The IPCC SRES scenarios
    The Skeptical Environmentalist                                                                                                                       Cool it


The IPCC produced a Special Report on Emission Scenarios (Nakicenovic et al. 2000), abbreviated SRES; it may be downloaded here. The SRES report formulates 40 socio-economic scenarios for the world up to the year 2100. They are divided into four main types, called A1, A2, B1 and B2. Lomborg presents these four in The Skeptical Environmentalist (Figure 166) and in the British version of Cool It! (Figure 51). However, he more or less misunderstands or misrepresents what the scenarios signify.
    The scenarios are not predictions, and they are not policies or opportunities to be chosen among. They are just possible outcomes of what happens during the next 100 years. There is no likelihood associated with them. As said in the SRES report section 4.2.1: "All four SRES "futures" represented by the distinct storylines are treated as equally possible". It has also been said that they are equally "valid". Thus it is very incorrect of Lomborg to say in Cool It! that one scenario is the "likely" one, or that one scenario is "wrong". We do not know what the future brings until the future has come.
    Concerning the high-growth and high-equity A1 scenario, we do not know if it will be possible. As stated in the SRES report section 4.4.4.: ". . it is agreed that such scenarios of high productivity growth and smaller income-per-capita disparities cannot be ruled out, even if they certainly are very challenging from the perspective of recent growth experiences in a number of regions, most notably in Africa." "Very challenging" means that it will be very difficult to let this scenario happen.
    Of course, if one had a possibility for an active choice, one would choose a future (like A1) with a high economic growth rate, and with a narrowing of the inequality between rich and poor countries. Nobody would deliberately choose a future (like A2) where the economies of poor countries are not growing very fast and where they never catch up with the rich countries. On the other hand, nobody would say for sure that it is possible that the poor countries in just 100 years from now will catch up with the rich.
    An important feature of A2 is that population growth in The Third World continues at a high level for a long time. This retards economic growth there, and leads to relatively high numbers of poor. At the same time, the total emission of CO2 over the century is higher in A2 than in A1, so the relatively low economic growth in A2 does not benefit the climate. Clearly, A2 is hardly a scenario that anybody would want. Most people would want lower population growth. By various policies, the birth rates in The Third World can be pushed downward to some extent, but nobody has the power to set the birth rates. So the birth rate is an important driving parameter that "just happens" to be at a certain level. We cannot "choose" the parameter. So the birth rates may happen to be like in A1 or like in A2. We dont´t now for sure yet. Therefore A1 and A2 are possible futures that may or may not happen irreswpectively of our will. They are not policies or options that can be chosen among.
    Next, Lomborg fails to understand that the scenarios must be baseline stories without climate mitigation. All scenarios include some effort to prevent deterioration of the environment, such as endeavors against water and air pollution. As stated in the SRES report section 4.4.10: " . . a range of environmental measures is assumed in all four SRES scenario families, although the magnitude and timing differ . . . ". Even for the B1 scenario the report says explicitly in section 4.3.3.: "The storyline does not include any climate policies." Therefore, it may lead to misunderstandings that the B1 and B2 scenarios are labelled as having more emphasis on sustainability or the environment than A1 and A2. The report says in section 4.2.1: "Nevertheless, there was considerable resistance in the SRES writing team against such a simplistic classification of storylines, so it is presented here for illustrative purposes only. These distinctions  are, in a sense, artificial, for example, both economic and environmental objectives are pursued in all scenarios, albeit with different levels of relative emphasis".
    So, efforts against e.g. air pollution in cities are included in the models, but efforts against global warming are not included. Why? Because the scenarios are models constructed with the aim of having a basis to which the costs of climate mitigation can be compared. So, for this reason also, it is wrong of Lomborg to speak of policy choice - no climate policy is implemented in any of the scenarios.
    It is also important to know that most of the 40 versions of the scenarios have deliberately been "harmonized" with respect to population growth and GDP growth. That is, economic growth is not the result of a certain starting condition, from which the model is allowed to predict the further course of events.  The economic growth in the models are not results, but preconditions. This harmonization is done in order to make model runs with mitigation as conclusive as possible. If the harmonized models did not have the same population and the same GDP,  we would not know if any differences between models in the effectsa nd costs of mitigation were simply due to differences in population and GDP.
    If one wants to know what will be the cost of climate mitigation, one may consult the following IPCC TAR report of working group III:
B. Metz et al. (2001): Climate change 2001: mitigation. 752 pp. Cambridge University Press. It may be downloaded here.
    The results presented in this report are, of course, that mitigation costs will be very different in different scenarios. For instance, in the A1 scenario faimily, three of the main versions are called A1FI, A1T, and A1B. The first has a growth based on fossil fuels as energy source. A1T , on the other hand, implies technological innovation that  allows a  major shift towards renewable energy sources, and A1B  (which may just be written A1) is  a balanced version  midway between the two. Of course, in A1FI  with its high consumption of coal etc., mitigation to reduce CO2 output will be much more costly than in A1TI.     The main results are presented on pages 548 - 549 in the mitigation report. For instance, if we choose to reach a stabilization level of 550 ppm CO2 in the atmosphere, then the costs to reach this level in 2050 are predicted to be as follows, expressed as percent of world GDP:
A1B:  0.8 %
A1FI: 1.8 %
A1T:  0.4 %
A2:    0.9 %
B1:    0.1 %
B2:    0.6 %
So, the lesson from this is that there is, of course, not one figure that tells us what stabilization at 550 ppm will cost. The price will differ widely according to what the future brings in other respects.
   More details may be found in T. Morita et al. (2000): Overview of mitigation scenarios. . . Environmental economics and and policy studies 3(2).
    Lomborg has seen the above mentioned figures from the mitigation report (he discusses them in footnote 2579 in The Skeptical Environmentalist). If he had chosen to present these figures to his readers, then the readers would have received useful information. But it seems that Lomborg does not do that because these figures are the results of cost-effectiveness analyses, whereas Lomborg prefers cost-benefit analyses. That is, the studies made within the IPCC framework are based on a certain stabilization target (e.g. 550 ppm CO2), and then model runs are performed to find out how that target may most effectively be reached, and what it will cost in terms of GDP reduction. Those studies that use carbon taxes to reach the target, find of course that the correct tax rate depends on the stabilization target. With 550 ppm as target, the A1 scenario requires carbon taxes of about $50 - 200 per t C as an average over time. This is much, much higher than the level of $7.5 / tC which is presented by Lomborg as being optimal. No wonder, therefore, that Lomborg rejects the IPCC results of cost-effectiveness studies.
     At the same time as Lomborg argues for a low rate of carbon taxation, he argues that the costs of mitigation are much higher than generally accepted. He does that by postulating that the costs of climate mitigation may be read from the differences between A1 and the other scenarios. This is a complete misunderstanding. Lomborg´s  figures with scenario GDP´s as pies and differences between the pies shown as black slices have nothing to do with mitigation costs. His texts related to these figures are therefore nonsense.

The Skeptical Environmentalist                                                                                                                       Cool it