In this Issue



Issue no. 35, March 2001


NSIDC Archive of Ice Shelf Imagery Leads to New Study of Antarctic Ice Shelf Collapse

A paper by Ted Scambos and Jennifer Bohlander of the National Snow and Ice Data Center, Mark Fahnestock of the University of Maryland, and Christine Hulbe of NASA's Goddard Space Flight Center appeared in the January 2001 issue of the Journal of Glaciology and was selected as an "Editor's Choice" in the 9 February 2001 issue of Science Magazine. The study proposed a new model that relates climate warming to ice shelf collapse on the Antarctic peninsula.

Focusing on the Larsen Ice Shelf on the Antarctic Peninsula, the research team analyzed satellite images of meltwater on the ice surface and a sophisticated computer simulation of the motions and forces within ice shelves. Their findings indicated that warmer surface temperatures during summer can cause more ice on Antarctic ice shelves to melt into standing water ponds, which then flows into cracks and causes additional fracturing.

Analysis of hundreds of satellite images in the AVHRR Polar 1 km Data Set, archived at NSIDC, led to the development of the breakup model. The images track both seasonal appearance of melt ponding and ice shelf extent on a daily to weekly basis. NSIDC monitors most of the major ice shelves on the Antarctic continent.

Floating ice shelves, which account for about two percent of Antarctic ice, typically undergo cycles of advance and retreat over many decades. The Larsen Ice Sheet experienced major retreats in 1995 and 1998, including more than 775 square miles that disintegrated during a January 1995 storm. While scientists have known that meltwater can fill crevasses and enhance fracturing, this is the first study to explain the physics linking ice-shelf viability and meltwater ponds.

"Satellite observations of melt duration provide important clues to the water pressure theory," said Fahnestock. After analyzing passive microwave data of the Larsen Ice Shelf over the past two decades, he deduced that the years with the longest duration of surface melting corresponded to the years of major shelf breakup events. The melt season during the 1995 Larsen Ice Sheet retreat was more than 80 days long - about 20 days longer than average.

NASA's Hulbe used a computer model to simulate the thermodynamics of part of the Larsen Ice Sheet before and after the 1990 retreat events to determine whether meltwater "wedges" could split a crevasse to the bottom of the ice sheet. She found that, depending on the internal strength of the ice, water-filled crevasses as shallow as 5 meters could fracture through an ice shelf 200 meters thick.

The researchers believe the splintered remains are likely held together by bridges between crevasses until a combination of winds, tides and another season of melting lead to a breakup. "The findings provide a solid link between climate warming and the recent extensive disintegration of some Antarctic ice shelves," said Scambos. "The process can be expected to be more widespread if Antarctic summer temperatures continue to increase."

In the past, researchers thought most Antarctic ice shelves were stable, due to very low mean annual temperatures, according to Scambos. The recent research shows that summertime temperatures are more critical to shelf stability. Several ice shelves, while colder on average than the Larsen, have summer temperatures just a few degrees C below freezing.

Warmer summers on the much larger Ross Ice Shelf in Antarctica could have severe repercussions because that ice shelf is part of the "braking system" for some very large glaciers, Scambos said. "If we begin to get significant water ponding there, and the shelf is eventually destroyed, we would likely have ice flowing off the Antarctic ice sheet at a much faster rate. That would increase sea level significantly."

While some areas of Antarctica have warmed by as much as 2.5 degrees Celsius in the past 50 years, few records have been kept of seasonal temperatures over ice shelves, according to Hulbe. "We need to monitor the summertime temperatures to see what the future holds for these ice sheets," she said.

For more information, see Larsen Ice Shelf Breakup Events.

Larsen B Ice Shelf


Ice Core Data Gateway Established

A new Ice Core Data Gateway will enable scientists to locate and obtain data from large, multidisciplinary ice coring programs. The partnership between the NSF-funded Antarctic Glaciological Data Center (AGDC), the World Data Center for Paleoclimatology, and the International Ice Core Data Cooperative resulted from discussions at the June 2000 Ice Core Working Group meeting. The Ice Core Gateway provides increased visibility (and use) of many data sets, greater access to data across scientific disciplines, and an organized, long-term data archive. Under the partnership, each data center will continue its discipline-specific expertise and program-specific activities, but will utilize web-technology to provide a single point of entry for users. Visit the Ice Core Gateway site for more information.

Greenland Bedrock Elevation Grids Now Available

The Greenland 5 km DEM, Ice Thickness, and Bedrock Elevation Grids, compiled by Jonathan Bamber of the Bristol Glaciology Centre, United Kingdom, are now available from NSIDC. Data are in ASCII text format at a 5-km grid spacing in a polar stereographic projection. A 5-km DEM data set is included, derived from a combination of ERS-1 and Geosat satellite radar altimetry data, Airborne Topographic Mapper (ATM) data, and photogrammetric digital height data. Ice thickness data are based on approximately 700,000 data points collected via a University of Kansas airborne ice penetrating radar in the 1990s. In addition, nearly 30,000 ice thickness data points were collected in the 1970s by a Technical University of Denmark airborne echo sounder. Bamber subtracted the ice thickness grid from the DEM to produce a grid of bedrock elevation values. Data are available via ftp.

For more information, including links to full documentation and access to data subsetting, visit the data catalog summary.

1.25 km AVHRR Polar Pathfinder Data Now Available

The AVHRR Polar Pathfinder Twice-Daily 1.25 km EASE-Grid Composites are now available from NSIDC. These products cover both poles and consist of twice-daily gridded and calibrated satellite channel data and derived parameters. Data products include calibrated, geolocated versions of the five Advanced Very High Resolution Radiometer (AVHRR) channels, clear sky surface broadband albedo and skin temperature, solar zenith angle, satellite elevation angle, sun-satellite relative azimuth, surface type mask, cloud mask, orbit mask, time of acquisition, and ice motion vectors.

The data set supports regional studies of polar climate, snow cover, sea ice, and surface melting at the best possible resolution available for daily polar coverage during the previous decade. The products were developed in conjunction with the 5 km AVHRR Polar Pathfinder data sets and the passive microwave brightness temperatures contained in the SSM/I and SMMR Pathfinders available at NSIDC. Current applications of the data sets include surface melting studies, snow cover variability and extent, sea ice extent and dynamics, albedo variations, and climate change studies related to monitoring ice shelf extent.

Data are composited onto two grids per day based on common local solar times and scan angle. AVHRR local area coverage (LAC) and High Resolution Picture Transmission (HRPT) level 1b data are used to generate the Polar Pathfinder products at grid spacings of 1.25 km. AVHRR Polar Pathfinder data extend poleward from 48.4 degrees north and 53.2 degrees south latitudes, from August 1993 through December 1998 for the Northern Hemisphere, and from April 1992 through January 1996 for the Southern Hemisphere. Data are in 1-byte and 2-byte integer grid format. File sizes for non-subsetted data range from 52 MB to 104 MB for the Northern Hemisphere, and from 41 MB to 82 MB for the Southern Hemisphere. Ice motion vectors are in ASCII text format, with file sizes of 8 MB for the Northern Hemisphere and 6.3 MB for the Southern Hemisphere. Data are available on 8 mm tape or by ftp.

For more information, including links to full documentation and access to data subsetting, visit the data catalog summary.

1999/2000 NASA DAAC Yearbook Released

NSIDC has released the new edition of the NASA DAAC Yearbook, titled "Distributed Active Archive Centers: Supporting Earth Observing Science in 1999/2000." The issue highlights multidisciplinary scientific research conducted using satellite data from NASA's Earth Science Enterprise (ESE) Program. This year, topics range from remote sensing use in international law, to analysis of a dramatic Antarctic glacier tongue breakout captured in SAR imagery.

Complimentary issues of the DAAC Yearbook are available by contacting NSIDC User Services. Past issues may be accessed at the DAAC Alliance site. Watch NASA's Earth Observatory for new DAAC features.

NSIDC welcomes correspondence from researchers applying NASA DAAC data who are interested in having work featured in future DAAC Yearbook issues. Please contact Annette Varani

Passive Microwave FTP Site Reorganization

In an effort to make navigation more user-friendly, NSIDC will reorganize its public ftp site for passive microwave data. This reorganization will involve brightness temperature and sea ice data in the polar stereographic projection, along with supporting tools and ancillary files. Data will be organized in the following order: (1) data type -- sea ice or brightness temperatures, (2) instrument, (3) algorithm, (4) platform, and (5) temporal resolution.

All files will be located at the ftp site. Please contact NSIDC User Services for further assistance.

Error Notice

Users of the Former Soviet Union Monthly Precipitation Archive, 1891-1993 data set should be aware that the original documentation incorrectly stated that the K2 (wind) correction had been applied to the data in this archive. Although the K2 correction values are included in the corrections file, they were not applied to the data. Users who wish to make corrections for wind effect should apply the K2 values to the data. The data set documentation has been updated to reflect the correct information. See the data catalog summary for more information.


NSIDC to Celebrate 25th Anniversary in 2001

In the fall of 1976, the World Data Center for Glaciology transferred from the U.S. Geological Survey in Tacoma, Washington, to the University of Colorado, Boulder. Since then, NSIDC has acquired, processed, archived, and distributed hundreds of data sets related to ice, snow, and the cryosphere. To celebrate its 25th anniversary, NSIDC will host an open house in fall 2001. Watch for details in the next issue of NSIDC Notes (June 2001) and on our events site, and help us celebrate 25 successful years!


NSIDC welcomes Jennifer Bohlander (Scientific Programmer), Michael Gehmeyr (Sr. Software Engineer, Raytheon), Feng Ling (Research Scientist I), John Maurer (Operations Technician), Christoph Oelke (Research Scientist II), and Betsy Sheffield (User Services Representative), all of whom joined us this winter.

NSIDC bids farewell to Marianne Primett (Sr. Operations Technician), Michael Primett (Sr. Software Engineer, Raytheon), Sergey Sokratov (Research Scientist and CIRES Visiting Fellow), and Jing Ping Ye (Scientific Programmer). We wish them well in their future endeavors.