More Likely Than Not? This is Scientific?
By David Henderson
2nd UPDATE BELOW
Reason science correspondent Ron Bailey, in the January 2014 issue of Reason, digs into how the Intergovernmental Panel on Climate Change (IPCC) deals with the fact that the globe has not warmed for the past 15 years despite the fact that the various models had predicted global warming. I’ll leave you to read his whole piece, which is excellent.
Instead, I want to focus on one statement in the report. To do so, I need to quote Bailey extensively. Bailey writes:
The report’s “Summary for Policymakers” declares it “extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” Whether that is so can be probed by comparing observed temperature trends with the simulations of the U.N.’s computer climate models, which assume that human influences are driving climate change. According to the IPCC researchers, “There is very high confidence that models reproduce the general features of the global and annual mean surface temperature changes over the historical period, including the warming in the second half of the 20th century” (emphasis in original). So far, so good: Both the model’s projections and actual temperatures rose during the latter half of the 20th century.
As evidence that the models “reproduce the general features” of actual temperature trends, the new report provides a handy graph comparing projections made in the panel’s previous report with three different temperature records. The report says “the trend in globally-averaged surface temperatures falls within the range of the previous IPCC projections.”
But is that so? Most temperature records show that since 1998 the models and observed average global temperatures have parted ways. The temperatures in the models continue to rise, while the real climate has refused to warm up much during the last 15 years.
Throughout the report that Bailey cites, the authors use terms like “very high confidence,” “low confidence,” “medium confidence,” “very unlikey,” “very likely,” etc. I’ve read their reports before and they have often used these terms. One might like some confidence intervals or a statement that very likely means that the probability is greater than 0.8, etc., but I’m sympathetic to their desire to be somewhat scientific while still trying to address a lay audience.
But here’s the next paragraph. A term in there blew me away. Bailey writes:
The IPCC report acknowledges that almost all of the “historical simulations do not reproduce the observed recent warming hiatus.” Not to worry, it assures us; 15-year pauses just happen, and you can’t really expect the models to simulate such random natural fluctuations in the climate. Once this little slow-down passes, the report maintains, “It is more likely than not that internal climate variability in the near-term will enhance and not counteract the surface warming expected to arise from the increasing anthropogenic forcing” (emphasis in original). In other words, when the warm-up resumes temperatures will soar.
For a criticism of this, I recommend that you read Bailey’s article.
I want to focus on something different: the term “more likely than not.” It seems at odds with the rest of the report. “More likely than not” is awfully colloquial. If we take it literally, it means that the probability is greater than 0.5. But if that’s what they meant and they wanted to write terms that fit their style for the rest of the piece, they probably would have written “somewhat likely.” Notice the difference? Had they used that term, the authors would have left a lot of readers putting more weight on the 15-year hiatus than the authors appear to want us to. I think the authors want us to think the probability is much higher than 0.5, but they’re not willing to come out and say it, probably because they can’t be that as sure as they would like. So instead they use a misleading colloquialism.
In the comments below, Mark Bahner links to their use of language. It turns out that when they say “more likely than not,” they really do mean a probability greater than 0.5. My bad.
I hereby resign as an interpreter of what the authors “appear to want us” to believe. I overstepped. But here’s the interesting point: Given that “more likely than not” means, in their view, a probability greater than 0.5, that, in itself, is quite striking. They are admitting that there is a probability, potentially almost equal to 0.5, that the “internal climate variability in the near-term will counteract and not enhance the surface warming expected to arise from the increasing anthropogenic forcing.”