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In from the cold: July 2010 Archives

The Dutch Environmental Assessment Agency, PBL, has released the results of a minutely detailed search for errors in part of the second volume of the Fourth Assessment of the Intergovernmental Panel on Climate Change. The search focussed on the eight chapters that assessed regional impacts.

It turned up quite a number of errors, mostly too trivial to waste time over, but two of them glaring. One was about how much of the Netherlands is below sea level. The IPCC gave a wrong figure, 55%. A quarter is much closer to the truth. But in the words of the PBL, "the error was made by a contributing author from the PBL[!], and the [IPCC Coordinating Lead Authors and Lead Authors] are not to blame for relying on Dutch information provided by a Dutch agency." Really? My atlas shows land below sea level in a tasteful strawberry shade, so I would have thought that a glance at the atlas would make it seem unlikely that more than half of the Netherlands is below sea level. On the other hand, the Dutch have had their finger in the dyke for centuries, and so are unlikely to be misled for long by anything the IPCC, or for that matter their own government, says on this point.

The other error was about the water tower of Asia, but I don't want to revisit that one just now other than to repeat that blunders happen.

This is all so predictable, and in a cosmic sense so trivial. There is, however, a way to view these blunders in proper perspective, even though I know I shouldn't use the word "paradigm" in an article for popular consumption.

All of us in the sciences know what a paradigm is, because we have either read or been told about Thomas Kuhn's 1962 book The Structure of Scientific Revolutions. A paradigm is a big, governing idea, one that makes sense of a lot of other ideas that would be disparate without it. Kuhn argued that when a scientific discipline undergoes a revolution, it is actually undergoing a paradigm shift, in which an old paradigm is replaced by a new one.

I am not sure about the old paradigm. Kuhn says that a paradigm is a set of one or more past achievements that some scientific community acknowledges for a time as supplying the foundation for its further practice. The achievements have to be unprecedented enough to attract adherents, and open-ended enough to leave all sorts of unsolved problems for them to work on. I detest the relativistic sociology of "acknowledges for a time", but this paraphrase is important as a key to understanding the recent fusses about climatic change and IPCC mistakes.

I don't think there ever was an old paradigm in the atmospheric and neighbouring sciences. First of all, there has been no revolution. Climatologists have been doing what Kuhn calls "normal science" for centuries. The foundations in dynamics and thermodynamics are as they have been since the 17th century, and in radiation as they have been since the late 19th century. But atmospheric and oceanic dynamics, and radiative physics, describe systems that, though they change continually, always stay the same unless you mess with them.

In fact, the big unifying idea didn't burst onto the scene. It evolved. Fifty years ago and more, there weren't many "adherents" in the study of global environmental change because there was little to adhere to other than a vague idea that another Ice Age ought to begin any millennium now. But there is certainly something to adhere to today. These days, the big unifying idea is the greenhouse effect, and in particular the anthropogenic greenhouse effect. It unifies because it is triumphant at explaining the facts while generating more questions than it answers.

Denialists are fond of criticizing climatological claims that "The science is settled", or equivalently that "The debate is over". If any climatologists have ever used those particular words, then what they meant to say was that the paradigm is doing fine. It did not originate in a revolution, but it has adherents who see it as unprecedentedly successful and find it so open-ended that the science of climatic change is still growing explosively.

If you can find a big enough concept, like Kuhn's paradigm, even mistakes about Asian water towers and Dutch polders fall into intellectual place. They are storms in a teacup. Debate about Himalayan glaciers and the risk of flooding in the Netherlands will go on indefinitely, enlivened by the occasional howler. If and when something more intellectually powerful comes along, we will replace the paradigm, but for now it is firmly in place and there is no sign of a replacement.

The Zwally effect is an acceleration of the flow of marginal ice in the ice sheets due to lubrication of the bed by meltwater percolating from the surface. Up to a point, this phenomenon is not surprising. It is well documented on smaller, thinner valley glaciers. The surprise, first documented by Zwally and co-authors in 2002, is seeing the same phenomenon in ice as thick as 1,200 m.

The Zwally paper has stimulated a growing literature with two main threads. One thread tries to explain how meltwater can find its way through more than a kilometre of ice. The other tends to show that the Zwally effect is not the reason for dramatic increases in the speed of tidewater outlet glaciers, where the evidence favours, quite strongly, warm ocean water as the culprit. But that doesn't mean that seasonal acceleration is uninteresting.

Ian Bartholomew and co-authors report on more dramatic seasonal acceleration than has been measured hitherto. It still doesn't rival the speed-ups observed on some tidewater outlets, but the observations highlight the potential of GPS from a different angle, and suggest fascinating insights into how the surface meltwater does its subglacial work.

This new report relies on time series of positions obtained with four Global Positioning System receivers deployed along 35 km of a land-terminating flowline at 67.1° N in southwest Greenland. The data include not just horizontal but also vertical velocities, as well as near-surface air temperature. Averaged over the summer, the speed-up from winter background values was rather modest. But the fascinating bits are the details.

The further up-glacier, the later the onset of speed-up, by more than a month. The natural explanation is a later onset of melting at higher elevations. The highest site was at 1,063 m and the lowest at only 390 m above sea level.

More interesting is that the horizontal velocity correlates very nicely with the vertical acceleration, or in other words with the rate of uplift of the surface. The ice goes faster when the surface is uplifting rapidly. Or rather, rapid uplift seems to provoke speed-up. This is a subtle observation in more ways than one. For one thing, the amounts of uplift are a few decimetres at most. That we can detect such subtle vertical motions is a payoff for all the trouble it took to loft a couple of dozen GPS satellites into orbit.

More interesting still is the authors' subdivision of the summer into three phases. In phase 1, there is no particular surface uplift or speed-up: the meltwater, if any, has yet to reach the bed. In phase 2, the cumulative uplift increases towards a maximum, and so do the horizontal velocities, more or less. (You need the eye of faith to see these phases in the noisy data. But I buy them.) The concluding phase 3 sees repeated episodes of uplift and speed-up, but the course of the surface elevation is downward and so, more or less, is that of the horizontal velocity.

Phases 2 and 3 add up to another picture of an invisible world beneath the authors' feet. The meltwater, once it reaches the bed, pressurizes the ice and forces it upwards, filling and enlarging cavities and promoting basal sliding. But the enlargement proceeds at least in part by melting of roofs and walls, implying the creation of connections and, in short, of a network. The network grows steadily better at discharging the arriving meltwater. Phase 2 becomes phase 3 when the network becomes more than able, on average, to cope with the spate of water. Phase 3 ends when the supply of meltwater gives out, and the ice starts winning again, resuming its regular wintertime job of squeezing the summertime channels shut.

If you want real glaciological drama, visual or acoustic, you should probably go to tidewater terminuses, at which most of the ice leaves the ice sheet. But there is still plenty of land-terminating ice, and the main things about the Zwally effect, granting that it is real, are that it must be real everywhere; and that if the surface of the ice sheet gets warmer then the bed of the ice sheet is bound to get busier.

Last year I had occasion to take a 500-km trip by taxi. The taxi had a GPS unit — a talking GPS. I didn't pay much attention along the way, but I had to be impressed when the taxi pulled up on the main street at our destination and the GPS announced, smugly but correctly, "You have reached your destination."

The Global Positioning System has become part of our lives in the last decade or two, but it is much more than talking taxis. Some recent work illustrates dramatically the ability of accurate positioning devices to tell us things about how the world works.

The toothpaste in the Earth's mantle complicates attempts to measure glacier mass balance by the gravimetric method, and also by the geodetic method. (For the latter, you need two maps of surface elevation. Subtract the earlier map from the later, and divide by the time span. The result is nearly a map of the glacier's mass balance, the only missing ingredient being an estimate of the density of the mass gained or lost.)

But what if the elevation of the glacier bed has changed, in conflict with our assumption that surface elevation change equals thickness change, or equivalently that all of the gravity signal is due to the glacier? It can and does happen, and the glacier itself is often to blame. Loading the underlying bedrock, it forces the soft mantle material, at depths below about 100—200 km, away from where the ice is building up. A glacier that is shedding mass constitutes a "negative load", and the mantle material flows back. (The negative load ends up as a positive load spread more thinly over the ocean.)

The trouble is that the mantle deforms viscoelastically. The elastic deformation is instantaneous and reversible, just like that of an elastic band. Very crudely, it amounts to about a third of the equilibrium response to the load. The remaining viscous part of the deformation is what we think of as flow. To model it, though, we need an accurate model of the variation of viscosity (stiffness) throughout the 2,800 km thickness of the mantle. That is a formidable challenge.

The mantle flows so slowly that it is still responding today to the loss of ice at the end of the Ice Age, roughly 10,000 to 15,000 years ago. The toothpaste is pushing the bed of the glacier upward slowly, and before we can interpret a change in its surface elevation as a change in its mass we have to remove the bed-elevation component of the change.

But now Yan Jiang and co-authors offer an ingenious twist on the monitoring of elevation change with GPS. They have collected five or more years' worth of GPS readings of surface elevation from several fixed sites around the North Atlantic. The sites are all on bedrock, not on glaciers. (They wouldn't bear on this particular problem if they were on the ice.)

The surface's vertical velocity varies from place to place. The ingenious twist is to focus on the vertical acceleration of the surface, which turns out to be systematically greater near to large ice masses (in Greenland, Iceland and Svalbard). The authors argue persuasively that, while the vertical velocity will reflect delayed viscous adjustment, the acceleration is a signal of the Earth's elastic response to recent increases in the rate of glacier mass loss.

There are some rough edges: sites with large accelerations and not much glacier ice nearby, and one site not too far from the ice but with relatively low vertical acceleration. But I can't think of a mechanism to explain these observations other than elastic response of the solid earth to recent removal of glacier ice. The viscous response to this unloading has barely begun, and the viscous response to deglaciation cannot possibly change by so much over a period as short as a few years. The authors even have a go with an elastic-response model at estimating the mass balance that would account for the acceleration in west Greenland, and get plausible answers.

The talking GPS on my taxi ride demonstrated the power of positioning accuracy at the few-metre level. For measurements of glacier mass balance we would like millimetre-level (vertical) accuracy, but there are technical and conceptual problems to be ironed out before that becomes reality. For now, Yan Jiang and co-authors have shown that decimetre-level accuracy will do nicely to be going on with.

In a remarkable paper about Pine Island Glacier just published in Nature Geoscience, Adrian Jenkins and co-authors describe the latest step in maturation of an emerging glaciological technology: autonomous underwater vehicles — AUVs — or in other words unmanned submarines, for exploring the undersides of ice shelves.

The United States Navy has been sending manned submarines beneath the ice pack of the Arctic Ocean for more than 60 years. But pack ice, a few metres thick, is small beer by comparison with the ice shelves that fringe much of the Antarctic Ice Sheet. These are typically a few hundred metres thick, and there is no question of surfacing by punching your way through the ice if you run into trouble. Indeed, the cavities beneath ice shelves, between the base of the shelf and the sea floor, must be among the most inaccessible of all the theoretically accessible places near the Earth's surface. Being so hard to reach — until now — the sub-shelf cavity is also one of the least observed, but not necessarily least understood, parts of the climate system.

Climate, you say? Well, the sub-shelf cavity is where the ocean meets the ice sheet. Assuming (safely) that thermodynamic equilibrium prevails, the contact between shelf ice and seawater must be at the freezing point of the seawater, which depends on the pressure of the overlying shelf and the saltiness of the water. But the water and ice at some distance from the contact will not be at the freezing point, so there must be a heat source or sink, and therefore melting or freezing, at the base of the shelf.

If you add warm water to the sub-shelf cavity, you should expect additional melting. In 2002, Rignot and Jacobs inferred, from temperature profiles in the ocean offshore, astonishingly high rates of basal melting near several grounding lines: more than 50 m of ice per year. But this does not mean 50 m/yr of thinning of the shelf. A central part of their analysis was measurement of ice flow across the grounding line by radar interferometry. The inference of rapid melting was required to explain why the floating shelf was "pulling" ice so aggressively out of the grounded ice sheet.

So the contents of the sub-shelf cavity, and what goes on within it, are just as much part of the puzzle of glacier response to climate as are the grounding line itself, the shape of the floor of the cavity, the water at the base of the grounded ice, and of course things that happen in the atmosphere and the wider ocean.

Jenkins and his co-authors have contributed the first large set of in-situ observations from the sub-shelf cavity. What strikes me most forcibly about this dataset is how triumphantly it confirms earlier theoretical analysis of the way things ought to be down there. According to theory, warm water should be flowing inward at depth, melting the shelf base aggressively near the grounding line and — having thus become cooler, fresher and more buoyant — flowing upward and outward along the base. This is exactly what the autonomous underwater vehicle observed, confirming that we did know a thing or two even before measurements became possible on this scale in the sub-shelf cavity.

There are other noteworthy points about this study. For example the AUV found a ridge in the sea floor beneath the floating ice, in just the right place to explain why the grounding line of Pine Island Glacier has been retreating inland since the first observations in the early 1970s, and to confirm that this is something we ought to be worried about.

But perhaps the most noteworthy point of all, looking ahead, is the AUV. Buried in the Methods section of the paper is this, describing an incident part-way through the field campaign: "... the AUV lost track of the rugged ice-shelf basal topography, ascended into a crevasse, collided with the ice and executed avoidance manoeuvres that prompted it to abort its program and take a direct route to the recovery waypoint. After minor repairs, ...". Putting it another way, the world is now a little bit smaller, but not less dramatic, than it used to be.