31 January, 2006

More celestial pyrotechnics from NASA?

In the aftermath of the collision of Deep Impact with Comet Tempel-1, one of the scientists in the control room cried, "I can't believe they pay us to have this much fun!" If this report in New Scientist is anything to go by, the experience has given NASA scientists a taste for using their space probes as ordnance:

A plan to drop a quarter-tonne copper ball through Mars's atmosphere and study the ejecta it blasts away from the planet's surface on impact is to be proposed to NASA.

…The idea behind THOR (Tracing Habitability, Organics, and Resources) is to fly an observer spacecraft to Mars and, hours before it reaches the planet, release an "impactor" ball. It could be up to 230 kilograms in mass and would be aimed at a region about 40° north or south of the equator.

The impactor, likely to be a giant copper sphere, would crash to the surface at more than 4 kilometres per second, blasting a crater about 10 metres deep.

The concept is very similar to Deep Impact, in that the observer spacecraft would analyse the debris thrown up by the impact with spectrometers, allowing scientists to get a peek at the composition of the Martian sub-surface. The main aim of this experiment is to test for the existence (or not) of low-latitude ice, although it might also detect organic compounds which might give hints of life (which, if it’s there, will surely appreciate having a quarter-tonne of copper dropped on its head from orbit). It’s quite a cool idea, in mad sort of way.

30 January, 2006

Locational irony

Before people get the wrong idea about where I fit in the Earth Science spectrum, I should emphasise that I’m not an oceanographer – I’m a geologist. I just happen to work at the National Oceanography Centre in Southampton (NOCS, an acronym which still sounds faintly dodgy to me, although I can’t really pinpoint why). Another name would be 'The Institute formerly known as Southampton Oceanography Centre' (a long story, but basically our new Director thought it was a good idea). Harry Bryden also works here, which is one of the reasons that I’ve been inspired to write not one, but two, posts about the recent excitement over the thermohaline circulation. Not that I’m pretending that working in the same building gives me any particular insight into the research: in fact, the only meaningful conversation I’ve ever had with Harry was a couple of weeks ago, when we discussed microwave methods for jacket potatoes (he was waiting for me to finish heating my lunch in one of the canteen areas).

NOCS is jointly run by the University of Southampton (the School of Ocean and Earth Science is located in the building) and the National Environment Research Council (NERC), one of the UK government’s research bodies. By ‘jointly run’, I mean that we have both University and NERC staff in the building; I don’t mean to imply that they actually talk to each other or anything like that.

Courtesy of Google Earth, which I’ve been having a lot of fun with recently, lets have a look at the Oceanography Centre:

Southampton is on the south coast of England (if you're still lost, look here). You can find NOCS in the middle of Southampton Docks. The two rivers at the top, the Test (to the east) and the Itchen (to the west), combine into Southampton Water, which opens out into the Solent (the channel separating us from the Isle of Wight) off the bottom of the image.

Here's the NOCS building itself. Believe me, it's not any prettier at ground level. I've marked where my office is. It is from here I work, in the words of one of my colleagues, as a ‘skivvy for a higher power'.

I'd like to draw your attention to two features on this image. The first is that NOCS is located right on the dockside, which is, you are probably thinking, an ideal location for an oceanography research centre; research vessels can park right alongside between cruises, allowing scientific equipment to be easily transferred to and from the ship. Unfortunately, the lack of any such ships in the image isn't just an accident of when the satellite went over. When the building was commissioned no-one thought to negotiate with the port authority about berthing fees, so they charge our ships commercial rates - these are so high that it's actually cheaper to park the ships somewhere else and transfer everything by truck. Places like Hull.

There is something on our dock, however. See the red things at the northwest end? That's our remote operated vehicle(ROV), called ISIS. If you squint, you can see the launch cradle for the ROV and a container containing the control apparatus, which I've highlighted in the magnified image:

Again, and again unfortunately, is it no accident that the satellite has caught it on the docks, because that is where it has remained since arriving at NOCS in mid-2003. In another instance of stunning foresight, no-one thought to secure funding to do anything with our expensive new toy (which has led some wit to recast the acronym as Is Still In Southampton), so there it sits with nothing to do.

So if you want locational irony, I have it in spades: I'm a geologist based at an oceanography centre, a centre which has a dockside location but rarely sees a research vessel, and an ROV which hasn't touched water for almost three years. And if that's not enough for you, try the fact that although I've been based in the UK, I've spent the last four years studying the tectonics of New Zealand. Fortunately, most days I find this quite funny.

29 January, 2006

Have I made your eyes hurt?

I've been doing a bit of tinkering with my template to personalise things a bit more around here. The main changes so far are to the title bar and the colour scheme, the latter of which is always a risky proposition where I'm concerned. If I'm committing crimes against the colour wheel, the more graphically-inclined of you should let me know.

28 January, 2006

In memory of Nick Shackleton

Nick Shackleton, one of the pioneers of palaeoclimate research, died last week. By developing the methods required to measure oxygen isotope variations in ocean sediment cores (or, more correctly, in the shells of foraminifera which were preserved in them), he was able to construct a complete record of the waxing and waning of the polar ice sheets. He further showed that these changes could be linked to variations in the shape of the Earth's orbit and could thus be used as a basis for dating the cores. Isotope stratigraphy is now routinely used by scientists worldwide to date and correlate past climate changes.

One of the things that I really enjoyed as an undergraduate at Cambridge was the fact that lectures were given by people at the top of their field; if you want to to enthuse and excite the students, nothing beats someone who is enthused and excited themself telling you about their research. It was no therefore no surpise that the isotope stratigraphy lectures were handled by Nick himself. He was the archetypal academic, sandals and all, and prone to go off on a tangent in the middle of a lecture; but just being in the same room as someone of his stature was an experience. In fact, he and his cohorts almost convinced me to go down the palaeoceanography path, until I found how much I disliked picking forams.

The Cambridge Quarternary Group have a more detailed summary of his life and work here, and have posted some of the tributes which they have received since his death here.

It is sad news, but Professor Shackleton developed an entire subdiscipline of Earth Science and worked at the forefront of it until he died, made a number of fundamental contributions to our understanding of the planet's climatic history, and is warmly remembered by all. That's not a bad legacy.

27 January, 2006

IDiots in the UK

The BBC reports that a survey of public attitudes towards evolutionary theory, taken to coincide with an episode of Horizon about Intelligent Design (ID), found that only 48% of the people questioned thought evolutionary theory ‘best described their view of the origin and development of life’, whereas 17% and 22% opted for ID and creationism, respectively.

ID watchers over the other side of the Atlantic are scratching their heads at our apparent eagerness to join them on a ride down the slope of scientific illiteracy, but I’m not sure it’s as bad as all that, as I’ll discuss in a moment. First, however, I’ll briefly review the Horizon which prompted it. The piece was entitled ‘A War On Science’, and examined the ID movement mainly through the lens of the recent Dover Trial. We got to meet all of the design big guns: Philip Johnson, Stephen Meyer, Michael Behe and William Dembski (I was most entertained by the fact that Behe was interviewed in a fairground – an interesting choice!). Their arguments were mainly addressed by Ken Miller (whose simple rebuttal of Dembski’s post-hoc mathematical handwaving was especially good), with some support from people more recognisable to us Brits: Richard Dawkins came across much better than he did in his own programme (and had the best put-down, with his complaint that in all of science, it was only in biology that researchers had to waste time battling ‘the yapping terriers of ignorance’); and it was especially nice to have David Attenborough weigh in, and strongly too, on the side of empirically supported science.

This was all reasonably good, but problems arose towards the end of the programme, as we awaited the outcome of the Dover Trial and the narrative tried to ramp up the tension. As it did so it blurred the distinction between the effect the result might have on the opinions of the scientific community (i.e., none) and its wider cultural impact (given that many of the interviews seemed to have been done before or during the trial itself, I think the producers may have been expecting the reverse decision and were collecting material for ‘The Day Science Died’). The implication that the courts are the ultimate arbiter of things scientific is, of course, completely wrong. Nonetheless, the whole ID thing has much less exposure over in the UK than in the US (I’m just a strange Panda’s Thumb addict), and I think in that context it was a pretty good introduction.

With that it mind, what is the meaning of this survey? Personally, I feel there’s some room for ambiguity in these results, because I don’t think many people in the average UK street would actually know what ID is. How many of the 17% who opted for ID knew what it was before they were asked about it? The BBC definition (which may or may not have been the one used by the survey) is:

Intelligent design is the concept that certain features of living things are so complex that their existence is better explained by an "intelligent process" than natural selection.

This is fine if you already know what ID is, but to the less aware would be quite easy to mix up with some sort of ‘guided evolution/God probably had something to do with it somewhere’-type theism. Of course, without knowing the specific questions that were asked, it’s hard to know for sure.

Gloom can also be offset by comparison to two recent survey results from the USA, here (which asked specifically about views on human evolution) and here. It isn’t possible to compare the results from these surveys in a completely objective way, because they asked slightly different questions and allowed for slightly different responses, but I’ve given it a go in the chart below: the horizontal axis plots response groups from the three surveys according to how ‘sciency’ their views are (or how ‘godly’ they aren’t).

The distribution of opinion for the UK survey is almost a mirror image of that in the States – if the Stateside bloggers found 48% of their own citizens were suddenly evo-friendly, I think they’d be cracking open their beers in celebration…

The other reported part of the survey, which appears more worrying at first glance, asks the respondents, from the three alternatives, ‘which one or ones they would like to see taught in science lessons in British schools’ (my emphasis). While 69% wanted evolution taught, ID and creationism also garnered 41% and 44%, respectively. Because people could choose one, two, or all three, without further information on the distribution of responses it’s difficult to assess what’s going on. I strongly suspect that people who thought creationism should be included were also keen on ID (as an ‘at least’ option) and there will be very little overlap between evolution and ID/creationism voters. I could be wrong of course, and 40% of people wanting to stick apologetics into science class is still worryingly high (unless people were thinking along the lines of showing how Darwin overturned the creationist model - I covered heliocentrism in GCSE Physics in a similar fashion). Hopefully the pollsters will clear everything up at some point by publishing the results in more detail, including what they actually asked!

25 January, 2006

What, no polar bears?

A couple of weeks ago, I discussed Bryden et al.'s widely reported Nature paper, which claimed a significant slowdown in the thermohaline circulation, and suggested that we're going to have to wait to see whether the results reflect a real trend, or are just a combination of error and natural variability in a poorly described system. All very well, but then I concluded:

In a couple of years, maybe we'll know whether I'll retire to vineyards or polar bears in my back garden.
Flippant, perhaps, and as it turns out also rather foolish. Via Real Climate, I've just come across a nice discussion (subscription required, I'm afraid) of how, even if the conveyor is slowing, its impact is not quite as simple as on=vineyards, off=polar bears.

One interesting aspect of this article is the caution with which most climate scientists are treating these results. Some example comments:

  • "The story is appealing, but it is a very extreme interpretation of the data."
  • "Bryden's results are extraordinary, but this is exactly why they require extraordinary evidence."

If nothing else, this shows exactly how slanderous so-called 'critics' are being when they accuse climate scientists of being more interested in playing politics than objective scientific analysis. If all they were interested in was scaring people into their liberal, social-engineered worldview and securing research funding, such data is an open goal for spinning any number of apocalyptic scenarios, a la James Lovelock. Instead, they are quite open about their doubts, which stem from two main sources. The first is observational: a reduced thermohaline circulation should reduce heat transport into the North Atlantic, but there has been no drop in high latitude sea surface temperatures, and Europe has been warming, not cooling, in the last decade. Secondly, the estimated increase in fresh water added to the surface North Atlantic is lower, by an order of magnitude, than that required by modelling to stop NADW formation. Much more warming is required before the thermohaline circulation should be seriously affected.

But going back to the quote from my previous post, the other message from this article is that although reduced NADW formation can be linked to cooling of Europe in the past, it is by no means clear that a shutdown will also lead to cooling in the future, and in fact most modelling seems to indicate it will not. At the end of the last Ice Age many think that the presence of sea ice provided an important positive feedback (through both the familiar ice-albedo effect, and by preventing heat transfer between ocean and atmosphere) which exacerbated cooling in response to slowing of the thermohaline circulation; our present warming climate reduces sea ice cover and hence the effectiveness of this mechanism. As Wally Broecker, one of the pioneers of this whole idea, puts it:
"The notion that a collapse of the thermohaline circulation may trigger a mini ice age is a myth."
But even if the local effects we might have expected may not be on the cards, this does not mean that we should not be concerned: the thermohaline circulation also plays an important role in supplying nutrient rich bottom water to many parts of the world, and additionally enhances the ability of the oceans to absorb CO2 from the atmosphere (by transferring dissolved CO2-rich water to the deep ocean). A shutdown could still have serious consequences. But, whatever happens, it appears that my garden will remain polar bear free.

Newer developments:
The Case of the THC "Shutdown"
THC not as weak as we thought - most of the time

19 January, 2006

We're all dooooomed!

On Monday, James Lovelock, the originator of the Gaia concept, published this article in The Independent, in which he gives a rather gloomy diagnosis of the planet’s health:

The climate centres around the world, which are the equivalent of the pathology lab of a hospital, have reported the Earth's physical condition, and the climate specialists see it as seriously ill, and soon to pass into a morbid fever that may last as long as 100,000 years. I have to tell you, as members of the Earth's family and an intimate part of it, that you and especially civilisation are in grave danger…

…before this century is over billions of us will die and the few breeding pairs of people that survive will be in the Arctic where the climate remains tolerable.

These levels of doom-mongering are unfortunately far too easily caricatured as a typical outburst from the sandal-wearing eco-evangelist set, and unlikely to be taken that seriously by anyone who isn’t worried already. Which is a shame, because this article contains the kernel of a point which I think should be hammered home to those who refuse to take our effect on the planet seriously at every opportunity: we need to spend much more time talking about the fragility of human civilisation in the face of rapid climate change. Whose crops fail when rainfall patterns are altered? Whose cities are flooded when sea-levels rise, or flattened by increasingly severe storms? Whatever damage we’re doing to the rest of the planet, our own house is also tottering on increasingly rotten foundations.

You might be surprised to learn that I’m not particularly worried about the long-term health of the planet, but that’s because I’m using long-term in the geological sense, and palaeontology teaches us that life as a whole can endure and bounce back from even extreme calamities. However, it also teaches us that change is the key to this recovery – new species supplant the old, and despite our technological prowess there’s no guarantee that we can buck that trend. In 50 million years, if Lovelock’s fears are realised, the planet might be host to another sentient species, who will puzzle over the apparently self-inflicted extinction of a tool-using, hairless race of primates which clearly had ideas above its station. They’ll probably also curse us for burning all the oil.

The obstacles to healing Gaia’s fever lie not in means, but in motive, in finding the will to possibly compromise our short-term comfort for a long-term goal. Sadly, this is not a defining characteristic of our species, as Lovelock discusses in his own hyperbolic way:

…sadly I cannot see the United States or the emerging economies of China and India cutting back in time, and they are the main source of emissions. The worst will happen and survivors will have to adapt to a hell of a climate.

To the cynic, the reasons for the pessimism become clear when we are informed that Lovelock has written a whole new book, ‘The Revenge of Gaia’. Ah-ha. Interestingly, if the synopsis if accurate, this expands his opinion from ‘We’re all dooooomed!’, to “We’re all doooomed! Unless you do what I say!” Even he thinks we have some hope, he’s just suppressing it for promotional purposes (in fairness to The Independent, they do let someone else argue that the problem is not yet insoluable).

Self-interest is a powerful force if we can harness it, trumping altruism every time. As more people realise that it’s not just the whales and rainforests which are at stake, but their own necks, they are more likely to start worrying, and forcing the hand of our spineless leaders.

16 January, 2006

Stardust doesn't crash

The capsule from the Stardust probe, carrying particles collected from the halo of Comet Wild-2, has landed intact in the the Utah desert. This must have come as a bit of a relief for the scientists involved after what happened to the similar Genesis mission, which returned solar wind particles to Earth in 2004 with a much bigger bump than expected, because the deceleration sensors which were meant to trigger the parachutes were put in backwards (it seems, incidentally, that not all has been lost from that mission).

So why the obsession with chasing dust? It's all to do with trying to work out how the solar system formed 4.6 billion years ago, a process which is still poorly understood. We have a general theory: a cloud of interstellar gas began collapsing under its own gravity into a solar nebula, triggered by a nearby supernova or other disturbance. This collapse heated up the centre of the nebula enough the trigger nuclear fusion, forming the Sun. Meanwhile, material in the cooler outer parts of the nebula condensed and coalesced into small protoplanets, which through numerous collisions eventually formed the larger planets we see today. Knowing the end result - small rocky planets closer to the sun, icy gas giants further out - provides important constraints on how this process occurred, but not enough to really understand what happened. You can produce several quite different models which give similar end results, mainly because we have to guess our starting conditions - the initial composition of the solar nebula is poorly known, and which model works best in producing a familiar solar system depends on which initial composition you assume.

This is where Comet Wild-2 (pronounced "Vilt-2", which I personally think makes it sound far less exciting) comes in. Comets are left over bits of material from the outer parts of the solar nebula, which were too far away from each other to coalesce into anything bigger; their composition is therefore likely to be very close to that of the original nebula itself, because only in large bodies can mixing and differentiation of elements (such as in the Earth, where heavy elements such as iron have been concentrated in the core) occur. Wild-2 has spent most of it's life in the outer reaches, meaning that has been kept in pristine condition; only recently, when a close encounter with Jupiter in 1974 slung it into the inner solar system, has the sun been close enough to start boiling off its volatile materials. This change in orbit also placed it within much easier reach of Earth, making a mission to return samples of material unaltered since the birth of the solar system possible (for the curious, in the case of Genesis, the outer layers of the sun from which the solar wind originates are also thought to contain material which is as yet unaltered by nuclear fusion from its initial state).

As well as constraining our models of planetary formation, knowing the composition of the early solar nebula can also answer other important questions. One thing scientists want to know is, is our sort of solar system commom, or rare? Almost all of the extra-solar planets we know about are gas giants orbiting their stars closer than Mercury does. Is this due to present limitations with detection techniques (after all, it was less than 10 years ago that no-one could say for sure that there were any extra-solar planets), or is our home system a weird aberration? By comparing our own solar nebula to other planet-forming nebula (for example, in the Orion Nebula), we might get some clues. Additionally, people will be looking for the presence (or not) of organic molecules, which may indicate that collisions with comets billions of years ago may have provided the raw materials for the emergence of life on Earth.

10 January, 2006

Richard Dawkins' The Root of All Evil?

You have to love Channel 4. On the one hand, this is the channel that is currently bringing us Celebrity Only (But In Their Own Minds) Big Brother, and is responsible for Space Cadets, the current low water mark for 'reality' (or 'make the proles look stupid') TV. But then last night it gave Richard Dawkins, the atheist Christians love to hate, the first of two prime-time slots to put forth his...erm...passionate views on religion (go here for the Channel 4 blurb). Now that's diversity.

I must admit straight away that I like Richard Dawkins. His writings about evolutionary biology are models of clarity, and in his more general writings on science he articulates an awe and wonder at the Universe revealed by science makes you realise that he is a long way from the nihilistic caricature often called up by his opponents. However, he makes no secret of his disdain for religion, and his use of his position in the public consciousness to make negative pronouncements on 'the process of non-thinking called faith' have earned him censure even from those who have sympathy for his views. But let's face it, religious bigotry is hardly soley the purview of the atheist; indeed, to paraphrase one of Dawkins' more famous quotes (which he used himself in the programme last night), we are all condescending and bigoted about most of the religions that humanity has ever believed in; he just goes one religion further.

That said, I'm not really sure what this program was trying to achieve. The actual scientific content was quite thin (what was there was well-explained), and as a meditation on why people reject reality in favour of ancient holy texts it wasn't particularly illuminating either. This was, I feel, the fault of both the interviewer and also the choice of interviewees. Dawkins was disinclined or unable to conceal his distaste for the views of those he was talking to - I suspect the former, because he also made several statements which were bound to inflame the discussion. For example, when talking with the Ted Haggard, president of the National Association of Evangelicals, he told him,'I was impressed by your show, it reminded me very much of...' paused, waited for everyone to mentally fill in '...rock concert', then finished '...the Nuremberg rallies'. Not likely to go down well. And it's really not much of a revelation that Haggard is not a great believer in the scientific method - I'm more interested in the why. Because this stuff is not restricted to the American Bible Belt; I personally know many evangelical Christians, both in the UK and abroad, who are surprisingly (and disturbingly) sympathetic to notions of Biblical literalism, and who are therefore rather uncomfortable when I talk about things geological. Why do they reject the majestic, strange and mind-blowing cosmos revealed by science, far more worthy of their God, in favour of the limited and petulant deity perceived by our ancestors? I don't think you're not likely to gain much insight into this question when you compare your subject to the Nazis (the interview with Haggard did, however, provide the most entertaining moment in the whole hour, when he said that he knew scientists 'who did not believe that structures like the eye and the ear could ever have arisen by accident'. Dawkins' own eyes almost exploded in the face of this misrepresentation of evolutionary theory).

Trawling the blogosphere for other posts which discussed this programme, two stand out. There's a much fuller review than I have time for at leyton.org; and A Very British Dude points out that Dawkins, rather oddly perhaps, omitted to talk about how the religious 'meme' from an evolutionary perspective (it might, for example, confer some advantage in terms of creating united, expansionist societies). I wonder whether Dawkins' second programme, unprovocatively entitled 'The Virus of Faith', might talk about that. Dude's conclusion is a bit dystopic for my taste, but it's well worth a read.

08 January, 2006

What does 'Highly Allochthonous' mean?

There are two different answers to this question, the first of which is the technical one. An allochton is a chunk of landscape which has been superimposed by faulting on top of rocks representing a completely different depositional environment; for example, a sequence of deep marine sediments which has been thrust over shallow marine or continental deposits. Therefore, a 'highly allochthonous' sequence is one that has been transported a large distance, usually by thrust faulting (I want to put a graphic in the title bar which expresses this concept, and who knows, maybe I'll actually get round to it at some point).

The second answer (which I know doesn't really flow from the question except in a highly tenuous way - deal with it) is the 'so what's this blog all about then?' one. I could tell you that the name 'poetically reflects my aspirations to cover the vast diaspora of disciplines which make up Earth Sciences', but in truth it's just a cool phrase which formed part of the banter when I was a geology undergrad, and it just seemed appropriate. On balance, I think the geeky true answer is preferable to the poncy false answer. Probably.

As for why I'm doing this - well, I like writing and I like science (my interest just about surviving exposure to the National Curriculum), so I've always been interested in science communication. More recently, as my PhD submission loomed, I've been looking for jobs in that sort of area, but with very little success. So, inspired by some of the excellent science blogs out there, and the fact that Geology appears to be a little underepresented in the blogosphere (from my searching thus far,at least), I thought this would be a good way to practice and develop my writing. It's good for me scientifically too, forcing me to read more widely that I might otherwise; for example, whilst writing my last post, I had to learn about aspects of oceanography I hadn't really covered in detail before. And I'm sure my teaching can only be improved by thinking more about ways to explain technical details of my subject more accessibly. Perhaps fortunately, none of these things specifically require my outpourings to be read! If people do visit and get something from my postings, that will just be a nice bonus (and of course is the thing that I hope for deep in my heart).

I started this blog in a blaze of enthusiasm last September, and realised quite quickly that producing regular posts was not quite as easy as I thought it would be. My so-called 'technical' job has involved a lot of teaching and other tasks strangely reminiscent of an academic position, on top of running the paleomagnetic laboratory and processing data for people. This has given me little enough time to do anything about my own research, so finding the time to write for this blog when I actually felt like writing has proven tricky. And writing each post seems to take much longer than I expect it to, although I'm hoping to get faster with more practice. I'm probably not helped by the fact that for the most part I don't really see the point of posting short links to news stories published elsewhere without adding my own analysis of my own. I'm not sure that will change, but I am hoping to post more regularly than the monthly spurts I've managed up to now. As for sustaining it - well, I have several ideas for posts stored in my brain (and even occasionally partially begun on my laptop), and more seem to pop up all the time, so it bodes well, or ill, depending on your perspective!

The Decade After Tomorrow?

Authors note: This post was mostly written before Christmas, but I then got distracted and didn't get the chance to finish it. I guess it's a bit less topical now but I don't like to waste hard-written words...

What does the the paper published in Nature last month by Bryden et al.[1] have in common with a recent Hollywood disaster movie? Answer: before The Day After Tomorrow descended into 3000km-wide superstorms sucking down air from the stratosphere and turning everybody into icicles (casually breaking the laws of thermodynamics in the process), both are concerned with a possible weakening of the thermohaline circulation in response to climate change.

However much we moan about the weather here in the UK, we should bear in mind that we are located roughly as far north as Hudson Bay. Which, it could be argued, is a tad cooler than here. The reason for this is that we (and the rest of Western Europe) benefit from the Gulf Stream, a strong current of warm surface water that flows from the Gulf of Mexico to the northeast Atlantic, bringing a lot of heat from the tropics with it. A large part of this north-eastward flow is wind-driven, but not all of it is. In the current climate system, heat loss and evaporation causes seawater in the northern Atlantic, particularly in the Nordic and Labrador Seas, to become so cold and salty, and therefore dense, that it sinks into the deep ocean and moves south back towards the equator below the surface currents, at depths of several thousand metres. About 20-30% of the total northward flow of water in the Gulf Stream is moving to replace the downwelling water in the North Atlantic as part of this 'thermohaline circulation'; the additional heat transfer from equatorial regions, and the penetration of warm water to much higher latitudes than would result from wind-driven flow alone, combine to produce much higher seasonal temperatures on our side of the Atlantic.

Looking back into the past, we can see a link between the strength of the thermohaline circulation and high latitude temperatures in the Northern Hemisphere. The sinking water in the Nordic and Labrador seas - often referred to as the North Atlantic Deep Water (NADW) has chemical properties which make it distinct from, for example, water from the oceans around Antarctica, or the Pacific. By taking sediment cores from the South Atlantic and chemically analysing shells or minerals that formed in ancient seas thousands of years ago, the penetration of the NADW into this region, which is greater when the thermohaline circulation is stronger, can be estimated (e.g. [2]). Such studies show that at the end of the last Ice Age, about 19,000 years ago, when glaciers covered much of northern Europe, the thermohaline circulation was much weaker than today, and strengthened over the next 10,000 years as we moved into the present (warm) Holocene period. More significantly, the sediment cores also indicate rapid oscillations in the penetration of NADW, where the thermohaline circulation suddenly got much stronger or weaker - oscillations which can be correlated to abrupt warming or cooling events in the northern hemisphere. These variations apparently occurred over very short timescales - certainly not three days, as a certain movie would have you believe, but in the course of decades, still extremely fast by geological standards.

The current warming trend is making surface waters in the North Atlantic less saline (and therefore less dense) by adding large amounts of fresh water - for example, from the melting of the Greenland icecap. This can potentially reduce NADW formation by weakening downwelling. Models of the thermohaline circulation suggest that once enough meltwater is added, the circulation abruptly collapses into a much weaker current. Furthermore, this change is not reversible - the density of surface waters has to increase substantially before vigorous NADW formation restarts.

Whilst this is all very worrying, up until now there has been no real indication that the increased supply of meltwater to the North Atlantic has been having the predicted effect. Now, however, Bryden et al. report the results of the latest of a series of east-west transects (starting in 1957, with the most recent in 1998 and 2004), measuring variations of water temperature and salinity with depth at various points, across the Atlantic Ocean at 25 degrees North. These measurements can be used to calculate the 'geostrophic flow': Coriolis forces caused by the Earth's rotation mean that north- or south- flowing currents in the Atlantic oceans generate east-west pressure gradients, which can be calculated from the temperature and salinity profiles (in case that made no sense whatsoever - not unlikely - this post at realclimate.org is a good starting point). Compared to the previous transects, results from the latest one in 2004 appear to show a 50% reduction in the southward flow of water between 3000 and 5000 m depth, which is sourced from downwelling water in the Norwegian Sea.

However, before you start investing wholesale in your uncle's woolly hat company, it's worth reading the small print. Because the current strengths are being estimated by an indirect method, the potential errors are quite large; in fact, the authors estimate that the error is about the same size as the apparent decrease in NADW flow. They also argue that because the observed reduction is associated with a particular component of the flow, rather than distributed at all levels, the effect is real, which is a fair point but not a conclusive one. The other problem is that there is virtually no data on the natural variability of the thermohaline circulation, so it is also possible that the most recent survey is just detecting this rather than a sustained reduction.

These caveats make it hard to really assess the significance of these results. But watch this space - the cruise which collected the data for this study was part of a programme to deploy a series of buoys along the 25N transect, which will continuously monitor the thermohaline circulation. This system will give a much better idea of the seasonal variability of the thermohaline circulation, and also reduce the errors in estimates of its strength. In a couple of years, maybe we'll know whether I'll retire to vineyards or polar bears in my back garden.

Newer developments:
The Case of the THC "Shutdown"
THC not as weak as we thought - most of the time
What, no polar bears?

[1] Bryden et al (2005). Nature 381, p655-657.
[2] Piotrowski et al. (2004). Earth and Planetary Science Letters 225, p205-220.