Glaciology - Greenland Ice Sheet (2) Hydrodynamics

‘Greenland’s glaciers speed up and slow down at a variety of temporal scales ranging from hourly to annual.’ Evaluate the evidence in support of this statement and the theories that have been advanced to explain the phenomenon.

Fiona Fang, Trinity Hall, 11/2025, 1989 words

The flow speed of Greenland’s outlet glaciers exhibits remarkable variability across timescales from hours to decades, reflecting the complex interplay between surface melt, basal hydrology, and terminus dynamics (Davison et al., 2019). Observations from GPS, satellite feature tracking, and remote sensing reveal that velocity changes can occur within hours of melt or drainage events, evolve seasonally as the subglacial drainage system reorganises, and fluctuate over years in response to long-term hydrological and oceanic forcing. Understanding these variations is critical because they regulate ice discharge to the ocean and thus Greenland’s contribution to sea level rise (Flowers et al., 2018). This essay evaluates evidence supporting the statement that glaciers speed up and slow down on a variety of temporal scales by examining mechanisms across three regimes: (1) short-term variability driven by transient basal pressurisation, (2) seasonal to annual evolution governed by drainage-system efficiency and self-regulation, and (3) marine-terminating glacier behaviour shaped by oceanic and terminus processes. In doing so, it considers both the empirical evidence and the theoretical frameworks that explain why glacier velocity responds nonlinearly to melt and boundary forcing.

At sub-daily to event timescales, fluctuations in ice velocity primarily reflect transient perturbations in basal water pressure. These can be induced by diurnal melt cycles, the formation or reactivation of moulins, or rapid lake-drainage events. Each mechanism operates through the same physical principle: a temporary increase in basal water pressure reduces effective pressure (N = Pi - Pw), thereby lowering basal traction and allowing faster ice motion. The first and most common control is the diurnal cycle in surface melt input. Meltwater routed through pre-existing moulins modulates subglacial pressure and ice motion on an hourly scale. Near North Lake, Shepherd et al. (2009) observed that ice velocity approximately doubled within ~2 h of peak melt input, followed by a return to near-winter levels within ~12 h. The mean daily speed-up was about 55 %, reaching > 100 % at the lowest site, and was accompanied by 1–4 cm of surface uplift, which provides a clear evidence of pressure-driven basal separation. A second process involves the abrupt establishment of new hydraulic connections between the surface and bed. The formation or reactivation of moulins can inject large volumes of high-pressure meltwater into an otherwise inefficient drainage system, generating localised accelerations independent of meteorological forcing. For instance, Joughin et al. (2008) documented a sharp speed-up near South Lake on day 217, coinciding with the opening of a moulin rather than any temperature anomaly, confirming that the response was hydraulically rather than thermally driven.

A third, often more dramatic mechanism is surface-lake drainage, which can trigger rapid basal pressurisation through multiple pathways. When the water level within a supraglacial lake exceeds the tensile strength of surrounding ice, a fracture can propagate to the bed, draining the lake catastrophically within minutes to hours. At North Lake, Das et al. (2008) and Joughin et al. (2008) recorded the drainage of ~4–5 × 10⁷ m³ of water in < 2 h, causing several centimetres of uplift and a doubling of ice velocity. Other lakes drain more gradually by overtopping into pre-existing moulins or shallow englacial channels, producing longer-lasting but smaller accelerations. Tedesco et al. (2013) compared such contrasting cases: the rapid hydrofracture drainage of Lake Ponting produced an ice-velocity increase of ~200 %, whereas the slow-spilling Lake Half-Moon generated a response less than half that magnitude. This contrast demonstrates that drainage rate, and hence the rate of basal pressurisation, strongly governs the intensity of dynamic response. Importantly, these events may also interact across large spatial scales. The tensile-stress pulse generated by one drainage event can propagate through the ice sheet, initiating hydrofracture and subsequent drainage at lakes up to ~80 km away (Christoffersen et al., 2018). Maier et al. (2023) further demonstrated a wintertime cascade of lake drainages at high elevation, producing widespread acceleration over ~5,200 km² despite minimal surface melt. This indicates that short-term velocity variability can arise from internal hydraulic reorganisation of the subglacial system, not solely from contemporaneous surface meltwater supply.

At seasonal to annual timescales, velocity variations reflect the evolution of the subglacial drainage system in response to sustained surface melt forcing. As meltwater inputs rise in early summer, water pressure increases within a distributed drainage network, reducing effective pressure (N = Pi - Pw) and enhancing basal sliding. With continued input, however, the drainage system reorganises into efficient, channelised conduits that restore high effective pressure and re-establish basal traction. This progressive adjustment gives rise to the characteristic pattern of early-season speed-up followed by late-season slowdown, and underpins the self-regulating behaviour of Greenland’s land-terminating glaciers. The onset of distributed drainage and short-lived speed-up is well captured by GPS observations. Zwally et al. (2002) showed that near-surface melt onset at Swiss Camp triggered velocity increases of ~35–40 cm d⁻¹ (≈50 % above winter) accompanied by measurable uplift, confirming that accelerations arise from hydraulic pressurisation rather than direct thermal softening. Yet the subsequent decline in velocity despite continued melt during, observed in the same region by Bartholomew et al. (2010), demonstrates that the enhancement is transient. Their multi-site GPS array documented an inland migration of speed-up during early summer, followed by a systematic slowdown in late summer, which is consistent with the evolution from a distributed to a channelised drainage network that restores higher effective pressures. The relationship between melt intensity, drainage efficiency and dynamical impact is further quantified by Sundal et al. (2011), who identified a runoff threshold of ~1.4 cm d⁻¹, above which additional meltwater promotes faster channel development and hence reduces, rather than enhances, mean summer velocity. In high-melt years, the period of enhanced flow was roughly 3 times shorter and late-summer speeds were 62±16 % lower than in cooler years. Collectively, these findings demonstrate a threshold-dependent response of ice motion to meltwater forcing, contradicting the early assumption that more melt inevitably means faster flow.

Beyond intra-seasonal dynamics, recent studies emphasise carry-over effects across years, linking summer hydrology to winter and multi-year flow. Sole et al. (2013) combined GPS records over multiple years to show that while summer velocities scale positively with melt intensity, whereas winter velocities scale negatively, as the persistence of efficient channels into winter maintains low basal pressures. Consequently, despite ~70 % interannual variability in melt, annual velocities changed only by less than 7.5 %, providing direct evidence for hydrological self-regulation. The same mechanism extends to decadal timescales. Using Landsat feature-tracking, Tedstone et al. (2015) reported a 12 % decadal slowdown (1985–2014) in a sector where surface melt increased by ~50 %. They attributed the trend to progressive expansion of efficient drainage pathways that lower mean basal water pressure. In other words, repeated warm summers appear to strengthen the hydraulic coupling between surface and bed, stabilising long-term flow. Yet this behaviour is not unidirectional: Williams et al. (2020) observed renewed acceleration after 2013 when melt decreased by ~15 %. The increase, concentrated near the margin and within thinner ice, was consistent with re-pressurisation of the bed as channels contracted.

While temporal variability in ice motion reflects the seasonal evolution of subglacial hydrology, its spatial expression depends on local catchment geometry, ice thickness, and surface hydrologic connectivity. A central question is whether the hydrological feedbacks identified at low-elevation land-terminating sites operate uniformly across the ice sheet, or whether spatial differences in melt supply and hydraulic potential produce contrasting responses. Palmer et al. (2011) studies feature-tracked satellite velocities across a 50 × 100 km sector of southwest Greenland and showed that the magnitude of late-summer speed-up is strongly modulated by surface catchment size: glaciers fed by large supraglacial drainage basins exhibited velocity increases up to ~48 % above winter rates, whereas smaller catchments displayed far weaker or no responses. This is because larger catchments generate higher meltwater volumes and support well-connected supraglacial stream systems that funnel water efficiently into moulins and the bed. The study by Tedstone et al (2014) combining InSAR, MODIS, and GPS data also confirmed such melt-season velocity contrasts between individual catchments, but they also they found that the proportional contribution of summer motion to total annual motion remained remarkably uniform (≈ 44–50 %) across the ablation zone. Elevation further modulates this behaviour. Doyle et al. (2014) extended GPS measurements to the upper ablation zone (~ 1840 m elevation, ~ 140 km inland) to test whether channelised drainage can form beneath thicker ice. They recorded summer velocities only ~ 8 % above winter, and a modest multi-year acceleration (~ 2.2 % yr⁻¹ between 2009–2012) during warm summers. The absence of strong late-summer slowdown, coupled with limited melt input, suggests that high-elevation drainage systems remain inefficient and predominantly distributed. Together, these studies reveal a vertical gradient in hydrological control, with self-regulation strongest near the margin and progressively weaker toward the interior.

In contrast to land-terminating glaciers, where velocity fluctuations are primarily governed by subglacial hydrology, tidewater glaciers are influenced by both meltwater inputs and marine boundary conditions that control terminus stress balance.

Meltwater can still act as an important trigger. For instance, at Ryder Glacier in North Greenland, Joughin et al. (1996) reported a 300 % acceleration over a seven-week period following the drainage of a supraglacial lake, implying rapid basal pressurisation even beneath thick, grounded ice. Yet at shorter timescales, ice flow variability is more commonly dictated by tidal forcing and calving events. At Jakobshavn Isbræ, Podrasky et al. (2014) showed that calving accounted for 94–99 % of observed velocity fluctuations, with tidal motion explaining 10–90 % of the remaining signal. Here, abrupt speed-ups associated with calving reflect the loss of terminal back-stress: the detachment of a large ice block reduces resistive stress at the grounding line, causing transient accelerations up to 50 × 10³ m d⁻¹ that decay within minutes to hours (Amundson et al., 2010). The tidal response, by contrast, is cyclic. At Helheim Glacier, ice motion lags tidal height by ~8 h, with peak velocities occurring near low tide when seawater pressure at the terminus is lowest, effective pressure highest, and basal traction reduced (Voytenko et al., 2015). This tidal signal decays upstream over a characteristic length scale of ~0.5 km, consistent with the localised propagation of stress perturbations near the grounding line.

Superimposed on these short-term oscillations are pronounced seasonal variations linked to mélange and sea-ice dynamics. During winter, a rigid mélange and reduced calving suppress ice velocity, while summer retreat of sea ice and mélange margins enables renewed calving and flow acceleration. At ~4 km upstream of Jakobshavn Isbræ, Guo et al. (2019) recorded seasonal velocity fluctuations of ±8 km a⁻¹ by 2012, with pronounced summer maxima. Over longer periods, thinning and retreat of the terminus amplify these effects: Howat et al. (2005) documented a ~40 % increase in Helheim Glacier’s peak velocity (from ~8 to ~11 km a⁻¹ between 2000–2005), coincident with ~7.5 km of frontal retreat and > 40 m thinning (2001–2003). Similarly, Jakobshavn Isbræ accelerated from 5.7 km a⁻¹ in 1992 to 12.6 km a⁻¹ 2003 (an increase of ~120 %) before later slowing during 2014–2016 as ocean waters cooled. To sum up, across Greenland, ice-flow variability reflects a continuum of interacting processes rather than discrete timescale-dependent mechanisms. On sub-daily to event scales, transient accelerations result from sharp increases in basal water pressure due to diurnal melt, moulin formation, or lake drainage. On seasonal and interannual scales, the evolution from distributed to channelised drainage systems governs an initially positive, then negative, relationship between melt and motion, leading to hydrological self-regulation over longer periods. At the marine margin, these hydrological controls intersect with oceanic forcing and terminus dynamics, such as tides, mélange rigidity, and calving cycles, producing retreat and re-advance from daily to annual scales. Collectively, these observations support the view that Greenland’s glaciers can both speed up and slow down across a wide range of timescales, yet they also reveal that such behaviour arises from the interdependence of hydrological, mechanical, and oceanic feedbacks.

 

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