Recently A. Sheperd and others used ERS-altimetry to measure thickness changes of Pine Island Glacier (PIG), one of the largest ice streams in the region. They observed a thinning of 10m at the grounding line from 1992 to 1999. To obtain a complementary time series of long-term ice velcoity of Pine Island Glacier we used a feature tracking method along the central flowline. Input are pairs of SAR intensity images separated by 2 months to two years. We observe a 10% increase of ice flow on the lower glacier during 1992 and 2000 (see Figure 1). The velocity increase between 1992 and 2000 is significant and cannot be attributed to seasonal variations. Our results are consistent with the thinning observed by Sheperd and others.
An additional peculiarity of the motion field on the floating part of PIG becomes apparent with differential radar interferometry (D-InSAR) using ERS-tandem data. A single interferogram covers a 1-day period. The two differential interferograms shown in Figure 2 were genetrated by differencing two single interferograms acquired several months apart in 1995 and 1996, respectively. Only topographic and non steady motion features are visible in these products. The two differential interferograms show wave-shaped velocity variations (Figure 2). The effect is less distinct in the ascending scene because the flow direction of PIG is almost perpendicular to the look direction of the ERS satellite. We estimate the spatial velocity variations to be about 30 m/yr over a scale of 10-15 km. If we assume an ice thickness of about 1000m at the grounding line, an acceleration of 30 m/yr would cause a glacier thinning of about 10m and a surface lowering of 1m on the floating part.
Our study shows temporal as well as spatial velocity variations. A correlation between the glacier thinning published by Sheperd and others and the observed temporal velocity increase is reasonable. We can speculate that the spatial velocity variations are indicative of a quasi-periodic flow acceleration, the latest cyle of which we see at the moment.