But skeptics misuse this statement to claim there has been no warming
In an interview with the BBC, Phil Jones, the embattled director of the British Climatic Research Unit, said that an observed warming trend of 0.12 degrees C per decade between 1995 to 2009 was “not significant at the 95% significance level.” On the other hand, he said, it was quite close to being statistically significant.
Predictably, the deniosphere jumped all over this. For example, here was Marc Morano’s headline at Climate Depot:
The Jig is Up! Climategate U-turn as Phil Jones admits: There has been no warming since 1995.
Either Marc knows nothing about statistics, or he is deliberately twisting the facts — or both. Phil Jones simply did not say that there has been no warming since 1995.
A 95 percent significance level simply means there is actually a 5 percent chance of a particular finding occurring purely by chance. So here’s what Jones is saying, in essence: There is a very slightly greater than 5 percent chance that the measured warming of 0.12 degrees C per decade between 1995 and 2009 was a statistical fluke — in other words, not real.
Or flop it around: There is a slightly less than 95 percent chance that the observed warming actually happened.
By convention, 95 percent significance often is considered “good enough to be believed.” But this is purely arbitrary, and it does not mean that something with a 94 percent significance level is categorically untrue. If a doctor told you that there was a 94 percent chance that you would die of cancer unless you underwent a particular treatment, what would do? Would you say, “Well doc, if there was a 95 percent chance, I’d accept the treatment, but since it’s just a 94 percent chance, I’ll decline”?
Somehow, I doubt it. I think you’d probably take the treatment.
The problem with the temperature record between 1995 and 2009 probably is not that there has been no warming during that period. The problem, as Jones told the BBC, is this: “Achieving statistical significance in scientific terms is much more likely for longer periods, and much less likely for shorter periods.”
You probably won’t hear that important statement from skeptics like Marc Morano, or from the news media for that matter. But it’s important, because it emphasizes a crucial fact: Climate change is best documented over the course of decades, not years. And over the course of decades, Jones told the BBC, the a chart Jones provided to the BBC at their request shows a clear trend: During four distinct periods since 1860, the climate warmed at a rate of approximately 0.163 degrees C per decade. The global climate has warmed at a rate of approximately 0.163 degrees C per decade since 1860.
Here’s the chart:
I don’t know why the BBC asked only about these specific periods, and I also don’t know off the top of my head what the trend per decade was during the missing time in between 1880 and 1910, and 1940 and 1975. I will try to track those numbers down and post them when I can. (If any readers of CEJournal have the numbers, plus a reference to a reliable primary source, please leave a comment.)