On Oct. 1, David G. Victor and Charles F. Kennel wrote an opinion piece that appeared in the journal Nature, Ditch the 2 C warming goal [1]. The provocative title, which accurately conveyed the point of view of the authors, led to several responses, two from Joe Romm at Climate Progress (here and here), one from Stefan Rahmstorf at Real Climate, one from William Hare at Climate Analytics and one from David Roberts at Grist. Victor wrote a long reply to the Romm and Rahmstorf pieces that appeared on Andy Revkin’s New York Times Dot Earth blog.
For those interested in digging in, I found the longer Victor response to be clearer than the very condensed Nature article. The Roberts response is the easiest read for those who are less technical, while the Romm, Hare and Landowsky pieces go into a lot more detail about the problems with the Nature article, which are many and varied.
I’m not going to get into a blow-by-blow analysis of the discussion. Instead, I’d like to explore some key aspects of the 2 C limit that Victor (and others) seem to misunderstand, because of the importance of this concept to making the case for urgent action on climate.
Let me begin by saying that Victor is an acquaintance of mine from when he worked at Stanford, and I’ve always been impressed by his keen intellect. I invited him to lecture in my class when I was first a visiting professor there in 2003-4. He also graduated from Harvard with an undergraduate degree in History and Science, as did I, so I have a deep understanding of his early training. I would call him a friend, though not a close one. But that doesn’t mean I agree with the arguments he made about abandoning the 2 C limit.
The 2 C warming limit is more than just a number (or a goal to be agreed on in international negotiations). It embodies a way of thinking about the climate problem that yields real insights [2]. The warming limit approach, which can also be described as “working forward toward a goal,” involves assessing the cost effectiveness of different paths for meeting a normatively-determined target. It has its origins in the realization that stabilizing the climate at a certain temperature (e.g., a warming limit of 2 Celsius degrees above pre-industrial times) implies a particular emissions budget, which represents the total cumulative greenhouse gas emissions compatible with that temperature goal. That budget also implies a set of emissions pathways that are well defined and tightly constrained (particularly now that we’ve squandered the past two decades by not reducing emissions).
The 2 C limit is a value choice that is informed by science. It should not be presented as solely a scientific “finding,” but as a value judgment that reflects our assessment of societal risks and our preferences for addressing them.
The warming limit approach had its first fully-developed incarnation in 1989 in Krause et al. [3] (which was subsequently republished by Wiley in 1992 [4]). It was developed further in Caldiera et al. [5] and Meinshausen et al. [6], and has recently served as the basis for the International Energy Agency’s analysis of climate options for several years running [7, 8, 9].
Such an approach has many advantages. It encapsulates our knowledge from the latest climate models on how cumulative emissions affect global temperatures, placing the focus squarely on how to stabilize those temperatures. It places the most important value judgment up-front, embodied in the normatively determined warming limit, instead of burying key value judgments in economic model parameters or in ostensibly scientifically chosen concepts such as the discount rate. It gives clear guidance for the rate of emissions reductions required to meet the chosen warming limit, thus allowing us to determine if we’re “on track” for meeting the ultimate goal, and allowing us to adjust course if we’re not hitting those near-term targets. It also allows us to estimate the costs of delaying action or excluding certain mitigation options, and provides an analytical basis for discussions about equitably allocating the emissions budget. Finally, instead of pretending that we can calculate an “optimal” technology path based on guesses at mitigation and damage cost curves decades hence, it relegates economic analysis to the important but less grandiose role of comparing the cost effectiveness of currently available options for meeting near-term emissions goals [2].
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