Source: skew-t, http://en.wikipedia.org/wiki/File:Topographic_map_of_Greenland_bedrock.jpg. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.
While assembling an article on glacial isostatic readjustment last month, the above map of Greenland created by Wikipedia user ‘skew-t’ caught my attention. As you can see, there’s something missing; namely the massive ice sheet that covers the world’s largest island. The ice sheet is so massive that it compresses the interior of the island downward. Typically, any topographic map of Greenland assumes the ice sheet to be part of the land, resulting in a uniform depiction of the island’s interior as being thousands of metres above sea level. Ignoring the ice sheet and going down to the bedrock, however, reveals a vastly different Greenland than the one we know. Taking strictly the soil into account, a large amount of Greenland actually lies under sea level. While the tall mountains of the south and east already stand out above the ice sheet and would experience little change in conditions were the ice sheet to disappear (which it’s rapidly doing already at a rate of 385 cubic miles per annum and growing), it is feasible that a massive lake would form in the centre of the island until such time as the surface below finally rebounded. The amount of land lying below sea level is comparable to the size of the Caspian Sea, and a lake covering that area below sea level would be in the running for the largest on Earth.
The prospect of a great Greenlandic lake in the centre of the island assumes that there would be no commensurate rise in sea level. Of course, as the ice sheet melts, the release of water into the oceans will greatly increase the sea level. As one can see on the map above, there are many areas rimming the portion below sea level that are themselves only 50 metres or so above the ocean. A rise in ocean elevation could theoretically flood these areas and infiltrate the low-lying interior, dividing the island and turning it into a massive archipelago. Already, one land previously thought to be attached to the mainland has been discovered to be an island as the ice sheet melts away; the revelation of Uunartoq Qeqertoq (Warming Island) made international headlines in 2007.
The above animation comes from a 2010 study led by Denmark’s National Space Institute. Using data from GPS measurements and from NASA’s GRACE (Gravity Recovery And Climate Experiment) satellite, the study shows how ice sheet loss is accelerating at the edges, especially in the narrower southern half of Greenland. Incidentally, there’s actually been a slight increase in ice sheet thickness in the interior at elevations above 2 000 metres, but this is due to the increased snowfall brought by warmer temperatures. As the edges of the ice sheet calve away and temperatures continue to warm, this, too, will be overturned. And even if it didn’t, the amount of ice sheet growth in the interior pales in comparison to the losses at the edge. For example, just last summer, a 260 km2 iceberg broke off from the Petermann Glacier in the northwest part of Greenland. The iceberg contained enough water to run all public tap water in the United States for 120 days.
This 2004 image from NASA shows rates of ice sheet elevation change in Greenland from 1997-2003. We can already discern the rapid calving along the edges that has now moved further inland as shown in the previous video. The Map Room directed me last week to the latest NASA imagery which shows that 2010 was a record year for ice sheet melting in Greenland. Image source: http://sas.ivv.nasa.gov/centers/goddard/earthandsun/thinningice.html.
The depression of the bedrock underlying the Greenland ice sheet is nothing compared to the depression underlying the Antarctic ice sheet, where much of the land underneath lies over 2 500 km below sea level. While East Antarctica is still breaking even with regards to ice sheet volume change, the ice sheet loss over West Antarctica is rapidly accelerating (although the rate at which it is doing so is unclear). As in Greenland, interior volumes are growing due to increased precipitation, but quickening glacial movement at the edges is eating away at the ice sheet overall. Since the ice sheet is literally the size of Europe, melting in Antarctica has a much greater impact on global sea level than melting in Greenland; 61% of Earth’s fresh water is held in the Antarctic ice sheet.
If we were to hypothetically remove the ice sheet from Antarctica as we talked about earlier with Greenland, the result would be shocking:
An elevation map of the bedrock underlying Antarctica. Source: P.W. Heinrich, http://commons.wikimedia.org/wiki/File:AntarcticBedrock.jpg?uselang=en-gb.
Showing only the bedrock above sea level really drives home just how drastically the Antarctic ice sheet pushes down the underlying continent. Source: Cristellaria, http://commons.wikimedia.org/wiki/File:AntarcticBedrock2.jpg?uselang=en-gb. Licensed under the Creative Commons Attribution 3.0 Unported licence.
There’d be no ambiguity about landlocked lakes like there would be with Greenland. Most of West Antarctica would be well underwater, with only the Transantarctic Mountains remaining attached to the mainland. The Antarctic Peninsula, the Ellsworth Mountains, Victoria Land, and the Executive Committee Range, among others, would form the bases of huge offshore islands. East Antarctica would be riddled with new bays and offshore archipelagos, and the South Pole would be lying on a small island in a Southern Ocean bay.
BBC News (2010). Huge ice island breaks from Greenland glacier. BBC News Science & Environment, 7 August 2010. Available at http://www.bbc.co.uk/news/science-environment-10900235. Accessed 18 February 2011.
BEDMAP Consortium (2000). BEDMAP: A new ice thickness and subglacial topographic model of the Antarctic. Available at http://www.antarctica.ac.uk//bas_research/data/access/bedmap/. Accessed 18 February 2011.
Cole, S. (2006). Greenland Ice Sheet on a Downward Slide. Goddard Space Flight Center, 19 October 2006. Available at http://www.nasa.gov/centers/goddard/news/topstory/2006/greenland_slide.html. Accessed 18 February 2011.
McCarty, M. (2007). An island made by global warming. The Independent, 24 April 2007. Available at http://www.independent.co.uk/environment/climate-change/an-island-made-by-global-warming-445966.html. Accessed 18 February 2011.
Ramanujan, K. (2004). Greenland’s Ice Thinning More Rapidly at Edges. Goddard Space Flight Center, 15 December 2004. Available at http://sas.ivv.nasa.gov/centers/goddard/earthandsun/thinningice.html. Accessed 18 February 2011.
Rignot, E. et al. (2008). Recent Antarctic ice mass loss from radar interferometry and regional climate modelling. Nature Geoscience 1: 106-110.
ScienceDaily (2008). As Ice Melts, Antarctic Bedrock Is On The Move. 18 December 2008. Available at http://www.sciencedaily.com/releases/2008/12/081215091013.htm. Accessed 18 February 2011.
University of Colorado at Boulder (2006). Antarctic ice sheet losing mass, says University of Colorado study. 2 March 2006. Available at http://www.eurekalert.org/pub_releases/2006-03/uoca-ais022806.php. Accessed 18 February 2011.
University of Colorado at Boulder (2010). Greenland Ice Sheet Losing Ice Mass on Northwest Coast, Says New International Study. 23 March 2010. Available at http://www.colorado.edu/news/r/f595fae00e6b451d4016ab9a43a049f8.html. Accessed 18 February 2011.