Tuesday, 28 January 2014

Antarctica Beneath the Ice

This winter has been a particularly brutal one in my part of the world (south-eastern Canada), with snow coming earlier than usual, extremely cold temperatures persisting for prolonged periods of time, and brutal ice storms causing massive power outages. With all this cold and snow, I thought it would be appropriate to talk about another cold place on Earth, Antarctica.

Sitting at the southern pole, Antarctica is almost completely (98%) covered by an ice sheet. Reaching thicknesses of up to 3km in places, this glacier flows under its own weight and is estimated to hold more than 50% of the world's fresh water. It is anticipated that melting of this great ice sheet, due to global climate change, will contribute significantly to sea level rise. But it is still not clear exactly how the glacier will react to climate change, because the glacier and the bedrock it sits upon is poorly understood.

Ice / No Ice
Antarctica is almost completely covered by a large ice sheet. A new data set, called Basemap2, uses over 26 million data points to determine the surface elevation of the ice (right image), the thickness of the ice, and the topography of the underlying bedrock (left  image). The vertical scale in these images has been exaggerated 17 times, to make the mountains and valleys easier to see.
To learn more about these data sets, go to the NASA Feature website. There you will also find an interactive map of the two datasets, which lets you switch between them, making them easier to compare.
Image Credit: NASA's Goddard Space Flight Center
 Luckily, scientists have recently produced a new data set that, in addition to revealing the stunning topography hidden beneath the glacier, will help modellers resolve these questions. Led by Dr. Fretwell at the British Antarctic Survey, an international consortium of scientists has released the Bedmap2 dataset. Bedmap2 builds on a previous data set called Bedmap (produced in 2001), providing surface elevation, ice thickness, and bedrock topography for all of Antarctica, south of 60° S.

To produce the new data set, the researchers incorporated an additional 25 million measurements and processed these using modern GIS techniques and hardware, which made manipulation of such large data sets possible. For the earlier Bedmap data set, which used only 1.4 million data points, this number had to be reduced for processing to be manageable. In addition, the more recent data points were collected using modern Global Positioning Satellite (GPS) technology, which helped to pin-point the data more precisely. The original Bedmap data didn't always have this level of precision.

The new data comes from a wide variety of sources, representing 83 different survey campaigns, run by various nationalities, and collected using ground, air, and space platforms. A large part of the data comes from the Operation IceBridge campaign. This airborne mission was flown from Punta Arenas, Chile, collecting laser altimeter and ice-penetrating radar data between 2009 and 2011. The radar instrument, called the Multichannel Coherent Radar Depth Sounder (MCoRDS), was operated by the Center for Remote Sensing of Ice Sheets at the University of Kansas. MCoRDS sent radar signals down through the ice and recorded the returning signals, which gave information on the ice surface, the internal layering within the ice, and the bedrock below. This data was processed to determine the surface elevation of the ice and the ice thickness, which were used to calculate the bedrock topography. However, ice-penetrating radar instruments, which tend to work best in flat areas, don't do so well in steep mountainous regions.  In such areas, surface elevation data from NASA's Ice, Cloud, and Land Elevation Satellite (ICESat) proved useful. The laser data from Operation IceBridge was used to verify the accuracy of the surface elevation data from these sources. In some cases, ground-based data was available, including over-snow radar, seismic sounding, surface elevation, bathymetry, rock-outcrop, grounding line, and ice-extent datasets. Also, when the density of the data was particularly low, satellite gravity data was used to determine ice thicknesses.

The result of all this data is that Bedmap2 provides higher resolution, greater coverage, and improved precision over the original Bedmap product. The large amount of data points allows the data to be interpolated over a 1 km grid, but the uncertainties can be high; up to 130 m in ice surface elevation and up to 1000 m in ice thickness.
Even so, Bedmap2 highlights the beauty of the bedrock under the ice. The better resolution shows off smaller features that could never be seen before, revealing the full scale of the mountain ranges, valleys, basins, and troughs. The new data has also found that the deepest bedrock elevation is actually deeper (by 15%) than previously believed. In addition, several other deep points (about 2.5 km below sea level) have now been identified on the Antarctic continent. It is not clear how accurate the numbers for these deep points are, but it is certain that the deepest point for any continent on the Earth is located somewhere in Antarctica. No other continental areas even come close to such depths.

The Bedmap2 data set also tells us a lot about the Antarctic ice sheet. Volumes calculated from the data indicated that 27 million km3 of water are stored in the ice sheet, which can potentially contribute 58 m to sea level rise if the Antarctic glacier should melt. These estimates are very close to the values that were determined from the original Bedmap product, but now our confidence in them is much greater. Also, we now know that the ice sheet is on average 4.6% thicker than was previously believed and that a much greater volume of ice exists below sea level.

Bedmap2 / Bedmap
The Basemap2 data set (right image) builds upon a previous version, called Basemap (left image).  The higher resolution and greater coverage of Basemap2, gives us better precision, making it easier to see the spectacular mountain ranges, valleys, and rugged terrain.
To learn more about these data sets, go to the NASA Feature website. There you will also find an interactive map of the two datasets, which lets you switch between them, making them easier to compare.
Image Credit: NASA's Goddard Space Flight Center
Mapping the thickness, volume, and bedrock of the Antarctic glacier helps us to understand how ice sheets respond to changes in ocean and air temperatures. Specifically, the shape and structure of the bedrock below the ice controls how the ice sheet moves, affecting its shape and thickness. For example, ice will flow faster downhill, be thinner at the top of the hill, and thicker at the bottom. Conversely, uphill slopes and bumpy terrain in the bedrock can slow down an ice sheet, or even hold it in place temporarily. Bedmap2 provides the level of detail that is necessary to understand these effects, allowing researchers to build more realistic and accurate models that simulate ice motion.

But the task in still not quite finished. There are still many places in the Bedmap2 data set where the amount of data is very poor or even completely non-existent. Dr. Fretwell and his colleagues have identified what they call 2 "poles of ignorance", regions where no data exists for several hundred kilometers. Clearly, the need for more data gathering exists. Which means we can all look forward to a Bedmap3 sometime in the future.

NASA's IceBridge Mission Contributes to New Map of Antarctica, NASA News, July 4, 2013.

Fretwell, et al. 2013, Bedmap2: Improved ice bed, surface, and thickness datasets for Antarctica, The Cryosphere, 7, 375-393, doi:10.5194/tc-7-375-2013.