Trends, Breaks, and Biases in the Frequency of Reported Glacier Lake Outburst Floods

Thousands of glacier lakes have been forming behind natural dams in high mountains following glacier retreat since the early 20th century. Some of these lakes abruptly released pulses of water and sediment with disastrous downstream consequences. Yet it remains unclear whether the reported rise of these glacier lake outburst floods (GLOFs) has been fueled by a warming atmosphere and enhanced meltwater production, or simply a growing research effort. Here we estimate trends and biases in GLOF reporting based on the largest global catalog of 1,997 dated glacier-related floods in six major mountain ranges from 1901 to 2017. We find that the positive trend in the number of reported GLOFs has decayed distinctly after a break in the 1970s, coinciding with independently detected trend changes in annual air temperatures and in the annual number of field-based glacier surveys (a proxy of scientific reporting). We observe that GLOF reports and glacier surveys decelerated, while temperature rise accelerated in the past five decades. Enhanced warming alone can thus hardly explain the annual number of reported GLOFs, suggesting that temperature-driven glacier lake formation, growth, and failure are weakly coupled, or that outbursts have been overlooked. Indeed, our analysis emphasizes a distinct geographic and temporal bias in GLOF reporting, and we project that between two to four out of five GLOFs on average might have gone unnoticed in the early to mid-20th century. We recommend that such biases should be considered, or better corrected for, when attributing the frequency of reported GLOFs to atmospheric warming.

Automated summary

Thousands of glacier lakes have formed in high mountains since the early 20th century following glacier retreat. Some of these lakes have released large amounts of water and sediment with disastrous consequences downstream. However, it is unclear whether the reported rise in glacier lake outburst floods (GLOFs) is due to a warming atmosphere or simply increased research efforts. This study finds that the number of reported GLOFs has decreased since the 1970s, coinciding with changes in annual air temperatures and number of field-based glacier surveys. The findings suggest a weak coupling between temperature-driven glacier processes and GLOF occurrences, or the possibility of overlooked outbursts.