A draft summary of the Fifth Assessment Report from the Intergovernmental Panel on Climate Change has been leaked to the press. Although I don't have access to the draft itself, the reporting alone is interesting, but also potentially confusing to the average layperson. The New York Times tells us that "An international panel of scientists has found with near certainty that human activity is the cause of most of the temperature increases of recent decades," and quotes the document as saying, "It is extremely likely that human influence on climate caused more than half of the observed increase in global average surface temperature from 1951 to 2010." We are also told that whereas in 2007, "the chances were at least 90 percent that human activities were the cause" of climate change, saying that now in 2013 "the odds are at least 95 percent that humans are the principal cause." Reuters words this slightly differently: "it is at least 95 percent likely that human activities - chiefly the burning of fossil fuels - are the main cause of warming since the 1950s." But what do the phrases "near certainty," "extremely likely," "95 percent odds," "95 percent likely" mean? [the emphasis is mine in all of the above quotes]. In this context, I think they all mean the same thing, but that's not entirely obvious.
First, we have to decode which sense of probability is meant. As philosophers of probability have discussed, there are a number of interpretations of probability. Some are objective, such as frequentist and propensity interpretations, while others are epistemic. Although I've defended the propensity interpretation in certain evolutionary contexts, I happen to think that the major interpretations of probability are all used in one context or another. In this context, I don't think scientists are saying that in 95 out of 100 universes like ours, humans would have caused global warming (roughly, frequentist), or that our universe has a 95% tendency to exhibit human-caused global warming (roughly, propensity). Rather, it's more plausible that they are expressing their degrees of belief about the claim that humans have caused the observed global warming and climate change (roughly, an epistemic understanding of probability).
Second, having decoded the meaning of probability at work, we have to decide what that 95% number means. 5% of doubt might sound like a lot of doubt. But scientists are a cautious lot, especially when dealing with huge complex models with many factors and a politically charged atmosphere. Given that context, their apparent (draft) claim that they think the odds are 95% that humans have caused global warming does mean "near certainty," or what non-philosophers would simply call "certainty."
[Acknowledgements: Thanks to Jonathan Kaplan and Rachel McKinnon for Facebook discussion].
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