Question

he Jakobshavn Glacier in
Greenland melts significantly during
the summer and is known as the
fastest-moving glacier in the world.
Construct an explanation for how
the water cycle impacts the melting
and movement of the glacier.

Answer 1

Brief answer
The seasonal water cycle — snowfall, summer melting, routing of meltwater, and exchange with the ocean — controls much of Jakobshavn’s summertime melting and its variable speed. Surface meltwater both increases melting (through changes in albedo and heat transfer) and changes how the glacier slides by reaching its bed, and ocean-warmed water increases calving and undercutting at the terminus. Together these processes cause the strong summer speed-ups and help drive the glacier’s long-term retreat.

Step-by-step explanation

1. Supply and melt
- Winter snowfall adds ice mass; summer solar heating and warm air cause surface melting. Jakobshavn experiences large summer melt volumes because it sits at relatively low elevation on the west coast of Greenland.
- Melt converts bright snow/ice into water and exposes darker ice or sediment (cryoconite), lowering surface albedo so the surface absorbs more solar energy and melts more — a positive feedback.

2. Surface water storage and routing
- Meltwater pools in supraglacial lakes and streams on the ice surface. These lakes can absorb heat and grow rapidly in summer.
- Streams and lakes focus water into holes (crevasses and moulins) that drain the surface water vertically into the ice.

3. Passage through the ice (englacial drainage)
- Water entering moulins travels through englacial conduits and crevasses to the bed. This can happen quickly during intense melt events, delivering a large pulse of water to the glacier bed in hours or days.

4. Basal lubrication and sliding
- When meltwater reaches the bed, it raises water pressure at the ice–bed interface and can reduce effective normal stress, lowering friction between ice and bedrock/sediment.
- High basal water pressure produces basal lubrication and rapid sliding of the ice over its bed — this is the main cause of summer speed-ups at Jakobshavn.
- The character of the subglacial drainage system matters: early in the melt season a distributed, inefficient drainage system keeps basal pressures high and promotes sliding; later, channels can form that drain water efficiently, lower basal pressure, and slow the glacier again. That seasonal switch causes the typical summer peak in speed followed by slowdown.

5. Hydrofracture and calving
- Water-filled crevasses and supraglacial lakes can drive hydrofracture (water forcing cracks to propagate), which can cause pieces of the terminus to break off (calve). Hydrofracture is an important mechanism for iceberg production from Jakobshavn’s steep tidewater terminus.
- Increased calving shortens the glacier and can lead to retreat of the grounding line, which changes ice geometry and can increase ice flow rates further.

6. Ocean interactions
- Warm fjord water melts the floating or submerged terminus from below (submarine melting) and undercuts the ice front, making it more prone to calving.
- Fjord circulation, supplied by surface fresh meltwater and ocean heat, modulates how much ocean heat reaches the terminus.

7. Longer-term feedbacks
- Thinning and retreat of the glacier change the ice stress balance so ice flows faster (dynamic thinning), which exposes lower, warmer ice and can increase surface melt further — another positive feedback.
- Some meltwater is retained in firn or refreezes, reducing immediate runoff; however, as firn becomes saturated or converts to ice (firn densification), more meltwater runs off directly to the bed and ocean.

Why Jakobshavn is especially affected
- Jakobshavn Isbræ is a fast, steep outlet glacier terminating in a deep, warm fjord. Large amounts of summer melt and efficient routing to the bed, combined with ocean warming and strong calving, make the glacier especially responsive to the water cycle. Observations show repeated summertime speed-ups tied to melt pulses and long-term acceleration associated with sustained increased melt and ocean forcing.

Summary
The water cycle supplies heat and liquid water that increase surface melting and reduce reflectivity, routes that water to the glacier bed where it lubricates and speeds sliding, promotes hydrofracture and calving, and delivers freshwater that changes fjord circulation and submarine melt. These linked processes explain why summertime meltwater strongly controls Jakobshavn’s seasonal accelerations and contributes to its rapid long-term retreat.