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In from the cold: November 2009 Archives

I have a sort of avuncular interest in Abramov Glacier, high in the Pamir Mountains of Kyrgyzstan. I once published a paper that relied on a set of detailed measurements from Abramov to learn more about the error bars on mass-balance measurements in general. It is just as well that I published the paper in 1999, because later in that year the research station that supported the field workers who made the measurements was destroyed by militants in a civil war (or it might have been an overspill from a civil war in neighbouring Tajikistan).

That was the end of the long, valuable series of glaciological measurements on Abramov Glacier. I hope the Kirghiz people, or the Tajiks, ended up better off for their civil war, but it certainly wasn't a good thing for glaciological understanding.

Nor was it the only occasion on which glaciers have got caught up in the machinery of human conflict. One of the oldest conflicts of which we know was the one in which Ötzi, the Iceman of the Ötztal valley in the Alps, died in about 3,300 BC. We know little of the argument which ended with Ötzi suffering an arrow wound that was to prove fatal, but in this instance we have the glacier – the Niederjochferner, to be precise – to thank for preserving the fragmentary and poignant evidence, and for allowing us to communicate with Ötzi. Although it is a shame that we cannot speak to him, he has certainly taught us a lot.

Beside the pathos of Ötzi's final hours, and the stupidity of what was done on Abramov Glacier, there is a certain grandeur about the highest battlefield of all, Siachen Glacier in Kashmir. In 1949, the ceasefire line between India and Pakistan, which both claimed Kashmir, was laid down only as far north as a point about 50 km from the border of Kashmir with China. In 1984, India occupied most of a triangular void between this point and the Chinese border, containing Siachen and its drainage basin. That quarrel was put on ice, literally, with a new de-facto ceasefire line, across which thousands of Indian soldiers on Siachen now stare at thousands of Pakistani soldiers on neighbouring glaciers.

Every few years, one side gets nervous and shoots at the other, and things threaten to get out of hand. In all, several thousand have died, but many more have died from avalanches, hypothermia and the like than from human violence. Fortunately the violent episodes have been growing less frequent, but 25 years seems like an awfully long time for two armies to stare at each other across a bleak snowfield.

Of course, you might be able to think of a word other than "grandeur" to describe this high-level conflict. If so, I probably wouldn't argue. I don't want to offend either of the parties to a 60-year-old quarrel, but I would like to point out that Siachen Glacier is a rather grandiose object – there aren't many valley glaciers as large as 987 km2 – and would be more valuable to us all if we could study it in safety.

There have, in fact, been a few studies of Siachen Glacier. For example M R Bhutiyani, writing in the Journal of Glaciology for 1999 (volume 45, pages 112–118), estimated its mass balance during 1986 to 1991 by the admittedly less than ideal "hydrological" method. Over the five years it lost the equivalent of 2.5 metres depth of water, with unknown but large uncertainty.

Two telling points about Bhutiyani's work, however, are that he is at the College of Military Engineering in Pune, and that he required clearance for publication from the Directorate of Military Intelligence in New Delhi.

One idea for the triangular void where India, Pakistan and China meet is to turn it into the Siachen Peace Park. It is an idea that has been around for a while. As yet it does not seem to have gained much traction, but it is the best idea I have seen so far. Maybe one day I will be able to write an article headed "Peace and glaciers".

When the Ice Melts is a remarkable story by Anjali Nayar in a recent issue of Nature. It centres on Thorthormi Glacier, which helps to feed the Pho River in north-central Bhutan. Like most other glaciers, Thorthormi is shrinking, and faster now than formerly. Much of the meltwater finds its way into a growing lake, dammed behind the moraine that Thorthormi built when it was larger than it is now. The moraine is nothing but a heap of rocks and dirt, delivered by the flowing ice to the glacier terminus.

The trouble is that, as moraines go, it is a rather flimsy barrier. As the meltwater accumulates behind it, the pressure on the moraine is building up. On the other side of the moraine lies Raphstreng Lake, itself a temporary store for glacier meltwater at the foot of the neighbouring Raphstreng Glacier. Eventually the meltwater finds its way into the mainstream of the Pho, and finally to the Brahmaputra and the Bay of Bengal.

Taking the long view, the fairly small Thorthormi and Raphstreng Glaciers, respectively about 17 and 4 km2 in area, are likely to cease to exist some time in the next hundred years, which raises long-term questions about the water resources of the Pho basin. But if you are a Bhutanese villager, with your home and your fields on the banks of the Pho, the water still ponded behind the Thorthormi moraine is not so much a long-run problem as a short-term menace. Nobody can tell whether, and if so when, the dam will collapse. If it does, the flood will take with it not just homes, livestock and crops, but hospitals, schools and other impedimenta of a decent modern society that Bhutan has managed to provide for itself in recent decades.

I have found that, in the face of adversity, you can't beat thinking of something to do and getting on with it. If it works, so much the better, and the way Bhutan is tackling the Thorthormi problem sounds as workable as anything I can think of. They are widening and deepening the outlet by which the Thorthormi meltwater bypasses the moraine. They are doing this with shovels, supplemented with hammers to smash the large boulders in which the moraine abounds and ropes to drag the pieces away. Of course this requires elbow grease, but bringing in heavy construction machinery would be impractical, apart from being too expensive.

This story is repeated with variations across the breadth of the Himalaya, as far as the menace is concerned. As far as the elbow grease is concerned, Bhutan is far ahead of other Himalayan nations. But even elbow grease costs money. The cost of the Thorthormi operation comes from scraps of first-world funding that amount to a few million dollars in all. Multiply by several hundred or thousand dangerous glacial lakes, and think about all of the other hazards conjured into being by global warming, and reflect that most of the total menace is not being tackled at all. It is not hard to accept that 100 billion dollars per year is a reasonable figure for the cost of adaptation to climatic change, or that the cost of failing to adapt will be greater still.

I can't help suspecting, though, that $100 billion worth of shovels and ropes is not the whole solution. Surely, for example, some of the cost has to go into high technology as a way of understanding the problem and deciding how to allocate the available human energy to the most effective solutions. In the Himalaya, satellite images of the glaciers and the glacial lakes have already been put to use in pinpointing the hazards. The leading contributions of this kind have come from ICIMOD, the International Centre for Integrated Mountain Development in Kathmandu, Nepal. Here is another example of self-help working wonders, and an example that shows that self-help doesn't necessarily mean low-tech. But we have to face the fact that self-help doesn't mean zero-cost either. The ICIMOD inventories of Himalayan glacier and glacial lakes are already out of date. As Pradeep Mool, the chief remote-sensing specialist at ICIMOD, says, "All the lakes need attention. But we have to prioritize ... ."

More often than not, reports about new measurements in remote parts of the cryosphere show that things are either worse than we thought or about as bad as we thought. A while ago, I was describing the out-of-the-way glaciers in the remote southern Indian Ocean. Things down there, it turns out, are about as bad as they are everywhere else. Sometimes, though, we learn that things are a bit better than we thought. A paper by Evan Burgess and colleagues, to appear soon in Journal of Geophysical Research – Earth Surface, shows that more snow has been accumulating in southeast Greenland than conventional estimates suggest.

The Greenland Ice Sheet loses mass through by melting and the calving of icebergs. There has been more of both of these in recent years. The accelerated loss is cause for concern, but we mustn't forget that the ice sheet also gains mass by snowfall. Working out that gain is difficult, because the measurements are so sparse. The in-situ facts about snowfall on the ice sheet come from a few hundred snow pits and ice cores, in which investigators have counted and weighed annual layers. Over large tracts, no measurements have ever been made, and until recently we had to rely on spatial interpolation to make educated guesses about these data voids. What is worse, we want to know the total accumulation, and we can't measure that around the edge of the ice sheet, where much of the snow melts and runs off.

Unfortunately, spatial interpolation won't manufacture surprises for you. (And, if it did, you wouldn't know whether to believe in them or not.) The contribution of Burgess and co-authors is to bring the skill of a climate model, called Polar MM5, to bear on this problem of missing information. The model stays close to the behaviour of the actual atmosphere by restarting itself from an observed state once a month, and its estimates of precipitation are calibrated by comparison with the sparse observations of accumulated snow. This yields much more spatial detail than could ever be got by trudging around on the surface of the ice sheet and digging holes.

The result is a net accumulation rate of 337 mm/yr of water equivalent, roughly the same as the annual precipitation in Wyoming or Andalusia. Other recent estimates for Greenland have ranged between 290 and 310 mm/yr so, accepting the Burgess number as more reliable, the older estimates were biased low by 10–18%. One of these estimates was by me. I reckoned the accumulation rate as 299±23 m/yr. The Burgess rate carries an uncertainty of ±48 mm/yr, so if you wanted to be picky you could claim that it does not represent a "significant" increase.

But we are looking at two different kinds of error. That plus-or-minus symbol, "±", stands for an uncertainty due to random factors that cannot be controlled for – a give-or-take kind of error. What Burgess and colleagues have done is to correct a systematic error, or bias. The older estimates were off mainly because in southeast Greenland, where there are hardly any measurements, they were based on guesswork.

The authors managed to obtain a single field measurement from this data void. Where I estimated 950 mm/yr and another worker had 880 mm/yr, their model gives 2,780 mm/yr, but the measured accumulation over two years was 3,760 mm/yr. So it is an open question whether all of the bias has yet been eliminated.

The increased gain is not nearly enough to offset the losses the ice sheet has been suffering of late. It is still shedding mass and contributing to sea-level rise. But we always want the best estimate we can get.

One of the nice things about this work is that it shows that we are making progress. In the first place, we now have estimates of the uncertainty in the numbers, which is a quite new development. And in the second place, the uncertainties, give-or-take plus bias, are not really all that large. When it comes to uncertainty, less is better, but random errors of the order of 15% are bearable, and less than they were a few years ago.

You have to be a subscriber to read the Burgess paper while it is in press. But if you want to keep an eye out for it, its document-object identifier is doi:10.1029/2009JF001293.

ASTER, the Advanced Spaceborne Thermal Emission and Reflection radiometer, has been looking at the Earth since it was launched in 1999 on a satellite called Terra. A joint Japanese-American venture, ASTER is the foundation for all sorts of environmental monitoring efforts, but in particular for the GLIMS initiative. GLIMS stands for Global Land Ice Measurements from Space, and while we are doing acronyms I may as well tell you that DEM is short for digital elevation model.

Glaciologists need good maps, which nowadays means DEMs derived from air photos or satellite images, but we have a special interest in up-to-date maps because nearly all of our glaciers are getting smaller. This includes getting thinner. An accurate map of surface elevation tells us the state of the glacier at the date of the image. If we have an accurate map or DEM from some earlier date, we can subtract the newer DEM from the older to measure the change in elevation.

There are complications. Many older maps turn out to be much less accurate than modern ones. Often, air photos of glaciers taken in the golden age of mapping, roughly from the 1940s to the 1960s, show featureless white expanses. Snow just isn't photogenic. Surprisingly often, there simply aren't any accurate maps from the olden days.

Nevertheless, GLIMS was conceived in the 1990s as a way of bringing our knowledge of glacier surface elevations up to date. This is where ASTER comes in. It has two cameras, one looking straight down and one looking backwards. The two cameras see the same patch of surface, but several seconds apart because the satellite is moving. So ASTER can see in stereo, and can work out the surface elevation using trigonometry.

Participants in GLIMS have been documenting glacier changes with images from ASTER and other satellite sensors. ASTER, though still working, is beginning to feel its age, but something very notable happened in June of this year: the ASTER Global DEM was released. This staggering product contains more than 250 billion land-surface elevations, every 30 metres over nearly the whole world.

ASTER DEMs are composites from images obtained between 1999 and 2008, so they may prove not to be well adapted for measuring glacier elevation changes. But there is one way in which they are sure to excel: the mapping of regions that have never been mapped accurately before.

The ASTER GDEM had a predecessor in the form of SRTM, the Shuttle Radar Topography Mission of February 2000. Interpreting radar imagery presents problems of its own, but SRTM DEMs have already been put to good use in monitoring glacier elevation changes. One problem is that the shuttle orbit limits coverage to between 60°N and 60°S, so that most of the world's glaciers are not covered. Aggravatingly, however, it was decided that the South Sandwich Islands, the only significant land between 56°S and 60°S, weren't worth the trouble.

They are more than ordinarily troublesome as far as conventional mapping is concerned: windy, cliff-girt and ice-covered, therefore nearly impossible to land on by boat, helicopter or any other conveyance; and too far away to justify sending a plane equipped for photogrammetry in the faint hope that they won't be cloud-covered.

Montagu Island, the largest (101 km<sup>2</sup>) of the South Sandwich Islands. Exposed land is green and brown. Ice is purple and blue, with the glacier margin in red.
Montagu Island, the largest (101 km2) of the South Sandwich Islands. Exposed land is green and brown. Ice is purple and blue, with the glacier margin in red.

The ASTER GDEM team doesn't think the South Sandwich Islands are too much trouble, even though they had to contend with clouds, ice floes that look like land, and, in the case of the island in the map, a continuing volcanic eruption. You can see one of their problems in the map: the south coast is greyed out because the GDEM masked off the surrounding ocean by relying on the best previous estimate of Montagu Island's position, which was 4 km too far to the northeast. Less obviously, the largest of the pale blue patches is the summit of Mount Belinda. It rises only to 1070 m, a full 300 m lower than previously believed on the basis of a shipboard measurement made in 1930.

1930 was the date of the first of only three "serious" visits to Montagu Island. You can see the top of Mount Belinda from sea level, if it isn't cloudy. But even on the clearest of days you can't see the rest of the summit plateau, on which no human eye has ever gazed. So this map is exploration in the modern mode, that is, from orbit. Thanks to ASTER, it is the first ever contour map of Montagu – including the bits we have never seen before.

Chances are you will never need to think about Montagu Island again. But it holds 50 times as much glacier ice as the iconic and much better known Kilimanjaro, and it is a part of the global glaciological picture that doesn't deserve to be left out. If Montagu Island should happen to cross your path in the future, remember that you first saw it contoured courtesy of the ASTER GDEM.

Amazingly, the Romans managed to create an empire that lasted 500 years without having a word for glacier. Amazing to me as a glaciologist, that is. I can see that there would not be much call for such a word in ancient Greek – all those sun-drenched islands – but the Romans needed to cross the Alps regularly, and on one vividly-recorded occasion to cope with Hannibal and his elephants.

A few articles ago, I was able to trace the word glacier back to 1332 on the strength of documentary evidence, and more conjecturally to some date in the post-Roman period when some unknown speaker of Franco-provencal first uttered a word from which our modern form could have descended.

The notional Latin word ancestral to French glacier is glaciarius or glaciarium, but its first known appearance was in London in the 1870s, when an entrepreneur used it for his skating rink. The name didn't take, as we know. The Oxford English Dictionary's next quotation is about the closure of the Southport Glaciarium in 1889.

The closest thing to a description of a glacier in writings preserved from antiquity is Polybius' account of the crossing of the Alps by the Carthaginian general Hannibal in September 218 BC ( Histories, III, 55). He wrote in Greek, some 70 years after the event, but he emphasized that his account was based on interviews with participants. He says of Hannibal's descent on the eastern side "The new snow which had fallen on the top of the old snow remaining since the previous winter was itself yielding, both owing to its softness, being a fresh fall, and because it was not yet very deep, but when they had trodden through it and set foot on the congealed snow beneath it, they no longer sunk in it, but slid along it with both feet, as happens to those who walk on ground with a coat of mud on it." This is the Loeb translation. The verb translated by "congealed", sunestekuian, might be better rendered as "compacted".

Evidence of absence is always harder to find than absence of evidence. It appears that, although they had words like glacies, ice, and glaciare, to freeze, the Romans simply did not have in their minds the idea of a glacier. One possibility, of course, is that Hannibal and his elephants were just slithering down a steep snow-covered slope. Perhaps he had more sense, assuming he had the choice, than to march his army onto a glacier. We will never know the facts, I suppose.

Some of the possible solutions, notably my not knowing enough Latin or Greek, are very plausible. Another plausible answer is that Hannibal travelled through a pass that was not glacierized. Polybius is not very specific about which valley Hannibal exploited, and two thousand years of follow-up investigation have failed to identify it. A good bet seems to be the route from Gap to Turin via Briançon, the Col du Montgenèvre and Susa, and he could have gone that way without coming across any glaciers. There are several other candidates, though.

But there is something here that I really do not understand. How could Polybius, who writes proudly of how he made the passage of the Alps so as to see for himself the terrain he was immortalizing, not have noticed the weird whitish things hanging from the ridges and creeping down the valleys? Surely the local inhabitants, if they had a word for them either in Latin or, more probably, a Celtic language akin to modern Breton and Welsh, would have given him the word? And in later centuries the Alps were well inside the boundaries of the larger Roman empire. Apparently neither politics nor trade nor even curiosity provoked the Romans into inventing the word glaciarium or finding an equivalent.

Perhaps ancient history is like palaeoclimatology, a subject about which I am slightly less ignorant. If nearly all of the evidence has disappeared irretrievably, is it a waste of time to wonder about Hannibal and glaciers, or about Roman-era climate and glaciers? Somehow I cannot manage to think so.