Why are the glaciers on Mt Everest more sensitive to climate change?

Researchers are still collecting temperature data to analyse and understand how the highest glaciers in the world will be affected by climate change

Often when the topic of glaciers and climate change is discussed, focus shifts to those in Greenland and Antarctica. But there are glaciers elsewhere too, such as in the Himalayas, which play a vital part in supplying water to people who live downstream. Now, our research has found that these glaciers may react more sensitively to predicted future climate change than previously thought, which could lead to them melting at a faster rate.

In 2017 and 2018, our EverDrill research team travelled to Nepal, to measure ice temperatures (using a converted pressure washer) on the Khumbu Glacier. Khumbu, whose ice is sourced in the Western Cwm of Mount Everest, flows down the flanks of the mountain from around 7,000 to 4,900 metres above sea level. Along with many other glaciers in the Everest region and across High Mountain Asia, the meltwater from Khumbu contributes to the water resources of huge populations in the mountain foothills.

We spent two field seasons (six to eight weeks each) drilling boreholes using a jet of hot, pressurised water to incise into the ice. In total we drilled 27 boreholes, ranging between 1-192m deep, across five sites. These boreholes are the first, deepest, and most spatially extensive achieved to date in the Himalaya using hot-water drilling.

Once the boreholes were drilled, we measured ice temperatures by installing strings of pre-built thermistor sensors linked to data-loggers located at the surface. We left them for six months and collected the data on a return trip in November 2017.

The main finding from this research was that the ice was warmer than we expected, with the coldest ice measuring –3.3°C. As the ice is formed on the flanks of Mount Everest, where the mean annual air temperature is –13°C at 7,000 metres elevation, we might have expected the ice to be at this temperature. Our borehole data reveal that this was not the case. Not only was the ice not this cold in our boreholes, but ice temperatures also increased towards the glacier terminus.

Cold and warm ice

Why does it matter that we found warmer ice temperatures than we expected? Glaciologists acknowledge two thermal types of ice: “Cold” ice and “warm” ice. Cold ice is below the melting-point temperature, so when additional heat is applied (from the sun or warmer air temperatures), the ice simply becomes less cold (going from –15°C to –14°C, for example). Warm ice, alternatively, is at the melting-point temperature, so any heat input melts the ice to become water.

In short, this means that Khumbu will respond more sensitively to any future additional heat inputs, such as warming air temperatures. It has been shown that air temperature increases are amplified at high elevations. For example, a global temperature rise of +1.5°C has been predicted to result in a +2.1°C warming across High Mountain Asia and a significant loss of ice mass across the region. As these glaciers melt and recede, their contribution to water resources will initially increase. However, as the volume of ice mass remaining decreases, this contribution will steadily decline. While the timescale over which this might occur is unknown, predictions suggest that “peak water” may be reached as soon as the middle of this century. Our finding of warm ice within Khumbu supports the predicted sensitivity of such glaciers to warming air temperatures.

But it’s not all bad news for the region’s glaciers. First, we don’t know if the temperatures we measured on Khumbu are representative of all glaciers in the area. More measurements are needed on other glaciers to determine this. Second, Khumbu and many other Himalayan glaciers are  debris-covered glaciers, which contain a surface layer of rocks and boulders that typically increases in thickness towards the terminus, up to several metres depth. This debris layer complicates the amount of ice surface melt that is produced: where the layer is thick, it acts like a blanket and insulates the glacier from warmer air temperatures, reducing melt rates.

However, this insulation of the lower glacier also results in the location of maximum melt shifting further up the glacier, to where the debris cover is thinner. In this area, the glacier melts by surface lowering, which in an extreme future scenario could lead to the detachment of the lower glacier, forming a new terminus at this higher elevation. While the lower, detached ice would become stagnant, it would still be protected from instant melting by its debris blanket. The new terminus above this would not, and melt rates could increase. Yet, our deepest borehole in this area of surface lowering was 192 metres, and did not reach the bed so the ice thicknesses remains unknown – but it could be that there is plenty of ice left in Khumbu to stop this happening.

These ice temperatures will now be fed into an ice flow model to better predict how the Khumbu Glacier will respond to climate warming and contribute to river discharges in the future. Meanwhile, we are still collecting temperature data to analyse and better understand how the highest glaciers in the world will be affected by climate change.

Katie Miles, PhD Researcher, Aberystwyth University

The EverDrill project is funded by the UK Natural Environment Research Council.