Global Warming Hazard – Seismology, Antarctic Ice Sheet Instability, and Abrupt Sea Level Rise
Edward Cranswick
21DEC2006
Recent reports of glacial earthquakes (Ekstrom et al.,
2003; Ekstrom et al., 2006) in Greenland and other localities and recent
evidence of Antarctic icesheet instability suggest that there is the potential
for abrupt global sea level rise – this indicates that one of the top priorities
of global warming hazard assessment is the regional and local seismic monitoring
of the Antarctic icesheet.
For three decades, there has been concern that increasing carbon dioxide concentrations in the atmosphere could lead to global warming which could in turn cause collapse of the Western Antarctic Ice Sheet and consequent seal-level rise of 5 m (Mercer, 1978; Overpeck et al., 2006). The collapse of the Larsen B Ice Shelf in Antartica in 2002 (3,250 square-km of shelf area disintegrated in a 35-day period beginning on 31 January 2002 <http://nsidc.org/iceshelves/larsenb2002/>) is an example of very rapid and unanticipated destruction of the Antarctic icesheet, but it did not contribute to sea level because an ice shelf is already floating on the sea as opposed to being supported by the sea floor, i.e., “grounded”, or resting on the land surface above sea level.
A New Zealand-led ice drilling team have recently recovered three million years of climate history by drilling through the ice and into the marine sediments below, and their initial results raise questions about the stability Ross Ice Shelf, "If the past is any indication of the future, then the ice shelf will collapse … If the ice shelf goes, then what about the West Antarctic Ice Sheet? What we've learnt from the Antarctic Peninsula is when once buttressing ice sheets go, the glaciers feeding them move faster …You go from full glacial conditions to open ocean conditions very abruptly. It doesn't surprise us that much that the transition was dramatic” (Naish, 29 November 2006).
Though the contribution of the global ice sheets to sea-level rise is usually thought of in terms of melting – a slower, more predictable process – the rapid mass movement of land-based ice sheets into the sea would produce rapid sea-level rise. Glaciers or ice streams transport ice to the sea at rates as fast as 400–800 m/yr (ice stream C, West Antarctic; Anandakrisknan and Alley, 1997) – the phenomena described below, glacial earthquakes, provide a mechanism that could potentially transport ice to the sea several orders of magnitude faster and, hence, produce abrupt sea-level rise.
Glacial earthquakes were first recognized by Ekstrom et al (2003) during a
search for peculiar teleseismic events that did not trigger the conventional
detector algorithms and, hence, did not appear in the standard earthquake
catalogues – their waveforms are severely depleted in high frequencies relative
to typical tectonic earthquakes. About 500 events were detected worldwide,
mostly occurring at plate boundaries and other tectonically active areas. A
subset of about 50 were located in glaciated regions, mountain glaciers in
Alaska and at the edges of ice sheets in Greenland (see Fig 1 which depicts an
expanded data set from a subsequent study) and Antarctica, and all these events
had moment-magnitudes within the range 4.6-5.1. The source mechanism of glacial
earthquakes can be modelled by a volume of glacial ice that slips down slope
(e.g., 10 cubic-km by 10 m) over a period of 30-60 s.
Fig. 1. Topographic map of southern Greenland and vicinity. The locations of 136 glacial earthquakes defining seven groups are indicated with red circles: DJG, Daugaard Jensen Glacier (5 events); KG, Kangerdlugssuaq Glacier (61); HG, Helheim Glacier (26); SG, southeast Greenland glaciers (6); JI, Jakobshavn Isbrae (11); RI, Rinks Isbrae (10); NG, northwest Greenland glaciers (17). Owing to the tight clustering of the earthquakes, many of the individual symbols on the map overlap. (from Ekstrom et al., 2006)
Further study of 182 glacial earthquakes in Greenland by Ekstrom et al (2006) indicates that the rate of events per year has been increasing steadily since 2002 and that they show a seasonal variation, increasing during the summer months when air temperatures cause surface melting of the ice sheet (see Fig 2; see also <http://www.earth.columbia.edu/news/2006/story03-23-06.php>, <http://www.agiweb.org/geotimes/dec03/NN_glacialeq.html>). The onset of glacial earthquakes and their increasing rate of occurrence during seasons of warm air temperatures that promote surface ice melting which also contributed to the collapse of Larsen B Ice Shelf suggest that a new mechanism for mass transport of ice sheets has been activated and that it is very sensitive to global warming.
Fig. 2. (A) Histogram showing seasonality of glacial earthquakes on Greenland. Green bars show the number of detected Greenland glacial earthquakes in each month during the period 1993 to 2004. Gray bars show the number of earthquakes of similar magnitude detected elsewhere north of 45-N during the same period. (B) Histogram showing the increasing number of Greenland glacial earthquakes (green bars) since at least 2002. No general increase in the detection of earthquakes north of 45-N (gray bars) is observed during this time period. (from Ekstrom et al., 2006)
For two decades, microearthquake surveys have been
conducted in Antarctica to investigate the details of ice-sheet deformation,
slip and flow (Blankenship et al, 1987;
Anandakrishnan and Alley, 1994, 1996, 1997).
In particular, in May 2006, a group from Pennsylvania State University and the
British Antarctic Survey (Anandakrishnan, Smith, Alley, Pollard and
Carlsen, 2006; <http://igloo.gsfc.nasa.gov/wais/pastmeetings/PPT06/anandakr.pdf>)
have proposed to monitor both glacial earthquakes and microearthquakes to
investigate WAIS instability and to disseminate these results to the public in
near-realtime and put them in the context of low-lying communities especially at
risk from sea-level rise.
In conclusion, with respect to assessing global warming hazard in near-realtime, microearthquake investigations in Antarctica offer a means of monitoring the details of WAIS instability and potential consequent abrupt sea-level rise.
References
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