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.