Jakobshavn Glacier can travel nearly the length of an Olympic swimming pool in a day. Near its fastest seaward section, measured velocities have at times approached 50 meters per day. That figure is remarkable, but the glacier’s scientific importance comes from how often the pace has changed—accelerating, slowing, and accelerating again as its front, thickness, fjord, and ocean conditions evolved.
Known in Greenlandic as Sermeq Kujalleq and in Danish as Jakobshavn Isbræ, the glacier drains a substantial portion of west-central Greenland into Ilulissat Icefjord. It is a major producer of icebergs and one of the ice sheet’s largest sources of dynamic ice loss.
From Relative Stability to Record Flow
Measurements from the 1980s and early 1990s showed annual speeds near the lower glacier of roughly 5.7 to 6.7 kilometers per year. In the late 1990s, the floating ice tongue that had occupied the fjord began to thin and break apart. The calving front retreated into deeper water, and resistance at the terminus decreased.
Acceleration followed. By 2012, mean annual speed at a point near the front was almost three times its mid-1990s value, while peak summer motion was more than four times faster. During the summers of 2012 and 2013, parts of the glacier exceeded 17 kilometers per year and briefly approached 46 meters per day, establishing record rates for Greenland at the time.
The seasonal cycle became pronounced. The front tends to retreat during summer, reducing support and allowing the lower glacier to speed up. Winter sea ice and a rigid mixture of icebergs called mélange can suppress calving, letting the terminus advance and the glacier slow. The strength of this response depends on the depth and shape of the bed beneath the front.
A Temporary Slowdown Reversed the Trend
Between about 2016 and 2018, Jakobshavn thickened, its front advanced, and its speed decreased. Cooler water in Disko Bay and Ilulissat Icefjord helped reduce submarine melting at the glacier terminus. The cooling was linked to changes in North Atlantic ocean circulation rather than a reversal of long-term global warming.
This episode showed that a fast outlet glacier can respond quickly when local ocean conditions change. It also demonstrated why a few years of thickening should not be interpreted as complete recovery. The glacier remained far inland from its position before the late-1990s retreat, and the fjord geometry left it sensitive to renewed thinning.
Reacceleration began in 2018. Mean annual velocity near the terminus increased by about 49 percent between 2018 and 2021. The timing suggests that warmer ocean water was not the only trigger. Surface lowering and dynamic thinning brought the terminus closer to flotation, reducing effective pressure at the bed and making the lower glacier easier to accelerate.
Speed Is Set by Several Forms of Resistance
Ice moves through internal deformation and by sliding over its bed. Gravity supplies the driving stress, which grows with ice thickness and surface slope. Friction beneath the glacier, drag along valley walls, and back pressure from the terminus oppose that motion.
When the calving front retreats into deeper water, thicker ice may stand at the terminus. Higher driving stress and reduced contact with the bed can increase flow. Faster motion then stretches and thins the glacier, encouraging additional retreat. A shallower bed ridge can interrupt the feedback by supporting the front and increasing resistance.
Ocean heat promotes submarine melting, but calving does not respond to temperature alone. Crevasses, buoyancy, fjord circulation, and mélange determine when blocks detach. Observations during the record-speed summer of 2012 showed that a relatively small retreat could produce a large acceleration, while some larger calving events caused little immediate change. The glacier responds to the stress configuration left after an event, not simply to iceberg size.
Surface meltwater can also reach the bed and change water pressure. Early in the melt season, extra water may enhance sliding. As drainage channels enlarge and carry water away more efficiently, basal pressure can fall and partly offset that speedup. On Jakobshavn, changes at the marine terminus remain especially important to the dramatic multiyear variations.
A Moving Target for Measurement
Synthetic-aperture radar satellites can track the glacier through clouds and polar darkness by comparing surface patterns in images taken days apart. Optical imagery maps the calving front during clear conditions, while airborne and satellite altimetry measures thinning. Instruments in the fjord record water temperature, and ice-penetrating radar reveals the deep trough beneath the glacier.
Velocity must always be tied to a place and interval. A marker near the terminus can be lost when the front calves, and speed generally decreases inland. A one-day summer maximum cannot be compared directly with a glacier-wide annual average. Maps produced from 2014–2024 radar observations show that velocities still reached close to 50 meters per day at times, while also preserving large spatial and seasonal differences.
Jakobshavn’s history rejects a simple assumption that glacier acceleration proceeds smoothly. The major speedup, temporary slowdown, and renewed acceleration were all real responses to changing boundary conditions. Its motion records how quickly an outlet glacier can adjust when ocean heat, ice thickness, bed contact, and calving-front resistance shift—even while the inland ice feeding it changes much more slowly.