This thinning could be taking place in two ways: melting due to warming from higher air temperatures above or from hotter ocean waters below. But little is known about rates of submarine melting as the only previous measurements took place in North America.

With this in mind, a team from the US and Canada has measured water conditions in fjords by outlet glaciers in the west of Greenland; its results revealed that the oceans could be playing a major role.

"A couple of studies conducted in Alaska suggested submarine melt rates were huge," Eric Rignot of the University of California, Irvine, US, and Jet Propulsion Laboratory, US, told environmentalresearchweb.
"We wanted to find out for ourselves if that was the case in Greenland too."

In August 2008, Rignot and colleagues from the University of California, Irvine; Jet Propulsion Laboratory, US; and the University of British Columbia, Canada, measured ocean currents, temperatures and salinity near the calving fronts of the Equip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland, as well as ice-front bathymetry.

"We did find that the rates are large, that the results collected in Alaska were correct to indicate that this is an important interaction," said Rignot. "This changes completely the way we should look at ice-sheet evolution. We have now mounting evidence that the ocean plays a major role that we have not yet fully accounted for and which is calling for more detailed studies."

By looking at water-mass and heat budgets for each fjord, the team calculated mean summer submarine-melt rates for the different glaciers of 0.7, 2.6 and 3.9 metres per day. These rates are around two orders of magnitude larger than surface-melt rates but comparable to rates of iceberg discharge.

Writing in Nature Geoscience, the researchers conclude that "ocean waters melt a considerable, but highly variable, fraction of the calving fronts of glaciers before they disintegrate into icebergs." They also suggest that submarine melting must have a profound influence on grounding-line stability and ice-flow dynamics.

The submarine-melt rates are likely to be lower in winter because there is less meltwater discharged from beneath the glacier to drive convection of warm ocean water up in front of the glacier.

"The high rates of submarine melting that our survey revealed is the missing link between the acceleration of glaciers in South Greenland and the intrusion of warmer-than-usual waters in the glacial fjords," said Rignot. "It highlights a fundamental way ice and climate interact – via the ocean – that is capable of explaining why these glaciers sped up so abruptly."

According to Rignot, the increase in surface melt of these glaciers has not been able to explain their acceleration. "The ocean waters are – in my opinion and based on our study, numerical models and other results – the main vector of change," he added.
Rignot classes his work as "a reconnaissance study, a seed study that is clearly calling for more detailed observations." The team will return to Greenland in August to survey other fjords, and it is also incorporating its finds in a numerical model of ice–ocean interactions.

"By publishing this study our hope is that many other researchers will be interested in this issue, from a numerical model point of view or an in-situ measurement point of view, so we can gain a better understanding of these processes," added Rignot. "This is calling for a multi-disciplinary effort, not just glaciologists; we need physical oceanographers, mathematicians, physicists [and] glaciologists to look at the issue of ice–ocean interactions together if we are to make major progress in our understanding of their impact on Greenland and Antarctica."

The researchers reported their work in Nature Geoscience. In the same issue, Straneo et al. found that subtropical waters are entering fjords in East Greenland and promoting melting of outlet glaciers.