonal timescales. Tracking of calving-generated icequakes reveals that calving of large icebergs is significantly more likely to occur during falling and low tides than during rising and high tides. We also observe that calving fluxes are greater during the late summer and fall than during winter, suggesting that, on the coast of Alaska, submarine melt of glacier termini is likely a dominant control on the calving rate (Bartholomaus and others, 2013). Background seismic noise (i.e., tremor) also offers glaciological insight. Tremor amplitude rises and falls seasonally and after storms, synchronously with subglacial discharge. Thus, subglacial discharge variations can be quantified at tidewater locations where discharge has been previously unknown.

At Yahtse Glacier and Kennicott Glacier, also in Alaska, we use GPS to observe contrasting responses in glacier motion to melt, rain, and lake-drainage events (Bartholomaus and others, 2008). At Kennicott, speedup responses are short-lived and glacier motion quickly returns to background levels. Yahtse Glacier’s response to hydrologic events is long-lived and leads to progressively slower flow over the course of the summer, demonstrating that in some cases changes in subglacial water routing are not reversible on daily to weekly timescales.

Together, seismic and GPS data offer views of glacier responses to environmental change with temporal resolution that is not available through approximately weekly satellite images. These highly resolved observations allow physical insight that improves our understanding of glacier physics, eventually allowing for better inclusion of glacier dynamical processes in ice flow models. Going forward, Earthscope’s Transportable Array in Alaska expands on the present opportunity to remotely track iceberg calving across coastal Alaska.­ New terrestrial radar interferometers offer a more complete view of ice flow variability by combining the spatial resolution of satellite imagery with the temporal resolution of GPS.

 

Ice sheet model implements extension of Bartholomaus et al. (2011)

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In work from several years ago, co-authors and I developed a model of subglacial and englacial water storage that reproduces some of the characteristic patterns of glacier flow observed in the field.  This work, published in the Journal of Glaciology, was motivated by observations from Kennicott Glacier in Alaska, where a large, ice-dammed lake drains beneath the glacier annually.  During the flood, water delivery to the glacier temporarily overwhelms the glacier’s ability to convey that water through and beneath the glacier, and the glacier flow speed increases by a factor of 5.

Now, in recent work by Ed Bueler, head of the PISM ice sheet modeling group at UAF, the model described in Bartholomaus et al. (2011) has been extended in a manner that allows it to be implemented efficiently at a large scale.  This distributed version  of the hydrology model can now be used within PISM and other ice sheet models to simulate water flow at the base of, for example, the Greenland and Antarctic ice sheets.  You can read more about Ed’s advance, published in the Journal of Glaciology, here.

Bueler 2014- Extending the lumped subglacial-englacial hydrology model of Bartholomaus and other (2011)

Novel views of iceberg calving presented at PARCA

by | posted in: Field work, Greenland, Research | 0

At the annual meeting of the Program for Arctic Regional Climate Assessment (PARCA) hosted at the NASA Goddard Space Flight Center, I’ll be presenting new data that allow a more complete view of the iceberg calving process.  These data include ground-based radar interferometry, seismic, and ocean current observations that reveal how major calving events proceed over 10s of hours before, during and after an iceberg detaches from the terminus.  This PARCA presentation will be the first view of these data, collected during the 2014 summer at the terminus of Rink Isbrae in West Greenland.  Further analyses of these data, taken together, will contribute to our understand of how and why calving occurs at Greenland’s largest outlet glaciers, and what the effects of these events are on the glaciers and adjacent ocean.

The PARCA meeting takes place on January 27th in Greenbelt, Maryland.  You can read more about the project that supported collection of this data here.

Radar interferogram from Rink Isbrae, Greenland
Unwrapped radar interferogram from Rink Isbrae in Greenland, following a major calving event. The colors represent relative rates of motion towards the radar interferometer. Data is shown here in a polar reference frame, with distance between the radar shown along the x-axis and look direction shown along the y-axis.

Presentations and sessions at AGU

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This year, I’ll be giving an invited talk in one of the ice/ocean interaction sessions, and convening another session focused on iceberg calving and submarine melt at the termini of tidewater glaciers. My talk, at 11:20 on Wednesday in MW 3007, will describe how we can use seismic noise to observe subglacial discharge at tidewater glaciers (C32B-05). My convened session is co-chaired with Ellyn Enderlin and covers a wide range of oceanographic and glaciological observations and models.  For this session:

  • The talks will be on Thursday at 4pm in MW 3007 (C44B).
  • Posters are on Tuesday afternoon in MW (C23A).  The posters for two other similar sessions are at the same time, so I’m expecting that we’ll have a lively, well-attended poster session.

This is also the first year for which I have scheduled the glaciology program on behalf of the AGU Cryosphere focus group.  The planning for this meeting took place over the spring, summer and fall of this last year. I’m wishing everyone a great meeting, and that conflicts in the schedules of glaciologist conference attendees are kept to a minimum!

Tim participates in international workshop to guide Greenland ice-ocean research

by | posted in: Greenland, Outreach, Research | 0

During the last several days, I have taken part in an international workshop to identify the major gaps in the scientific community’s understanding of interactions between the Greenland Ice Sheet and its surrounding ocean.

The workshop on Greenland Ice Sheet-Ocean Interactions, under the acronym GROCE, was hosted by the Alfred Wegener Institute, in Bremerhaven, Germany.  Over the two day meeting, ~28 scientists from Germany, Norway, the UK, Poland, Japan, Canada, the US and other countries framed the questions we considered most essential for understanding Greenland’s rapid changes, as well as the strategies and resources necessary to respond to those questions.  It was interesting and exciting to hear the commonalities and differences in research priorities from the broad cross section of glaciologists and oceanographers in attendance.

The report produced to summarize our workshop will be used to help guide funding agencies and the proposal efforts of the broader scientific community.

Next stop for me: San Francisco.  The annual meeting of the American Geophysical Union starts there on Monday.

Workshop attendees
Attendees at the 2014 GROCE workshop

New perspective on calving in press at Nature Geoscience

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A new study, of which I am a co-author, examines globally-collected observations of iceberg calving and novel model results to present a new framework for understanding this important mass loss process.

Our study explains why calving rates vary so greatly over time, and how small changes in the environment can lead to tremendous changes in calving activity.  We show for the first time that calving belongs to a class of processes termed “self-organized critical.”  These processes occur in the same manner over many orders of magnitude such that there is no single, characteristic event size.  The calving terminus self-organizes to the point where it is always at the cusp of collapse. This property makes iceberg calving very challenging to predict.  However, in our manuscript, we demonstrate one potential solution for addressing this challenge and including self-organized critical calving in ice flow models.

Our paper will be published in a forthcoming issue of the journal Nature Geoscience.

Watch a presentation of my research

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I recently presented an overview of my research program to date at the weekly seminar of the Univ. of Texas Institute for Geophysics.  You can view it here.

In this talk, I describe my past and ongoing research into how ocean-terminating glaciers can rapidly lose mass through their termini.  This week, I’ll be traveling to Purdue University to make a similar presentation.  An abstract for this seminar is below:

The largest and most rapidly changing glaciers on Earth flow into the ocean. Ice loss from these glaciers will be the largest contributor to sea level rise in coming centuries and is also the least certain component of the sea level budget. These uncertainties are driven in large part by the poor understanding of two processes by which tidewater glaciers and ice sheets lose ice at their termini: submarine melting by warm ocean water and mechanical iceberg calving. 
The fronts of tidewater glaciers are among the most active and inaccessible geological environments.  These challenges have limited the long duration, high resolution calving and melt measurements that yield insight. Using seismology and oceanography, I identify the magnitudes and variability of submarine melt and iceberg calving at Yahtse Glacier, a major tidewater glacier in southern Alaska. I find that the submarine portion of the glacier terminus melts at over 10 m/d during much of the year. In addition, cavitation of icebergs beneath the sea surface can generate seismometer-recorded “icequakes,” revealing that calving varies seasonally and in response to ocean tides. Seismic tremor also offers the first ever view of subglacial discharge from a tidewater glacier.  Discharge increases during late summer, which promotes submarine melt.  Together, these multidiscipline observations improve our understanding of the geophysical processes responsible for rapid ice loss across the cryosphere.