The Synthesis Report

After the Berlin meeting, my work shifted to the Synthesis Report and its approval session in Copenhagen. Since I was now working with many natural scientists as well as social scientists, there was less room for philosophy. At the meetings, I saw myself, more than anything else, as an advocate for sensible ideas from outside science.

Here is an example. Traditionally, the IPCC expresses its scientific conclusions in terms of likelihoods: it is extremely likely that human beings have caused observable climate change, it is likely that warming will be less than two degrees if cumulative emissions remain below one trillion tonnes of carbon, and so on. But elementary decision theory, which is not part of science, is that decision-making should not depend on likelihoods alone. The right decision to make is not necessarily the one that is likely to have the best result. For example, a ship should carry lifeboats, even though it is unlikely that they will ever be used to save lives. The reason is that, in the very unlikely event of the ship’s sinking, the result will be dreadfully bad if it has no lifeboats. In determining whether to carry lifeboats, this badness should be discounted by its small probability, but even so it outweighs the cost of the lifeboats. This example is no more than common sense, but decision theory goes further. It tells us that decisions should be based on mathematical expectations of value. The likelihoods stated in IPCC reports are insufficient for good decision-making.

To calculate expectations of value we need to know two further things. First, we need to know whole probability distributions, not just the likelihoods of particular events. For example, we need to know, not just what degree of global warming is likely, but what probability to attach to each degree of global warming that might occur. Second, we need values. For example, we need to know how bad each particular degree of warming would be, were it to occur. This second requisite can be met only by deploying a theory of value, which is in the domain of philosophy and far beyond science. Science can in principle satisfy the first requisite, by determining probability distributions for degrees of warming and other variables. However, full distributions are hard to extract from the available climate data. This explains why distributions did not appear in IPCC reports before AR4.

At Synthesis Report meetings I often pressed the point that we need to go beyond likelihoods. And, indeed, the Synthesis Report of AK5 recognizes this point, at least to the extent of giving attention to the possibility of unlikely but very bad events. The unlikely possibility that climate change will destroy our civilization may be more important than the less bad consequences that are much more likely, because it would be so very bad (see Weitzman, 2009). The Synthesis Report says, “Because risk involves both probability and consequence, it is important to evaluate the widest possible range of impacts, including low-probability, high-consequence impacts that are difficult to simulate” (IPCC, 2014b, p. 58). This is a step forward for the IPCC. I wish I could claim credit for it, but I cannot; the IPCC was taking this step anyway without my pressing it.

On matters of ethics, I carried into the Synthesis Report as much as I could from the SPM of WG3. Since this text had already been approved in Berlin, delegates were not able to object to it in Copenhagen. For instance, the SPM of the Synthesis Report includes the sentences: “Mitigation and adaptation raise issues of equity, justice and fairness. Many of those most vulnerable to climate change have contributed and contribute little to GHG emissions.” What the delegates could do is dilute the messages by inserting anodyne words and sentences. The substance survives in the report, but to find it you have to cut through the dross.

Conclusion: Lessons for Philosophers

I am pleased the IPCC decided to give a small place to philosophy in the AR5. The need for it is obvious to philosophers, but many non-philosophers among the IPCC authors and delegates were puzzled by the presence of philosophers. Indeed, many were unclear what philosophy is.

I was surprised at just how alien the methods of philosophy seemed to many natural and social scientists. They lack our patience with argument. We take it for granted that questions should be thought through with high analytic precision, and we go where the arguments lead us. But they often think we are pointlessly picky about precise meanings and points of logic. For example, many philosophers are concerned about the implications of the nonidentity effect— the fact that changing government policies toward climate change will change the identities of the people who will come into existence in the future. The report of WG3 contains a brief discussion of the relevance of the nonidentity effect for the theory of justice toward future generations. But more than one social scientist took me aside to tell me that no one would take this discussion seriously. They found no fault with the arguments; they just did not want to hear them.

Nevertheless, I think we managed to demonstrate that we have a contribution to make. The SPM of WG3 says that, “Social, economic and ethical analyses may be used to inform value judgements.” ‘Ethical analyses’ are explicitly recognized as useful.

In part, I think we achieved some recognition because I was able to talk easily to economists. It helped a great deal to be able to speak a language that was familiar to my colleagues. I hope this allowed me to bring some accurate, sensible thinking to our discussions—the sort of thinking philosophers are devoted to—quite apart from the small amount of substantive philosophy I was able to introduce.

In sum, as philosophers we think in a different and more concentrated way than other people do. This is exactly why we can make an important contribution. But it sets us apart from our colleagues and gives us a hurdle to overcome. To work effectively in the public domain, we have to find a way of overcoming it.


Research for this chapter was supported by Australian Research Council Discovery Grants DP140102468 and DP180100355.


  • 1 I apologize for all the abbreviations I use.
  • 2 The quoted phrase comes from Article 2 of the United Nations Framework Convention on Climate Change (1992), which specifies the convention’s aim. It says, in part, “The ultimate objective of this Convention ... is to achieve ... stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”


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IPCC (1996), Climate Change 1995: Economic and Social Dimensions of Climate Change: Contribution of Working Croup III. Cambridge: Cambridge University Press.

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IPCC (2007), Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II. Cambridge: Cambridge University Press.

IPCC (2014a), Climate Change 2014: Mitigation of Climate Change: Contribution of Working Croup III. Cambridge: Cambridge University Press.

IPCC (2014b), Climate Change 2014: Synthesis Report. Cambridge: Cambridge University Press.

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Weitzman, Martin (2009), “On modeling and interpreting the economics of catastrophic climate change,” Review of Economics and Statistics, 91, pp. 1—19.

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