Heliophage


All that is solid…
August 6, 2010, 8:23 am
Filed under: Earth history, Geoengineering, Nature writing, Published stuff

The Economist has an occasional column called Green View which looks at all sorts of environmental issues, though with a preponderance of climate stuff: in the past few months we’ve looked at arctic ice, business and biodiversity, tuna farming, Svalbard (of course), Climategate, malaria and climate change, the Hartwell paper, future urbanisation and a bunch of other stuff. Since I’m the Energy and Environment Editor I sort of own this slot, though I don’t write every one of the pieces that goes in. And since there’s a lot less blogging around these parts than there used used to be, I thought some of you might like to know this.

This page lists a whole lot of the columns (and a few other things that have strayed in by mistake), but as of a few weeks ago it is probably not being updated any more due to a change in the way we publish things on line. A couple of weeks ago there was a piece on what geoengineering could mean for different regions that might be of some interest to readers of this blog. Excerpt:

Uncertainty about who might do best from what sort of project allows discussions of geoengineering to take place without the parties to the debate knowing in any detail where any nation’s specific interests might lie. This introduces what the philosopher John Rawls called a “veil of ignorance”; making decisions as if such a veil existed, Rawls thought, was a good basis for justice. (If regional outcomes could be predicted accurately, a different Rawlsian idea, that of the difference principle, might come into play. This states that just action consist not just of improving things for everyone, but specifically for improving things for the worst off, and would give the effects of geoengineering on the least developed countries a particular importance.)

And this week, rather atypically, there’s a piece on the Earth’s core, and the way things you don’t expect to be transitory turn out so to be. Excerpt:

The Earth is a recycling scheme that has been running for a third of the age of the universe. Microbes and plants endlessly pull carbon, nitrogen and oxygen from the atmosphere and pump them back out in different forms. Water evaporates from the oceans, rains down on the land, pours back to the seas. As it does so it washes away whole mountain ranges—which then rise again from sea-floor sediments when oceans squeeze themselves shut. As oceans reopen new crust is pulled forth from volcanoes; old crust is destroyed as tectonic plates return to the depths from which those volcanoes ultimately draw their fire.

Anyone who likes that second piece might want to check out the essay in Seeing Further (Amazon UK) which I blogged about here, or the Earthrise piece I did for the Times a few years ago, which also covers some similar ground. (Out of ideas, or following a ceaseless process of re-creation? You decide…)



Apologies
November 22, 2009, 5:00 pm
Filed under: Earth history

There have been some rogue postings here as I have tried to get to grips with an alternative blogging system for other purposes; I have deleted them, and apologize for any confusion. If anyone found some windfarm stats and a list of Copenhagen side events useful, I’m glad, but this ain’t the place for that.

Secondly, but of more lasting significance, belated apologies to David Schwartzman for mispelling his name in Eating the Sun. He is a man of single n. He is also a man with some interesting views about the origin of photosynthesis derived, in part, from his belief that the early earth was a very warm place indeed. If you want to catch up with these beliefs, try

“Cyanobacterial Emergence at 2.8 Gya and Greenhouse Feedbacks” David Schwartzman, Ken Caldeira & Alex Pavlov, Astrobiology 8, 187-203 | doi:10.1089/ast.2006.0074
The idea of a hot early earth was questioned in a recent Nature paper

“Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago” M. T. Hren, M. M. Tice & C. P. Chamberlain, Nature 462, 205-208 | doi:10.1038/nature08518

but I wouldn’t expect that to stand as the last word…


For those about to tip…
September 3, 2009, 3:43 pm
Filed under: Earth history, Global change, Warning: contains molecules

This week’s Nature has a review article in it I have been waiting for for some time, and which I suspect may become something of a classic:
Marten Scheffer et al, Early-warning signals for critical transitions Nature 461, 53-59 (3 September 2009) | doi:10.1038/nature08227.  Here’s the abstract

Complex dynamical systems, ranging from ecosystems to financial markets and the climate, can have tipping points at which a sudden shift to a contrasting dynamical regime may occur. Although predicting such critical points before they are reached is extremely difficult, work in different scientific fields is now suggesting the existence of generic early-warning signals that may indicate for a wide class of systems if a critical threshold is approaching.

One of the problems with tipping points in complex systems is that straightforward analysis is incredibly unlikely to be precise and accurate about the tipping point’s threshold. A model may happily tell you that a system has a tipping point, bit it will not tell you where it is. In climate terms, you can be sure that there is a point at which the Greenland ice sheet will collapse, but you don’t know how far we are from it. This article reviews work which suggests a way round this. The system itself may tell you when it is getting close to a tipping point through subtle changes in the way its behaviour varies over time — in particular changes associated with “critical slowing”. What follows is my interpretation of the paper, which seems impressively approachable for a piece of mathematics, but which I may nevertheless be getting wrong; any real mathematicians in the audience should feel free to chip in in the comments.

A key symptom of critical slowing are that the system becomes lower to restore itself to its usual state after being perturbed. This means that if the system is fluctuating, its present state will become more closely determined by previous states — its “memory” will increase. In mathematical terms, this is an increase in autocorrelation. At the same time, and seemingly contradictorily, its variance may also increase because it becomes less able to recover from external shocks; that effect appears not to be as well grounded as the autocorrelation, but it does turn up in a lot of models.

As well as slowing down in this way the system will also start to get assymetric, because fluctuations that push it towards the tipping point and those that push it away will not be responded to in quite the same ways. It may also start to “flicker” as it moves back and forth across the boundary between two states before plumping firmly for one or the other. There are also ways of looking at differences over time, which I can’t really sum up: instead I’ll quote an example from the paper dealing with desertification:

Models of desert vegetation show that as a critical transition to a barren state is neared, the vegetation becomes characterized by regular patterns because of a symmetry-breaking instability. These patterns change in a predictable way as the critical transition to the barren state is approached, implying that this may be interpreted as early-warning signal for a catastrophic bifurcation [that being is one of the types of tipping point under discussion]

More on the desert stuff in this fascinating Science paper from 5 years back, one of the authors of which, Max Rietkerk, is also an author on the Nature paper.

How does this work in practice? Here’s an example using data from a 2008 PNAS paper by Dakos et al, on which many of the review’s authors worked.

The top plot is showing calcium carbonate percentages in a deep sea sediment core as a marker for the influence of the carbon cycle on the climate, as discussed in this 2005 Nature paper. The bottom plot is the measure of autocorrelation. As you can see, the time series goes from being not autocorrelated at all to being highly autocorrelated, and then bang. Similar autocorrelations can be seen in front of seven other abrupt climate shifts the PNAS authors looked at. The review looks at similar patterns in the onset of asthma attacks, ecological events, epileptic seizures and sudden stockmarket surges, and as the authors conclude

Flickering may occur before epileptic seizures, the end of a glacial period and in lakes before they shift to a turbid state; self-organized patterns can signal an imminent transition in desert vegetation and in asthma; increased autocorrelation may indicate critical slowing down before all kinds of climatic transitions and in ecosystems; and increased variance of fluctuation may be a leading indicator of an epileptic seizure or instability in an exploited fish stock.

So these processes really do seem to have a lot in common, much of which is related to the mathematical treatments reviewed in the paper. This is not to say everything is settled:

More work is needed to find out how robust these signals are in situations in which spatial complexity, chaos and stochastic perturbations govern the dynamics. Also, detection of the patterns in real data is challenging and may lead to false positive results as well as false negatives.

There are a lot of complexities here to do with how you filter the data, what data you choose, whether all the bifurcations that cause critical slowing are really catastrophic tipping points, and more. There are probably people who think it is all hogwash (and people should feel free to point me towards them). But I must say that after reading the review and feeling I have come a little way towards understanding what is going on, I look forward over the next years to people with climate models that show tipping-point behaviour getting stuck into this sort of analysis looking for precurssors. (Here’s a topical question: what does an autocorelation on the year-by-year arctic sea ice minimum show?)  The idea that such mathematical work will ever reach a level where you would feel justified in saying “the Greenland ice tipping point is ten years away” may be far fetched. But you don’t know til you try.

Image copyright Nature Publishing Group



More ocean mixing
August 3, 2009, 2:35 pm
Filed under: Earth history, Geoengineering

A sperm whale, off to stir up the depths

A sperm whale, off to stir up the depths

Following on from the post about artificial ocean mixing, last week’s Nature has a new development in a story I’ve always thought rather fun: the role of animals in stirring up the ocean. The problem that this addresses is what can be called the “missing mixing”. For the oceans to circulate from top to bottom, water has to sink — as it does in the North Atlantic and round Antarctica — but it also has to be lifted up; deeper, denser waters have to return to the surface. (If sinking was the whole story the ocean would simply remain stratified.) Considering how vast the ocean is, and what a huge and climate-crucial amount of heat it pumps around, the magnitude  of the mixing required is remarkably small: two or three terawatts, or as William Dewar strikingly puts it in a News and Views piece accompanying the Nature article,

Roughly speaking, all the energy needed to mix a cubic kilometre of subsurface ocean could be provided by a single hand-held kitchen mixer.

The problem is that, small as it is on a planetary scale, no one is sure how that couple of terawatts of mixing comes about. Astronomy offers a figure for the dissipation of energy by tides of almost 4TW, but a lot of the work done there is done in shallow seas not deep oceans. Carl Wunsch and others have estimated the amount of  work attributable to wind at about 1TW. But there’s no guarantee that all that work actually cashes out as mixing — you can do work on water that doesn’t mix it up much. So winds and tides alone might be enough — but its quite possible that they are not.

Dewar has been arguing for a while that some of the missing mixing might be attributable to the movement of animals. In an endearing order of magnitude calculation a few years ago he suggested that the world’s 360,000 sperm whales, which spend 80% of their time at depth expending about 5kW on their swimming, contribute more than a gigawatt of mixing. Dewar argues on various grounds that there could be a terawatt of biogenic mechanical energy in the oceans, about half of it due to the biosphere to fish and other swimmers and the other half to wigglers and splashers and squelchers of the prawny/jellyfishy/whatever persuasion.

But how much does that matter? Viscosity damps down turbulence,  so the fact that something little is thrashing about doesn’t necessarily mean it’s contributing to large scale mixing. This is where the new paper from John Dabiri’s lab at Caltech comes in. Dabiri and his student Kakani Katija have looked at a mechanism of mixing first discussed by Charles Galton Darwin (grandson of the more famous CD, head of Britain’s National Physical Lab for a while, and a gloomy eugenicist with whom I believe Robert Heinlein was much taken) fifty years ago, which probably shouldn’t be called entrainment, but sort of looks like it. This is the mixing due to water carried along with the mixer, and depends on the shape and volume of the creature involved. The paper concentrates on jellyfish, which seem to be a Dabiri specialty, and backs up its theoretical analysis with some nice videos demonstrating what goes on in practice which I don’t currently seem to be able to embed.

The paper shows  that the Darwin entrainment effect is distinctly different from the wake turbulence previously considered; it also seems to operate at larger scales than the turbulence created by say the flapping legs of a shrip. Katija and Dabiri estimate that taking it into account could increase estimates of the animals’ possible contribution from 1TW to at least 2TW, though as Dewar cautions “translation of Katija and Dabiri’s results from anecdotes to assessments of possible global impacts remains to be carried out.”

In an accompanying news piece, Roberta Kwok quotes Carl Wunsch on the “forbidding challenge” the work might pose to climate modellers, since it could require them to revisit estimates of the ocean’s diffusivity, a key modelling parameter, and quite possibly vary it from place to place and time to time. An earlier bit of Nature journalism went into this, and the whole missing mixing story, in further detail (and both point out that some people, such as Andre Visser, aren’t at all convinced there’s anything of interest going on here, fish-stirring-wise).

The scientific debate looks set to run on for some time, and to be quite a lot of fun: if climate and people influence fish (jelly and otherwise) and fish influence mixing and mixing influences climate and people there could be all sorts of fun and important stuff to learn there. But it struck me that it was worth seeing whether there was an engineering issue, too. A couple of terawatts is, in geoengineering terms, pretty small potatoes (the effects of something like a stratospheric aerosol veil would be in the hundreds to thousands of terawatts) and if an intervention on such a small scale could have an effect on the rate at which the oceans transport heat, or replenish their nutrients, or transport cold carbon-dioxide-saturated waters into the deep, that could be interesting (I am *so* not advocating rearranging the ocean currents here: I’m just thinking about what sort of leverage there might be).

A rough estimate shows that this isn’t an issue for the purportedly hurricane-thwarting pumps I wrote about last week. Though in terms of heat transfer such systems might clock in at 10GW or 20GW, that’s because water has a large heat capacity. In terms of the mechanical work they would do my schoolboy physics suggests that it’s on the order of a megawatt. But then those pumps aren’t designed to do mechanical work: who knows what the inventive mind of Steve Salter would come up with if that was the primary objective. If we wanted to mix up the oceans as much as the fish or the winds do, it might not be all that hard.

[In an experiment with a new posting format, here are the main references to papers and aricles, corralled together]

The new paper: A viscosity-enhanced mechanism for biogenic ocean mixing, Katija, K and Dabiri, J, Nature 460, 624-626 (30 July 2009) | doi:10.1038/nature08207

Dewar’s News and Views article

Roberta Kwok’s news story

Quirin Schiermeier’s earlier News Feature on the missing mixing: Churn Churn Churn Nature 447, 522-524 (31 May 2007) | doi:10.1038/447522a

Dewar et al’s paper setting out the terms of the debate:  Does the Marine Biosphere Mix the Ocean? Journal of Marine Research, 64, 541-561 (July 2006)

Carl Wunsch’s homepage: Munk W. and Wunsch, C., 1998, Abyssal Recipes-II: Energetics of Tidal and Wind Mixing (pdf) is of particular relevance

Sperm whale image from flickr user doublebug, under a creative commons licence: this version of the researchers’ video is from Wired Science



Filling Olivia’s shoes
June 17, 2009, 5:09 pm
Filed under: Earth history, Published stuff

Belatedly, I should point out that I am doing a few columns/posts over at the Times so that Olivia Judson can enjoy some time off. Turns out that they are about life and atmospheres, last week’s (Found in Transit) on detecting life by way of the atmospheres of other planets, this week’s (Heavy Weather) on a speculative mechanism by which life might influence the climate. One more to come next week…



A talk with Jim Lovelock

Here’s the video of a conversation I had with Jim at the Nature offices a few weeks ago. Also available in its full glory at the Nature site. And indeed apparently at The Guardian too. I’d post excerpts from a transcript if I had the time, or a transcript…



Jim Lovelock on oxygen and intelligence
March 31, 2009, 11:08 am
Filed under: Earth history

A bit of catch-up. Just before dashing off to Copenhagen I had the chance to spend some time with Jim (and Sandy) Lovelock, some of which was at and after a Nature event and some more of which will soon be making it to a Nature video, I hope. Jim is, among many other things, a master of analogy, and I was struck by a new one that he was road testing —  one that mixes catastrophe and optimism in a way that chimes with his new book, but which is not I think spelled out there.

The analogy is between oxygen and intelligence, and the creatures — cyanobacteria and humans — which brought these new and terrible entities to the world.  In both cases, there were precursors. There are non-biological processes which produce free oxygen, and there is intelligence of various sorts elsewhere in the animal world. But in both cases there was at a specific point a quantitative shift so huge as to be qualitative and then some. Cyanobacteria really did overturn the biosphere 2.45 billion years ago, and they and their descendents have shaped it ever since; human intelligence has done so too, from the Pleistocene die-offs onwards.

Lovelock’s point is that an evolutionary breakthrough in a single form of life can have global consequences and that such a breakthrough can be — probably must be — highly destabilising, even catastrophic. The presence of free oxygen was a huge change in the terms on which life prospered on the earth, since everything that came before was based on anaerobic metabolisms: many niches and perhaps species were wiped out. Similarly, intelligence of the sort demonstrated by humans is proving far from benign on a global level, with ever increasing stress on ecosystem services and, as with oxygen, wholesale rewiring of various biogeochemical cycles.

Yet in time, for all the disruption it caused early on, oxygen became the basis of something far grander than what had come before. The amount of free energy available to the biosphere increased spectacularly, with reasonable levels of oxygen facilitating complex multicellular life in way which may well be impossible in an anaerobic world (Catling et al [pdf]).  All the life you see and care about is made possible by that oxygen (though not all the life that you depend on — at the deep biogeochemical level the microbes, including the anaerobic ones, still rule). Similarly Jim suggests that, in time, intelligence may make possible a more wonderful planet in ways we can hardly guess at today and bring forth a “wise, thoughtful world”.

A stromatolite that looks like a brain. Coincidence...?

A stromatolite that looks like a brain. Coincidence...?

There are what seem to be weaknesses to the analogy. Many (including, FWIW, me) continue to believe that oxygenic photosynthesis was around for a long time before oxygen was able to build up in the atmosphere — hundreds of millions of years, maybe even a billion — and that the eventual breakout of oxygen in the Great Oxidation Event of 2.45 billion years ago was not quite the catastrophic holocaust that it has previously been portrayed as, and which Jim still feels it was. But a) we may be wrong (Joe Kirschvink certainly thinks so [pdf]) and b) that may not matter too much. This is an analogy, after all.

And as an analogy, Jim takes it one intriguing stage further. While cyanobacteria still abound, they are no longer the sole source of oxygen, nor even the dominant one. The cyanobacteria not only made new life forms possible — they also incorporated theselves into them, in the form of chloroplasts. The ability to make oxygen was disseminated into creatures radically unlike the original cyanobacteria — into kelp forests and elm trees and cactuses and camelias. Perhaps, Jim suggests, the same is true of intelligence — that its destiny is to be spread far beyond the species in which it first originated, into new achitectures of life and thought.

It’s an idea that may sit uneasily with Lovelock’s current pessimism, which sees human activity as bound to lead to a fairly massive die-back. But it flows easily from the tradition of thought that Lovelock (and Freeman Dyson, and the late Arthur C. Clarke) drank from as a young man, a tradition that combines a respect for thermodynamics (there’s a free-energy/information level to Lovelock’s analogy that would probably be interesting to tease out)  with a cosmically-contextualised yearning for transcendence. It’s a tradition that retains its power to move today, and it’s a thought-provoking pleasure to hear him give it voice.

Images: Death in Biscay by Christian Darkin on the basis of MODIS imagery  commissioned for a great feature by Nick Lane that Nature ran last year, all rights reserved; brainy stromatolite from mjwy’s guide to stromatolites on eBay