Nature Letters on PWG

May 22nd, 2008

Posted by: Roger Pielke, Jr.

The 8 May 2008 issue of Nature published 4 letters in response to the Pielke, Wigley, and Green commentary on IPCC scenarios (PDF). This provides a few excerpts from and reactions to these letters.

Vaclav Smil of the University of Manitoba writes:

I largely agree with the overall conclusion of Pielke et al. that the IPCC assessment is overly optimistic, but I fear that the situation is even worse than the authors imply.

Smil is realistic about the challenge of mitigation:

The speed of transition from a predominantly fossil-fuelled world to conversions of renewable flows is being grossly overestimated: all energy transitions are multigenerational affairs with their complex infrastructural and learning needs. Their progress cannot substantially be accelerated either by wishful thinking or by government ministers’ fiats.

But pessimistic about action:

Consequently, the rise of atmospheric CO2 above 450 parts per million can be prevented only by an unprecedented (in both severity and duration) depression of the global economy, or by voluntarily adopted and strictly observed limits on absolute energy use. The first is highly probable; the second would be a sapient action, but apparently not for this species.

Christopher Field, from Stanford University agrees with our analysis and its implications:

The trends towards increased carbon and energy intensity may or may not continue. In either case, we need new technologies and strategies for both endogenous and policy-driven intensity improvements. Given recent trends, it is hard to see how, without a massive increase in investment, the requisite number of relevant technologies will be mature and available when we need them.

Richard Richels, of the Electric Power Research Institute, Richard Tol, of the Economic and Social Research Institute (Ireland), and Gary Yohe, of Wesleyan University support our analysis and our interpretation of its significance:

Pielke et al. show that the 2000 Special Report on Emissions Scenarios (SRES) reflects unrealistic progress on both the supply and demand sides of the energy sector. These unduly optimistic baselines cause serious underestimation of the costs of policy-induced mitigation required to achieve a given stabilization level.

This is well known among experts but perhaps not to the public, which may explain why some politicians overstate the impact of their (plans for) climate policy, and why others argue incorrectly that ‘available’ off-the-shelf technologies can reduce emissions at very little or no cost.

They also make an absolutely critical point about climate policy – it is necessarily incremental and adaptive:

The focus of policy analysis should not be on what to do over the next 100 years, but on what to do today in the face of many important long-term uncertainties. The minute details of any particular scenario for 2100 are then not that important. This can be achieved through an iterative risk management approach in which uncertain long-term goals are used to develop short-term emission targets. As new information arises, emission scenarios, long-term goals and short-term targets are adjusted as necessary. Analyses would be conducted periodically (every 5–10 years), making it easier to distinguish autonomous trends from policy-induced developments — a major concern of Pielke and colleagues. If actual emissions are carefully monitored and analysed, the true efficacy and costs of past policies would be revealed and estimates of the impact of future policy interventions would be less uncertain.

Such an approach would incorporate recent actions by developed and developing countries. In an ‘act then learn’ framework, climate policy is altered in response to how businesses change their behavior in reaction to existing climate policies and in anticipation of future ones. This differs from SRES-like analyses, which ignore the dynamic nature of the decision process and opportunities for mid-course corrections as they compare scenarios without policy with global, century-long plans.

Ottmar Edenhofer, Bill Hare, Brigitte Knopf, Gunnar Luderer Potsdam of the Institute for Climate Impact Research (Germany) suggest that the range of rates for the future decarbonization of energy in the IPCC reports is in fact appropriate:

Over the past 30 years, the decrease in energy intensity has been 1.1% a year — well above the 0.6% a year assumed in 75% of the energy scenarios assessed by the IPCC.

Developments in China since 2000 do raise concerns that the rate of decrease in energy and carbon intensity could slow down, or even be reversed. However, similar short-term slow-downs in technical progress have occurred in the past, only for periods of more rapid development to compensate for them. India, for example, does not show the decreasing trend in energy efficiency seen in China.

The figure of 75% of scenarios of the IPCC assuming 0.6% per year decrease in energy intensity is difficult to interpret. But here is what the IPCC itself says on this (WGIII Ch. 3, p. 183 PDF):

In all scenarios, energy intensity improves significantly across the century – with a mean annual intensity improvement of 1%. The 90% range of the annual average intensity improvement is between 0.5% and 1.9% (which is fairly consistent with historic variation in this factor). Actually, this range implies a difference in total energy consumption in 2100 of more than 300% – indicating the importance of the uncertainty associated with this ratio.

So if 5% fall below 0.5%, it is hard to understand what the authors mean by “0.6% a year assumed in 75% of the energy scenarios assessed by the IPCC.” Contrary to the other letters Edenhofer et al. conclude:

The IPCC’s main policy conclusions stand: present technologies can stop the rise in global emissions.

The final letter is from Joseph Romm, of the Center for America Progress. He chooses to parse what is meant by the term “climate policies” in the vernacular of the IPCC:

They criticize the IPCC for implicitly assuming that the challenge of reducing future emissions will mostly be met without climate policies. But the IPCC’s Special Report on Emissions Scenarios makes clear that, although the scenarios don’t technically have climate policies, they can and do have energy efficiency and decarbonization policies, which amount to the same thing

It is not clear why this semantic point matters for interpreting our analysis as it has no implications for either our technical analysis or its interpretation. Of course, the IPCC defined the notion of “climate policies” quite precisely for a reason — because the policies that relate to improved energy efficiency and decarbonization assumed by the IPCC to occur in their scenarios in the absence of climate policy mean that these other policies would be implemented with no effort focused on the stabilization of greenhouse gases in the atmosphere (no cap and trade, no Kyoto, no carbon tax, etc. etc.). These policies, whatever they are, would happen spontaneously or automatically without any concern for climate. This assumption was explicit in the terms of reference for the IPCC SRES exercise for the purpose of clearly identifying the marginal benefits and costs of climate-specific policies.

Romm then simply repeats the conclusions of the IPCC:

the IPCC report makes clear that we have the necessary technologies, or soon will, and focuses on creating the conditions for rapid technological deployment

Interestingly, with a letter in Nature Romm, who has been a strong critic of our paper on his blog, had a perfect opportunity to explain what might have been incorrect in our technical analysis, and did not. We can assume that he was unable to find any flaws and thus chose to focus on the implications of the analysis, which he does not enagage, choosing simply to restate a position that he held before our paper came out.

As can be seen clearly in the letters, there is not a consensus among energy policy experts on the role of technological innovation in efforts to mitigate climate change. This is a debate which has only just begun, and for which there are a range of legitimate and informed points of view, despite the efforts of some to demagogue anyone who disagrees with their views.

15 Responses to “Nature Letters on PWG”

  1. jromm Says:

    Once again Roger you have too many misstatements for me to respond to. First off, with this post, I think it is rather clear that your support for 450/500 ppm is in name only, since you spend all your time explaining why it can’t be done or quoting others who agree with you.

    Nature asked me for a letter — that is the only reason I sent one in. They limited me to a few hundred words — and then refused to publish any direct quotes from the scientific literature. Indeed, they said my core critique of yours was obvious and they didn’t want to run it. So the letter ended up eviscerated. I would have withdrawn it if I thought you would have made such an absurd leap as to its meaning.

    I have published over 10,000 words explaining the myriad flaws in your paper — most of which you never rebutted. So I can only assume you agreed with my analysis.

    Smil is a smart guy — but he doesn’t know energy technologies. If he thinks the world is doomed, he is entitled to his opinion. But quoting him ardly counts as a rebuttal to those who do understand his choice is a false one and that 450 ppm is technologically and economically achievable, but it is probably not politically achievable — especially if people listen to him (and you).

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  3. Roger Pielke, Jr. Says:

    Joe- We’d be welcome to publish your original letter as submitted to Nature … send it along.

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  5. Roger Pielke, Jr. Says:


    Also, you write, “I have published over 10,000 words explaining the myriad flaws in your paper”

    And I think about 4,337 of those 10,000 words were “delayer” and “denier”;-)

    Seriously, if you’d like to explain in 1,000 words our less the TECHNICAL flaws in our paper (i.e., not your disagreement with our interpretation of its policy significance, which we’ve aired quite comprehensively), I would be happy to run it as a main line guest post here. And if you can do it without using the word “delayer” that’d be great ;-)

    Just let us know.

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  7. Richard Tol Says:

    Joe — saying that Vaclav Smil does not understand energy is a bit of stretch.


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  9. Lupo Says:

    Roger you might be forgetting that any questioning is denial even for those in policy and implementation or in technology matters or by those involved in the economic world related to energy or environment who have a job to question these things. Even statisticans looking into the statistical and those tasked to look into the social aspects of a field of climatology are not allowed to question the work of climate scientists. Those who are science climate people that question are also deniers. Does it not seem that way to you by behavior and attitude?

    These are not the same worlds and those in the know do not trust the motivations or understand the terms or the other viewpoints?

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  11. jromm Says:

    Now Roger — it is clear for all to see that I didn’t say “Vaclav Smil does not understand energy.” He does — but he doesn’t understand “energy technologies.” What he understands about energy is that the energy system changes very slowly on its own. Left to its own devices, obviously the energy system won’t prevent 450 ppm.

    The question is whether humans are sapient enough to take matters into their own hand and speed things up. He thinks it would be sapient if we “voluntarily adopted and strictly observed limits on absolute energy use.”

    I think it would be sapient if we “adopted an aggressive strategy to deploy energy efficient and low carbon technologies, some version of the 14 or so wedges” here:

    I think that would be more sapient — and more plausible — than his plan. But I am fully aware it is unlikely to happen. But as long as it is not too late — and it pretty clearly isn’t — I remain an optimist, I prefer to see the glass as 5% full.

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  13. jromm Says:

    Oops, I see that was Richart Tol who made the Smil remark, not Roger.

    BTW, I’ll probably post the original letter — and the full e-mail exchange with Nature — on my blog.

    1000 words? Let me think if that does the critique justice.

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  15. Richard Tol Says:


    Your technical objections to Pielke would be welcome indeed. I searched your blog, but found little material than can be described as an academic argument.

    A number of us have done the sums, and found results that are very similar to those of Roger, Chris and Tom: The IPCC SRES scenarios are peculiar in many ways.


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  17. jromm Says:


    The technical objections to Pielke can be found here — I can see why it was hard to find since I left Pielke and Nature out of the title:

    Interestingly, while I have problems with Pielke’s analysis — my far bigger concerns are with his primary conclusions.

    The IPCC SRES scenarios are very peculiar. I’d even say they are not terribly useful and a largely counterproductive. But, as I explain in the above post:

    The historical decarbonization data strongly suggests that the two major conclusions of the Nature article are wrong:

    1. The IPCC scenarios are not filled with “Dangerous Assumptions,” as the title of the Nature article asserts.
    2. The recent carbonization data does not support the central conclusion of the article “Enormous advances in energy technology will be needed to stabilize atmospheric carbon dioxide concentrations at acceptable levels.” In fact, if anything, it supports the reverse conclusion.

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  19. jromm Says:

    Oh, I forgot to post Part 2 of that:

    That is the post where I explain why this statement from Pielke et al is somewhere between “very misleading” and “completely untrue”:

    … the scenarios assume a certain amount of spontaneous technological change and related decarbonization. Thus, the IPCC implicitly assumes that the bulk of the challenge of reducing future emissions will occur in the absence of climate policies. We believe that these assumptions are optimistic at best and unachievable at worst, potentially seriously underestimating the scale of the technological challenge associated with stabilizing greenhouse-gas concentrations.

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  21. Richard Tol Says:


    You seem to have entirely missed the main point of PGW: The SRES scenarios assume very substantial technological change. A number of people in the field, particularly engineers and politicians, do not know that — and they present the difference between “frozen technology” and “business as usual” as “climate policy”. This has led people to believe that climate policy is effortless and costless.

    As a minor point, the SRES scenarios are very peculiar in their assumptions about population (too little migration, fertility falling too fast, aging too little), economy (too fast in Africa, too slow in Asia), and energy efficiency (too fast) — their energy supply looks in line with history, but they may have overlooked the current dash for coal.


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  23. jromm Says:


    I suppose after reading the piece a dozen times and writing maybe 10,000 words about it, I could have “entirely missed the main point of PGW.” Had you read the two links I provided, though, you’d see that I haven’t. Since you didn’t, let me repeat the point here:

    As I explain, the key word in PGW, which you omit from your comment is “spontaneous.” In paragraph three they write,

    “… the scenarios assume a certain amount of spontaneous technological change and related decarbonization. Thus, the IPCC implicitly assumes that the bulk of the challenge of reducing future emissions will occur in the absence of climate policies.”

    PGW repeats the word “spontaneous” several times, implying that the technological improvement happens on its own. And in Paragraph II, they write the IPCC reference scenarios “assume no policy interventions directed toward reducing greenhouse-gas emissions.”

    But those statements are so misleading as to be completely inaccurate.

    As I wrote, In fact, in its scenarios, the IPCC assumes energy and environmental policies, but just isn’t allowed to call them “climate policies.” I kid you not.

    Here is what the Special Report on Emission Scenarios (SRES), which the Nature article cites, says about the scenarios:

    “As required by the Terms of Reference, however, none of the scenarios in the set includes any future policies that explicitly address additional climate change initiatives, although GHG emissions are directly affected by non-climate change policies designed for a wide range of other purpose.”

    Let me give a very specific example. Most of the IPCC scenarios are of little interest because they result in global warming of much more than 2°C — and thus they make catastrophic climate impacts likely. The B1 scenario, however, is worth examining because it keeps warming close to 2°C through energy efficiency and decarbonization. Does this happen spontaneously? Quite the reverse:

    “The central elements of the B1 future are a high level of environmental and social consciousness combined with a globally coherent approach to a more sustainable development. Heightened environmental consciousness might be brought about by clear evidence that impacts of natural resource use, such as deforestation, soil depletion, over-fishing, and global and regional pollution, pose a serious threat to the continuation of human life on Earth. In the B1 storyline, governments, businesses, the media, and the public pay increased attention to the environmental and social aspects of development.”

    In other words, in B1, humanity aggressively pursues sustainable development.

    “… Technological change plays an important role. At the same time, however, the storyline does not include any climate policies, to reflect the SRES terms of reference. Nevertheless, such a possible future cannot be ruled out.”

    Semantically, the scenario writers are not allowed to include climate policies — but they are allowed to include policies that would lead to a great deal of decarbonization and energy efficiency, which is the same thing.

    “Particular effort is devoted to increases in resource efficiency to achieve the goals stated above. Incentive systems, combined with advances in international institutions, permit the rapid diffusion of cleaner technology. To this end, R&D is also enhanced, together with education and the capacity building for clean and equitable development. Organizational measures are adopted to reduce material wastage by maximizing reuse and recycling. The combination of technical and organizational change yields high levels of material and energy saving, as well as reductions in pollution.”

    That does not sound like “spontaneous technological change and related decarbonization.” In fact, it sounds like an aggressive and coherent climate policy to me.

    “The B1 storyline sees a relatively smooth transition to alternative energy systems as conventional oil and gas resources decline. There is extensive use of conventional and unconventional gas as the cleanest fossil resource during the transition, but the major push is toward post-fossil technologies, driven in large part by environmental concerns.”

    And it also sounds like B1 is imagining a future where conventional oil and gas production peaks and declines, which, as we’ve seen, appears to be our likely future.

    “Given the high environmental consciousness and institutional effectiveness in the B1 storyline, environmental quality is high, as most potentially negative environmental aspects of rapid development are anticipated and effectively dealt with locally, nationally, and internationally. For example, transboundary air pollution (acid rain) is basically eliminated in the long term. Land use is managed carefully to counteract the impacts of activities potentially damaging to the environment. Cities are compact and designed for public and non-motorized transport, with suburban developments tightly controlled. Strong incentives for low-input, low-impact agriculture, along with maintenance of large areas of wilderness, contribute to high food prices with much lower levels of meat consumption than those in A1. These proactive local and regional environmental measures and policies also lead to relatively low GHG emissions, even in the absence of explicit interventions to mitigate climate change.”

    Pretty amazing, no? A primary conclusion of the PGW article, embodied in its title, “Dangerous Assumptions,” is simply wrong.

    You, and PGW, claim “This has led people to believe that climate policy is effortless and costless.”

    I have never met anyone who believes the IPCC says that. Let’s see, what did IPCC head, Rajendra Pachauri, tell the NYT when the Synthesis report was released:

    “If there’s no action before 2012, that’s too late. What we do in the next two to three years will determine our future. This is the defining moment.”

    Does that sound like the head of a group that has underestimated the scale of the climate challenge?

    The IPCC report says,
    “There is high agreement and much evidence that all stabilisation levels assessed can be achieved by deployment of a portfolio of technologies that are either currently available or expected to be commercialised in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers.”

    Again, 1) doesn’t sound to me like they are assuming spontaneous technological change. They are assuming technological change driven by strong policies. And yet PGW ends by saying:

    “IPCC plays a risky game in assuming that spontaneous advances in technological innovation will carry most of the burden of achieving future emissions reductions, rather than focusing on creating the conditions for such innovations to occur.”

    What “risky game” are they playing? The IPCC scientists are begging the world to stabilize below 450 ppm starting NOW. We’ve seen the IPCC does not assume “spontaneous” advances — it assumes humanity will take strong action.

    “… rather than focusing on creating the conditions for such innovations to occur.” This is similar to the earlier phrase “diverting attention from policies that could directly stimulate technological innovation.” It is similarly nonsense.

    Have you read Working Group III report? I’m looking it over right now. It lays out in great detail all of the technology strategies needed to stimulate innovation, including a carbon price of $50 a ton of CO2, which is hardly a low price.

    It talks about the GDP cost If we adopt the least-cost trajectories. The numbers they use are certainly in the range of the numbers I’ve seen in the studies I believe. They talk about “changes in lifestyle and behavior patterns can contribute to climate change mitigation across all sectors” the need for “new energy infrastructure investments in developing countries, upgrades of energy infrastructure in industrialized countries.” They write “initial estimates show that returning global energy-related carbon dioxide emissions to 2000 levels by 2030 would require a large shift in the pattern of investment.”

    And on and on an on. Therefore, 2) it doesn’t sound to me like they are saying this would be effortless and costless. Every climate scientist and energy expert I know is increasingly desperate in begging the world to take strong action immediately. I wrote a big piece on that in Salon.

    I just think PGW created a strawman and then destroyed it.

    Richard — please point me in the direction of links to politicians and other opinion leaders who
    “believe that climate policy is effortless and costless” or to articles written about the IPCC report that draw such an inference. I have seen neither.

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  25. Richard Tol Says:

    Joe: I’ve made it a rule that I always study the person I’m discussing with. You seem to be unaware of the fact that I am a veteran of the IPCC and intimate with SRES.

    PGW have not uncovered some hidden assumption in SRES. Rather, they point at an assumption that is either misunderstood or ignored by large number of SRES users.

    Engineers who believe in costless action: Blok, Krause, Laitner.
    Politicians who confuse baselines with policy: Bush, Dimas.

    IPCC WG3 in its AR4, by the way, strengthens this misinterpretation of the baseline. If you look at the cost curve implied by the tables in the technical summary, then you’ll find that it bends the wrong way. If you wonder why, you’ll eventually find a footnote that will tell you that the deeper targets can only be reached from lower baselines. The material in the summary suffers from severe sample bias, significantly underestimating the costs of emission reduction.

    And yes, this was no mistake: At least one author and one referee pointed to sample bias and the potential for incorrect interpretation.


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  27. jromm Says:


    I am extremely well aware who you are (and if I didn’t, I know how to use Google, so please drop the patronizing tone).

    That’s why I was stunned you defended Smil, who, as Pielke noted, wrote in his Nature letter: “Consequently, the rise of atmospheric CO2 above 450 parts per million can be prevented only by an unprecedented (in both severity and duration) depression of the global economy, or by voluntarily adopted and strictly observed limits on absolute energy use. The first is highly probable; the second would be a sapient action, but apparently not for this species.”

    This is NOT what the IPCC report that you helped author concludes in the least bit, nor is it what most major economic models that I have seen concluded, and it is certainly not what bottom-up studies show.

    You defended Smil. Do you agree with him?

    Second, do you mean “George W” Bush is confused? And you are blaming the IPCC for that? You cannot be serious. I doubt he could spell IPCC.

    Third, you need to do more than just drop names of people who “believe in costless action” or “confused baselines with policies” (whatever that means) — you need to provide links.

    I know Laitner very well, if you mean Skip Laitner, a resource economist, not an engineer. He certainly does NOT believe in costless action. He does believe there is a very large energy efficiency resource, as does the National Academy of Sciences and the U.S. national laboratories (in a report I oversaw, which you know doubt know since you’ve studied me), and the IPCC itself!

    As you well know, the 2007 Synthesis report says, “Bottom-up studies suggest that mitigation opportunities with net negative costs have the potential to reduce emissions by around 6 GtCO2-eq/yr in 2030.”

    Wow! A 20% reduction in global emissions might be possible in a quarter century with net economic benefits!

    Is that costless action? I suppose so. Is it true? Again the U.S. DOE and the NAS and McKinsey thinks so, among others. Guess they are all wrong.

    But in any case, they don’t have their positions BECAUSE of the IPCC, they have them because that is what the literature says, because they did the analysis themselves, and/or because many of them, many of us, have direct experience which makes clear that the energy efficiency potential exists.

    Just because the mainstream economic community does not accept the existence of a large efficiency potential 1) does not mean you are right and 2) does not mean everybody who does is wrong and wrong because of the IPCC.

    In any case, one of the IPCC’s job is to reflect the scientific literature — not just the view of part of one academic discipline. So again, I am still waiting to see what the dangerous assumptions in the scenarios are.

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  29. Richard Tol Says:


    Laitner is indeed Skip. Their AMIGA model is a wonderful example. They use a baseline scenario with lots of technological progress, and then in their policy analysis they use the difference between frozen technology and baseline again to find that 550 ppm can be reached at the stroke of a pen!

    The bottom-up studies in the IPCC and elsewhere tend to make the same double-counting error.