16 December 2009

AGU Day 2

Posted by Dave Petley

NB: I have posted a review of the session on Scientists’ communication of critical global environmental change issues, in which both Michael Mann and Richard Alley spoke here:
Communication of critical global environmental change issues

There wasn’t much in the Natural Hazards area of interest to me on Day 1, so I decided not to blog yesterday. There was a little more today – and lots later in the week – so here goes.

I spent the first part of the morning in the underground geoscience session, for which I gave the first (8 am) invited paper on our Boulby Geoscience project. The rest of the papers were pretty good, but beyond the scope of this blog. After coffee I went to a paper by Karnawati and her colleagues from Gadjah Mada University, the topic being the landslides triggered by the recent earthquake in West Sumatra. This was pretty interesting, although I was a little frustrated by the lack of detail. She noted that the area around Maninjau lake, to the north of the epicentre, was most seriously affected by landslides. The lake is the remains of a volcanic crater, so I guess the materials may well be pretty susceptible to failure, but she noted that the main landslides all lay close to faults or lineaments, which is an ingteresting observation in this setting (I wonder whether essentially everything is close to a lineament in such a dynamic setting?). There is now a serious threat from post-seismic re-activation of the slides – not a surprise – and the author gave some examples showing that this has already started to happen. The rainy season starts about now. Dr Karnawati noted that a key task is therefore identifying safe areas for relocation of affected villages – it appears that this is going well – and community engagement with disaster risk reduction.

I then moved sessions to a presentation on the Slumgullion landslide by Schulz from USGS (see my earlier post on this work here). The first part of this talk reiterated the extraordinary observation that the daily movement of the slide is correlated with atmospheric tides, as per their recent paper. In the second half Schulz noted that the processes and morphology of large landslides and faults are similar, but that it is much easier to study landslides. To gain a better understanding of movement processes (presumably relating to both systems) they had positioned an array of seismometers on the landslide for week in the summer. Although they had power problems early on (so often the case with seismometers in my experience), the later data showed that the seismic events also correlated with the movement events and thus the atmospheric tides. The initial analysis has focused on the harmonic events that they recorded – which are likely to be related to fluid flow. As these occurred during periods of low atmospheric pressure it may be that their hypothesis about atmospheric tides inducing pore water movements is on the right lines.

This was followed by an entertaining talk by Dick Iverson, also from the USGS, who sought to explain stick-slip movements on landslides. These movements – in which the slide appears to move and then stop repeatedly – are something of a conundrum. The problem is that stick-slip in faults is usually explained by the storage of energy in elastic deformation, but in a slope with no cohesion it is hard to do this. Iverson demonstrated mathematically that the behaviour is related to the relationship between pore pressure and movement – basically when the slide starts to move diffusion allows the pore pressure to drop, which slowly reduces movement. When the slide stops pore pressure builds again until movement restarts. I think that the idea that this is the key mechanism has been around for a while, but this was the first time I have seen it modelled properly.

After lunch I went to the Bjerknes Lecture – one of the big set pieces – which was given by Richard Alley from Penn State. He brought a geological perspective to the link between atmospheric carbon dioxide and temperature. He started the lecture by noting the ongoing harassment of climate scientists by the denialist camp – giving an example of a demand made to his university that he be fired for continuing to claim that carbon dioxide causes temperature change. He presented this with great humour and grace, but the underlying message about the way that scientists are being treated was clear, and was a great concern. At a time when the denialist scientist de jour, Iam Plimer, has been embarrassed in a debate with a journalist, and the high profile campaigner Christopher Monckton has been caught on video calling campaigners “The Hitler Youth”, and then has brazenly denied it, the sense that the science community is under siege is clear.

Anyway, back to the lecture. Alley reviewed climate changes in the geological record, going back 4.5 billion years. His central point was that in almost every case carbon dioxide has emerged as the smoking gun in terms of causation, and indeed that it is essentially impossible to explain the observed changes without carbon dioxide acting as the key forcing. This is true for the “faint young sun paradox” (4.6 billion years ago), the snowball earth period, the late Permian extinction period, the mid-Cretaceous “Saurian sauna” period and the Palaeocene-Eocene thermal maximum for example.

Alley did note that although his own data shows that for the last 500,000 years carbon dioxide and temperature have moved in lockstep together, sometimes that CO2 lags behind temperature. He observed with great amusement that this has led some denialists to claim that CO2 is a thermometer not a cause of temperature change. Of course the glacial – interglacial cycle is driven by the cyclicity in the orbit, but Alley noted that the magnitude of the temperature changes can only be explained using a carbon dioxide feedback mechanism. He ridiculed the idea that because CO2 sometimes lags temperature it cannot be the cause of warming. He compared this to debt associated with credit cards, saying that if he went out and spent a modest amount on his card he would end up owing the credit card company money. If he didn’t pay this off then punitive interest rates would mean that his debt rapidly spiralled out of control. In the end his bankruptcy would be the result of the debt increasing due to the interest rates. The application of the interest rates lagged behind the spend, but still caused the insolvency. He likened the orbital cycle to the initial spend but the carbon dioxide to the interest. So, clearly carbon dioxide can drive the temperature, even if it didn’t act as the initiator of the process. Of course you need an explanation as to why this would be the case – but that explanation is simple as its is an application of straightforward radiative physics, as demonstrated two centuries ago.

Next he noted the huge progress that has been made in understanding the link between temperature and CO2 in recent years. He noted that when he was a young scientist there were several examples in the geological record in which there was global warmth but no apparent corresponding high CO2 level in the atmosphere. As sampling and measurements have improved it has been shown that almost all of these cases do in fact have high carbon dioxi
de concentrations. Just a few anomalies remain – most notably the warm period in the Miocene. However, very recent (peer reviewed) research is now showing that CO2 levels were high, although more work is needed.

Finally, he took a few minutes to show that arguments for other causal factors for global temperature change in the geological record just don’t hold water, He used the example of cosmic ray flux, a favourite of the denialist blogosphere a year or two ago. He showed that the geological record globally demonstrated a huge spike in cosmic ray flux at 40,000 years BP, but that temperature did not respond at all.

He finished with a very simple message – the geological record shows that carbon dioxide is the key factor that controls temperature. Other factors do operate, but the CO2 signal consistently dominates. He noted that the geological record shows that over a timescale of few centuries timescale a doubling of CO2 results in a warming of about 2.8 C, which is consistent with the IPCC figure for climate sensitivity. He made a very bold statement that if the key factor that explains the temperature record in the geological record is CO2 – without carbon dioxide concentration changes it is impossible to explain the observed behaviour.

Overall it was a great lecture – highly entertaining, informative, challenging and coherent. The audience for the talk was huge, and greatly appreciative of the quality of the presentation.

The lecture should be available as a podcast in due course here:


Do take a look – you won’t regret it (not least for the amusing first five minutes).

NB: I have posted a review of the session on Scientists’ communication of critical global environmental change issues, in which both Michael Mann and Richard Alley spoke here:
Communication of critical global environmental change issues