Ice Loss, Gravity, and Asian Glacier Slowdown

A recent international study found that glaciers in high mountain Asia (HMA) are actually slowing down. Researcher Amaury Dehecq of NASA’s Jet Propulsion Laboratory and his co-authors analyzed 17 years of data from 2000 to 2017, attributing their results to widespread glacial thinning. They found that 94 percent of variability in glacial flow rates could be explained by ice thickness changes.

The study asserts that glaciers are thinning worldwide and at an increasing rate from the start of the 21st century. However, according to Dehecq, exactly how glaciers respond to mass loss on a regional scale was previously not well understood. This uncertainty highlighted the necessity of understanding the consequences of glacier thinning in a warming world and catalyzed this innovative study.

Nyenchen Tanglha mountains Tibet on GlacierHub
The Nyainqêntanglha mountains of Tibet, where some of the most significant glacier slowdown is occurring as a result of ice loss (Source: randomix/Flickr).

Trending: Velocity Over Time

Dehecq and his co-researchers measured glacier surface velocity changes with 1 million satellite image pairs from Landsat-7, obtaining annual velocities by comparing images taken one year apart over the same area. This was done through feature tracking; the researchers identified specific, recognizable features (i.e. crevasses, dirt patches), then measured how far they moved from one picture to the next. They did this over and over again, millions of times, to translate the image pairs into usable velocity data.

In all, the researchers calculated velocity changes over time for 11 subregions in high mountain Asia. The most significant glacier slowdowns were seen in the Nyainqêntanglha mountains of Tibet and in the Himalayas in India (Spiti Lahaul), with 37.2 and 34.3 percent velocity decreases per decade, respectively.

Lesser but still significant slowdowns were observed for glaciers in the following regions: West Nepal, East Nepal, Bhutan, Hindu Kush, Pamir, Tien Shan, and the inner Tibetan Plateau.

velocity map for high mountain asia glaciers on GlacierHub
Velocity changes from 2000-2016 for glaciers in high mountain Asia (Source: NASA Earth Observatory).

“It is only recently that big data crunching has allowed this hypothesis to be tested on such a grand scale,” said William Colgan, a research climatologist at the Geological Survey of Denmark and Greenland, in an interview with GlacierHub.

Noel Gourmelen, a co-author of the study and professor of glaciology at the University of Edinburgh, Scotland, emphasized the importance of data availability in allowing studies like this one to be successful in the future. “This research was possible because of sustained and open Earth Observation programs,” he said, also calling for continued support, maintenance, and expansion of programs like NASA’s Landsat and ESA’s Sentinels satellite series.

On Thin Ice

Dehecq and his co-researchers matched calculated velocity trends with observations of glacier thickness from 2000 to 2017. Data on glacier thickness is obtained by using remote sensing to create a model of glacier elevation change over time. This comparison showed a strong relationship between the two; each region that observed a decreasing velocity trend also observed a corresponding trend in ice thinning over the same time period.

Colgan also spoke to GlacierHub about the trending relationships this study revealed. “This study is some of the clearest evidence to date of the link between climate forcing and ice dynamics in land-terminating glaciers,” he said. “Based on these Himalayan observations, the study is telling us to expect widespread slowdowns in ice flow in regions where glaciers are experiencing widespread thinning; that’s most regions of land-terminating glaciers.”  

Karakoram region China on GlacierHub
Muztagh Ata and Lake Karakul in the Kunlun region, one of the few places where glaciers are advancing (Source: dreamX/Flickr).

Despite the strong relationship between thinning ice and decreasing velocity, each subregion had a slightly different magnitude of change. The researchers suggested that “regional differences in climate and glacier sensitivity to temperature,” could influence small spatial variations in the overall trend.

Accordingly, this study also found that regions with advancing glaciers are speeding up. Two adjacent regions in high mountain Asia, Karakoram and West Kunlun, experienced a positive mass balance along with slight velocity increases from 2000 to 2017.

The Glacial Pace

Mountain glaciers have a simple motivation for their downhill progression—gravity. Gravity causes the surface ice on a glacier to creep, slowly deforming and thinning the glacier as it moves down the mountain.

How fast glaciers travel on this journey is controlled by two factors: gravitational driving stress and glacier thickness. Driving stress is dependent on slope. The steeper the mountain, the stronger the gravitational force. Since surface ice moves faster than the ice underneath, ice thins as a glacier travels, meaning a glacier will get progressively slower the further it goes. That is, until it reaches an elevation where the surrounding climate is warm enough to rapidly melt the ice.

Dehecq and his co-researchers concluded that these two factors alone can be used to effectively calculate glacier surface velocity. “The strength of the link between mass loss and change in flow was surprisingly strong,” said Gourmelen. “One might have expected that changes at the base of the glacier would have played a role and impacts basal sliding of the ice, but this does not appear to be the case when looking at the HMA region as a whole.”

A Warning for Warming

Mount Kailash on GlacierHub
The sacred Mt. Kailash in Tibet and surrounding mountains are home to several glaciers that feed major rivers, lakes, and communities. Read more about Mt. Kailash on GlacierHub (Source: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team).

A warming world means more glacial surface melting, and at higher elevations. However, surface melting also means ice thinning, which slows down the flow of glaciers due to gravity. So although less ice exists overall, there is also less ice reaching the elevation where it will melt.

The findings of this study improve knowledge of glacier feedbacks in the context of anthropogenic climate change regarding sea-level rise and the hydrology of certain regions. Glacial slowdown in high mountain Asia could potentially impact the availability of freshwater for communities in surrounding countries like Kazakhstan, Pakistan, India, Nepal, Bhutan, Tibet, and China.

Gourmelen gave GlacierHub an apt overview of the importance of understanding climate-glacier feedbacks:

“This is yet another sign of the impact of climate change on glaciers, the machine is slowing down. Glacier flow is a fundamental component of the glacier machine, it is the conveyor belt bringing ice from high elevation where it forms to lower elevation where it melts. This process impacts glacier met rates and glacier extent and is a key component of glacier modeling and hydrology. By providing a relationship between mass loss and flow change, parameterising model predicting future of glaciers and water availability will be made easier and more precise. It will also help interpreting some of the changes in glacier shape that we have observed in the last decades.”

Mauri Pelto, professor of environmental science at Nichols College and director of the North Cascades Glacier Climate Project, spoke to GlacierHub about the global implications of this study. “The key takeaway is the same we see for alpine glaciers around the globe, warming temperatures lead to mass balance losses, which is the key driver in glacier response,” he said. “A sustained negative mass balance leads to thinning, which leads to a glacier slow down whether the glacier is in the Himalaya, Alps, or Cascade Range.”

Pelto further explained his considerations for both the short and long-term implications of glacial slowdown in high mountain Asia. “In the short run the slow down will increase retreat rate. In the long run less dynamic mass transfer to lower elevations will lead to a reduction in glacier retreat,” he said.

In all, glacial slowdown could help preserve ice mass in the foreseeable future. However it could be at the cost of abundant freshwater for mountain communities.

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