One of the largest uncertainties in predicting sea level rise in a warming climate is the iceberg calving rate, i.e. the rate at which marine-terminating glaciers will discharge ice to the ocean. Previous work has assumed that calving is controlled mainly by the speed of the glacier.

However new research, published today (Friday 9 October) in Nature Communications, shows that in many glaciers it is actually the temperature of sea water in contact with the glacier that controls the rate of ice loss.

The research team includes scientists from Swansea University, the University Centre in Svalbard (UNIS), and the Scottish Association for Marine Science (SAMS). Close collaboration between glaciologists and oceanographers was key to making this discovery.

Marine-terminating glaciers gain mass from snowfall and lose it through surface melt and iceberg calving. The balance between these processes determines the rate at which ice on land contributes to sea level rise. The new research shows that where submarine melt of ice in direct contact with the ocean can keep pace with ice flow, the calving part of the balance is controlled by the water temperature deep in the fjord.

In a process known as ‘melt undercut calving’, the ocean erodes the glacier beneath the waterline and the ice above simply crumbles into the fjord under its own weight. This process dominates glacier discharge in Svalbard, where the study took place, and may well do so elsewhere. Wherever melt-undercut calving is the dominant process, future predictions of ice discharge under climate warming can be made using models of ocean temperature and circulation.

Professor Adrian Luckman from the Department of Geography at Swansea University, lead author of the paper, said:

“We now understand for the first time what controls iceberg calving rates in Svalbard. We anticipate that deep fjord water temperatures also control ice discharge in many other Arctic glacier settings”

Professor Doug Benn from the University Centre in Svalbard (UNIS), the leader of the project that funded the research, said:

“This detailed satellite record has allowed us to make a huge advance in understanding how calving glaciers react to ocean warming”

Dr. Finlo Cottier from the Scottish Association for Marine Science (SAMS), the lead oceanographer on the study, said:

“Using a unique Arctic data network, this study resolves the debate over the competing influences of ocean versus air temperature on glacial calving”.

The study used an unusually dense series of very high resolution images from the TerraSAR-X satellite. Data at a resolution of two metres acquired every 11 days for 18 months was analysed to measure both the seasonal advance and retreat of the glacier terminus, and the speed at which ice was replenished from further up the glacier. Three glaciers with diverse flow rates were studied.

Water temperature at 20-60 meters depth was measured by a moored instrument in the adjacent fjord; part of a network of Arctic observatories operated by SAMS with collaborators in University of Tromsø and UNIS. Supported jointly by the UK and Norway, the network supports international science projects ranging from fisheries to glaciers.

For over a decade, oceanographers Dr Cottier and Prof Inall from SAMS, in collaboration with Prof Nilsen from UNIS, have investigated processes by which coastal water is exchanged, and how warm ocean waters are drawn towards the glaciers.  Recent growing awareness of the role of the fjord oceanography on glaciers has made this a critical area of research.  This work offers insight into how the regional bathymetry and proximity to warm ocean currents influences glacier response.

Apparently simple relationships such as the one revealed by this study allows for more accurate prediction of future marine-terminating glacier melt in a warming world.

The research was conducted as part of the CRIOS project (Calving Rates and Impact on Sea Level), funded by the ConocoPhillips/Lundin Northern Area Program.

Glaciers and the ice-sheets that feed them cover 10% of the Earth surface and lock-in 75% of the Earth’s fresh water. As the Earth surface and oceans warm, ice melts; partial melt and runoff from this vast frozen reservoir will dominate future global sea level rise. Nowhere is ice-sheet behaviour more uncertain than where it meets the ocean.

• Department of Geography at Swansea University:  In the latest Research Excellence Framework (REF), Geography’s research at Swansea was ranked 6th in the UK for research impact and 11th in the UK for research environment.  The Department hosts a large community of postgraduate researchers studying PhD degrees, and runs one year MSc courses. Geography at Swansea is top in the UK with 100% student satisfaction (National Student Survey 2014). For more information go to http://www.swansea.ac.uk/geography/undergraduate 
• University Centre in Svalbard (UNIS) is the world's northernmost institution for higher education and research, located in Longyearbyen, Spitsbergen at 78°N. UNIS offers high quality courses at the undergraduate, graduate and postgraduate level in Arctic Biology, Arctic Geology, Arctic Geophysics and Arctic Technology. For more information go to http://www.unis.no
• Scottish Association for Marine Science (SAMS): SAMS is Scotland’s leading marine research centre, delivering education at all levels in marine science under the auspices of the University of the Highland and Islands; including a joint honours degree with the University Centre in Svalbard (UNIS). 80% of SAMS research was graded as ‘internationally excellent’ in 2014. For more information go to http://www.sams.ac.uk

Picture: Kronebreen, Svalbard, one of the glaciers studies in the paper.