This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

IOP A community website from IOP Publishing

Powered by Movable Type 4.34-en

In from the cold: February 2010 Archives

The idea that a super-enormous volcanic eruption — or hypereruption — would alter the climate dramatically has been around for a long time. It fits the facts about the biggest historical eruptions we know of, and also our understanding both of how volcanoes work and how the atmosphere works. But could the drama extend to tipping the climate from an interglacial state to a full-blown ice age?

The answer, as has long been believed, is still No, according to Alan Robock and colleagues in a paper published last year. They added several new kinds of potential cooling mechanism to two climate models, and were unable to trigger an ice age.

When a volcano goes off, it is always unpleasant for those in the immediate neighbourhood. The climatologist's concern, however, is with the broader consequences. A violent enough eruption can loft its products into the stratosphere, where they can persist long enough to spread around the world.

The main culprit is sulphur dioxide, SO2. It reacts with water vapour to form a haze of sulphuric acid droplets. The droplets increase the scattering of incoming solar radiation, making the atmosphere more reflective and cooling the Earth slightly. The more SO2, the more cooling.

The snag is that the haze doesn't last. The atmospheric effects of Pinatubo in 1991, the largest eruption of recent times, were detectable for a few years at most.

Krakatau in 1883 was bigger than Pinatubo. Tambora in 1815 was even bigger, and still stands as the largest eruption in the historical record. If we turn to the geological record, the largest eruption we know of is that of Toba in Sumatra, in about 72,000 BC. Toba yielded a quantity of stratospheric SO2 hundreds of times that of Pinatubo, which was about 20 megatonnes.

Robock and colleagues injected 300 "Pinatubos" of SO2 into the baseline run of their models, but also tried amounts as great as 900 Pinatubos. With a dynamic vegetation module, they explored the feedback on global temperatures of widespread death of vegetation due to the volcanic cooling. The feedback was not very impressive. Precipitation dropped markedly, but cooling reached about 10 degrees at most, and recovery was nearly complete after about a decade. Coupling the climate model to an interactive model of atmospheric chemistry, they found that the SO2 reaction products persist for longer and produce greater total cooling — as much as 18 degrees — but still no permanent, ice-age-like change in the climatic state. The cooling was partly offset by warming influences, such as more water vapour and ozone in the stratosphere, and more methane in the troposphere. All of these are greenhouse gases.

One thing that bothers me about the Robock study, which is a step forward, is that it still may not cover all the bases. For example the model runs may not have been long enough to pick up delayed responses of the ocean to reduced inputs of heat during the cooling episode. And the climate models were unable to follow the behaviour of the other sluggish players in the drama, the glaciers themselves.

On the other hand, look at what actually happened. In an older paper, Zielinski and co-authors found a signal from Toba in an ice core drilled in Greenland: about six years of strongly enhanced deposition of sulphate, followed by a 1,000-year long "stadial". Stadials, identified by looking at ratios of the isotopes of oxygen, are relatively short cool episodes within ice ages. However Toba was preceded by 2,000 years of more moderate cooling, which suggests that the stadial proper might have happened anyway. What is more, the oxygen isotopes repeat a very similar pattern in the 2-3,000 years after the end of the "Toba stadial": rapid warming, moderate cooling, rapid cooling, with no evidence for volcanism at all. In fact, these two excursions look rather like Dansgaard-Oeschger events.

So we have a plausible but not compelling link between our only known hypereruption and a limited amount of long-term cooling. If a Toba happened tomorrow, it might presage a short stadial, but not a long one, and anyway stadials ought not to be at the top of your list of things to worry about. But on the purely intellectual side, the effort to understand Toba nevertheless bears on an important question. How hard do we have to hit the climate system before it really gets upset, or, putting it another way, what does "tipping point" mean?

We have made some astounding intellectual advances in the past few millennia, and we do right to honour our fellows who make these forward leaps. It is proper to regard Isaac Newton, for example, as one of the most important human beings ever. But the 1% of inspiration would be nothing without the necessary 99% of perspiration, and most intellectual advances have been anonymous.

The oldest recognizably modern ideas about glaciers are no older than a couple of centuries, but the way for them was paved by a lot of preparatory observation and thought. Although neither the Greeks nor the Romans had a word for glacier, I cannot believe that nobody had observed or thought about glaciers before the eighteenth century.

Ötzi, the Iceman who died at the main drainage divide of the Alps about 5,300 years ago, probably crossed the divide regularly for pastoral purposes. Surely he must have had a word for the thing, now called the Niederjochferner, on which he died. Besides glacier and its relatives, there are several other words for the thing in alpine languages: vedretta and vadret in Romansch and Friulian, ferner in the dialect of the Tyrol, and kees, another Tyrolean dialect word. Several small neighbours of Ötzi's glacier are called kar. We do not know whether Ötzi used an ancestor of one of these words, but it would have been hard for him to move around and do his work without some such token.

I have not found any information about the history of vadret. The -et may be a diminutive suffix, or a relic of some meaning that has now been lost, but is it naive to wonder whether the vadr- part is a relative of English water? You have to allow a certain slipperiness in the meanings once attached to these tokens. The evidence consists of seeming parallels between the tokens. If you accept the parallelism, you may uncover evidence of thinking. In this case, the implied intellectual achievement is the recognition that ice and water are different aspects of the same thing. Somebody had to be the first to work this out.

Of course the tokens themselves are not unchanging. They gain and lose bits from time to time, which is why the linguistics experts are satisfied that water and Greek hudor, the ancestor of our prefix hydro-, are the same. For some reason speakers of Greek are not keen on the w sound.

kar, kees and ferner seem also to have no known history before the last few centuries, although ferner is interesting because it resembles firn, a German word for compacted snow, and perhaps fonna, Norwegian for an ice cap or snowfield.

Perhaps we can get somewhere by looking for the most basic idea. Latin glacies, ice, is traceable to a reconstructed Indo-European root *gel-, cold, freezing, with descendants in the Italic, Teutonic and possibly Slavic branches of Indo-European. English belongs to the Teutonic branch, and according to The American Heritage Dictionary of Indo-European Roots modern descendants of *gel- in English include chill, cool and cold itself.

When the root acquired its -k suffix, and what it signified, are unknown. Pokorny, in his monumental 1959 Indogermanisches etymologisches Wörterbuch, suggests that it was in fact -g and was simply an example of reduplication of the initial consonant. Pokorny also thinks that *glag became glacies under the influence of other Latin nouns such as facies, appearance, and acies, edge. Some say there is a connection with Greek galaktos, milk, explicable because milk and bubbly ice can resemble each other in colour. If this is correct, which word influenced the other is not clear, and anyway the independent status of Latin *gel- is demonstrated by gelu, frost, and gelidus, frosty, icy cold.

If we have not lost the track, the deepest layer of meaning in the word glacier is the idea of cold. It makes sense to me. Even Isaac Newton showed early promise as a glaciologist. The second sentence of his Mathematical Principles of Natural Philosophy, published in 1687, was about the compaction of snow. It is true that he then lost the track, for which we should be grateful because of the new path he opened up for later glaciologists. But we should also be grateful for all of the thinking that went on before Newton. Somebody had to be first to notice that you only get snow (Indo-European *sneigwh) and ice (Indo-European *eis) where it is cold.

Climatic change in Antarctica is complicated. The northernmost part of the continent, the Antarctic Peninsula, is warming at extreme rates, while elsewhere the pattern is mixed and in some parts there appears to be little or no warming. Up to a point, we glaciologists don't mind whether Antarctica is warming or not. It is so cold that even an implausible temperature increase wouldn't come close enough to the melting point to affect the mass balance.

Indeed, there is a plausible argument that warming would make the mass balance more positive. The Antarctic interior is extremely dry because the capacity of the intensely cold atmosphere to deliver water vapour, and therefore snow, is minimal. Warmer air can carry more water vapour, so snowfall should increase in a warmer Antarctica.

The evolving mass balance of Antarctica is most interesting around the edges, though. Warmer ocean water is increasing melting at the bases of ice shelves and pulling grounded ice across the grounding lines at increasingly scary rates. A modest increase in interior snowfall would not make this picture less scary.

Ice-stream dynamics is not the only interesting thing about the periphery of Antarctica. Here, in the least cold latitudes, we observe what little melting does happen. Spread over the continent, it amounts to a few mm of water-equivalent loss per year, against gains by snowfall of about 150 mm/yr. Losses by discharge across the grounding line are much greater. But melting, if negligible in the big picture, is still interesting.

In a recent paper, Tedesco and Monaghan update a standard measure of melt intensity in Antarctica, the so-called melting index. They watch the ice sheet's emission at microwave wavelengths (8 to 16 mm) and exploit one of the most useful radiative attributes of water. At these wavelengths, the emissivity of frozen water is low, and as conventionally presented in imagery it looks bright, but when it melts its emissivity rises dramatically and it looks black. An intermittently wet snow surface flickers between bright and dark, and we can keep track of melting by noting, in twice-daily overpasses by the imaging satellites, whether the image pixels are bright (cold) or black (warm).

The melting index, summed over a glacierized region for a span of time, is measured in square-kilometre-days, an odd-sounding unit but one that captures what we want to know. For each pixel it is just the number of days on which the pixel was black times the area of the pixel. For the whole region it is the sum of these pixel counts.

The Antarctic melting index has averaged about 35 million km2 days per year (October to September, to be sure of keeping the austral summer months together) between 1980 and 2008. Here comes the intriguing feature: in 2009 it was only 17.8 million km2 days, which is not only a record low but also continues a trend towards lesser annual indices that began in 2005. The melt extent (the area experiencing at least one day of melting) was the second lowest recorded, reaching only half the average of 1.3 million km2.

Tedesco and Monaghan account for this oddity in terms of slow organized variability in how the atmosphere behaves. Two patterns of multi-annual variation in the circulation of the southern atmosphere, the Southern Oscillation and the Southern Annular Mode, together correlate rather well with the melting index. But the authors acknowledge that the correlation breaks down in some Antarctic regions, and that the common variance does not point to a clear-cut physical explanation. (Translation: we don't understand what is happening.)

Antarctica is a happy hunting ground for climate denialists, but they need to be ignored because they are on a wild goose chase. In the first place, anomalous patterns of temperature change haven't stopped melting rates from accelerating, and ice shelves from disintegrating, in the warmest part of the continent. Second, global warming is global. Regional non-warming, and even regional cooling, don't invalidate the main conclusion. The fact that we don't understand why Antarctica is anomalous doesn't invalidate it either. Finally, when it comes to Antarctic change it's the ocean that we need to worry about. From the glaciological standpoint, warmer water is the problem, not warmer air.

It seems that if you want a bunch of comments about your blog you have to say something controversial. At least, that is what happened to this blog last week. I hope that we can get back sooner rather than later to tranquil consideration of the pleasures of studying glaciers, but in response to some of the comments there is more to be said about "denialists".

I will stick with that term, which some do not like, because it emphasizes a valuable distinction between denial and doubt. I was not talking about doubters, and in fact as an antidote to breach of trust I have lately been plugging the wisdom of Bertrand Russell as encapsulated in his first commandment: "Do not feel absolutely certain of anything". There is a world of difference between denial on the one hand and healthy scepticism, or even just asking questions because you don't know what to think, on the other.

There is also a world of difference between genuine ignorance and culpable ignorance. It is a capital mistake to suppose that I know a lot more than you do. I remember a long-ago field trip to look at glacial sediments during which we managed to get, er, lost. I was sitting at the front of the bus and by a fluke managed to get us unlost. One of the students said "How did you do that?!", at which point the bus driver interjected": "That's why you're a student and he's a professor." True, but superficial. There is an infinity of subjects about which I am genuinely ignorant.

I do know a superficially greater amount about glaciers than you (probably), which is why I am the blogger and you are the reader, but that is not important. One of the points I tried to make last time was that the denialists who commented on the news stories about the Himalayan-glacier fiasco are culpably ignorant.

I admit that "trust me, I'm a scientist" makes a lousy sales pitch, but nearly all of the denialist comments that I was deploring boil down to "trust me, even though I'm a dope". Seen from one angle, what I have just put down is a terrible thing for a scientist and university professor to say. It is rude and probably hurtful. It breaks elementary rules about how to make conversations work. (Don't rile your adversary. Give him a way out.) So it cannot possibly advance the discussion. Or can it? I have been worrying a good deal about this recently.

First of all, I am not selling anything. My scientific contributions about glaciers are just contributions, aimed at pushing the frontier of knowledge and understanding forward by a little bit. They are intended to be read critically and accepted or rejected according to the best judgement of the reader. Second, and more fundamentally, there is an awkward attribute of "trust me, even though I'm a dope" that I can't shake out of my mind, namely that whether or not it is helpful or kind or sensible it is a true paraphrase of the denialist comments.

To put it as diplomatically as I can, there is a problem at the core of the debate about climatic change, and the problem is the uniformly low calibre of the arguments on one side. The arguments on the other side vary from pretty good to compelling. There are loopy environmentalists, of course, but none of them contributed to the newspaper discussions I am talking about. I don't know how to solve this problem, but winking at it doesn't make it go away.

One thing about glaciers that doesn't get a lot of attention is that they are independent indicators of the state of the atmosphere. The river of reasoning has the spectral absorption bands of carbon dioxide at one of its sources, but further downstream it is braided. The information from weather stations is one of the channels, but the information from glaciers is a different channel. Even if, against all probability, the denialists were to succeed in knocking out my colleagues at the Climatic Research Unit at the University of East Anglia, they would still have succeeded at most in blocking one of the channels temporarily. The carbon dioxide molecules would still be absorbing and re-emitting infrared radiation. The consequent feedbacks would still be at work. The atmosphere would still be getting warmer. Most awkwardly for the denialist cause, the glaciers would still be shedding mass at an accelerating rate.

If you forget for a moment about the weather-station channel and about the carbon dioxide molecules at the headwaters of the stream, and try to explain why the glaciers are shrinking, and shrinking faster now than formerly, you come up against the considered judgement of the scientific community. Science has agreed that you can't answer these questions satisfactorily if you forget about the carbon dioxide molecules. And even if you persist in forgetting, you still have no coherent basis for tackling the question "What should we do about this?". The denialist answer is "nothing", but that brings me to my last point.

I want to emphasize that my comment-eliciting remarks last time were a direct criticism of those members of the public who can be described accurately as denialist, as opposed to sceptical or doubting. I haven't got a satisfactory answer for Clif Carl's poser about how to have his doubts addressed or for Steve Carson's thoughtful analysis of how best to bring travellers back from the borders of denial. I am not attacking the shadowy "vested interests" that are often blamed for climatic misinformation. Nor am I saying that the denialist citizens who comment on the newspaper articles are the dupes or stooges of these vested interests – which would be truly insulting. I am saying that we have to do something about improving the calibre of the debate, and I have no idea what. When it comes to the study of how the public makes up its mind, I am just another member of the public.

What I am saying seems to lead us to the absurdity of requiring ordinary citizens to spend their evenings and weekends boning up on glaciology, spectroscopy and a long list of other special subjects. The alternative seems to be for them to trust somebody, to which, as we have seen, there are objections. That is why I prefer to write about jam jars, baskets of eggs, fiords that turn out to be astonishing and stuff like that. Boning up on glaciers can be a lot more fun than it sounds like.