Where the Yala Glacier’s Ice is Going

Sublimation, the process by which a solid changes phase to gas, is a largely unquantified component of glacier mass loss worldwide. A study on Nepal’s Yala glacier, recently published in Frontiers In Earth Science, quantified the glacier’s loss of ice to the atmosphere during the 2016-2017 winter. Researchers found approximately 21 percent of Yala’s annual snowfall was returned to the atmosphere via sublimation, a rate higher than most glaciers on Earth’s tallest mountain ranges.

Like classroom demonstrations with dry ice, sublimation can occur from a static surface. Snow sublimation is the loss of water from the snowpack directly to the atmosphere. Though Yala is one of the world’s most studied glaciers, a complete understanding of water balance and glacier mass has been limited. In addition, complex terrain and dynamic conditions often inhibit models from accurately estimating sublimation.

Yala (5,350 meters) is one of the world’s most studied glaciers (Source: Mark Horrell/Flickr).


The process to measure the rate of sublimation is complicated: sublimation varies based on the time of year, hour of the day, cloud cover, complex terrain features, altitude, and specific atmospheric conditions like humidity and wind speed. Even in a static environment, these components are difficult to measure. Add dynamic environmental factors like drifting and blowing snow, ice that melts and refreezes (skewing energy balance calculations), and remote fixed instruments that rise and fall with the glacier itself, and you get a vague idea of the quantification problem faced by scientists.

Researchers utilize two primary methods to measure sublimation: the gravimetric method, which continuously monitors the weight at a specific part of the snowpack, and the eddy covariance method, a process of direct observation to measure and calculate atmospheric factors. The gravimetric method can incorrectly interpret wind-induced erosion of the snowpack as sublimation. The researchers, which were comprised of a team from Utrecht University and the International Centre for Integrated Mountain Development, were able to measure turbulent fluxes at Yala’s surface using the latter technique. Turbulent fluxes act on frozen water molecules the same way wind might affect leaves scattered on a surface; some are lifted and become airborne, while others remain grounded, depending on the wind strength, direction, and location. Through extensive and careful post-processing of the water vapor, air temperature, and vertical wind, the research team was able to accurately estimate sublimation.

The eddy covariance system measures sublimation (Source: Walter Immerzeel).

Out of the myriad components affecting sublimation, the team condensed Yala’s sublimation rate into two primary determinants, wind speed and humidity, which vary depending on the time of year and day. Daily sublimation rates were separated into humid days and non-humid days. Less sublimation occurs on humid days, due to colder surface temperatures and a weaker vapor pressure gradient. When humidity is low, winds increase, resulting in a well-mixed atmospheric layer above the surface and a vapor pressure gradient ideal for sublimation. Sublimation varies greatly from location to location on the glacier.

The project required two trips: one to install equipment and a second to retrieve the data. Emmy Stigter, a doctoral student at the University of Utrecht in the Netherlands and principal author of the study, led the research team. “The fieldwork involves quite some hiking and a lot of logistical challenges,” she told GlacierHub. Yala is a four-day hike from the start of the Langtang Valley, which is a day’s drive from Kathmandu. The instruments required so much energy to power that the team had to lug a car battery up the glacier to ensure it would have sufficient energy to run during the research. Though the equipment was in place all winter, a data card was corrupted, limiting some of the team’s observations to just over a month in autumn.

Base camp for the Yala Glacier team, October 2016 (Source: Walter Immerzeel).


During the 32-day study period, which occurred from October to November 2016, Yala lost 32 millimeters of water equivalent. This represents a significant share of the glacier’s net loss during the period (70mm). Yala’s one millimeter per day rate of sublimation is a pace higher than the Swiss Alps, Colorado Rocky Mountains, and Spain’s Sierra Madre. Due to the low atmospheric pressure, sublimation is most prolific at high altitudes, like that of the Himalaya. Only Kilimanjaro and the Andean peaks exhibit comparable rates of sublimation, according to the authors.

Sublimation on Nevado Salcantay in the Peruvian Andes (Source: Peter Deneen)

The researchers found that sublimation rates are highest in November and December and peak around one o’clock in the afternoon. Sublimation rates also differed depending on wind at the locations on the 1.5-square kilometer glacier; the faster the wind, the faster the rate of sublimation. Stigter’s team observed that rates were 1.7 times higher on ridges and .8 times lower at the bottom of the glacier.

Blowing snow, which was not accounted for in this study, may be a consequential factor leading to underestimation of mass loss to sublimation. Suspended particles sublimate on an order several times greater than the surface sublimation, as there is more ventilation and supply of dry air. One study showed that up to 30 percent of annual snowfall was removed in the Canadian prairie and Alaska due to blowing snow sublimation, while Antarctica lost up to 85 percent of its precipitation. Stigter is currently involved in a new study quantifying sublimation during wind-induced snow transport events.

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East African Glaciers at Risk from “Global Drying”

In the tropical climate of East Africa, glaciers are an unexpected, yet vitally important part of the ecosystem. Since 1900, African glaciers have lost a staggering 80 percent of their surface area, contributing to regional water shortages.

While rising temperatures may seem like an obvious cause of global glacier retreat in many regions, the glaciers of east Africa are a unique exception. A study published in Cryosphere earlier this year has found that the largest glacier on Mount Kenya, the Lewis Glacier, is melting because of decreasing atmospheric moisture rather than increasing temperatures.

Snow-capped peaks of Mount Kenya (Source: Valentina Strokopytova)
Snow-capped peaks of Mount Kenya (Source: Valentina Strokopytova)

African glaciers have all but disappeared, except for three locations in East Africa: Mount Kilimanjaro in Tanzania, Mount Kenya in Kenya, and the Rwenzori Range in Uganda. Scientists have been studying the few remaining African glaciers in hopes of preserving what is left of the rapidly melting ice. While headway had been made in understanding the causes of melting on Kilimanjaro, the melting on Mount Kenya, Africa’s second tallest mountain, has remained a mystery until now.

The complex climatic features of Mount Kenya, combined with the lack of observational data, has made it difficult to pinpoint an exact cause of Lewis Glacier’s retreat. Lindsey Nicholson, a researcher at the Institute of Atmospheric and Cryospheric Sciences, led a study in 2013 that concluded a combination of causes was responsible for the melt, rather than one factor in particular.

Building on  her previous work, the team, led by University of Graz’s Rainer Prinz and Lindsey Nicholson, set out to collect the data they needed to gain a more accurate understanding of why Lewis Glacier was melting. They installed an automatic weather station on the glacier at an elevation of 4,828 meters, and collected 773 days of data over the course of two-and-a-half years.

Glacier lake on Mount Kenya (Source: Cheyenne Smith)
Glacier lake on Mount Kenya (Source: Cheyenne Smith)

In conjunction with the data from the weather station, the team used a model to predict how much Lewis Glacier would melt under a range of different scenarios. By manipulating variables, including precipitation, air temperature, air pressure, and wind speed, in the model, the team was able to see which factors played the biggest role in glacier melt.

The team found that moisture had the biggest impact on Lewis Glacier’s surface area, rather than air temperature or a combination of other climatic factors. Despite differences in location and elevation, the glaciers of Mount Kenya and Kilimanjaro are melting for the same reason: East Africa is getting progressively drier, and the lack of water is impacting much more than just the glaciers.

The glaciers on the peak of Kilimanjaro lie significantly above the regional freezing point—year round, the peak is cold enough to maintain its ice levels, even as surface temperatures in East Africa have steadily increased. Yet, Kilimanjaro’s glaciers continue to retreat and are projected to disappear completely by 2020. Temperature changes fail to explain the severity of the mountain’s glacier retreat.

Observational studies have showed that Kilimanjaro is receiving less cloud cover that leads to increased radiation from the sun, and less precipitation, causing infrequent snowfall. The IPCC has projected a 10% decrease in rainfall during the already dry season from June through August, amplifying the impacts of regional dryness and drought.

crop fields at the foot of Mount Kenya--the mountain serves as a major watershed for surrounding agriculture and livestock (Source: Cheyenne Smith)
crop fields at the foot of Mount Kenya–the mountain serves as a major watershed for surrounding agriculture and livestock (Source: Cheyenne Smith)

The impact of a drying climate has greatly impacted Kilimanjaro, and caused its glaciers to retreat from sublimation–a process by which the ice changes directly into water vapor rather than melting into water. The theory that moisture is the main factor impacting glacier melt on Kilimanjaro has, up until now, been assumed to be a product of the mountain’s height and not generalizable to all East African glaciers. Prinz and Nicholoson’s findings suggest that drying may be the main reason for glacier melt throughout the region as a whole.

Mount Kenya’s glaciers are at lower elevations compared to Kilimanjaro’s, and lie much closer to the regional freezing level. It was therefore expected that rising temperatures would affect the glaciers of Mount Kenya, and no scientific studies had proved or disputed this assumption.

Droughts, desertification, and crop failure have become increasingly common in tropical Africa, and according to the study this is primarily caused by shifting ocean conditions that are preventing moisture from circulating over East Africa. The lack of moisture means there is not enough precipitation—either as rain over the savannas or snow on the mountain peaks—to sustain the glaciers or the populations that rely on them. In order to preserve the last remaining African glaciers, it will be necessary to understand and prevent changes in water, rather than only changes in temperature.

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