High-consequence outcomes and internal disagreements: tell us more, please

This article is one of several in this special double-issue that reports the views of “users” of IPCC reports. I am a user in the sense that I advise the policy-making community and rely on the IPCC reports to provide me with authoritative views on the state of the science. My principal recommendation for making the IPCC more helpful to the policymaking community is to strive in the Fifth Assessment Report (AR5) to communicate fully what the climate science community understands and does not understand about highconsequence outcomes. This will require the AR5 authors to provide vivid information about future worlds where high-consequence outcomes have emerged. It will also require the AR5 authors to reveal any disagreements persisting among them after the give-and-take of the writing process has run its course. In the Fourth Assessment Report (AR4) the presentation of high-consequence outcomes had shortcomings that can be rectified in AR5. 1 Helping the policymaker decide how important climate change is Users like me, who read IPCC reports in order to give better advice to others, are probably a small fraction of all the users. Many of us, including me, are self-appointed. We publish (including articles like this one), lecture, and serve on committees. In our own minds, we do not “represent” any interest group, and our reward for doing well is to be invited to serve again. Our principal target is the policy-making community, including office holders, their staffs, and those opinion-formers whom they take seriously. In giving advice, we hunt for the issues that matter a lot but are poorly formulated or are buried out of sight. The better the IPCC does its work, the less work we have. Inevitably, those with a role like mine make assumptions about what policymakers want to know. I assume that Objective Number One for anyone in the policy-making community is to form an independent judgment of how urgent the climate change problem is. Does the world need to drop everything else and assign climate change the highest priority, or is Climatic Change DOI 10.1007/s10584-011-0187-5 R. H. Socolow (*) Department of Mechanical and Aerospace Engineering, Princeton University, Guyot 139, Princeton, NJ 08544, USA e-mail: [email protected] climate change one of several important problems? Does the world have the luxury of being able to take a whole century to accomplish decarbonization, or must the job be accomplished in just two or three decades? Policymakers understand that a fast pace for emissions reduction will generate large disruptions as “solutions” are implemented, but that a fast pace will also produce large benefits to the extent that disruptions from climate change itself are avoided. Thus, to make an informed decision about pace, a policymaker needs help answering two questions: How disruptive are the “solutions”? How disruptive is climate change? Disruptions associated with solutions, and ways for the IPCC to discuss them, are topics for a different essay. Suffice it to say that policymakers prefer to go slow. They are inured to the sales pitches they receive from all quarters. In considering extensive reshaping of landscapes, global expansion of nuclear power, albedo management, and other problematic responses to climate change, they can imagine cures worse than the disease. In my view, the Fifth Assessment Report (AR5) could distinguish itself, relative to its predecessors, by the depth of its probing of the problematic aspects of “solutions.” But the objective of this essay is to address how the IPCC could better help the policymaking community in its search for deeper understanding of disruptions due to climate change itself. I set aside the well known conundrum that the policy maker seeks spatial detail (especially related to the region he or she represents) while the modeler is constrained by coarse resolution. Rather, my focus here is on the policy maker’s interest in understanding high-consequence outcomes. The phrase “high-consequence outcomes” has become a term of art in IPCC literature. Highconsequence outcomes arise in a world of very strong positive feedbacks, a world that spirals out of control. For specificity, I have in mind a five-meter rise in sea level by the end of this century, or major alterations of the global hydrocycle, or major changes in ocean biota, or yet another of what my colleague, Steve Pacala, calls “monsters behind the door.” The key recommendation of this article is this: In AR5 the IPCC should communicate fully what the science community does and does not understand about high consequence outcomes. The rest of this article is intended to shed light on what this task entails. 2 Making high-consequence outcomes vivid The policy-making community understands that there are two complementary ways to think about damage from climate change. There are outcomes that are best guesses, and there are outcomes that are either much more benign or much more alarming than the best guesses. Policymakers know they need to understand the full story, the full distribution—both the middle and the “tails.” They wonder whether, in reaching a judgment about whether to act forcefully now, they should give more weight to the best guesses or to the tails, and they want to understand the relative weights. Alas, earth systems science right now cannot provide much guidance about these relative weights, yet they matter greatly. Martin Weitzman has shown that the problem of relative weights for mean vs. extreme outcomes can be formulated mathematically using a damage function that sums over outcomes, and that whether the mean or the extreme dominates (whether the sum converges) depends on currently unknowable details of the probability distribution for the worst outcomes. [Weitzman 2009] Accordingly, the policy-making community needs information about both probable and improbable outcomes. One can imagine that, for many policymakers, the priority given to climate change is strongly dependent on what the IPCC thinks about high-consequence outcomes. Climatic Change