Dirt on glaciers
Dirt is more or less ubiquitous on glaciers. Even when there isn’t much, it makes the ice darker, a climatically important fact that can also be put to intriguing uses. Sometimes the dirt gives us striking visual effects in the form of multiple medial moraines. When there is enough dirt to cover the surface nearly completely, we call the glacier a debris-covered glacier.
The glacier isn’t necessarily buried entirely. In the accumulation zone, fresh snow will mask older debris, and usually not all of the ablation zone, where all of the snow melts, is debris-covered. Indeed the debris typically covers only a few to perhaps 20 percent of the surface, at the lowest elevations. But this part is critical, because it is where we expect to observe most of the mass loss that is supposed to balance the mass gain in the accumulation zone.
Debris-covered glaciers are a nuisance from a number of standpoints. Probably the biggest nuisance is that we don’t know how to judge the effect of the debris on the mass balance. Conventional wisdom has it that thin debris increases, and thick debris reduces, the melting rate of the underlying ice. The debris, being darker than the ice, absorbs more of the incoming radiation, but the more debris there is the less radiative heat reaches the ice.
In a recent laboratory study Natalya Reznichenko and co-authors reinforced the conventional wisdom. In an interesting twist, they showed that the daily cycle of radiation makes a big difference. If you irradiate a sample of debris-covered ice continuously, then after a delay of some hours, increasing with the thickness of the debris, you get the same rate of meltwater production as if there were no debris at all. But if you cycle your lamps with a 12-hour period, to mimic day and night, the underlying ice never gets a steady input of heat. The night undoes much of the work accomplished during the day.
The melt rate is slower beneath debris more than 50 mm thick, about half a handsbreadth, and faster beneath thinner debris. This is the “critical thickness”, at which the debris has no net impact on the melting rate. In an elegant graph summarizing the field measurements, the Reznichenko study shows that the critical thickness varies with altitude or equivalent latitude. On higher-latitude glaciers, or equivalently at lower altitude, the critical thickness can be as low as 20 mm, but it can exceed 100 mm at low latitudes or high altitudes.
So the real world is a bit more complicated than the laboratory when it comes to the effect of real debris on real glaciers. What about real-world melting rates? There are distinct signs that real debris-covered glaciers are even harder to understand than ideal ones.
For example, Akiko Sakai and co-authors pointed out that on debris-covered glaciers in Nepal there are lots of small meltwater ponds. One such pond absorbed seven times more heat than the debris cover as a whole, accelerating the melt rate and producing a knock-on effect because the departing warm meltwater enlarged its own conduit, a phenomenon known as “internal ablation”. And on a debris-covered glacier in the Tien Shan of central Asia, Han Haidong and co-authors showed that exposed ice cliffs, accounting for only 1% of the debris-covered area, produced about 7% of all the meltwater generated across the debris-covered area.
It would be nice if we knew the fractional debris cover of every glacier, and even nicer if we could say by how much the mass balance of each glacier is altered by its debris cover. This is a pipe dream for the moment, but the careful small-scale measurements on debris-covered ice seem to suggest that ignoring the debris, as we have to do now when estimating mass balance on regional and larger scales, could overestimate the mass loss quite seriously.
The trouble is that although we have very few measurements of the whole-glacier mass balance of debris-covered glaciers, they seem not to be consistent with the detailed laboratory and field studies. One of the most careful regional-scale measurements, by Etienne Berthier and co-authors, found that Bara Shigri Glacier, in the Lahul region of the western Himalaya, thinned by —1.3 m/yr over five years, a rate significantly faster than the —0.8 m/yr determined by photogrammetry for all the glaciers in the region.
“Bara Shigri” means “great debris-covered glacier” in Hindi. Clearly we don’t know enough about the behaviour of debris-covered glaciers, great or small, and as there are lots of them, and lots of people depend on them, they need continued study.
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