Glacial lake outburst floods, GLOFs for short, are expected to increase in frequency over time as global temperatures warm. These floods can be very sudden, fast-flowing, and powerful enough to form their own seismic signatures. They carry water, rocks, trees, and debris down valleys, destroying homes and sometimes killing people and livestock.
Many glaciers such as ones in the Hindu-Kush, Karakoram, and Himalayas are shrinking rapidly, forming glacial lakes and causing potentially catastrophic floods for tourists and nearby communities. Understanding the influence of climate change on the frequency and intensity of GLOFs will help disaster risk managers in developing early warning systems and disaster response plans.
Although experts expect these moraine-dammed glacial lakes to grow in size with the addition of glacial meltwater, the risk of GLOFs doesn’t necessarily increase everywhere. In a recent article published in Nature Climate Change, Georg Veh and several of his colleagues from the University of Potsdam and the GFZ German Research Centre for Geosciences examined historical flood occurrences in the the Himalayas that were considered to be hotspot regions for glacier retreat. They aimed to observe GLOF activity for the last few decades, assessing changes in frequency and trend.
Some climate scientists hypothesize that dangerous GLOFs will become more frequent with the growth of moraine-dammed glacial lakes. According to Veh and his colleagues, testing this hypothesis is confounded by incomplete data. Historical reports on GLOF activity are selective, and the researchers speculated that 40 reports on GLOFs in the Hindu-Kush, Karakoram, and Himalayas since 1935 only accounted for large and destructive cases. This suggests that a significant portion of the data might be missing.
To account for reporting bias, the team examined changes in GLOF frequency through a systematic inventory of activity in the Hindu-Kush, Karakoram, and the Himalayas. They were able to identify moraine-dammed lakes and activity in Landsat images from the late 1980s to 2017. Researchers used a random forest model, which was able to generate land-cover maps. These maps provided probabilities for water, cloud, shadow, ice, and land cover across the image tiles. During GLOFs, lakes would abruptly decrease in size, changing from a water to land classification in the Landsat image.
The research team mined over 8,000 Landsat images of the region. In addition to the 17 GLOFs reported since the 1980s, the researchers added 22 newly detected occurrences. They found that despite increasing rates of meltwater entering glacial lakes, particularly in the central and eastern Himalayas, which observed rates of up to six times higher than the northern basin, GLOF abundance remained low.
The average annual rate of 1.3 GLOFs in the region remained unchanged over the last three decades. The fraction of GLOFs per unit of meltwater area, however, has declined since the 1990s.
“We infer that climate-driven rates of glacier melt and lake expansion may be unsuitable predictors of contemporary outburst potential,” stated the researchers.
Their findings were consistent with research on glacial lakes in the Patagonian Andes.
The scientists inferred that their result may indicate a sort of resilience to climate-driven triggers such as glacier calving and ice avalanches, the most frequently reported cause of GLOFs. Unfortunately the team was unable to identify triggers for the 22 newly identified outburst floods, although 16 of them came from pro-glacial lakes within proximity of their parent glaciers. GLOFs generated by calving and avalanche events become less relevant as glaciers retreat from the lakes they have formed.
They also mentioned the importance in perceiving the role of alternate triggers such as earthquakes and landslides in the formation of outburst floods. They give the example of the 2015 Gorkha earthquake in the Nepalese Himalayas. The 7.8 magnitude earthquake did not provoke GLOFs, but it generated landslides which hit glacial lakes.
Veh said the research demonstrated that climate as a sole driver did not change GLOF frequency over the last decade, but that does not mean that frequency will remain unchanged in the future.
“Reliably projecting the future frequency of outburst floods remains an open issue, given that our current knowledge of triggers is quite vague today,” Veh said. The updated inventory of outburst floods will allow for further examination of these cases in more detail.
“Better knowledge of the processes involved in glacial lake outburst floods will ultimately reduce current uncertainties in hazard and risk assessment,” he added.
The researchers believe new generations of optical and radar sensors may be effective in better recognizing GLOF triggers and determining when the next glacier lake outburst flood might occur.
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