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In from the cold: July 2009 Archives

It sounds like something they might do to you at a health spa, doesn't it? But to students of glaciers, basal lubrication is the key that unlocks a long list of puzzles.

Why do precise measurements of glacier motion often show stick-slip behaviour, that is, hours and hours of near motionlessness punctuated by half-hours of rapid movement? Why do some glaciers surge, that is, accelerate suddenly every few decades, flowing rapidly for a year or two before returning, sometimes suddenly but more often gradually, to normal? Why does the landscape of southern Ontario, which I can see from my window, undulate? Why, in the sediment of the northern Atlantic Ocean, are there occasional layers of sand, interrupting the blanket of ultra-fine-grained mud?

The layers of sand beneath the Atlantic are spaced irregularly, 10,000-15,000 years apart, according to the Principle of Superposition, at depths below the sea floor that correspond to the last ice age. They are thin on the European side, thicker towards the northwest, and thickest of all in the neighbourhood of Hudson Strait, which separates Quebec from Baffin Island. The simplest explanation of this pattern is that every so often the bed of the Laurentide Ice Sheet, that covered most of Canada, became much more slippery. Much of its interior was drained by the Hudson Strait Ice Stream, which accelerated occasionally and discharged icebergs in huge numbers. With the icebergs came the sand. All of the plausible accounts of this instability have variations in basal meltwater supply, or possibly just its behaviour, as a critical ingredient.

Around where I live, we are rather proud of our drumlin field. Somebody counted these egg-shaped hills and got up to about four thousand. But geomorphologists now reckon that the tunnel channels are even more interesting. Tunnel channels are drainage networks shaped by subglacial meltwater at the end of the last ice age, after the ice had shaped the drumlins and indeed not long before the ice disappeared altogether. For a long time I simply could not see these things, and I still suspect that the geomorphologists are asking for more meltwater than is probable, but recent evidence from beneath the modern ice sheets is vindicating their interpretations. Now I can see the ancient tunnel valleys in the light of modern ones, apparently hard at work, beneath the Antarctic Ice Sheet.

I don't know why most glaciers do not surge but a few do. Nor does anyone else. Surging is a phenomenon that has eluded explanation over several decades of concentrated observation and analysis. But we are all positive that subglacial hydrology contains the answer if we can only put together the pieces of the puzzle. The most recent instance of a surging glacier, detected by the U.S. Geological Survey on 3 July 2009, happens also to be a famous glacier - Malaspina Glacier in Alaska.

Many glaciers go faster in summer, suggesting that meltwater supply has something to do with glacier speed. Where the ice is observed to move in short bursts, there is usually also a suggestion, from one line of evidence or another, that it spends most of the time frozen - that is, stuck - to its bed. Slip happens when that immobile state is disturbed, in other words when the bed is lubricated upon the arrival of meltwater. But where does the meltwater come from? And go to?

It might not go anywhere, if the stuff that is moving around is not water but heat. That is, stick-slip may be telling us not about patterns of meltwater flow but about patterns of thawing and freezing. In fact, there may not be any heat moving around either. The melting temperature depends, slightly but measurably, on the confining pressure. So the thaw-freeze patterns could actually be patterns of subtle fluctuations of pressure, not just squeezing the water from one place to another but determining which of the two states, solid or liquid, it is stable in.

It is all very complicated, at scales from sticky patches up to the width of the north Atlantic and beyond. Great fun for glaciologists, but not without consequences for society - for example, if the Antarctic or Greenland Ice Sheet should decide to do what, according to the lesson from the sand under the Atlantic, the Laurentide Ice Sheet did repeatedly.

James Thurber taught us that the world is divided into two kinds of people: those who think the world is divided into two kinds of people and those who don't. I belong to the former group, believing that the world is divided into two kinds of people: those who think words are interesting and those who don't. Again, I belong to the former sub-group. If you belong to one of the other sub-groups, you might want to read something other than this article, because an obsession with words can be tiresome.

One of my jobs recently has been to coordinate the compilation of a glaciological glossary for the International Association of Cryospheric Sciences. The Glossary of Mass-balance and Related Terms is likely to be of interest mainly to specialists, but one of the words we had to define was "glacier". We had a lot of fun with this, it being the sort of thing about which specialists find it easy to disagree. But where did the word come from?

The Oxford English Dictionary says that a glacier is "a large accumulation or river of ice ...", and quotes William Windham and Peter Martel as the first persons to use the word in English, in a letter printed in 1744 in which they describe excursions from Geneva to the glaciers around Chamonix in 1741 and 1742. They spelt it glaciere, apparently conforming to the spelling and pronunciation of the local inhabitants. The French speakers of Savoy seem to have been undecided about whether the noun should be feminine or masculine. In modern French a glacier is a glacier and a glacière is an icehouse or cold cellar. That the English word comes from French will not surprise anyone, but where did the French, or the Savoyards, get it from?

The word may first have appeared in print in 1574, in Josias Simler's De Alpibus Commentarius: "... it is called Gletscher by our people". Gletscher is one of the words for glacier in modern German, and it looks like a fairly obvious borrowing from a French or Italian dialect of the Alps. The modern Italian word is ghiaccaio. The Trésor de la langue française in my university's library tells me that glacier or glacer is first recorded from the west of Switzerland in 1332.

You can read Windham and Martel if you can find a library with a subscription to Eighteenth Century Collections Online. It is an intriguing insight into the enquiring outdoor 18th-century mind, and is the first serious work of glaciology ever written in English. One of the intriguing things about it is that the authors' understanding of glacier has all the ingredients that we settled on a few months ago as essential for our 21st-century glossary definition: low temperature, frozen water on land and, most intriguing of all, motion: "the Glaciere is not level, and all the Ice has a Motion from the higher Parts towards the lower".

Windham and Martel were relying for these facts on the local inhabitants, who not only understood that the ice itself "has a motion", but knew from their own observation that the glaciers were extending progressively further into the valleys. We have the opposite problem in the 21st century.

So the idea that a glacier is ice in motion has a long history. I suspect that the idea goes back long before 1332, to the unknown first coiner of the word. The glac- with which it begins is from glacies, the Latin for ice, but the ending -ier is more suggestive. Speakers of French have made heavy use of this suffix. The Trésor de la langue française takes several pages to list all the work it does, but a leading part of its job is to describe the idea of carrying or delivering. Just as a pear tree is a poirier or carrier of pears, so it seems reasonable to guess that a glacier is a deliverer of ice.

We like to think that our new Glossary of Mass-balance and Related Terms is an up-to-the-minute summary of glaciological thinking, but this kind of thinking has evidently been going on for longer than we are apt to think.

Sorry for yet another acronym, and sorrier still that the odds are against your guessing correctly what it stands for. No, it is not a file in Portable Document Format, nor yet a Post-Doctoral Fellow, but a Probability Distribution Function. These PDFs are an essential tool in modern efforts to grapple with environmental risk, uncertainty and just plain ignorance.

Suppose you knocked on a lot of doors, asking to measure the height of the person answering the door. (This is just a thought experiment. Actually doing it would not be a good idea.) At each door you would have a fair chance of getting a measurement equal to the average height of the human population. In fact, averaging your large sample of heights would be the best way to estimate that height, but the sample would also tell you a lot about how often different actual heights crop up. Once in a while the door would be opened by a dwarf or a giant, but the odds are in favour of the person who opens the door being average.

That is the essence of a probability distribution function. The PDF is just the frequency with which anything that varies is spread across the range of its possible values. The idea works just the same for the outcomes of climate model runs as for measured human heights.

We know, from previous large samples, that the PDF of human height is symmetrical. Short people are less likely than average people, and so - with about the same odds - are tall people. This is not true of climate model runs. Their PDFs have fat tails.

All credible climate models predict warming over the next century. The most common outcomes are those in which, by the time the carbon dioxide concentration reaches twice the value it had in about 1850, the temperature will have risen by between 2.0 and 4.5 °C.

The snag is that "most common" is not the same as "average". A rather small proportion of the model runs predict much greater warming than do the commonest ones, and there is no compensating fatness of frequency on the less-warming side. The average of the predictions is noticeably warmer than the commonest prediction.

How do we respond to this undoubted, unpleasant fact? It is telling us something about the real climate of the future, but in an unsatisfactory way. I think that it is at least a guide to right action. There are actions that will make an improbable catastrophe even more improbable, and if we would not be unhappy with either their costs or their benefits then these actions become more apparently prudent. Policies founded on this reasoning are called "no-regrets policies". We probably need more of them.

This is supposed to be a blog about glaciology, and I haven't yet said anything about glaciers. Well, consider the West Antarctic Ice Sheet. Most of it is grounded below sea level, and there is a chance that continued warming will cause it to drain catastrophically into the ocean, starting perhaps in the next hundred years. Our best understanding is that not only is this chance extremely slim but also it would take several hundred years for the catastrophe to play out. But we cannot be certain on either of these points, or on whether the other tail of the PDF (ice-sheet growth because of increased snowfall in a warmer world) is equally probable.

In fact, we glaciologists are so far behind the climatologists that we haven't even got a PDF yet. One course of right action is therefore obvious: study the problem intensively. The PDF either will or will not turn out to be fat-tailed, and if it is fat it will be either because of physics or because of ignorance. The first exploratory efforts to model the vulnerability of ice sheets are just beginning to appear, and the rush is on to be able to say something quantitative about the likelihood of ice-sheet collapse in time for the next major assessment by the Intergovernmental Panel on Climate Change, due in 2014. At the moment, though, all we can say is "Watch this space".

Arctic sea ice has become one of our bellwethers, in part because for the past 30 years we have been able to watch it expanding and retreating nearly in real time, but also because it definitely obeys the laws of physics.

Recently the extent of Arctic sea ice has decreased fairly steadily. It is also notable that 2007 was a year of record minimum extent (in September), prompting conjecture about the Arctic becoming ice-free soon, with an ice-free September perhaps as early as 2013. However 2008 did not quite match the 2007 record. This year, and the next couple of years, will show us whether 2007 was a blip or the start of something even more serious than the 30-year trend.

I still remember our geography teacher at school starting to teach us climatology by saying "The climate is the average weather". He went on to tell us about the convention of presenting the climate as "normals", or averages, over 30 years. One bad summer simply doesn't add up to a climatic change. But it seems that it is human nature both to get rattled when a record is broken and to forget about the problem when the record-breaking behaviour is not repeated.

Equally, and unfortunately, it is not in human nature to worry much about systems like the climate that evolve over 30-year time scales. It is too easy to make the mistake of thinking that bad things aren't going to happen for 30 years.

Knowing that sea ice obeys the laws of physics, we should expect replacing bright sea ice with dark open water to illustrate the idea of feedback. The less reflective the surface, the more the radiative heating, and the more the loss by melting, and the less reflective the surface ... . In other words, the future of sea ice could start to look much, much worse quite suddenly. However, knowing too that we don't know everything, we should be concerned about the inability of climate models to agree on the future of sea ice.

Sea-ice predictions come from a couple of dozen large-scale climate models. They diverge wildly over the next century. Many of them don't even reproduce the recent evolution of sea ice, a clear indication that the modellers have development work to do. Two recent analyses show just how unsure we are about what is coming, but both also offer interesting ideas about how to cope with uncertainty as manifested in poor model performance.

Boé and colleagues point out that the models that do the best job of simulating the past 30 years are also the models that predict the earliest disappearance of sea ice. Using the A1B scenario for greenhouse-gas emissions, considered middle-of-the-road, this happens in about 2060-80 if we agree that disappearance means dropping below 10% of the 1979-2007 average in September. Boé and colleagues explain this observation in terms of the models' accuracy in describing the proportion of the ice that was thin to begin with, and therefore more at risk of disappearing altogether.

Wang and Overland selected the best-performing models by requiring them not only to come within 20% of the September observations for 1980 to 1999, but to beat the same target for the range of extents observed during the whole year. The thinking is that a model is likely to be more trustworthy if it matches more of the firm evidence. They find, with the six models that qualify, that September sea-ice extent is most likely to drop below about 10% of the recent average in 2030-2050.

Comparing the two studies, Mat Collins prefers to put his money on the later Boé estimate than on Wang and Overland. His reasons are cogent, but I think that the crucial point is to exclude the models that obviously get the recent history of sea ice wrong, something Boé and colleagues do not do.

Our understanding of the laws of physics already gives us the message "Sooner or later", but focusing on the models that are not obviously wrong - not the same thing as obviously right, of course - the message becomes "Sooner rather than later".