Ellesmere Island, Northern Canada

The Arctics largest ice shelf is breaking up. The Ward Hunt Ice Shelf is a remnant of the compacted snow and ancient sea ice that extended along the northern shores of Ellesmere Island in Northern Canada until the early twentieth century. Rising temperatures have reduced the original shelf into a number of smaller shelves, the largest of which was the Ward Hunt Ice Shelf on the northwest fringe of the island. The Ward Hunt Ice Shelf encompasses Ward Hunt Island and covers the mouth of the Disraeli Fiord. Until recently, fresh melt water formed a 43-meter deep lake on top of almost 400 meters of seawater in the fiord. Called an epishelf lake, the relatively fresh water dammed by the 3000-year-old ice shelf became the basis of a rare ecosystem. Disraeli Fiord was the largest remaining epishelf lake in the Northern Hemisphere. Between 2000 and 2002, the Ward Hunt Ice Shelf began to crack and eventually broke in two, allowing the lake behind it to drain rapidly into the Arctic Ocean. Derek Mueller and Warwick Vincent, of the Centre détudes nordiques at Université Laval in Quebec, Canada and Martin Jeffries of the Geophysical Institute at the University of Alaska Fairbanks in Fairbanks, Alaska described the event in a paper published in Geophysical Research Letters on October 18, 2003. This series of true-color Moderate Resolution Imaging Spectroradiometer (MODIS) images captured by the Terra satellite show the later stages of the break-up of the ice shelf and changes in the Disraeli Fiord between 2001 and 2003. The first image, taken on July 22, 2001, shows the Ward Hunt Ice Shelf as it appeared in all previous images. Though the shelf had already cracked and the lake was draining, the changes were not visible in MODIS images. In 2002, the ice shelf began to show signs of deterioration in the MODIS images. On August 7, 2002, the ice shelf appears much as it did in 2001. The next MODIS image, taken on August 16, shows that a large chunk of ice is missing from the northern side of the shelf, to the right of the island. Mueller, et al. note that RADARSAT images likewise show that a section of the shelf broke away sometime between August 6 and August 11, though they suspect that a number of small ice islands calved off the shelf rather than one large iceberg. The MODIS image taken on August 7 narrows the time window of the event even further. The 2003 images show dramatic changes in the Disraeli Fiord. On in early July, an aqua pool of melt water formed on the surface of the ice in the fiord. By July 23, 2003, the water had drained off the ice into the fiord below, leaving no visible signs that it had ever been there. But it may have weakened the ice beneath. The ponded water could have absorbed more energy from the sun, heating and weakening the ice it rested on. It is also possible that, because the epishelf lake had drained, the ice is now mostly made up of salty seawater instead of fresh water. Salt in sea ice makes it weaker than fresh-water ice. Whether from ponded water or salt contamination, by August 29, the ice in the fiord had broken up for the first time in recorded history. This and later images also distinctly show the crack running from the fiord across the shelf to the ocean. Image interpretation provided by Derek Mueller and Warwick Vincent, Centre détudes nordiques, Université Laval in Quebec, Canada and Martin Jeffries, Geophysical Institute, University of Alaska Fairbanks in Fairbanks, Alaska.
Source: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC

Por Mapas Owje