Glaciers worldwide are highly sensitive to climate change, though they respond on a range of timescales (typically a few decades to a century or more for temperate alpine glaciers) [1
]. As a result, they are excellent, albeit complex, indicators of climate change [3
]. One of the biggest controversies of the last decade over the false claim of impending disappearance of Himalayan glaciers by the year 2035 [6
] based on errant information [7
] resulted in an intense and continuing research focus that has greatly improved our knowledge of Himalayan glaciers [10
]. In the past decade, the glaciological community has learned, for example, that glacier mass loss rates in High Mountain Asia (HMA) are comparable with the global average mass loss rates [15
], and that mass loss, thinning, length, and area retreat rates of glaciers in the region are heterogeneous [11
]. Also, documented better than ever before, is the spatiotemporal frequency and magnitude of changes in Himalayan glacial lakes. Climate model projections driven by high emissions scenarios suggest that HMA will lose as much as 65% of its ice mass this century [1
], which could result in the formation of thousands of new glacial lakes.
Recent studies [18
] have shown that enhanced meltwater production and glacier retreat coupled with increased area to hold meltwater in newly exposed glacial over-deepenings leads to moraine-dammed proglacial lakes. In the Nepal Himalaya, the majority of present-day large moraine-dammed lakes did not exist before the 1950s [22
]. Many of these lakes started forming in the mid-1950s to 1960s as small supraglacial lakes, which coalesced and started growing rapidly in the 1970s [23
]. According to one estimate, there are over 1466 glacial lakes in the Nepal Himalaya alone [26
] and it has been suggested that most of these formed in response to warming temperatures during the second half of the 20th century [26
]. However, one recent study assigns a longer sequence of response times, such that lakes growing between the 1950s and 2010s may be long-delayed responses to climatic shifts that occurred over the previous one to two centuries, including but not necessarily dominated by the anthropogenic warming of recent decades [28
Out of the known 1466 glacial lakes, 21 have been identified as posing exceptional risk of glacial lake outburst floods (GLOFs), including Imja, Lower Barun, and Thulagi [26
]. A more recent study, however, classified both risks and hazards for these lakes and determined Lower Barun as a high hazard and high risk, Thulagi (locally known as Dona Lake) as a moderate hazard but high risk, and Imja (locally known as Imja Tsho) as both a moderate hazard and risk [29
] partly due to a recent lake-lowering project [29
]. However, the study pointed out that the continued expansion of Imja Lake could increase its hazard profile due to the high susceptibility of avalanches entering the lake. The approach to classifying GLOF risk (exceptional, very high, high, or moderate) is different in these studies. Overall, the variability in risks and the rate of lake growth, both in Nepal and the broader HMA region, depends on a number of factors including glacier microclimate, topography, glacier dynamics, glacier debris cover, unstable mass around a lake, proximity to downstream infrastructure, potential flood volume, and peak discharge [32
]. Specifically, the intrinsic hazards, risk exposure, and vulnerabilities from glacial lakes are great in the high relief areas around the Mount Everest and Makalu massifs in Nepal, which contain numerous large lakes and observe frequent slope failure and flash floods associated with monsoon rain.
Glacial lakes with moraine dams are metastable. Those with ice cores are subject to melting, gravitational collapse, or buoyant flotation; even if the dams hold up, mass movements—debris flows, rock slope failures, and avalanches—into the lakes can produce massive overtopping waves [33
]. As a result, there is a potential for many of these lakes to produce sudden GLOFs, which can devastate downstream areas causing possible loss of life and damage to local infrastructure. Although not all glacial lakes are hazardous, some will inevitably produce damaging GLOFs given the glacial lake system dynamics, topographic setting, and downstream populations and infrastructure. While glacial lakes have been controlled by engineering to reduce their GLOF potential, most of these are in Peru; in HMA, similar efforts are limited [31
GLOF events from glacial lakes are a worldwide phenomenon [34
]. Although there have been suggestions that the GLOF frequency in the Himalayan region has increased in recent decades [5
], a statistical assessment of moraine-breach GLOFs shows that the number of GLOFs have declined there and around the world following an increase in the mid-20th century [28
]. The GLOF rate trends are attributed to glaciological and limnological response times following climatic fluctuations, including the end of the Little Ice Age and more recent warming. Reference [51
] examined all types of GLOFs, including ice-dammed and moraine-dammed lake outbursts, and identified a similar early 20th-century rise of global GLOFs followed by a stabilization over the past three decades. To date, no systematic account for the long response times between climatic perturbations and lake formation and drainage has been established. This presumably includes timescales needed for the critical thinning, retreat, and change of the glacier surface relief to allow for the pooling of water and lake growth, as well as stochastic periods between triggering events such as landslides or extreme weather [28
]. While GLOFs occur relatively infrequently, because many glaciers are undergoing the type of thinning and flow stagnation that creates metastable lakes, it is likely that the risk of GLOFs may increase in the future. Nonetheless, they pose a severe flood risk in the high mountains [53
], even though the full impact of anthropogenic warming has not yet been reflected in GLOF activity. To understand these risks, we selected three major glacial lakes in Nepal (Imja, Lower Barun, and Thulagi) where we have detailed field and historical records to develop a broad regional understanding of glacial lake evolution. Specifically, we investigated a satellite time-series of the lake area, ice velocity, lake volume growth, moraine and ice elevation changes, and the evaluated hazards resulting from these changes.
6. Concluding Remarks
Detailed field surveys and Landsat satellite image and DEM analyses were used to document and evaluate the evolution of Imja, Lower Barun, and Thulagi moraine-dammed proglacial lakes in the Nepal Himalaya. All three lakes have expanded from initial clusters of small supraglacial lakes in the 1950s/1960s to large hazardous, ice-cored, moraine-dammed proglacial lakes with water volumes of 78.4 × 106 m3, 112.3 × 106 m3, and 36.1 × 106 m3, respectively. The growth rates of the three lakes follow different trajectories, an observation attributed to differing ice/water contact areas and other characteristics (e.g., geometry) of the glaciers and lakes and the surrounding topography. These lakes currently drain through relatively stable, boulder-armored channels, but the condition of these channels is different at each location. Relatively high surface irradiance over the lower portion of each glacier will ensure that meltwater production will remain high and promote future lake enlargement. Furthermore, numerous studies have concluded that these glaciers are responding to climate change—both ongoing anthropogenic change and natural climate change stemming from the end of the Little Ice Age. This will lead to further glacier retreat, downwasting, lake formation, and expansion. Additionally, coupled system responses—including potential for mass movements into each of the lakes—will continue and contribute to increasing high hazard potential in the future.
Lower Barun Lake is one of the largest, deepest, and most rapidly growing lakes in Nepal, and appears to have a relatively unstable outlet channel and numerous hazard triggers around the lake. The growing size of the lake (~0.5 km2 in the last decade) will likely increase the load on the moraine, reducing its ability to contain the impoundment, hence increasing the probability of rapid lake drainage. Whether a lake drainage produces a major GLOF would depend on the type of breach. We recommend continuous monitoring and possible lake-lowering intervention after further detailed surveys. Imja Lake has recently undergone engineered channelized drainage, further stabilizing its outlet, while a relatively stable, vegetated end moraine surrounds the Thulagi Lake outlet. However, Thulagi’s outlet is extremely precarious, as a small lake level rise of a couple meters could suddenly erode and enlarge the outlet.