Ice sheet - glacier - ocean 

A complex problem

The interactions between the ice sheet and the ocean are far from being sufficiently understood to be able to make reliable predictions for the future contribution of the ice sheets to sea level changes. The interaction between the Greenland Ice Sheet and the North Atlantic or the Arctic Ocean takes place via a large number of small-scale peripheral and outlet glaciers that are in direct contact with the ocean. The latest studies, including southern Greenland, suggest that the warmer North Atlantic could play a decisive role in destabilizing the ice sheet through this contact. A connection between oceanic warming and increased calving of icebergs is also supported by paleostudies from sediment cores ^[30,31].

According to the International Climate Council, the dynamics of marine terminating glaciers play a significant, underestimated role in the loss of mass [6]. In April 2016, the IPCC therefore decided at its 43rd meeting to draft a special report on the topic of “Ocean and Cryosphere in Climate Change”. The simultaneous occurrence of the loss of mass along the edges of the Greenland ice sheet, the acceleration and retreat of the glaciers, and the warming of the North Atlantic indicate an interaction at the interface between glaciers and ocean. Little research has been done on this interaction so far, so that new interdisciplinary research fields are developing internationally ^[18,29] (see GRISO network http://web.whoi.edu/griso/).

At the interface between glacier and ocean 

Warm ocean water is present at the calving front and (if existing) on the floating underside of marine terminating glaciers. Observations show that the acceleration of glacier melt in recent years has been accompanied by a warming of the fjord water ^[20,21]. If the water temperatures increase, the thickness of the glacier ice decreases. In addition, the glaciers are retreating further and further from the ocean inland and their flow speed increases. The latter in particular has an impact on the ice masses inland, which then accelerate towards the coast thanks to the powerful ice streams - and contribute significantly to the loss of mass of the Greenland ice sheet. Model studies also suggest the ocean as the most plausible driver of glacier changes ^[24], but the underlying mechanisms are so far insufficiently understood.

Meltwater - where does it melt?

Subglacial melt water, i.e. fresh water that brought into the ocean below the water surface, can have different origins:

1. Warm water temperatures lead to melting on the calving front and on the underside of a (possibly existing) glacier tongue that protrudes into the water. We refer to this as "basal" or "submarine" melting.
2. Water melts on the surface of the ice and drains through glacier mills to the bottom of the ice sheet, where it is carried into the ocean. We refer to this as "supraglacial" melting. With the supraglacial contribution, the atmosphere also comes into play as a decisive driver, which acts on the glacier surface through complex multiscale processes ^[28].

Regardless of its origin, the subglacial melt water mixes with the surrounding ocean water and can in turn modify the strength of the basal glacier melt [25,26,27]. It is currently not possible to quantify what proportion of the mass loss is due to supraglacial melting or dynamic ice mass loss due to the destabilization of glaciers [6].

Peripheral Glaciers

Not all glaciers on the Greenland coast are connected to the ice sheet. These so-called peripheral glaciers only make up about 5% of the area, but they also make a decisive contribution to the total loss of mass of the Greenland Ice Sheet - during the period 2000 to 2011 they accounted for about a quarter of the total mass loss ^[20,21]! This loss of mass also probably results from warmer water and air temperatures. Thus, also the peripheral glaciers intrude additional freshwater into the ocean.