Pine Island retreat on the radar

Pine Island is the largest glacier in the West Antarctic Ice Sheet and one of the fastest ice streams on the continent. About a tenth of the ice sheet drains out to the sea by way of this glacier.

At the margins of ice sheets, ice lifts off of the land and floats out over the water as an ice shelf. Measuring these ice shelves and following their evolution is important for pinpointing the mass of an ice sheet and its contribution to changing sea levels.

But measuring them is difficult. Drilling and submarine surveys can be performed locally, but do not provide the continuous and global measurements needed for accurate monitoring.

Satellites can gather information frequently and over large areas, providing an effective means to quantify the melting of ice shelves. Radars on satellites can ‘see’ through clouds and in the dark, making them particularly suited for measurements over polar regions prone to bad weather and long periods of darkness.

During the course of 20 years, the orbits of ESA’s ERS satellites were occasionally modified to make more frequent measurements over the polar regions. Although the ERS-1 and -2 missions ended in 2000 and 2011, respectively, their archived data are still being mined to make discoveries.

In a recently published study, the accelerated thinning and the retreat of the boundary between the ice and the ocean – the grounding line – in Antarctica’s Pine Island Glacier were documented using ERS radar images.

The results show that, between 1992 and 2011, the glacier’s grounding line retreated by 20 km on average, reaching a maximum of 40 km in the central section. The retreat remained constant while the ice thinned by over 200 m during the 20 years.

This acceleration is consistent with increased melting driven by warming ocean water in the cavity beneath the floating section of the ice sheet.

“A first challenge in tracking the ice thinning at the ocean-ice interface is to measure the displacement of ice with enough precision to be sensitive to the tidal flexing of the ice at the grounding line, the point at which the ice becomes afloat,” said Noel Gourmelen from the University of Edinburgh, co-author of the study.

“A second challenge is to discriminate between the flow of ice of about 4 km a year and the tidal flexure of about 1 m – which are both captured within a single measurement.”

To do this, the scientists used ‘radar interferometry’, combining two or more radar images over the same area to detect changes occurring between scans.

With the frequent coverage by ERS-2 towards the end of its life, scientists were able to extract tidal flexing from the maps of ice speed.

Combined with earlier observations made by both ERS-1 and -2, they mapped 20 years of thinning ice at the Pine Island Glacier with higher accuracy than ever before.

Because the glacier’s melting does not appear to slow down or stabilise at any point, the scientists believe that existing projections of sea-level rise may be too conservative: Pine Island may contribute more than predicted.

While radar imagery from the ERS satellites, as well as from the Envisat mission – which ended in 2012 – provided crucial information on the evolution of the Antarctic ice sheet, continued coverage is needed for monitoring glaciers and rising sea levels.

The upcoming Sentinel-1 mission of the Copernicus programme will routinely measure ice speed at frequent intervals, allowing for more accurate monitoring of the interaction between the oceans and ice globally.

Figure 1 above: This Envisat image shows the Pine Island Glacier in West Antarctica and reveals a crack in the glacier’s tongue about 25 km long. The image was acquired by Envisat’s radar on 6 April 2012, just two days before contact with the satellite was lost. Over its ten-year lifetime, Envisat’s radar imagery was constantly used to monitor ice in the Antarctic. Satellite radars are particularly useful over polar regions because they can see through clouds and darkness. Copryright: ESA

Figure 2 above: Radar interferometry is a remote sensing technique that combines two or more radar images over the same area to detect changes occurring between acquisitions. Using this technique with data from the ERS satellites, scientists were able to map the retreat of the Pine Island Glacier’s grounding line – where the ice stream lifts up off the land and begin to float out over the ocean. From 1992 to 2011, the grounding line retreated on average by about 20 km, reaching 40 km in the central section. Copyright ESA

Figure 3 above: The European Remote Sensing satellite ERS-1, launched in 1991, carried a comprehensive payload including an imaging synthetic aperture radar, a radar altimeter and other powerful instruments to measure ocean surface temperature and winds at sea. ERS-2, which overlapped with ERS-1, was launched in 1995 with an additional sensor for atmospheric ozone research. Copyright ESA

Source: European Space Agency (www.esa.int)