Gliding Across The Ice

ESA’s Earth Explorer CryoSat. Image courtesy of ESA/AOES Medialab.

ESA’s Earth Explorer CryoSat. Image courtesy of ESA/AOES Medialab.

There’s been a flurry of reports in the last couple of weeks, reporting melting ice and retreating glaciers in Greenland and the Himalayas respectively.

A paper by McMillan et al (2016), titled ‘A high-resolution record of Greenland mass balance’ and published in Geophysical Research Letters earlier this month, highlighted that Greenland’s melting ice has contributed twice as much to sea level rise than in the previous twenty years. The research used CryoSat-2 radar altimetry between 1 January 2011 and 31 December 2014 to measure elevation changes in the Greenland ice.

The main instrument on ESA’s CryoSat-2 satellite is a Synthetic Aperture Radar (SAR)/Interferometric Radar Altimeter known as SIRAL, although also carries a second version of this instrument as a back-up. The SIRAL instrument has been enhanced to detect millimetre changes in the elevation of both ice-sheets and sea-ice. It sends out bursts of radar pulses, with an interval of 50 μs between them, covering a 250 m wide strip of the Earth and measures the time of the return signal to determine the height of the satellite above the Earth. It requires a very accurate measurement of its position to calculate this, and so it also carries a Doppler Orbit and Radio Positioning Integration by Satellite (DORIS) instrument to determine its orbit.

The research team discovered that the Greenland Ice Sheet lost an average of 269 ± 51 Gt/yr of snow and ice during the investigative period, which compared well with other independent measurements from sensors such as the Gravity Recovery and Climate Experiment (GRACE) satellite and results from climate models. This snow and ice loss corresponds to a 0.75 mm contribution to global sea-level rise each year.

It was reported this week that research undertaken by the Indian Space Research Organisation, Wadia Institute of Himalayan Geology and other institutions have revealed that the majority of the glaciers in India are retreating; albeit at different rates. Using remote sensing data up to 2006, the study looked at 82 glaciers in the Bhagirathi and Alaknanda river basins and found that there had been an overall loss of 4.6% of the glaciers within the region. The Dokriani glacier in Bhagirathi is retreating between 15 and 20 metres per year since 1995, whereas the Chorabari glacier in the Alaknanda basin is retreating 9-11 metres per year.

It’s interesting to read the retreating glacial picture alongside the research published by Schwanghart et al (2016), titled ‘Uncertainty in the Himalayan energy–water nexus: estimating regional exposure to glacial lake outburst floods’, in Environmental Research Letters. Here the research team completed the first region wide risk assessment of floods from glacial lakes, even though this only covered around a quarter dams in the Himalaya’s. The study mapped 257 dams against more than 2,300 glacial lakes within the region and found that over 20% of the dams are likely to be overwhelmed with flood water as rock systems that surround glacier-fed lakes fail. Due to the hydro-electric power needs of the region, more dams have been built in recent years, putting them closer to glacier-fed lakes.

The potential danger of this issue is demonstrated by the collapse of Zhangzangbo, a glacier-fed lake in southern Tibet, in 1981 where 20 million cubic meters of floodwater damaged hydroelectric dams and roads causing damage of approximately $4 million.

These three reports also show the potential danger melting ice and glaciers pose both locally and globally. Remote sensing data, particularly from satellites such as CryoSat-2, can help us monitor and understand whether this danger is increasing.