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In from the cold: August 2010 Archives

In glaciology we like to distinguish between maritime glaciers and continental glaciers, because the climates that sustain these two kinds of glacier are quite different. The two adjectives make it sound as though the maritime ones ought to be near the sea and the continental ones oughtn't, but the reality is more complicated and more subtle.

Most glaciers consist of an accumulation zone at high altitude, where they gain mass, and an ablation zone at low altitude, where they lose mass. Roughly in the middle is the equilibrium line, at an altitude (the equilibrium line altitude or ELA) where loss and gain just balance.

From year to year the ELA varies through hundreds of metres, but on average over the decades it changes much more slowly, as the changing climate alters the balance. But what pins the equilibrium line to a particular average altitude? Why 1000 m, say, and not 2000 m, or 0 m?

A short but inaccurate answer is "Temperature". The hotter it is, the more ice you can melt. A slightly longer but much more accurate answer is "Temperature and snowfall". The more snow falls, the more heat you need to melt it. So the climate at the slowly-changing ELA is a measure not just of how hot it is, or of how snowy it is, but of how hot and snowy it is.

By definition, you need just the right amount of heat at the ELA to melt just the amount of snow that falls. Observations show that, although the ratio varies quite a bit, you get about 5 mm of melted snow for every positive degree-day, that is, for every degree Celsius sustained above the melting point for 24 hours. The snowier it is, the lower the ELA has to be, because it is warmer at lower altitude.

If the winter snowfall is equivalent to 10,000 mm of meltwater, you need about 2,000 positive degree-days in summer to melt it all. A snowfall of 1,000 mm of meltwater-equivalent requires only 200 positive degree-days, and 100 mm means only 20 positive degree-days. These numbers span very roughly the range of actual snowfall on real glaciers, from coastal Norway and Patagonia near the snowy end to the highest glaciers in Bolivia and Tibet near the dry end.

If your glacier has to keep flowing downhill to find an ELA that is hot enough, it eventually reaches sea level. It becomes a tidewater glacier, and icebergs start falling off. The ocean is doing some of the work (of adjusting the size of the glacier to the climate) that the atmosphere can't manage by itself.

Mountain ranges are traps for moisture, and potentially for snow, because they force the air to rise, cooling it and condensing out the moisture. What is more, once the wind has negotiated the mountain range it will be a lot drier, so we invariably find that across our glacierized mountain ranges the ELA rises to leeward.

This is the essence of what "maritime" and "continental" mean to glaciologists. We stretch the words so that maritime means "warm and snowy ELA" and continental means "cold and dry ELA". Some of the most "continental" glaciers are close to shorelines. There are not many maritime glaciers far inland, but the ones in the eastern Himalaya probably qualify, because the monsoon still packs a punch even after blowing over peninsular India.

Like all scientists, I get a lot of scientific papers to review. Between a fifth and a third of the ones I get start with "Glaciers are sensitive indicators of climate change", and I always cross it out, commenting that it is a boring cliché. The thing about glaciers is that they are insensitive indicators of climate change, because they integrate over temperature, precipitation, winter, summer, altitude, and possibly a significant horizontal extent. In the process they tell us things that no thermometer or rain gauge can.

They tell us, for example, that there is more to climatic change than rising temperatures. Because it had got snowier, the maritime glaciers of coastal Norway, but not the continental ones further inland, were gaining mass until about a decade ago. But more recent measurements show that rising temperature has now overcome this effect. And in the big picture, careful comparisons make it clear that less snowfall is definitely not why nearly all glaciers are losing mass. The message from the glaciers is that ELAs are rising, and rising because it is getting warmer. The reduction of snowfall that would be required to explain rising ELAs is enormous, and far beyond what we observe.

Strange and beautiful ice

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Rupert the Bear, a staple of my early literary diet, was regularly very impressed by the window art of his visitors Jack Frost and his sister Jenny Frost. We can explain much of this art rationally, but that enhances rather than diminishes its strange beauty.

At the Earth's surface, water ice crystallizes in the hexagonal system and snowflakes tend to have six arms or to grow as six-sided plates — which begins to account for their beauty. You can find beautiful snowflakes by the million in cyberspace. And of course when the weather is right you can just go outside, or even look at your window. But crystallography isn't the whole story.

What got me thinking about the beauty and strangeness of frozen water was a recent article by Toshiyuki Kawamura and co-authors. They describe spray ice, derived from the freezing of spray onto trees and shoreline structures. In bulk, the spray ice resembles "very large monster-like forms", as they put it.

They also describe ice balls, spheres (roughly) of ice, up to tens of centimetres across. The ice balls seem to form from lumps of slush on the surface of the lake. The lumps get rolled about by the waves and swell, frozen hard after a drop in temperature, then washed ashore by the waves and the wind.

The Kawamura photographs are technical rather than dramatic, but the freezing of liquid water onto solid objects can offer more drama than you need. Around where I live, we still remember the ice storm of 1998. Across eastern Ontario and Québec there are still trees, bent over in 1998, that haven't yet got back to something like vertical.

That ice was technically glaze, with a density near that of pure ice. The Kawamura spray ice is technically rime, less dense because it forms in irregular masses. Hoar is the equivalent, usually of still lower density, that forms by the deposition of water vapour as ice.

Rime or hoar often forms on our glaciological instruments when they are left out over the winter. If the instrument is a simple mass-balance stake, you just end up with an interesting photograph, like the one taken by my co-author Marco Möller on Austfonna in Svalbard. We like this one so much that we are hoping to persuade the publisher of our forthcoming Glossary of Glacier Mass Balance and Related Terms to put it on the front cover.

Rime on a mass-balance stake, Austfonna, SvalbardA mass-balance stake photographed by Marco Möller on the ice cap Austfonna in Svalbard. The sun has done just enough work to loosen the rime (or hoar?) with which the stake had been coated, and the rime has fallen to the surface as an intact body. (I also like the waves in the middle troposphere, picked out at the thinning edge of the cirrostratus veil by ice crystals condensing at the wave crests and sublimating in the troughs.)

But rime can be a glaciological nuisance. If your instrument is an automatic weather station or AWS, it will have been built for ruggedness and is likely to be still chugging away, recording the temperature, wind speed and other variables, even while more or less buried in the rime. How do you know how to interpret these records, or indeed whether you should?

For me, the weirdest frozen phenomenon of all has to be ice spikes. These protuberances grow out of confined bodies of water that are freezing from the top down. Their base-to-tip length can exceed 10 cm, and can be 10 or more times the base breadth. They taper towards the tip and grow upwards at seemingly random angles. According to Kenneth Libbrecht, the best way to study them is in ice-cube trays filled with distilled water and placed in an ordinary freezer with an air temperature near to —7° C. About half of the ice cubes will produce spikes in these conditions. A fan to promote air circulation also promotes spike formation. Although tap water doesn't yield as many spikes, plenty of spikes have been reported from out of doors, including the very first in 1921.

Water in a tray freezes at its free surface, starting at the confining walls and growing inwards. When the ice cover is all but complete, the water, under gentle pressure, has nowhere to go but up and into the little hole. If the liquid travels to the edge of the hole before entering the solid phase, you have a hollow tube that is evidently a working funnel for liquid water, which doesn't freeze until it reaches the propagating tip.

But why? Evidently there is an exquisite balance at the tip between the arrival of liquid, the removal of heat, the release of heat due to the freezing, and probably other factors. Nobody has yet managed to write down the algebra describing this balance.

Upside-down and inside-out icicles are somewhere beyond the borderline of relevance, but a lot of science is like that. The fact that the irrelevant is also the unexplained has a lot to do with why people get excited about ice spikes. And although they can't compete with Jack and Jenny Frost for beauty, they do show that strangeness can sometimes be strangely close to beauty.

By Graham Cogley

If you burrow in the literature of palaeoclimatology, climatic dynamics and glacial geomorphology, you can find all kinds of snowline: the transient snowline, the annual snowline, the climatic snowline, the regional snowline, the orographic snowline, and for all I know some others. Most of them are misnomers.

It is easy to see how the word "snowline" became popular. If you look at a mountain the line separating snow-covered from snow-free terrain is often the most striking feature of the view. This transient snowline is often straight and, as far as the eye can judge, horizontal. But it doesn't have to be. It can be messy, because of outlying disconnected patches of snow and patches above the snowline that are free of snow. What is more, it can and does wander up and down over the contours, which leads to uncertainty because we often say "snowline" when we mean "snowline altitude". On a single mountain or even a single glacier the snowline can range in altitude through hundreds of metres, so we really mean "average altitude of the snowline".

In glaciology, we have two further complications: we don't really want to know about the transient snowline but about the annual snowline, and we don't even want to know that so much as the annual equilibrium line.

The annual snowline is the transient snowline just before the first snowfall of winter, or whenever the mass of the glacier reaches its minimum in the annual cycle. Year upon year, comparing annual snowlines is much safer than comparing transient snowlines from random points in the cycle of the seasons. On the other hand, observing these annual snowlines is much trickier because of the likelihood that you won't get there in time or won't see anything if you do (because of the weather).

The annual snowline sometimes separates the glacier into an upper part that has gained mass over the year from a lower part that has lost mass, which makes it the same as the annual equilibrium line. But not always. If meltwater from the surface refreezes when it reaches the contact of the snow with the underlying glacier ice, we distinguish it as "superimposed ice", which represents mass gained by the glacier during the current mass-balance year. This is important for accurate book-keeping, but it also means that the snowline altitude can be tens of metres or more above the equilibrium-line altitude, with exposed superimposed ice in between.

In climatology, especially at regional and broader scales, these complications are usually set aside. This is where the word "misnomer" comes into its own, because except when they plot the snowline altitude on a graph against latitude the climatologists' snowline (regional, climatic, orographic, whatever — let's agree to call it the climatic snowline) is not a line at all. It is a two-dimensional surface. But we all know what the climatologists mean, and misuse of words is sometimes curiously unimportant as a barrier to the advancement of understanding.

The climatic snowline is a generalization, but a very valuable generalization, summarizing the state of the atmosphere near the Earth's surface in a very distinctive way.

Look at the climatic snowline in the graph. Set aside the complications and inaccuracies of usage, and ignore for a moment the colour scheme and the fact that the "line" is discontinuous — pretty fat at some latitudes, missing altogether at others. What we see is the altitude at which, if there were some land, there would probably be glaciers, or glacier equilibrium lines to be precise.

A global approximation of the climatic snowline A global approximation of the climatic snowline. South Pole on the left, North Pole on the right. Each little square is at an altitude which is the average of many "mid-altitudes", each of which is the average of one glacier's minimum and maximum altitude.

One curious thing about the climatic snowline is that it is nearly always assumed to be an isotherm, often the one representing a mean annual temperature of 0° C. The colour scheme shows that this is only a good assumption if you are willing to do a lot of ignoring. For example at latitudes between 55° N and 65° N the mean temperature of the snowline during the warmest month can be as high as +8° C (the lowest snowlines) or as low as —4° C (the highest snowlines).

The graph has been lying around on my hard drive for two decades. I only dug it out because I wanted to put Snezhnika, at 44.8° N and an altitude of 2450 m, into context. You can find small but stable glaciers like Snezhnika in mountain ranges where the climatic snowline is well above the highest peaks, but they are not a reliable reflection of the big picture. There is a lot more to be said about the big picture, but it will have to wait for another occasion.

What a bad idea it was for some layout person at New Scientist to label the photo of a Himalayan glacier with the caption "Himalayan glaciers will vanish by 2035". Putting "Some" before "Himalayan" would have made the story true, as opposed to false. Of course it would also have made the story boring, as opposed to attention-grabbing. That glaciers are vanishing is a commonplace of the journalists, and up to a point it is a truism.

But truisms need to be seen in true perspective. Some Himalayan glaciers have undoubtedly vanished already. When any regional inventory of glaciers is repeated, typically after a few decades, the count usually goes down slightly. Sometimes it goes up, if larger glaciers have fragmented into smaller ones. More often a few percent, or even just a fraction of a percent, of the glaciers have indeed vanished.

The true perspective on this is that the glaciers that have vanished were never very big in the first place. The last large-scale episode of glacier growth, the Little Ice Age, culminated 100—300 years ago depending on where you look. But wherever we look, the evidence is that nearly all glaciers have been shrinking since that time. It is likely that the ones that have vanished already are mostly the ones that came into existence during the few centuries leading up to the date of peak ice.

There is more to the true perspective, though. For a start, given the climatological evidence for warming, we need to know whether the rate of loss of ice is greater now than, say, a few decades ago. Here the glaciological evidence is unequivocal: it is. But there is still more to be said.

Plenty of small glaciers have failed to make it. South Africa lost its only glacier during the 1990s. Chacaltaya Glacier in Bolivia was highlighted as a disappearing glacier in volume II of the IPCC's Fourth Assessment, where you can see it dwindling from 0.22 km2 in 1940 to 0.01 km2 in 2005. It disappeared in 2009.

On the other hand, some tiny glaciers have survived. Recently Grunewald and Scheithauer reported on those of southern Europe (excluding the Caucasus). It might be a challenge to identify in some of them the flow that is required by most definitions of the difference between a glacier and a snowpatch, but all are the genuine article in the sense that they are still there at the end of every summer.

I bet you didn't know that there are two glacierets in Bulgaria. (I am betting on whether you knew, not whether you care.) The authors managed to retrieve ice cores from one of them, Snezhnika. It was 12 m thick at its thickest, and on average 3 m thick over an area of 0.01 km2, or 10,000 m2. Since the early-20th-century disappearance of Corral del Veleta in the Sierra Nevada of southern Spain, Snezhnika has been western Europe's southernmost glacier, at 41.77° N. Working their way northwards, Grunewald and Scheithauer document glaciers in Albania, Montenegro (this one a monster, five times the size of Snezhnika) and Slovenia.

These shrimps seem to be doing OK, although the picture is mixed elsewhere. For example in the Cantabrian mountains of northwest Spain all that is now left of the Little Ice Age glacierets is four buried lumps of ice, while Calderone Glacier, in the Apennines of Italy, split in two during 2009.

All this might provoke subdued mirth among more macho glaciologists, but glaciers that refuse to go away should elicit admiration for their pluck and stubbornness. They also remind us that gains by snowfall and losses due to sunshine are not the whole story of glacier mass balance.

Chris de Beer and Martin Sharp studied 86 glaciers smaller than 0.4 km2 in southern British Columbia and showed that between 1951 and 2004 a few disappeared and a few shrank, but most didn't change much. By careful analysis, they found that these objects have found sizes that are in equilibrium with their prevailing microclimates. Nearly all were in shadow for much of the time and were nourished significantly by snow avalanches from the surrounding terrain.

So the survivors offer a twist in the plot of the mass-balance story, but they do not point to flaws in our understanding of climatic change, and nor do the less fortunate ones. We should expect more and more disappearances as time passes, but should not panic when the journalists tell us that "Glaciers are vanishing".