Sentinels in constant watch on mighty polar glaciers

The European Union’s Sentinel satellite system has begun monitoring 6 mighty polar glaciers in near real-time. The sextet — Pine Island and Thwaites in Antarctica; and Jakobshavn, Nioghalvfjerdsfjorden, Zacharae Isstrom and Petermann in Greenland — are major contributors to ongoing sea-level rise. They are thinning and flowing faster, and scientists believe some of them have become unstable. The data is currently being gathered by the Sentinel-1a spacecraft, but it will be assisted soon by a sister platform, Sentinel-1b.

Both have radar instruments that are able to see the glaciers’s surfaces day or night, and in all weathers. They can track activity by keeping a watch on the velocities of crevasses as they move towards the ocean. The UK’s NERC Centre for Polar Observation and Modelling (CPOM) has just sent live a new web portal that all researchers can access.

Using only Sentinel-1a data for the time being, the service will provide a snapshot of behaviour at least every 12 days. “The widespread coverage and short repeat period of Sentinel-1a has revolutionised the way we can observe glaciers around the world,” said Anna Hogg from Leeds University.

“The first step is to focus on areas of known dynamical imbalance — and, certainly, these 6 glaciers are key contributors to sea-level rise,” explained CPOM colleague Prof Andrew Shepherd.

Nioghalvfjerdsfjorden and Zacharae Isstrom are 2 big glaciers that enter the ocean next to each other in the northeast of Greenland. Together they make up 12% of the Greenland ice sheet. Zacharae Isstrom hit the headlines in November last year. A report said it had broken loose from a stable position held in 2012 and was now in an accelerated retreat.

The northeast of Greenland has only recently attracted attention because the most vulnerable regions of the ice sheet were always considered to be much further south.

Jakobshavn is sited in southwest Greenland. Not only does it move very fast (at times over 17 km a year), but it is also retreating rapidly inland, at a rate of many hundreds of metres per year. Periodically, it displays spectacular calving behaviour. Billions of tonnes of icebergs are shed from its front every year and move out of the fjord towards the Atlantic.

Petermann is in the northwest of Greenland. It is another producer of spectacular icebergs. Some are so big they are referred to as “ice islands”. In 2010 it produced a block 260 sq km in area. In 2012, a 130-sq-km piece came away. Calving is part of the natural life cycle of ocean-terminating glacier. But ongoing monitoring and in situ research will establish if any of the drivers are changing, and how.

Pine Island Glacier in West Antarctica is contributing more to sea-level rise than any other ice stream on the planet. It drains an area of 160,000 sq km, which is roughly two-thirds the size of the United Kingdom. The geometry of the rock bed under Pine Island makes it unstable, and scientists believe it is now in a self-sustaining retreat that could contribute on the order of perhaps 3.5-10mm to global ocean rise in the next 20 years. Thwaites Glacier is in the same part of West Antarctica as Pine Island and of similar size. It too has experienced significant retreat since the early 1990s. Its grounding line — the point where its leading front lifts off the bed and floats — has gone backwards some 15 km.

Continuing retreat will eventually take this line over a sill, which would then expose the deep-seated interior mass of the glacier to a potential collapse.