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.


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…

Copenhagen: Has the Amazon tipping point tipped?
March 12, 2009, 12:01 am
Filed under: Global change, Trees

It appears that the action on Wednesday afternoon was where I was not: in the session on tipping points. Chris Jones of the Met Office’s Hadley Centre presented some studies of the Amazon (abstract in pdf) that have caused a big media stir. The studies suggest that a) there is a threshold level of warming beyond which much of the Amazon forest is committed to die back (probably being replaced by savanna) and b) that for significant parts of the forest that threshold is alarmingly low. Indeed it is quite possibly either unavoidable in the near future or already dwindling in the rear-view mirror. As I understand it from people who saw the presentation, models in which all the warming already in the pipeline (ie with no further emissions) is realised leave the forests pretty much committed to some dieback, and modest further warming seals  the deal. I wasn’t  able to check that with Jones himself, but it seems to fit with what he and his colleagues write:

We present results to show a possible climate threshold beyond which some dieback is committed and this commitment rises dramatically for global temperature rise above 2 degrees C, a threshold often used by policy makers in their definition of dangerous climate change. Any subsequent recovery is on such a long timescale as to make the dieback effectively irreversible on any pragmatic level.

Here’s the coverage from the Times and here’s some from The Guardian. Worth noting that it’s a single study, that there are error bars to consider and that people have in the past suggested that the Amazon is often more vulnerable in the Hadley Centre model than in most others. But still very worrying; all the more so if it were to be spun as a counsel of despair on efforts to stop deforestation on the basis that there’s no point preserving a forest that’s already doomed.

I’ll see if I can find Chris Jones, or some Brazilians, or both to talk about this with on Thursday.

Cross posted at Climate Feedback

Is it just about the energy?
February 16, 2009, 9:13 pm
Filed under: Global change, Uncategorized

Andrew Revkin has an interesting post on dot.earth. It’s common to talk (as I sort-of did back here) as if cheap energy solves, if not everything, all the big stuff. But might it just bring more problems in its wake — perhaps by pushing the human population so high that much of the rest of the natural world is pushed out, or perhaps through some other dire and unforeseen consequence? Andy:

On a finite planet, where would limitless energy, combined with humanity’s infinite aspirations, take us? This leads to a question that’s been touched on here periodically. Does a shift in values and aspirations have to accompany the technological leaps that will assuredly be made in the coming decades?

There have been heaps of warnings for a very long time about unintended consequences from a rush to new technologies. (If you haven’t read Bill Joy’s 2000 essay for Wired, “Why the Future Doesn’t Need Us,” I encourage you to do so — possibly with a stiff drink nearby.)

Jesse Ausubel of Rockefeller University has often asserted that technological advances will inevitably lead to more space for nature, allowing forests to expand, fisheries to rebuild and the last refuges for wild things to persist. But others warn that creeping deterioration of the world’s biological patrimony is happening in parallel with our creeping disinterest in the diversity of life and ecosystems. As Edward O. Wilson explained here in laying out “Wilson’s Law,” if we focus too much on the physical infrastructure that sustains us, without sustaining the planet’s variegated veneer of life, we’re in deep trouble.

It probably won’t surprise readers that with various caveats I am more of the Ausubel opinion. I don’t think that higher standards of living necessarily correlate with worse environmental outcomes. And I don’t think energy breakthroughs will have the same sort of demographic effect that fossil fuelled modernioty has But I am aware that it is an inductive argument. The history of the past few centuries suggests that the phenomenon of the demographic transition is real: from a situation where birth rates and death rates are both high, you move to a situation where death rates drop (leading to an expansion of the population) that is followed after a lag by a period where birth rates drop, too, and the population thus stabilises.

The demographic transition, from Keith Montogomerys online essay

The demographic transition, from Keith Montgomery's online essay

It seems likely to me that in the near term world population is going to play out this way, and that after nearly three doublings in the past few centuries we have less than a single doubling left to go. I don’t think a massive influx of clean energy will change that and lead to the sort of growth seen during the fossil-fuel era again. The precise height of the plateau, though, remains unknown, and important: things are probably a lot easier with a world of nine billion than of twelve billion.

And that which might lie in wait further on down the x axis should also give us some pause. It’s a bit end-of-history to think that humanity goes through millennia of premodernity, a couple of centuries of transition and population growth and then millennia of postindustrial population stasis, end of. This seems to be demographics a la Christopher Robin: “Now we are six we’re as clever as clever, and we think we’ll stay six for ever and ever”.

Not that any of this matters, people will say, if the planet’s carrying capacity is surpassed over the long term. But how many people the world can support depends on how they live and what technologies they live with: applying a simplistic ecological notion of carrying capacity that doesn’t take differential attitudes and technologies into account makes littlesense. Humans need to understand the environment and the ecosystem services it offers a lot better, but not so that they can discover some set-in-stone carrying capacity and adapt to it. They need to do it to understand how to engineer and sustain a carrying capacity that suits, within the limits of what is possible.

Which brings us to “Wilson’s law” , which Andy has quoted E O Wilson as defining thus:

If you save the living environment, the biodiversity that we have left, you will also automatically save the physical environment, too. If you only save the physical environment, you will ultimately lose both.

I can’t make up my mind whether this is trivial or obtuse, and I haven’t seen the argument that Wilson makes to back it up. The first part seems true by inspection, in that saving “the living environment” would necessarily entail massive action on the physical-environment side (curbing climate change). The second part sounds like something that Wilson would like to be true and that might be true but that can’t be proved true. A priori it seems unlikely that the current living environment is uniquely capable of providing non-substitutable ecosystem services without which the physical environment collapses, taking humanity with it. Other biospheres — perhaps differently or more simply arranged — might do as well. They might have less resilience, but a lack of resilience isn’t the same as certain doom, especially when there is active observation and course correction going on.

Geoengineering by the numbers

A very useful paper (abstract|pdf|discussion space) comes out today in Atmospheric Chemistry and Physics by Tim Lenton and his student Naomi Vaughan. Tim told me when I was reporting the Andy Ridgwell paper on leaf albedo (Nature story|blog entry) that he’d become pretty interested in evaluating geoengineering schemes, and was setting up a group at the University of East Anglia to assess them. This paper presumably represents the first fruits of that interest, providing a ranking of most of the geoengineering schemes proposed in the literature in terms of the amount of radiative forcing they can provide.

Radiative forcing is, more or less, the difference in terms of energy per square metre that’s associated with any given action that changes the climate; it’s a pretty routine way of expressing things in IPCC-land. The IPCC puts the radiative forcing associated with the greenhouse gas  industrial and industrialising societies  pumped into the atmosphere from 1800 to 2005  at about 1.6W/m², and the forcing for a doubling of CO2 at about 3.7W/m².

Lenton and Vaughan first divide geoengineering proposals into two sorts: shortwave and longwave. Shortwave schemes seek to reduce the amount of energy that gets into the earth system by reflecting away incoming sunlight. Longwave schemes seek to increase the amount of energy leaving the earth system by making the atmosphere more transparent to outgoing infrared radiation — that is, by reducing the greenhouse effect. Then they assess the two with some very simple modelling (well, for the longwave there are some wrinkles, but it’s all in principle pretty simple). They don’t claim that the figures they come up with are the best available in any particular case, just that they are all derived the same way, and so allow fairly straightforward comparisons. By standardising the techniques they also show up a few errors in previous analyses: for example, if you increase the total amount of light reflected back into space by clouds, you reduce the amount reflected by the surface, simply because less light gets there in the first place.

The first and most striking conclusion is that if you want to have a big effect, go shortwave. Sulphate aerosols in the stratosphere (which were the main topic of this piece and these Climate Feedback posts) and mirrors/refractors in space (also in that piece, and in this paper by Roger Angel) both have the potential to provide as much by way of negative forcing as a doubling of CO2 provides by way of positive forcing. Not surprising; if you’re not constrained by money or by concerns about environmental side effects, you can put mirrors in the sky and particles in the stratosphere until it’s darkness at noon.

When you leave these global technologies behind, the other shortwave interventions rank, unsurprisingly, more or less according to the area they affect. Increasing the brightness of marine stratocumulus clouds, as proposed by John Latham, would affect about 17% of the earth’s surface, and the Lenton-Vaughan analysis suggests that the whitening effect would have to be considerably more marked than previous work has assumed; but if that brightening could be achieved then a negative forcing that averages more than 3W/m² should be possible. Covering non-sandy deserts with aluminium and polyethylene (not an idea I had come across before, and a pretty silly one as far as I can see: more here if you want it) makes 2% of the surface a lot brighter, and gets you an average 1.7W/m² of negative forcing, obviously very unevenly spread. Increasing the brightness of the planet’s grassland as  Robert Hamwey has discussed (pdf) gets you 0.64W/m², and the Ridgwell et al idea of planting brighter crops gets you 0.44W/m² at best, croplands being smaller than grasslands. Lightening everywhere that people actually live (another idea from the Hamwey paper) gets you 0.19W/m²; increasing the area of plankton blooms that seed the creation of clouds in parts of the southern ocean gives you just 0.016W/m² (and that may be an overestimate) and restricting yourself to just creating shinier cities  gives you no more than 0.01W/m².

What of the longwave? In principle, capturing carbon dioxide from the air (pdf of the Keith et al paper) and burying it in the ground could give you whatever radiative forcing you wanted; the limits to such a scheme are entirely economic, rather than being imposed on the earth system. All the other schemes, though, which involve making changes in the natural carbon cycle, are quite constrained, with none able to counter a doubling of carbon dioxide, even given the most extreme assumptions.

The biggest effect comes from really aggressive planting of forests, as described in an essay (pdf) by Peter Read on his global gardening plans. This involves growing enough plant material in the next 50 years to more than completely make up for all the arbon dioxide lost through deforestation and land use change over the past few centuries, which is really remarkably ambitious, especially if people are still going to have some space to grow food. By 2050 this strategy gets you an effective 0.49W/m² of negative forcing thanks to 88 gigatonnes of carbon dioxide being stored away. A variant of the idea in which you grow the biomass and burn it in power stations fitted out for carbon capture and storage does even better: 0.69W/m² by 2050 and almost 2W/m² by 2100 (For the longwave calculations, the radiative forcing depends on how long the programme has been going on. It also depends on what assumptions you make about how effective carbon-emissions control is; Lenton and Vaughan calculate all the forcings in terms of what extra relief the carbon-dioxide drawdown provides in a world that is already making serious cuts in emissions).

A lower tech idea that Read is fond of, as for that matter am I, is turning biomass into biochar and ploughing it into the ground. Jim Lovelock, Lenton’s mentor and friend, was extolling this as a possible way of making things better in New Scientist last week, speaking to the in-this-case-aptly-named Gaia Vince. This may make sense for all sorts of reasons, and the fact that making the charcoal also provides you with fuel (see Johannes Lehmann’s commentary in Nature a few years ago) is obviously  a plus, but even a really aggressive campaign along these lines gives ou a negative forcing of only 0.40W/m² by 2100.

After that come a bunch of ocean fertilization schemes, using phosphorous, nitrogen and iron, all of which offer something in the region of 0.1-0.2W/m². A system of pumping nutrient-rich water up to the ocean surface sketched out by Lovelock and Chris Rapley (earlier blog entry) delivers a truly meagre 0.003W/m² by 2100.

None of this, as Lenton and Vaughan are at pains to make clear, counts as an endorsement; all the schemes have side effects and risks, as well as in some cases (ahoy there, vast fleet of  space parasols) quite remarkable costs. But looking at the options this way does allow a sense of what might be possible, and a way of seeing what might be done in a mix and match sort of way. And the fact that the paper is published in the discussion section of ACPD means that the various researchers whose work is discussed will have a chance to answer back, correct any poor assumptions, and carry the debate forward.

Cross-posted to Climate Feedback

Image from flickr user gianky, under a creative commons licence.

Eathrise @ 40
December 24, 2008, 5:04 pm
Filed under: Earth history, Global change, Nature writing, Published stuff

From today’s New York Times

It takes nothing from the beauty and power of the image, though, to point out that it was the photographer, far more than its subject, who was isolated, and that the fragility is an illusion. The planet Earth is a remarkably robust thing, and this strength flows from its ancient and intimate connection to the cosmos beyond. To see the photo this way does not undermine its environmental relevance — but it does recast it.

To substitute these flows for the fossil fuels poised to despoil our planet and also run out on us — worst of both worlds — is an epic task. But the message that frames all the other messages of “Earthrise” is that we can rise to epic tasks. Look where the photo was taken. “If we can put a man on the Moon …” quickly became shorthand for society’s failure to achieve goals that seemed far simpler. But still: we put a man on the Moon, and that does say something. Efforts on a similar scale aimed at harvesting the energy flowing about us are entirely appropriate, and could make things a great deal better. We cannot solve all problems; some climate change is inevitable. But catastrophe is not.

“Earthrise” showed us where we are, what we can do and what we share. It showed us who we are, together; the people of a tough, long-lasting world, shot through with the light of a continuous creation.

Happy Holidays