Over the last three decades, in response to a warming climate, glaciers across the globe have lost mass, retreating to higher elevations and fragmenting^{1, 2}. In many regions, depressions in the bedrock beneath these glaciers carved out through years of glacial movement have been uncovered as the glaciers retreat. In these hollows, melt water can collect, forming what is known as glacial lakes³ (Figure 1). These lakes can trigger a feedback loop, promoting further glacial ice loss and leading to lake expansion. Since 1990, the number, area and volume of glacial lakes globally has grown rapidly⁴. These glacial lakes represent a major hazard, as failure of their natural dams can lead to an unpredictable and sudden release of water and sediment known as a glacial lake outburst flood (GLOF).

GLOFs have a range of triggers, but critically, can result in significant loss of life and transnational impacts; in the last 70 years, several thousand people have been killed by GLOFs in the Cordillera Blanca alone⁵. As glacial lakes have been growing, downstream population, infrastructure, and hydroelectric power (HEP) schemes have also seen an increase, whilst agriculture has intensified^6,7. This is particularly true across developing nations in High Mountain Asia. As such, exposure to GLOFs has increased globally - more people now live-in close proximity to a major glacial hazard. Whilst some communities will have experienced GLOFs before, in many regions where glacial lakes have historically not existed communities and governments may be unaware of the hazard posed, inhibiting their ability to prepare for, and recover from, potential GLOF disasters, increasing overall vulnerability.  

In order to understand where globally is most in danger from GLOF impacts, it is important to analyse hazard, exposure, and vulnerability together (Figure 2). Whilst GLOF hazard is often the focus of scientific studies, until now, exposure and vulnerability have rarely been included in evaluations of glacial lake danger, with many focussing solely on the physical characteristics of the glacial lakes. We argue that all three components (hazard, exposure, and vulnerability) must be included in GLOF studies if an accurate representation of danger is to be determined.

Through analysing all three components, our research shows that globally, 15 million people could be exposed to impacts from potential GLOFs. Populations in High Mountains Asia (HMA) are the most exposed, and on average, live closest to glacial lakes with ~1 million people living within 10 km of a glacial lake. More than half of the globally exposed population are found in just four countries: India, Pakistan, Peru, and China. We identify the Andes as a region of particular concern. Here, some glacial basins have the highest potential for GLOF impacts globally however the region remains relatively unstudied, with few published research studies compared to the likes of HMA. We urgently need more detailed studies across the Andes in order to manage GLOF danger and prevent future disasters. Importantly, we find the higher danger basins are not necessarily home to the largest, most numerous, or most rapidly expanding glacial lakes. Instead, we show that it is the number of people and their proximity to a glacial lake, and their capacity to cope with disaster, that are key factors for determining the level of potential GLOF impact. Our findings therefore enforce the need to include exposure and vulnerability in future studies rather than focussing on hazard alone.

By identifying regions with high threat of GLOF, our findings could allow for more targeted GLOF risk management. This is particularly valuable for regions with economic constraints, as well as for regions where the threat of GLOF has not yet been fully considered or understood. How GLOF danger might change in the future remains subject to debate. As glaciers continue to recede with climate change, existing glacial lakes will expand and many new lakes will form, altering the spatial pattern of GLOF lake conditions. Locations where GLOF have been a major hazard, such as in Bhutan (Figure 3) may shift towards areas with comparatively little experience of GLOFs. Further research is required to evaluate temporal changes in lake conditions, exposure, and vulnerability to determine the relative roles of each for GLOF risk. Our work provides a base from which more detailed work could be undertaken to broaden our understanding of GLOFs. 

References

1. Zemp, M. et al. Global glacier mass changes and their contributions to sea-level rise from 1961 to 2016. Nature 568, 382–386 (2019).
2. Hugonnet, R. et al. Accelerated global glacier mass loss in the early twenty-first century. 2021 5927856 592, 726–731 (2021).
3. Carrivick, J. L. & Tweed, F. S. Proglacial Lakes: Character, behaviour and geological importance. Sci. Rev. 78, 34–52 (2013).
4. Shugar, D. H. et al. Rapid worldwide growth of glacial lakes since 1990. Clim. Chang. 10, 939–945 (2020).
5. Emmer, A. et al. 70 years of lake evolution and glacial lake outburst floods in the Cordillera Blanca (Peru) and implications for the future. Geomorphology 365, (2020).
6. Schwanghart, W., Worni, R., Huggel, C., Stoffel, M. & Korup, O. Uncertainty in the Himalayan energy–water nexus: estimating regional exposure to glacial lake outburst floods. Res. Lett. 11, 074005 (2016).
7. Immerzeel, W. W. et al. Importance and vulnerability of the world’s water towers. Nature 577, 364–369 (2020).