Ice Streams and Outlet Glaciers
The great ice sheets of Greenland and Antarctica do not lose their ice uniformly across their margins. Instead, the vast majority of ice discharge to the ocean is concentrated in relatively narrow corridors — ice streams and outlet glaciers — that move at speeds tens to hundreds of times faster than the surrounding slow-flowing ice. These features are the ice sheet's drainage network, equivalent in function to rivers draining a continental watershed, and they are the primary mechanism by which ice sheet mass is transferred from the accumulation interior to the ocean where it melts or calves as icebergs.
Understanding ice streams and outlet glaciers is inseparable from understanding sea level projections. The largest uncertainties in twenty-first century sea level rise projections come not from surface melt calculations, which are relatively well constrained by temperature and energy balance models, but from the dynamic behaviour of ice streams and outlet glaciers — their potential for rapid acceleration, grounding line retreat, and marine ice sheet instability. The difference between a few tens of centimetres and over a metre of sea level rise by 2100 depends largely on how the ice streams of West Antarctica and northern Greenland behave in a warming climate.
What Is an Ice Stream
An ice stream is a region within an ice sheet where ice flows significantly faster than the adjacent ice, typically defined as flowing more than 100 metres per year when surrounding ice moves at rates of tens of metres per year or less. Ice streams in the Antarctic Ice Sheet include the Siple Coast ice streams — Pine Island Glacier, Thwaites Glacier, Smith Glacier, Bindschadler Ice Stream, and Whillans Ice Stream — that drain the West Antarctic Ice Sheet into the Ross Ice Shelf and the Amundsen Sea.
Ice streams are generally underlain by soft, water-saturated sediments and maintained by a distributed pressurised water system at the bed that reduces basal friction dramatically. The discovery in the 1980s by Barclay Kamb and colleagues that Whillans Ice Stream was underlain by a deformable sediment layer several metres thick, maintaining the fast flow through sediment deformation, changed the understanding of ice stream dynamics fundamentally. The ice streams of the Siple Coast move at several hundred metres per year and drain an area of West Antarctica equivalent in size to Mexico.
Outlet Glaciers of Greenland
In Greenland, the fast-flowing ice corridors are called outlet glaciers rather than ice streams, though the physical distinction is sometimes one of convenience. Greenland's outlet glaciers are typically constrained within topographic fjords carved during previous glaciations, unlike the Siple Coast ice streams which flow across relatively featureless sediment plains. The fjord confinement means that their speed is partly set by the width of the fjord and the buttressing provided by any floating ice tongue extending into the fjord.
Jakobshavn Isbrae (also called Sermeq Kujalleq in Greenlandic) on the west coast is the fastest major outlet glacier in Greenland, historically moving at roughly 20 to 25 metres per day at its calving front — about 9 kilometres per year. It drains approximately 6.5 percent of the Greenland Ice Sheet's area. In the 1990s and 2000s, Jakobshavn accelerated dramatically, coinciding with the collapse of its floating ice tongue and the retreat of the calving front into deeper fjord waters. The floating tongue had been providing back-stress that slowed the flow; its removal allowed the glacier to speed up.
Helheim Glacier on the southeastern coast and Kangerdlugssuaq on the east coast are other major outlet glaciers that have been extensively studied and monitored. Together, Greenland's major outlet glaciers discharge the majority of the island's total ice flux to the ocean, with smaller glaciers in the northeast and northwest contributing proportionally less.
Pine Island and Thwaites: The Vulnerable West Antarctic Glaciers
Pine Island Glacier and Thwaites Glacier in West Antarctica represent the two outlet glaciers of greatest concern for sea level projections. Both drain sectors of the West Antarctic Ice Sheet where bedrock lies well below sea level and slopes downward toward the interior — the topographic geometry associated with marine ice sheet instability. Pine Island Glacier has been thinning and accelerating since at least the 1990s, when satellite altimetry first detected the drawdown signal propagating up the glacier from the grounding line.
Thwaites Glacier, sometimes called the Doomsday Glacier in popular press coverage, is broader and drains a larger area than Pine Island. Its grounding line retreated measurably between the 1990s and 2020s, and the geometry of the bed beneath it suggests that continued retreat could bring the grounding line into contact with a particularly deep and expansive overdeepened basin — potentially triggering accelerating retreat through marine ice sheet instability. The ITGC (International Thwaites Glacier Collaboration), a joint US-UK research program, has deployed instruments on, in, and beneath the glacier to characterise the ice-ocean interaction and ice dynamics at the grounding zone.
Ice Shelf Buttressing and Its Loss
Many of the largest outlet glaciers in Antarctica flow into floating ice shelves — thick platforms of ice that extend beyond the grounding line, supported by ocean water beneath. Ice shelves do not contribute directly to sea level when they melt, since they are already floating, but they play a critical role in regulating the flow of grounded ice behind them. The back-stress exerted by an ice shelf on its source glacier — called buttressing — can slow the grounded glacier flow substantially.
When ice shelves are thinned by ocean warming or lost through calving, the buttressing force is reduced or removed and the glaciers behind them accelerate. The Larsen B collapse in 2002 was followed by speed-ups in tributary glaciers of several times their pre-collapse rates. The warming of Circumpolar Deep Water intrusion under the ice shelves of the Amundsen Sea — Pine Island Bay in particular — has driven basal melt rates at the Pine Island and Thwaites ice shelf undersides to among the highest observed anywhere in Antarctica.
Monitoring Ice Stream Velocities
Modern ice stream monitoring relies primarily on satellite-based observation. Synthetic Aperture Radar interferometry (InSAR) from ESA's ERS and Sentinel-1 satellites and NASA's ALOS series has provided continuous velocity maps of the entire Greenland and Antarctic ice sheet margins since the early 1990s. Repeated passes separated by known time intervals allow surface displacement to be measured at metre to sub-metre precision, revealing the detailed velocity fields of ice streams, outlet glaciers, and their tributaries.
CryoSat-2 and IceSat-2 altimetry satellites measure ice surface elevation changes, detecting thinning signals that often accompany acceleration. Combined velocity and elevation change datasets enable calculation of dynamic mass loss — the component of ice sheet mass loss attributable to ice flow changes rather than surface melt.
Browse the map to explore the locations of Greenland's major outlet glaciers and the ice streams of the Antarctic Ice Sheet, placing these dynamic systems in their full geographic context.