Measuring glacier velocity

By Alex Hyde

Why is understanding glacier velocity important?

In the Himalayas, and world-wide, climate change is driving glacier retreat. In some mountain regions like the Himalayas, this climate change is also resulting in large scale changes in glacier ice-flow dynamics and velocity (1-2), which is impacting glacier response to climate change. Measuring these changes in glacier velocity is essential for understanding the impact of climate change on ice dynamics (Figure 1).

In this article, we focus on how glacier velocity is measured, with a specific focus on glaciers in the Himalayas.

Figure 1: Increasing surface melt during the spring can have a big impact on ice velocities in the Himalayas.  (Source: ICIMOD)

Trends in glacier velocity in the Himalaya

Measurements of long-term trends of glacier velocity in the Himalayas have indicated a trend in ice flow slowing down1. These changes have been attributed to climate change, and are primarily associated with glacial thinning, as thinner ice slides at a slower rate as it exerts less pressure on its bed(3).

We are also seeing greater seasonal variability in ice velocity. This can be linked to more surface melt water reaching the glacier bed due to higher melt rates in the spring and summer seasons. This decreases friction and causes more glacier sliding at the ice-bed interface, driving increased acceleration in ice velocity in the spring and summer, followed by decreases in the late summer, at the end of the ablation season(4-5).

Ice velocity and glacier mass balance

Changes in glacier velocity can influence the rate of glacier retreat. Accelerating glaciers, without a corresponding increase in snowfall to sustain them in their snowy accumulation areas, will transfer more mass to the glacier terminus, increasing melt and accelerating recession.

Alternatively, slower-flowing glaciers may downwaste in situ as more debris accrues on the ice surface. Changes in glacier velocity therefore impact the way in which glaciers respond to climate change, and influence rates of recession and thinning.

Local and regional trends on glacier velocity

The response of glaciers in the Himalayas to climate change has not been uniform across the range. Weather patterns can vary from valley to valley, glaciers on different slopes may differ remarkably in character and form (for example, see Figure 2) (6), and, in occasional cases, the formation of a meltwater lake at the snout of glacier can drastically affect a glaciers dynamic regime.

Figure 2: The ‘morphology’ of glaciers in the Himalayas can vary significantly over a short distance due to the huge local variability in climate and topographic setting. Glaciers in the range can therefore have very different flow regimes. (Source: Wikimedia Creative Commons)

Impact of proglacial lakes on glacier velocity and recession

By increasing the local water table, proglacial lake formation increases hydrostatic and subglacial water pressure at the terminus of the glacier. This reduces bed friction at the lower parts of the glacier, locally increasing glacier sliding (23, 24). Glaciers then lose mass into the lake through calving and basal melt.

As the lake level increases, ice at the glacier terminus can become buoyant, resulting in flotation, and a massive reduction in friction at the bed leading to acceleration and thinning (25). The formation of a lake at the glacier terminus can therefore act to accelerate glacier flow and enhance recession and thinning, accelerating rates of glacier retreat. An example of such rapid retreat can be seen at Thorthormi Glacier in Bhutan between 2014 – 2022(21).

Because of the varied ways in which climate can imfluence the flow and health of a glacier, as well as regional and local factors such as topography and terminal environment, it is important to measure glacier velocity as well as terminus recession or thickening/thinning of the glacier. This helps us to understand how glaciers are responding to climate change, and how this varies locally and regionally.

How do we measure glacier ice velocity and what are the challenges?

Measuring surface ice velocity should ideally account for the sizable variability in velocity along the length and breadth of the glacier (Figure 3).

On mountain glaciers, ice velocity is usually measured in meters per year (m yr-1), and the rate of flow can vary from anywhere from <10 m yr-1 to over 500 m yr-1 in exceptional circumstances(10)­­.

Glacier velocity map for a lake-terminating glacier in Bhutan, showing increase in ice velocity as the glacier ice meets the lake at the bottom.
Figure 3: A map of velocity for a large lake terminating glacier in Bhutan (Thorthormi), estimated using feature tracking. Ice flow direction can also be mapped using this technique. Note the big increase in ice velocity as the glacier ice meets the lake at the bottom.

Ice-flow measurements can be made through both field studies or through remote sensing, using satellites or drones/UAVs.

Measuring ice flow velocity in the field

In the field, researchers can install a network of stakes in the ice and measure the movement of these stakes over time (Figure 4). Traditionally this method used rope to measure the relative displacement of stakes; however, contemporary studies will use high accuracy Differential GPS systems, allowing for more precise velocity measurements(12).

While reliable, this approach only yields data for a limited number of points on the glacier surface, and is typically constrained to accessible parts of the glacier so lack the spatial coverage necessary to capture glacier-wide variability, while also being time limited by the duration of the fieldwork campaign/equipment lifetime.

Instaling stakes for the measurement of glacier velocity in the field.
Figure 4: Stakes installed in the glacier surface can be used to track ice motion over time using a GPS.

Remote sensing of ice flow velocity

High resolution satellite imagery allows researchers to measure the movement of Himalayan glaciers by comparing the changes in surface features over time.

Techniques such as ‘feature tracking’ compare the locations of features visible in the satellite imagery, such as crevasses and surface debris, between different images of the same glacier (Figure 5). By tracking the movement of these features we can calculate the relative displacement at a specific point over time(13-15). Glacier-wide values for displacement are converted into velocity to map the pattern of ice motion across the glacier surface.

Figure 5: Points indicating the change of crevasse position over a couple of months – a good example of a trackable feature.

Feature tracking, however, requires trackable features to be present; cloud cover, snow cover and shadowing can reduce the number of features available; likewise, snowline retreat and change in shadow position can be incorrectly detected as ‘ice motion’ complicating measurements. These variables can make velocity detection challenging in the Himalayas, particularly during the monsoon season.

Newer satellite platforms such as PlanetScope(16) that employ 1000s of small ‘cube-sats’ can provide daily to sub-daily imagery (Figure 6). These platforms offer a greater window of opportunity in being able to pick out the days with clear weather, while improvements in image resolution over conventional platforms such as Landsat and Sentinal allow for more trackable features to be detected (Table 1)(16-20).

Table 1: Comparison of satellite platforms for image resolution, sampling frequency, bands and service duration.
Planetscope images can be used to track glacier velocity
Figure 6: PlanetScope’s Dove satellites can capture short-term changes at a very high spatial resolution (Planet 2023).

Improvements in feature tracking software, image filtering and increasing image availability have opened up the potential for accurate high-frequency, glacier-wide mapping of ice velocity across the Himalayas, and in the last few years studies employing these techniques have revealed important new findings into how climate change is altering ice dynamics in the region(6, 21-22).

Precision in these estimates is vital in understanding the impact that climate change may have on these glaciers, and what this may mean for future water supply and downstream communities(7-9).

Glacier velocity in the Himalaya

The varying morphology of glaciers in the Himalayas results in a range of observable flow characteristics along the profile of the glacier.

As ice velocity is primarily controlled by ice gradient (surface slope), we typically see higher velocities on steeper slopes (Figure 7), and slower flowing-ice in lower slopes.

Glacier velocity maps for Tshojo Glacier in Bhutan.
Figure 7: A comparison of glacier surface gradient (left), and ice velocity (right) for Tshojo Glacier in Bhutan. For surface gradient and ice velocity, darker red corresponds to steeper slopes and higher ice speeds, whereas light green corresponds to shallower slopes and blue to slower ice speeds.

This variability in ice flow corresponds to distinct differences in the surface characteristics of a glacier. On steeper, fast-flowing ice we can expect to see more crevassing formed by the flexure of ice over steeper ground as well as stretching effect of accelerating ice forming surface fractures.

On slower-flowing, low-gradient ice, we may see increased surface debris cover and surface pond formation, as the relative lack of crevasing allows for water and debris to accumulate on the surface. Ponding water and surface debris cover can driver increased melt as they darken the ice surface, reducing the albedo (reflectivity) of the ice, increasing absorption of radiation (26-27)(Figure 8).

Ice velocity is therefore an intrinsic component in understanding the morphology of individual glaciers in the Himalayas, and how a changing climate may alter the distinct character of glaciers in this region.

Figure 8: The steep Khumbu icefall below Everest is fast flowing (>400 m yr-1), and highly crevased. This compares the flatter lower regions of Tshojo Glacier in Bhutan where surface lakes and thick debris cover dominate (<20m yr-1)(Image source: Wikicoms).

Further reading

About the Author

Alex Hyde is PhD student at Newcastle University. His project focuses on understanding how large lake terminating glaciers in Bhutan are responding to climate change, with a particular interest in why some lake terminating glaciers undergo catastrophic collapse, while others retreat steadily.

He undertook a BSc in Geography at the University of Sheffield, before taking a year out to work for the UN Development Program in Kyrgyzstan; here the importance of glaciers as a water resource motivated him to return to Sheffield to complete an MSc in Polar and Alpine change, before then moving onto Newcastle University.


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