The significance of 5 year trends

5 year trends from surface temperature are not very significant and are a bad measure of anything. As everyone should know. But it seems that some people don’t. So in tedious detail…

Pick up the HadCRU temperature series from here. Compute 5, 10 and 15 year trends running along the data since 1970 and get (black lines data, thicker black same but smoothed, thin straight lines non-sig trends; thick straight blue lines sig trends):

i-f52290941a698ea6e0aa2af09525f70d-5-year-trends.png

From which you can see (I hope) that the series is definitely going up; that 15 year trends are pretty well all sig and all about the same; that about 1/2 the 10 year trends are sig; and that very few of the 5 year trends are sig.

From which the motto is: 5 year trends are not useful with this level of natural variability. They tell you nothing about the long-term change.

10 thoughts on “The significance of 5 year trends”

  1. It perturbs me when supposedly respected scientists make statistically unsound claims like Pielke did about these 5 years series when they really should know better. The “It has been cooling since ’98” meme is another example propagated by Lindzen & Bob Carter among others.

    You don’t even need stats to see the claims are unsound – its pretty obvious just from eyeballing the relevant graphs.

    What is going on here? Have they really forgotten all they learned in Stats 101 or are they playing fast and loose with data to push an agenda or preserve their ego? It surprises me because it can only damage the credibility as scientists – but perhaps scientists aren’t the peer group they are trying to impress.

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  2. Significant as in 95% significant? Certainly important for scientific reasons, but in a potential bet I’m willing to put down money even when the odds are less than 95% in my favor. Still, five year periods look pretty dicey. Someone’s trying to get me into a five year bet, but I think I’ll need a longer period.

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