Stern report 3: sensitivities, sceptics, and Doomsday

Stern report on climate change: sensitivities, sceptics, and Doomsday, third post in a series

My previous post was a snapshot of Stern’s uncontroversial presentation of the story so far and the underlying science.

The next step is prediction. For this we need two pieces of information: the sensitivity of the climate (represented by average temperature) to greenhouse gas concentrations; and the path of future emissions of these gases under current policies. So we need two different kinds of model, of the climate system and the socioeconomic one. They have different problems and uncertainties. I’ll look at the latter in my next post.

I’m starting to wonder if I’ll ever finish, as I’m still on Chapter 1! Still, to travel in reality is better than to arrive.


Stern sums up the state of the art on climate sensitivity in his table 1.1 (p.12), using a 95% confidence range.

Stern table1.1.jpg

He summarizes (pp 10,11):

If annual emissions continued at today’s levels, greenhouse gas levels would be close to double pre-industrial levels by the middle of the century. If this concentration were sustained, temperatures are projected to eventually rise by 2 — 5ºC or even higher…. If greenhouse gas levels were to reach 1000 ppm, more than treble pre-industrial levels, the Earth would be committed to around a 3 — 10°C of warming or more, even without considering the risk of positive feedbacks.

I think Stern is not presenting the data in the best way here. First, there should be a summary averaging the studies. Second, a 95% confidence level is appropriate in science for establishing a causal connection, but is far too conservative for policy within an established causal framework. The criterion for policy is surely “a strong balance of probabilities”, not “beyond reasonable doubt”. I would present the data like this; the bands represent bands of one and two standard deviations, the confidence numbers are guesses because the distributions are not normal:


The studies Stern cites have strong modal peaks (chart 1.2, page 9). “Sensitivity ” here means the effect of a doubling of greenhouse gases from pre-industrial levels, say from around 275 to 550 ppm CO2 equivalent.

Stern 1.2.jpg

From this, we have a pretty strong central estimate of around 3°C for a doubled concentration. We’ll hit this concentration in under fifty years, and are already at 450 (chart 1.1).

Stern underplays the time aspect. The lags are large, and the climate takes centuries to reach equilibrium. However, according to this IPCC chart from the 2001 report, most of it comes by 2100, except for the highest emission scenario. I suggest policy reports should use this horizon, except for the doomsday risks. On the other hand, the lags mean that (p 12):

Climate models project that the world is committed to a further warming of 0.5° – 1°C over several decades due to past emissions

and ultimate stabilisation will involve unwinding a huge legacy.

The strong insight of the Stern report is that under uncertainty, rational people sum over the risk distribution, not just take the central value or – worse – say that as it’s uncertain, let’s do nothing. So he draws attention to the long upwards tails of all the models. This makes a big difference to the impact assessment. Stern p:9:

These sensitivities imply that there is up to a one-in-five chance that the world would experience a warming in excess of 3°C above pre-industrial even if greenhouse gas concentrations were stabilised at today’s level of 430 ppm CO2e.


What should we do about the sceptics? Ignore them. I propose to follow John Quiggin here. The findings cited by Stern represent an overwhelming consensus of qualified opinion, based (see previous post) on settled basic physics and chemistry. The professionals among the sceptics can publish their arguments in journals. If you are an expert, write to Nature. If you are an amateur not prepared to take the consensus’ word for it, go to to ask your questions and learn. Either way, a current affairs blog is not an appropriate forum for your doubts. Readers are therefore politely but firmly asked not to post here any comments of the form “But [insert name of sceptic] says that…”; I reserve the right to delete them.

If the science were settled, surely there wouldn’t be any qualified sceptics? This objection reflects a false picture of the sociology of science. Consider being a maverick as a professional strategy. As the odds on being right dwindle, so does the payoff to the bet if it wins. It’s like the answer to the puzzle why, when only a few bull seals get to mate, sex ratios are even at birth: the few male winners mate with many females. So there are always likely to be mavericks in academia (and they’re likely to be men). In fact, as the status of an area of enquiry converges on certainty, and the mainstream scholars move on, the proportion of publication by mavericks and cranks will increase. Who’s writing now about the age of the earth, or the authorship of Hamlet?

Another Baconian “idol of the theatre” is the demand for absolute certainty. This is a mistake in positive science, though increasing the assurance of belief is worthwhile up to a point. It is utter folly in praxis, when we are normally compelled to choose under pressure of time. We face not only statistical uncertainty but a wider ignorance of our environment. Remember that clinical trials of new medical treatments are routinely stopped when there is “overwhelming evidence of the benefit of the treatment”. It is unethical to pursue the experiment beyond that point aiming at absolute certainty.


Going back to Stern, the more recent models tend to show higher sensitivities, as they account better for complex feedbacks. But there are big unknowns, the potential positive feedbacks: weakening of carbon sinks in forests and oceans, and methane releases from bogs, peat and undersea hydrate deposits. (Box 1.3, p.11). As Stern calmly puts it (p.10):

It remains unclear whether warming could initiate a self-perpetuating effect that would lead to a much larger temperature rise or even runaway warming, or if some unknown feedback could reduce the sensitivity substantially.

Runaway warming? And it can’t be ruled out? The earth obviously hasn’t suffered this in 4 billion years, or we wouldn’t be here. But there’s a plausible – and controversial – claim that it suffered runaway cooling 700 million years ago.

This is the stuff of nightmares. Runaway warming reduced Venus into a Gehenna that could not support life as we know it. It would be worse than Jonathan Schell’s eerie (if improbable) vision in his 1982 tract The Fate of the Earth of the aftermath of all-out nuclear war, a vast prairie of grass and insects. Runaway warming would extinguish all life.

The Oxford philosopher Nick Bostrom – my kind of maverick! – has written about such low-probability, mega-catastrophic risks as “existential disasters“. Bostrom’s list includes asteroid impact, genocidal aliens, pneumonic HIV, self-replicating nanobots turning the planet into grey goo, and unwise experiments by alien physicists that may trigger

a breakdown of a metastable vacuum state that our part of the cosmos might be in, converting it into a “true” vacuum of lower energy density. This would result in an expanding bubble of total destruction that would sweep through the galaxy and beyond at the speed of light, tearing all matter apart as it proceeds.

Bostrom includes runaway warming, but is over-sanguine about it:

Hopefully, however, we will have technological means of counteracting such a trend by the time it would start getting truly dangerous.

We have a safe baseline technology already – that of the 17th century: horses, sailing ships and windmills, and our options increase all the time. The fear is that we wouldn’t have the time or the political nous to head off a Diamondian collapse.

There’s no point in losing sleep about the Klingons, but Bostrom is right to say we should pay attention to the risks we can do something about and

maximize the probability of an okay outcome, where an “okay outcome” is any outcome that avoids existential disaster.

The main concern of policy should of course be Stern’s conventional risk spectrum. But we also need hedges against runaway warming, just as we do against asteroid impact; initially, enough monitoring to give us timely warning, and a political framework for acting quickly if we have to.

Author: James Wimberley

James Wimberley (b. 1946, an Englishman raised in the Channel Islands. three adult children) is a former career international bureaucrat with the Council of Europe in Strasbourg. His main achievements there were the Lisbon Convention on recognition of qualifications and the Kosovo law on school education. He retired in 2006 to a little white house in Andalucia, His first wife Patricia Morris died in 2009 after a long illness. He remarried in 2011. to the former Brazilian TV actress Lu Mendonça. The cat overlords are now three. I suppose I've been invited to join real scholars on the list because my skills, acquired in a decade of technical assistance work in eastern Europe, include being able to ask faux-naïf questions like the exotic Persians and Chinese of eighteenth-century philosophical fiction. So I'm quite comfortable in the role of country-cousin blogger with a European perspective. The other specialised skill I learnt was making toasts with a moral in the course of drunken Caucasian banquets. I'm open to expenses-paid offers to retell Noah the great Armenian and Columbus, the orange, and university reform in Georgia. James Wimberley's occasional publications on the web

16 thoughts on “Stern report 3: sensitivities, sceptics, and Doomsday”

  1. While I am a firm believer in global warming, a physicist friend pointed out the ridiculousness of the "runaway warming" scenario. We're putting the carbon back into the air that was there during the Jurassic; that's where the plants that ultimately turned into fossil fuels got it. The planet has supported life capably for 4 billion years; adding back the carbon that was once removed is unlikely to change that, though it may have all sorts of adverse effects on Africans and those living below sea level.
    Venus is a lot closer to the sun, and has all sorts of other greenhouse gasses floating around in there; there is absolutely no reason to think it will happen here. One can't rule it out, but one also can't rule out the snowball earth scenario, or the not entirely unlikely possibility that our current carbon output is staving off the return of another ice age.

  2. You should refer all skeptics to this paper:
    Keller, CF (2003) Global warming: the balance of evidence and its policy implications. A review of the current state-of-the-controversy. 3:357-411.
    It considers all the data, their sources, alternative explanations, etc. Overall a good review.
    Even the most skeptical models, relying on sunspot cycles and so forth, have the result that 50-70% of the warming is due to carbon emissions.

  3. I appreciate the effort to think through the issue, on the level of good democratic citizen.
    I don't know if I can express my concern clearly enough, but I'll give it a go:
    I get that the time lags between atmospheric chemistry and climate are long, but doesn't that make the critical policy variable either a critical threshold or a sustainable pace of change?
    Is the aim of policy going to be to slow the rate of increase in emissions to a level consonant with a "safe" or sustainable pace of change? Or, is the objective of policy going to be to avoid some expected threshold, the risks of which are viewed as being too extreme?
    What I get, through the filter of journalism, is that the wide band of uncertainty on target (Y2100) values actually reflect a high degree of certainty that the pace-of-change cannot be usefully controlled, without identifying a companion threshold limit.
    Pace-of-change, by itself, imposes costs, which at higher rates of change, accelerate. Resources expended in amelioration or adjustment costs can be balanced against reducing the pace-of-change, by reducing the rate at which additional carbon emissions change the atmospheric and oceanic chemistry.
    Pace-of-change, however, can not be considered in isolation from threshold values. Above certain threshold values, the pace of change may not be controllable. That there is a substantial risk that the pace of change might become uncontrollable is the real meaning of the fairly wide range of Y2100 target values.
    A threshold, beyond which pace-of-change becomes uncontrollable, would seem critical to identify. Beyond that threshold, policy would be useless. Not only would the adjustment/amelioration costs accelerate, but the economic foundation providing the resources to meet those costs might be deteriorating uncontrollably. And, there would no longer be a policy option to restrain pace-of-change by restraining carbon emissions.
    The brakes must be applied to carbon emissions, with an eye toward not crossing that critical threshold. The threshold, not estimates of Y2100 conditions, ought to be the objective of policy-oriented analysis.

  4. Bruce,
    The pace-of-change you refer to is known in ecology as 'rate of change' and the threshold is the 'tipping point, best thought of as a sigmoid curve (sorry no HTML here to enhance my comments).
    Ecosystems that resist catastrophe with stress are 'resilient'. The issue is that we cannot control resilience with our current technology, so managing ecosystems to the tipping point is a poor strategy due to our current knowledge; managing this way in hopes of technology coming up with a solution may best be termed as crazy.
    Thus, your point of slow pace-of-change (wrt 'emissions') is, with our current knowledge set, the best [only] option in my view. And what I think James is getting at in his middle graph.

  5. Hmmm.
    My comments aren't appearing. I twice drafted a long reply to Jane Galt, which hasn't appeared.
    James, is there a problem?
    (drop the 'at' in email if you wish to email me)

  6. Jane
    I posted a long answer, complete with cites. It hasn't appeared, not sure why.
    The short answer is it took millions of years to sequester all the carbon in coal, oil and natural gas.
    We are releasing that, plus other greenhouse gases such as CFC, plus CO2 from biomass, in the space of less than 350 years, 1750 to 2150 roughly.
    In fact, it is much worse than that: something like half that CO2 release has been since 1945.
    The effect is like overfilling a bathtub. Carbon dioxide released sits in the atmosphere for 100 years.
    The planet has a capacity to absorb c. 4.5bn tonnes of carbon a year.
    We are emitting 7bn tonnes carbon pa, plus another 2bn tpa from land use changes, deforestation etc.
    This *could* become a runaway process, if there is massive methane release from permafrost melt, if there is massive die-off of the rainforests due to drought.
    It is also the case that plants become less efficient removers of CO2 as CO2 concentrations rise.
    Once we pass the 'tipping point' then nothing we do will halt the inexorable rise of CO2 in the atmosphere.
    We don't know what that tipping point is, but some scientists think it could be as low as 550ppm (v. 380ppm now) or even lower. At current growth rates of CO2 emission growth, that is less than 75 years away.

  7. Valuethinker, that makes no sense. The planet started off with a lot of carbon dioxide in the air; it's been progressively declining since the invetion of photosynthesis. We're putting some of the carbon, not all of it, that used to be there back. By what mechanism doesn't putting the carbon dioxide that used to be in the atmosphere, back into it, suddenly trigger a runaway effect?

  8. To be fair, the level of CO2 in the atmosphere has been generally declining at the same time that the Sun's output has been *rising*, so it's not totally implausible that we could see thermal runaway at a CO2 level which the planet endured in the *very distant* past.
    "Once we pass the 'tipping point' then nothing we do will halt the inexorable rise of CO2 in the atmosphere."
    Got a rather constricted view of what we're capable of doing if we really have to, don't you?

  9. Within the same long time lag we are now looking at, have we assumed the continuing same rate of CO2 contributions by GDP?
    Then how do we spend our money? More to hedge the "runaway warming", or reduce the growth rate of Co2 contributed by GDP(or production)?
    Can we control the "runnaway warming"? If we can, will it reduce the rate of CO2 growth rate contributed by GDP(or production)?
    If the latter is preferred, how much confidence should we put into the past Co2 growth rate?

  10. 2 (cont'd) — we emit 7 Gt of carbon pa, and estimated sequestration is half that. So a 180mt release pa of methane is going to do as much damage, again, as all our CO2 release.
    3. the Arctic heats up because as the snow cover lasts less time and there is less of it, there is less albedo (solar reflectivity). Again a positive feedback loop is created. The temperature swing in the Arctic is going to be much bigger than the 2 to 5 degree centigrade swing at the equator.
    The above 3 are the key 'tipping points' *that we know about*. But see 5.
    4. there is the Atlantic thermo-haline circulation (THC). This is also known as the Gulf Stream and the Atlantic Conveyor. We don't know what happens if increased icewater release interrupts that.
    What we do know is that the earth's climate goes through long stable periods, and then occasional sudden 'flippings' or 'pulsings'.
    The effect of 4 might be to cool Europe dramatically, or it might not.
    5. The Permian Extinction is the poster child of the rapid CO2 rise. The result was the extinction of 90% of animal life on the planet.
    (cont'd next post)

  11. So to sum up, we have a number of 'tipping points', arising from the fact that what we are doing is geologically unprecedented (almost), pouring CO2 into the atmosphere:
    – rainforest destruction — rainforest turns from a sequester of CO2 into a source (I've heard estimates of as little as 3 years of drought, certainly Australia is having the '1 in 1000 year drought')
    – permafrost accelerated melt and methane release*
    – decline in the earth's albedo due to alpine ice melt and Arctic warming
    – sudden climate change due to interruption of the THC
    – Permian Extinction effects as described: the 'redline' is 1000ppm, but we don't know at what point the positive feedback effects of global heating kick in (and then we can do, literally, nothing to stop it)
    – (one I didn't mention) – dying of oceanic life due to increased oceanic acidification. Already small hard-shelled creatures are dying off, because they cannot form their shells in the higher acidity. Kill enough ocean life, and the power of it to sequester CO2 is lost.
    This is a new threat, and I don't know much about it, but neither, I think, do the ocean biologists. But indeed, the ph of the ocean is changing.
    * my calculation is off due to confusing Carbon and CO2 (1 tonne emitted carbon is 3.667 tonnes emitted CO2). 1 tonne CH4 (methane) = 20 tonnes CO2 in greenhouse gas effects = 5.45 tonnes Carbon release. Of course, CH4 decays in the atmosphere relatively quickly, combining with oxygen to produce CO2 as a residual gas.
    The reality is we don't know what kind of planet we will have, and what kind of civilization, if the weather warms more than 5 degrees centigrade.
    Would this be the end of life on earth? I doubt it. Algae would evolve that would adapt to the higher CO2 levels, and eventually, perhaps over thousands of years, reduce them– there is precedent. James Lovelock says that a couple of hundred million of us will evolve a new, nuclear powered civilisation around the shores of a Meditteranean-like Arctic ocean.
    What chance does one need of effects like the Permian Extinction to justify radical action? 1%? half a per cent? 0.1 per cent?
    The bathtub metaphor is apt. We are already committed to a much warmer planet– the CO2 emitted up to now will last up to 100 years in the atmosphere. We have little or no control over the next 2 degrees centigrade of heating.
    I know some geophysicists who think we have already passed the tipping point.

  12. Brett
    We've argued the toss on geo-engineering before.
    1. I did some back of the envelope calculations about man-made efforts to change the planet's albedo. You get to enormous numbers: just covering 10% of the surface of the planet with tinfoil is a huge number.
    It would be much cheaper simply to replace all our coal-fired stations with IGCCs and sequester the CO2 output off the syngas synthesisers.
    2. SO2 injection into the atmosphere is only a short term solution: SO2 lasts weeks in the atmosphere, but CO2 over 100 years. And the acid rain could wind up killing more plant life.
    3. I don't know what other 'geoengineered' solutions you were thinking of but the generic problem with this class of 'solutions' is that injecting excess CO2 into the atmosphere and then injecting *something else* to offset the heating absorption effect is *not* the same thing as restoring the atmosphere to its original condition.
    4. we might be able to do something with genetically engineered algae. But an algae bloom like that could kill all other sea life. And the ocean also needs nutrients (there were experiments with salting the ocean with iron pyrites, but they weren't successful).
    So if we get desparate we might try it, but it has big risks and problems, and we certainly don't know how to do it now.
    5. I'll put orbital shades into the science fiction realm. At current costs per pound into orbit, we can't do it. And if we could do it, we should go whole hog and have solar power satellites.

  13. Sally
    The report looks into the CO2/GDP ratio in some detail.
    CO2 emission rises with GDP at a fairly constant rate *but*
    there are distinct structural shifts at certain levels of GDP/ head.
    one is at about $5000 per head, and another at about $25000 per head.
    When countries cross those lines, further GDP growth tends to produce markedly less CO2 emission (per unit of growth) than previously.
    The hypothesis of that Stern Report is that we can bring forward those points of structural reduction by better technology and by a system of carbon permits.
    It's worth remembering that if the world economy keeps growing at the rate it has done since 1950 (2.2% real pa) then in 2050 it will be 2.7 times the current level, and in 2100 7.7 times.
    If it keeps growing at the post 1990 rate (2.9% real) it will be 3.5 times the current level in 2050, and 14.7 times the current level in 2100.
    That underlines a couple of points:
    – spending 1%, or even 5%, of that GDP on CO2 emission control is not going to be an unbearable expense
    – the scale of the mountain ahead of us to climb, in terms of reducing CO2 emission relative to GDP
    We have many, but not all, of the technological and economic building blocks to do the latter.
    The fact that CO2 emitted *now* will affect the climate 100 years from now, underlines that we have to get *moving*, to stand any chance of preventing the more severe scenarios of global climate change.
    We are *now* deciding the climate our grandchildren and great grandchildren will face.

  14. You cannot have thermal runaway on earth because the top of the atmosphere is constrained to be below the boiling point of water. This is because we are that much further from the sun. Water vapor will always condense out and fall back to earth at some level of the atmosphere. Thus boiling off water vapor, which is what messed up Venus, is not allowed. It can get plenty unpleasant here tho.

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