The dark art of falling albedo

A story on Mother Jones last July by John Vidal described the world’s highest glacier, Khumbu, turning visibly darker as particles of fine dust, blown by fierce winds, settled on the bright, fresh snow. “One-week-old snow was turning black and brown before my eyes,” said American geologist Ulyana Horodyskyj. Unfortunately she lacked the presence of mind to make a photographic record of this startling event.

The story mentioned other high-altitude glaciers and described the polar ice sheet, including the Greenland ice cap, coming under increased pressure of melting from atmospheric aerosols given off by human activity and exacerbated by global warming, the melting contributing to dangerous sea level rise. But I find problems with this narrative, which strains to make its arguments.

The first problem is simply the accompanying illustration (above). I identified it as our own Mt Ruapehu, in the centre of the North Island, and so it proved. There in Google images I found the original image from a NASA story on aerosols that used a photo from the New Zealand GeoNet service.

It’s not the Himalayan mountains. It’s New Zealand’s tallest active volcano just hours after its latest eruption stained the summit snow with tons of volcanic ash.

The accelerated melting of snow when it’s covered by a dark dust can be dramatic. In the bright sunlight of even high latitudes, the areas covered by the dust quickly melt, sinking into the surrounding white snow. You can’t help but be impressed. But it’s totally misleading to compare it with airborne dust, because that is much more sparse than what you can spread on the snow by hand from a bucket. They needed our volcanic ash in picture for the drama—the real wind-blown dust would be almost invisible to the naked eye.

Let’s return to Mother Jones. John Vidal explains:

The phenomenon of “dark snow” is being recorded from the Himalayas to the Arctic as increasing amounts of dust from bare soil, soot from fires, and ultra-fine particles of “black carbon” from industry and diesel engines are being whipped up and deposited sometimes thousands of miles away. The result, say scientists, is a significant dimming of the brightness of the world’s snow and ice fields, leading to a longer melt season, which in turn creates feedback where more solar heat is absorbed and the melting accelerates.

Sounds reasonable. But the paper suggests:

the transport of dust from snow-free areas in the Arctic that are experiencing earlier melting of seasonal snow cover as the climate warms may be a contributing source of impurities.

That is fact-free speculation which they don’t bother to quantify. I submit that it’s singularly unlikely you would find substantial wind-borne dust just from whatever extra margins of ice-covered territory are exposed by warming that has not been occurring. If it had been occurring, the extra margins would be minuscule (I’m not surprised they declined to quantify them). But surely it was included only to help with the next funding round; they are fully aware most of the aerosols come from far away from the ice with no connection with global warming. They are informative in discussing these general impurities:

Light-absorbing impurities, hereafter referred to simply as impurities, such as soot, mineral dust or micro-organisms (cyanobacteria and algae) present in snow are known to decrease the albedo in the visible part of the solar spectrum. This usually results in a strong increase in the energy absorbed by snow even for a low impurity content.

In addition, these are airborne particles. Atmospheric aerosols have two major, counteracting effects. The flying dust intercepts the sun’s rays, warms up, the heat passes to the air and much of it goes back to space, thus cooling the ground, while the dust that settles absorbs solar rays and thus warms the ice and snow. I think that’s how it works, and I don’t think settling dust warms snow or ice-free ground. Aerosols are thought to have been responsible for some cooling in global average surface temperatures between about 1950 and 1990, though I’ve seen nothing about the warming effect of the grounded dust.

The authors ought to have allowed for the cooling effect of the airborne phase of the aerosols they studied as well as the warming effect on the snow and ice the dust lands on but they don’t appear to have done so. They cannot reasonably claim cooling or warming alone, as one inevitably follows the other—another example of the chaos and complexity of the climate.

Vidal describes the estimation of the hypothesised melting:

In a paper in the journal Nature Geoscience, a team of French government meteorologists has reported that the Arctic ice cap, which is thought to have lost an average of 12.9 billion tonnes of ice a year between 1992 and 2010 due to general warming, may be losing an extra 27 billion tonnes a year from airborne dust alone, potentially adding several centimeters of sea-level rise by 2100. Satellite measurements, say the authors, show that in the last 10 years the surface of Greenland’s ice sheet has considerably darkened during the melt season, which in some areas is now between 6 and 11 days longer per decade than it was 40 years ago. As glaciers retreat and the snow cover disappears earlier in the year, so larger areas of bare soil are uncovered, which increases the dust erosion, scientists suggest.

It’s disappointing to see yet another egregious example of lack of context in a scientific paper. What does it mean to the Arctic ice sheet? Perhaps they expect that workers in their field already know the sizes of the significant bodies involved? Still, it takes just a few words to remind the reader, so failing to compare the loss to the body being lost can be seen only as an attempt to deceive—and this is confirmed when I find the figure they should have used.

The total Arctic ice sheet mass is about 3.1 million Gigatonnes (Giga = billion), and the Greenland ice sheet by itself is about 2.8 million Gt. This tells us the extra ice loss from aerosol-induced melting of about 27 Gt/year is insignificant. It represents about 0.096% of the Greenland ice sheet and only 0.087% of the entire Arctic ice sheet. Let me know if you think these quantities should be considered significant.

In addition, the error margins could drastically affect these estimates. I don’t know how to discover the error margins, possibly buried deep in the references.

No connection is drawn in this study with carbon dioxide or other greenhouse gases, so to use it (as John Vidal at Mother Jones uses it) to imply we’re still wrecking the earth and must save it is tremendously misleading. Well, all right, it’s an untruth.

The authors end with a plea for further funding:

Adequate modelling of the deposition, fate and effects of light-absorbing impurities in snow along with future field and remote sensing observation programmes to monitor their evolution are therefore urgently needed.

Good luck with that.

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