Scientists built a computer model of the glacier fields near Mount Everest to test how they might react to future climate change—and in some of their future simulations, the ice fields had all but disappeared.
The model showed glacier mass loss ranging between 70 and 99 percent by the year 2100, according to the study, published on Wednesday in the journal The Cryosphere. "I was pretty surprised by how much volume reduction we saw in the models," said lead author Joseph Shea with the International Centre for Integrated Mountain Development in Kathmandu, Nepal in a phone call. "That to me is the most surprising result of the study—how sensitive the glaciers in the basin are to future climate change."
The outcome of the research should not be equated with a forecast or prediction, said Shea; there are still many uncertainties with the model, including the current actual depth for many of the glaciers. But the trend is "clear and compelling," according to the paper: "Glaciers in the region appear to be highly sensitive to changes in temperature, and projected increases in precipitation are insufficient to offset the increased glacier melt."
Glaciers matter because they impact ecosystems and climates in significant and complex ways. They are important contributors to local hydrological systems and can contribute to sea level rise as they melt. The Hindu Kush-Himalayan region—the focus of this particular study—is sometimes called the "third pole" because it contains the world's most glacial ice outside of the Arctic and Antarctic regions.
The international team of scientists—which included researchers from the Netherlands' Utrecht University, and Joseph Fourier University in France—modeled changes in the glacier fields of the Dudh Koshi basin of central Nepal from 1961 to 2007, which they tested against available data. The model glaciers "did really well" in approximating the known historical behaviour of glaciers in the field, said Shea. Next they plotted out predictions based on scenarios provided by the Coupled Model Intercomparison Project Phase 5, or CMIP5, which provides a framework for climate modeling scientists to conduct coordinated research. The project describes four potential trajectories for global greenhouse gas concentrations through 2100.
For this study, the researchers used the scenario with the most elevated greenhouse gas concentrations and the one with the second-to-least. For each of those two scenarios, the scientists ran the model for four possible outcomes in terms of change in temperature and precipitation in the area, ranging from relatively wet and cool conditions to relatively warm and dry. Predictably, the model glaciers fared better in wetter and cooler conditions, but even in that environment they lost a minimum of 70 per cent of their mass when projected out to the end of the century.
There are at least two significant potential impacts to glacier melt on the people of Nepal. First, glacier melt is an important water source for communities, especially in the driest time of year before the monsoons hit, typically in April and May. While retreating glaciers may provide increased water flow in the short term, in the long term water levels are likely to drop as the resource becomes depleted. "Against this backdrop, changes in glacier area and volume are expected to have large impacts on the availability of water during the dry seasons," according to the paper. This could affect agriculture, hydroelectric power generation as well as community water resources.
Secondly, increased glacier melt can lead to increased formation of proglacial lakes, dammed in by moraines, according to the study. These dams can burst, causing flooding in communities downstream. "The failure of moraine dams in the Koshi River basin has led to 15 recorded glacier lake outburst flood (GLOF) events since 1965, with flows up to 100 times greater than average annual flow," the paper states. The collapse of the glacial lake Dig Tsho on August 4, 1985, for example, destroyed a nearly completed hydropower facility, killed at least four people, caused damage "for tens of kilometres downstream," and took out large stretches of the main trekking route to the Mount Everest base camp.
However, relating the modeled glacier melt to the local hydrology is a step for future research, said Shea. Until that is done, it will be difficult to have a clear picture of how melting glaciers will affect river flows. It could be that the effect is minimal, as monsoons are such an important contributor to streamflow. However, glacier contributions during the dry season and at higher elevations could be quite significant. "I'm sure scientists say this all the time, but I think we really need just more data about the glaciers and about what they're doing," Shea explained.
The Dudh Koshi basin is currently covered with 410 square kilometres of glaciers, according to the paper. The area includes several of the world's tallest mountains, including Mount Everest (Sagarmatha), Cho Oyu, Makalu, Lhotse and Nuptse.
What will the basin look like if most of its ice disappears? It is the mid-elevation sections of the glaciers that are most at risk, according to study authors. The lower "tongues" of the glaciers are somewhat insulated by a layer of debris, causing them to melt at a slower rate. And at the highest elevations, melt is slower and regeneration of the glacier through snowfall is more significant. "Our scenarios suggest that future reductions in glacier area will occur mainly in clean ice regions between accumulation areas and debris-covered termini," according to the study. The model glaciers showed the lower edges eventually splitting from clean ice above. It's a trend that's already been observed at two nearby glaciers. "Model scenarios from this study are thus consistent with field observations and suggest this will become a familiar picture in the coming decades," the paper reads.
The study confirms the trend of global climate change research, said Shea. Climate warming is real, it's here, and even ambitious carbon emission-reduction strategies will slow but not reverse its effects: "I don't know if at this point there's much that can be done to overturn the signal of glacier change. The warming that we've seen so far is only part of it. There's still much more to come in the pipeline. Oceans are warming—taking up heat—atmosphere is warming, glaciers are retreating."