Heliophage


The Planet Remade

So, I meant to set up a new website and everything, but what with, well, you know, stuff, I haven’t quite got round to it. And right now, I am away on holiday. But this doesn’t mean I am not immensely proud of and excited by my new book, “The Planet Remade: How  Geoengineering Could Change the World”, published by Granta in the UK on October 1st and by Princeton University Press in the US on November 3rd. Here’s the Kirkus review, which is the only one out yet. It concludes:

An important account of cutting-edge research that will fascinate serious readers and demand the attention of policymakers.

But I should add that frivolous readers are utterly sure to find stuff in it for them, too.

Other people have said some nice stuff, too, when asked to by me or the publishers (a lot of it is at the publishers’ links above). I am fond of and grateful for this from Marek Kohn:

Written with the grace and clarity its subject demands, The Planet Remade offers just what the issue of climate change needs: fresh thinking about what can be done, based on deep respect for the planet, the science, and the concerns of people with differing points of view. It’s an enriching addition to the literature of possible worlds.

And the Samuel Johnson Prize, rather fabulously, has put the book on its longlist.

As with Eating the Sun, I’ll be posting links to all the reviews that I see. But not til we get back from our holiday.

You can pre-order/buy on Amazon UK, pre-order on Amazon US, or buy it from a bunch of other places.

If you’d like to hear me talk about this stuff, here are some places where I will be doing so in the UK over the next month; more will follow, along with US dates early in 2016. Or you could try this podcast from The Guardian



Some of what I think about geoengineering

I recently had the great pleasure of attending this year’s Breakthrough Dialogue at Cavallo Point, an event at which the Breakthrough Institute brought together kindred spirits of disparate views to hash out some of the many issues that that Institute takes an interest in. On the basis of this Economist special report I was invited to talk about nuclear power, but in the many fruitful interstices of the meeting found myself talking about geoengineering quite a lot, because this is the sort of crowd where that sort of discussion makes sense, and because I am working on a book on the subject.

Towards the end of the meeting, a friend mentioned to me that perhaps I should be more careful in such conversations – people seemed to be getting the wrong idea about what I believed. This may be the case – I can’t really vouch for what message people were picking up, and I’ll admit that I sometimes run off at the mouth and that jet lag when drink has been taken doesn’t always help matters.

That said, I think there is a danger to being too careful in talking about geoengineering. If all the people who know about geoengineering are meticulous in the care that they take in talking about it, they will create no new misapprehensions – but they may do little to dispel old misapprehensions, and they may pass up the opportunity to carve out for geoengineering a more central place in our ongoing discussion on climate. I think it deserves that place; if I didn’t I wouldn’t be writing a book about it.

But while there may be good reason to be expansive in one’s talk, there’s no good reason for being careless, or even sloppy, in one’s reasoning. I have tried to be pretty careful in published stuff in the past, such as this 2007 piece in Nature and this 2010 piece in Prospect. Some time in the future I hope to provide all the clarity and nuance one could wish for in the book. But for the time being, here are a few key points in my current thinking, expressed with what I hope is appropriate care. Continue reading



Some catching up: Climate science
April 7, 2010, 6:58 am
Filed under: Global change, Published stuff

There’s been a range of climate stuff in The Economist recently. Biggest was Clouds of Uncertainty, an overview look at climate science and its uncertainties. Here’s the set up

Increased antsiness about action on climate change can also be traced to the recession, the unedifying spectacle of last December’s climate-change summit in Copenhagen, the political realities of the American Senate and an abnormally cold winter in much of the northern hemisphere. The new doubts about the science, though, are clearly also a part of that story. Should they be?

In any complex scientific picture of the world there will be gaps, misperceptions and mistakes. Whether your impression is dominated by the whole or the holes will depend on your attitude to the project at hand. You might say that some see a jigsaw where others see a house of cards. Jigsaw types have in mind an overall picture and are open to bits being taken out, moved around or abandoned should they not fit. Those who see houses of cards think that if any piece is removed, the whole lot falls down. When it comes to climate, academic scientists are jigsaw types, dissenters from their view house-of-cards-ists.

The defenders of the consensus tend to stress the general consilience of their efforts—the way that data, theory and modelling back each other up. Doubters see this as a thoroughgoing version of “confirmation bias”, the tendency people have to select the evidence that agrees with their original outlook. But although there is undoubtedly some degree of that (the errors in the IPCC, such as they are, all make the problem look worse, not better) there is still genuine power to the way different arguments and datasets in climate science tend to reinforce each other.

It ends up like this

Adding the uncertainties about sensitivity to uncertainties about how much greenhouse gas will be emitted, the IPCC expects the temperature to have increased by 1.1ºC to 6.4ºC over the course of the 21st century. That low figure would sit fairly well with the sort of picture that doubters think science is ignoring or covering up. In this account, the climate has natural fluctuations larger in scale and longer in duration (such as that of the medieval warm period) than climate science normally allows, and the Earth’s recent warming is caused mostly by such a fluctuation, the effects of which have been exaggerated by a contaminated surface-temperature record. Greenhouse warming has been comparatively minor, this argument would continue, because the Earth’s sensitivity to increased levels of carbon dioxide is lower than that seen in models, which have an inbuilt bias towards high sensitivities. As a result subsequent warming, even if emissions continue full bore, will be muted too.

It seems unlikely that the errors, misprisions and sloppiness in a number of different types of climate science might all favour such a minimised effect. That said, the doubters tend to assume that climate scientists are not acting in good faith, and so are happy to believe exactly that. Climategate and the IPCC’s problems have reinforced this position.

Using the IPCC’s assessment of probabilities, the sensitivity to a doubling of carbon dioxide of less than 1.5ºC in such a scenario has perhaps one chance in ten of being correct. But if the IPCC were underestimating things by a factor of five or so, that would still leave only a 50:50 chance of such a desirable outcome. The fact that the uncertainties allow you to construct a relatively benign future does not allow you to ignore futures in which climate change is large, and in some of which it is very dangerous indeed. The doubters are right that uncertainties are rife in climate science. They are wrong when they present that as a reason for inaction.

Some people liked this article; others not so much



Glaciers and the IPCC
January 22, 2010, 3:35 pm
Filed under: Global change, Published stuff

There’s a piece in The Economist this week on the mistakes about glaciers in one part of one volume of the fourth assessment report of the IPCC. An extract, with source links:

The [IPCC] WG-II case study cites a report by the WWF [pdf][see note below], an environmental group, as the source of the date 2035. The WWF in turn cites a study presented in 1999 to the International Commission on Snow and Ice (ICSI) [pdf] by Syed Hasnain, chair of ICSI’s working group on Himalayan glaciers.

But the passage about 2035 that the WWF report quotes comes not from that ICSI report (which was unpublished) but from an article that appeared around the same time in Down to Earth, an Indian magazine. This article was based in part on an interview with Dr Hasnain, who was also quoted by New Scientist as saying it was possible the glaciers would be gone in 40 years. The article in Down to Earth claims that the area covered by glaciers would drop from 500,000km2 to 100,000km2 by 2035, a claim found in the IPCC report but not in the WWF report. This suggests the Down to Earth article was itself a source for the IPCC, though Murari Lal, a retired Indian academic, now a consultant, who was one of the four co-ordinating lead authors of the chapter, says this was not the case.

There are two further problems with the area figure. One is that the research in question [“Variations of snow and ice in the past and present on a global and regional scale” UNESCO, ed V M Kotlyakov, 1996 pdf] was looking at all the world’s glaciers, not just the Himalaya’s. The other is that the research was looking at the prospects for 2350, not 2035.

Since that piece was written, it has been pointed out to me that the error in the WWF report was apparently corrected by the WWF in 2005.

Image from flickr user Bernt Rostad, used under a Creative Commons licence



Earth monitoring: Damn it’s frustrating
November 18, 2009, 11:45 am
Filed under: Global change

There’s a fascinating paper by Corinne Le Quéré and many other hands out in Nature Geoscience on what’s up with sources and sinks of carbon dioxide. The authors claim that the sinks are getting weaker, and that a greater proportion of the carbon dioxide emitted is staying in the atmosphere, which is a pretty big thing. They are not absolutely sure of this, and when they spoke to the press earlier this week they recognised that another recent analysis by Wolfganag Knorr suggested the sinks were sucking up just as great a proportion as they ever did. The datasets are not great, which is one reason for the uncertainty; another, as I understand it (haven’t dug into the matter) turns on how you filter out variability when looking for the trend. This matters not least because variability may be driving some of the trend: El Ninos, for example, decrease the tropical land sink because they dry places out, and if El Nino frequency is part of the long-term climate signal then removing them from the data means removing some of the trend too. Anyway, the paper doesn’t say for sure that the sinks are beginning to slow down, but the authors do find it likely-in-the-ipcc-sense, which means they assess the probability as 2 to 1 odds on or better. (Update: John Timmer at Ars Technica has more on the differences between the papers.)

SMAP also looks pretty cool

Now there’s some fascinating stuff in this, which I may come back to, but there’s also cause for deep frustration. A real sense of what the sinks are doing would be hugely helped by having better measurements of carbon dioxide, both from global monitoring networks and from satellites. But that just doesn’t seem to be a priority. Global ground based monitoring of carbon dioxide capable of showing regional effects is still underfunded — this summer in Boulder Pieter Tans was telling me that there was a lot more that could be done for relatively little money. And satellite measurements took a terrible blow when the Orbiting Carbon Observatory, OCO, didn’t make it to orbit in February. OCO could have been rebuilt and relaunched quickly: the team was all still there,the designs were good, it was as shovel-ready a piece of spending as you could wish for. But it wasn’t funded as such, and doesn’t seem to have any new money associated with it. So as I understand it the money to refly OCO will have to be gouged out of NASA’s existing earth science budget (8% of its total budget, because it’s not like the earth is a sexy supernova or galaxy or anything like that). I have to assume that that means it will fly later than it could have (and cost more); that probably means delays for SMAP, the soil moisture mission currently due to launch in 2013. SMAP would provide unparalleled data on one of the key parts of the planet — the part just beneath our feet that contains the root zones of plants and the water that those roots require.

Not to be treating such missions as a global priority strikes me as simply crazy. Now I know that there is an argument that the need for new climate science is overstated — that we know the outlook is bad, and that there is no science that is going to change the degree of action or inaction that that badness is held to merit. I understand that stressing a need for more data and science might be seen as offering reasons for delay. But the OCO and SMAP data really matter. They matter to understanding the processes going on — where the carbon is coming from and going to, how ecosystems are being effected, and so on. In SMAP’s case they even have operational implications — better soil moisture data means better forecasting. But despite the fact that great things turn on these issues, they don’t get the money. And their ground-based correlates, which offer even more bang for the buck, get penny pinched too.

Scientific monitoring isn’t going to save the world — but it will tell us what is going on as we try to adapt and to prioritise mitigations. Which means we should be taking it seriously.

Refs:

“Trends in the sources and sinks of carbon dioxide” Corinne Le Quéré, Michael R. Raupach, Josep G. Canadell, Gregg Marland et al. Nature Geoscience (2009) | doi:10.1038/ngeo689

“Is the airborne fraction of anthropogenic CO2 emissions increasing?” Wolfgang Knorr Geophys. Res. Lett., 36, L21710, (2009) | doi:10.1029/2009GL040613



Pen Hadow isn’t news
October 15, 2009, 9:59 am
Filed under: Global change, Media

Yes, its melting

Yes, it's melting

Having been a little disobliging about Pen Hadow, an explorer, in a previous post about “Heroes of the Environment” I now feel moved to return, briefly, to the subject. This morning the BBC is putting data gathered during Hadow’s recent Arctic adventure — a trek with two companions, called the Catlin Arctic Survey, that was meant to get to the North Pole but didn’t — into its main radio news bulletins at the moment. As Professor Peter Wadhams of Cambridge puts it in the BBC’s online version of the story and many other places:

The Catlin Arctic Survey data supports the new consensus view – based on seasonal variation of ice extent and thickness, changes in temperatures, winds and especially ice composition – that the Arctic will be ice-free in summer within about 20 years, and that much of the decrease will be happening within 10 years.

Which seems fine: but not news. What we have here is a consensus for which, because it is a consensus, we must assume there is already a lot of evidence, being backed up by some incremental unpublished data (pdf of science findings) that will later be presented at the Copenhagen meeting and submitted to Cold Regions Science and Technology, a journal I am pretty sure that the BBC does not often use as a source. What Wadhams says seems true and sensible, but reporting stuff that is widely accepted as news doesn’t.

To be fair, the BBC further reports that

Pen Hadow admitted that the expedition had not led to “a giant leap forward in understanding” but had been useful as an incremental step in the science of answering the key questions about the Arctic.

but it doesn’t go on to say why it reports this incremental step. The answer, obviously, is that it thinks the public likes explorers, and will find science done by explorers interesting, even if the equipment broke down and the observations are not yet of any proven value; it also probably thinks that raising awareness about global warming is a good thing. Personally I find people who “explore” a planet that has already been pretty thoroughly explored  and do so in deliberately over-challenging and attention-seeking ways (Hadow first came to fame walking to the North Pole on his own without resupply) a little off-putting, even, in my more misanthropic moods, distasteful, though on the only time I met one he seemed a nice enough chap (that said, we were both talking to Edwina Curry, so contrast may have had something to do with it). If such people want to try to contribute to science and advertise a reinsurance company as they do so, fair enough (though the apparent shennanigans with data on their website suggest that this most recent undertaking was not utterly rigorous in its approach). But following HRH the Prince of Wales, who says this is a “remarkably important project”, in treating this data gathering as news seems to be a pretty straightforward mistake, even before you get on to the mistakes in headlining that “news”, such as “North Pole ice cap gone in 10 years“.

The generally grumpy tone of this post, and the unaccustomed act of linking to Watts, brings to mind a point a friend recently made to me — that I am more critical of  climate-change claims that tilt beyond science into propaganda in private than I am in public. I think that to the extent this is true it is only because most of what I do in public isn’t about that sort of thing; I don’t spend much time attacking climate sceptics in public, either, and that certainly shouldn’t be seen as suggesting support. But while I am tapping away I will take this opportunity to say that I think people who talk about a climate sensitivity of more than 4ºC as remotely likely need to  explain why they think this when others such as James Annan have argued well against it*. And I think when people talk about multi-metre sea-level rises, as  Jonathan Porritt did on Radio 4 recently (“I have increasingly less time for those whose nimby-ist sentiments persuade them that somehow the best route to defending their cherished landscapes is by letting it be drowned by a huge amount later on in life. Which particular bit of the landscape do you want to defend James if what we’re threatened by is a seven metre rise in sea levels? ” — rtf transcript), they need to make clear that they are talking about the situation that’s not “later on in life” for anyone who doesn’t expect to live for a few centuries.

*hint: explanations that include phrases such as “I don’t use the classical Charney measure of sensitivity because I think one also has to take into account biosphere and other long-term feedbacks to which it pays no heed” seem to me to be headed in a plausible direction; I may also show sympathy to “unlikely events in the long tail dominate the risk assessment”, though rather less so…

Not terribly relevant image from A Davies/Greenpeace, used under a Creative Commons licence



Terraforming the Sahara

Spot the dune...

An interesting paper in Climatic Change: Irrigated afforestation of the Sahara and Australian Outback to end global warming by Leonard Ornstein, Igor Aleinov and David Rind Doi: 10.1007/s10584-009-9626-y. (Mason Inman has a nice write up with some background and comment over at ScienceNow; [update] and corresponding author Len Ornstetin chronicles the idea’s rocky research road on his own site). The central idea is that with enough irrigation you can turn big deserts into big forests: forests big enough to suck up a large part of total carbon dioxide emissions for decades or even centuries. I think that you can take this notion as a serious plan, a thought experiment, a jeu d’esprit, a warning or a jumping off point, depending on predisposition. Aspects of all that in what follows.

Here are the basic numbers: The Sahara is about a billion hectares in area, on which you could fit a trillion eucalyptus trees. Those trees, if working flat out, could each put on twenty kilos of biomass a year. If roughly half that biomass is carbon, that would mean a net annual sink on the order of ten billion tonnes of carbon. That’s about the amount that humans currently emit.

To create such a forest in a century, you would have to plant as many hectares of trees every year as are currently lost to deforestation worldwide. And, even harder, you’d have to provide them with what they need to order to grow. You need a great many things to turn a desert into a forest — soil nutrients, microbiota, possibly pioneer plants, a compelling reason for doing the work, and so on — but the biggest hurdle, pretty obviously, is water. Eucalyptus, the authors say, needs about a metre of rainfall  a year. For a billion hectares, that’s 10 trillion tonnes of water. The authors assume, reasonably for all that I know, that if you have smart irrigation getting the water to just where it is needed you can get away with half that amount. Even so, even the vast aquifers beneath the Sahara don’t contain the amount of water required, so it will have to come from desalination plants on the and be pumped it up to where it is needed (the average elevation of the Sahara is about 450m). The size of this undertaking — more than 50 new Niles, flowing in reverse — may explain why the authors feel they need to use that fine old-school term “terraforming” for their undertaking.  The power requirement, if I’m reading their figures right (4.04kWh/m^3 fresh water delivered), is a bit to the north of 2.2 terawatts, about 40% of it for desalination by reverse osmosis and about 60% for pumping.

The world’s electricity generators currently provide about 18,000 TWh of energy, which averages out at 2TW of constant supply. So in energy terms the desalination and pumping needed for the Sahara forest would use a bit more electricity than the world currently generates for every other purpose.  This unavoidably sounds nutty. But that is at least in part because of the nuttiness of the situation, rather than its proposed solution — the nutty situation in which we burn fossil carbon at tens or hundreds of thousands of times the rate at which it is sequestered over geological time. If humanity insists on putting so much carbon dioxide into the air every year that it would take a brand new forest the size of the Sahara to suck it all up, then that’s where the madness starts. That creating such a forest would have to be a large undertaking — large in terms of  the whole world economy — is just a consequence of the initial folly.

And in practice the investment would be smaller. A nice thing about forests is that they can go some way to creating their own weather, and the authors have looked at this effect with some climate modelling work. If a forest with irrigation dampened soil is imposed on the Sahara, rain begins to fall, in some places as much as a metre of it every year. This rainfall doesn’t obviate the need for irrigation, because it is strongly seasonal — basically an extension of the West African monsoon of April to November. But it might significantly reduce the irrigation requirements. Maybe you could get away with just a terawatt…

The Sahel, to the south of the Sahara, also gets damper in those enhanced and extended monsoon rains, which is definitely a plus, I’d guess, and the African Easterly Jet, a feature which is driven in large part by the temperature contrast between the desert and surrounding land, seems to more or less vanish. Since a large number of Atlantic hurricanes get their starts as kinks in the AEJ, that might be a pretty significant change, too. Beyond that, the rest of the world seems pretty much unaffected. In particular, the authors say that their models show no additional warming that might be laid at the door of the change of albedo which comes with replacing light desert with darker trees. (I think this fits with the 2007 Bala et al paper in PNAS, which suggested that warming associated with afforestation would be due to changes in boreal, rather than tropical, forest cover).

The Bodele depression

The Bodélé depression (pic by Charlie Bristow)

There is, however, a fly in the ointment. The Bodélé depression in Northern Chad is only a small part of the Sahara, but it is the world’s greatest source of mineral dust, with the winds drawing some 700,000 tonnes a day off the surface. According to  Koren et al in ERL, 2007 40 million tonnes of dust a year travels from the Bodélé to the Amazon rain forest, half the total annual mineral inputs into the forest basin (the dust fertilises the mid Atlantic, too, and it may play a role in abating hurricanes too — Jim Giles wrote a lovely piece on this for Nature some time back). There’s a real chance that this dust is crucial to maintaining the soil fertility of the forest, and even if the Bodélé itself were left unirrigated and unforested, the increase in precipitation all round it, and the wetter atmosphere downwind of it, would probably shut it down as a dust producer. If growing a forest in the Sahara hurts the one we already have in the Amazon it obviously becomes a less attractive proposition (though if we are going to lose the Amazon forest anyway, things might look different…). That said, if you are pumping trillions of tonnes of water across continental scales, thenpaying to air dump a few tens of millions of tonnes of fine-particle mineral fertiliser upwind of where you want it is hardly going to break the bank.

Something the authors don’t look into is that the higher the CO2 level in the atmosphere gets, the easier this all becomes. Higher carbon dioxide levels make plants more water efficient, all other things being equal. All other things are not, necessarily, equal — higher CO2 also makes things hotter, which plants don’t much care for. In a world with some solar radiation management, though (such as aerosols in the stratosphere) all things might indeed be kept equal, or at least temperature might be. Martin Claussen has been working for some time on the idea that the Sahara is a “tipping element” in the climate regime, one that can be pushed from a dry state to a wetter one relatively easily. In a more carbon rich but not-too-hot world the circumstances might be right for it to tip the other way, and it might take rather less than a 50-Nile terraforming project to nudge it over.

In the final analysis, I don’t think I take this paper very seriously as a practical proposition. Doubling global electricity generation for a single project seems far fetched. For such a thing to be put anywhere near the top of one’s list of African infrastructure investments would require that a great many other large and important development initiatives (provision of power, water, roads, cold chains, vastly improved agronomical advice, etc to the vast majority of the population, for starters) would already have had to have been put in place. But it’s kind of nice to imagine a world in which we were wealthy and together enough to have actually taken the pressing need for those changes to heart, and were thus in a position to consider greening a great desert too.

Bahrains Tree of Life -- a limited pilot project...

Bahrain's Tree of Life -- a limited pilot project?

And regardless of practicalities I think there’s real value in taking the analysis further. A big idea like this throws off many fascinating questions that force you to look at the earth, and what we know about it, in new ways (or old ways but with a new twist):

What polycultures would you build the new forest with? (all-eucalyptus-all-the-time is fine for first calculations, but doesn’t sound like anyone’s idea of a proper landscape. Baobabs? Laurels? And what fauna might be good, or bad?)

What  genetic engineering — reduced flammability, higher albedo leaves, more refractory soil carbon, who knows what else — might help?

How much bioenergy with carbon capture could be built into the scheme, perhaps initially to power some of the inland the pumping stations?

Can biochar help? (and a million other soil-creation questions)

What are the best silvicultural ways to make the new woodlands pay, as that is something people by and large like their environments to do, and can there be room for some agriculture too?

How could local people best be convinced this was a good idea? And what are the property title reforms that would be prerequisite?

If the AEJ stops, do hurricanes stop too? Or does some other mechanism initiate them, maybe somewhere else? And does the dust really have an effect?

When the Sahara was wetter and less dusty in the past, did the Amazon actually suffer from lack of nutrients? (I think there is actually some research already out there on that — but can’t offhand think where)

How can the transformation be made stunningly beautiful?

What regions and landforms do you want to keep as monuments/heritage sites/national or world parks? There  would undoubtedly be a real aesthetic/biodiversity loss in the removal of the desert, not to mention risks to some utterly wonderful buildings.

How to stop the Fremen becoming soft and decadent now that Arrakis has become a land of milk and honey?

and so on.

In particular, it would be nice to see some analysis of halfway houses; where in the Sahel and points north might merely huge, as opposed to planet-sized, afforestation be attempted, and what would be the costs and benefits? It is possible to transform land on very large scales, if not quite this large: 40m hectares of the Brazilian cerrado have been brought into agricultural production over the past fifty years. Can afforestation/silvicultural interventions on such scales ever make sense? And where else might be suitable for such things?

And on the topic of where else: My apologies to any Australian readers for not going into the paper’s analysis of foresting the Outback in addition, or as an alternative, to the Sahara. Basically the arguments are largely the same but the costs and effects are a bit smaller. There’s also a risk of interfering with El Nino that would definitely merit further attention. If anyone wants to blog more on that aspect of the subject send me a link and I’ll post it up here.

Image credits: Eucalyptus trees at the top from Big Lands Brazil, who would like to sell you some…; Bodele  from Charlie Bristow, reused with permission; Tree of life from Flickr user Solvo under Creative Commons license



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



Eating up the carbon

The last of my filling-in-for-Olivia columns at the NYT is now up, a quick run through some points from the later parts of Eating the Sun and subsequent stuff. It’s a carbon-climate crisis, energy is about flows not stocks, many wedges needed, yadda yadda yadda:

Given that humans are changing the atmosphere at an unprecedented rate, what responses should we expect from the biosphere? And is there anything that we can do to make those responses work to human benefit? For those in a hurry, the answers in brief: a) complicated ones; b) yes, at least a bit.

More…



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…