GRL - Changes In W. Antarctic Ice Stream Dynamics (Pine Island Glacier Speed Up 42% 96-07)

[hatrack]

Eric Rignot

Earth System Science, University of California,
Irvine, California, USA

Jet Propulsion Laboratory,
Pasadena, California, USA

Abstract

<1> The Advanced Land Observation System (ALOS) Phased-Array Synthetic-Aperture Radar (PALSAR) is an L-band frequency (1.27 GHz) radar capable of continental-scale interferometric observations of ice sheet motion. Here, we show that PALSAR data yield excellent measurements of ice motion compared to C-band (5.6 GHz) radar data because of greater temporal coherence over snow and firn. We compare PALSAR velocities from year 2006 in Pine Island Bay, West Antarctica with those spanning years 1974 to 2007. Between 1996 and 2007, Pine Island Glacier sped up 42% and ungrounded over most of its ice plain. Smith Glacier accelerated 83% and ungrounded as well. Their largest speed up are recorded in 2007. Thwaites Glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed. Total ice discharge from these glaciers increased 30% in 12 yr and the net mass loss increased 170% from 39 ± 15 Gt/yr to 105 ± 27 Gt/yr. Longer-term velocity changes suggest only a moderate loss in the 1970s. As the glaciers unground into the deeper, smoother beds inland, the mass loss from this region will grow considerably larger in years to come.

Received 21 January 2008; revised 27 February 2008; accepted 10 March 2008; published 28 June 2008.

EDIT

17] Model studies showed that the acceleration of Pine Island Glacier is controlled by its ice-shelf buttressing and the position of its grounding line within the ice plain region , which is an area grounded only 40–50 m above hydrostatic floatation upstream of the 1996 glacier grounding line . Progressive un-grounding of the ice plain caused by dynamic thinning of the glacier (anywhere from 1–2 m/yr to 5 m/yr) reduces back-stress resistance to flow, increases the longitudinal strain rate of ice, so ice accelerates and thins vertically, which in turn enhances grounding line retreat. The 4-km grounding line retreat of 1996–2000 was sufficient to explain the 10% acceleration of the glacier . Using the same model, Thomas et al. <2004b> predicted that the glacier velocity will reach 3.6 km/yr in the 11 years it will take to thin the ice sufficiently for complete floatation of the ice plain assuming a thinning rate of 2 m/yr. Results presented here show velocities increasing to 3.75 ± 0.1 km/yr and probably ungrounding of most of the ice plain by late 2007, consistent with the more rapid thinning observed since 2002.

<18> We have no InSAR data to confirm the new grounding line position, but an examination of the mottled appearance of the glacier surface in 2002–2007 MODIS data (see the auxiliary material) suggests a grounding line retreat of 15 ± 6 km between January 1996 and December 2007 at the glacier center, nearly the entire ice plain region. Surface bumps visible in 2002 are no longer visible in 2007, which is indicative of a transition to floatation.

<19> If confirmed, near-floatation of the ice plain has major implications for the glacier evolution because the grounding line would be at the edge of a smoother sea bed, well below sea level, and extending more than 250 km inland. If the grounding line retreats in that sector, Thomas et al.'s <2004b> model predicts a further doubling of the glacier speed, with the glacier now becoming afloat over seabed 1200 m below sea level, following breakup of the ice shelf to seaward of the ice plain (R. Thomas, personal communication, 2008). This would cause a sea-level rise by up to 1 mm/yr from this one glacier alone. Even though breakup is probably not imminent, continued ice-shelf thinning by about 4 m/yr would increase its likelihood.

EDIT

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