How long before a trend is 'statistically significant'? How questions to Jones were cherry-picking

muriel_volestrangler

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Tue Feb-16-10 06:26 PM
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How long before a trend is 'statistically significant'? How questions to Jones were cherry-picking

Edited on Tue Feb-16-10 06:46 PM by muriel_volestrangler

A blog posting written in December 2009, talking about the length of time for global temperatures to become 'statistically significant' (note this is with the GISS data up to November 2008, so dates aren't precise compared with the CRU data that Jones would be asked about). You can see why they asked (a) about the period from 1995 up to the present, (b) 2002 up to the present. They needed to find the date before which warming has been statistically significant, and then add a touch (nothing else special about 1995, after all); and the date since which some kind of cooling trend can be found, even if a warming trend of 0.1, or 0.2, degrees per deacde still fits comfortably in the confidence limits.

Time and time again, denialists try to suggest that the last 10 years, or 9 years, or 8 years, or 7 years, or 6 years, or three and a half days of temperature data establish that the earth is cooling, in contradiction to mainstream climate science. Time and time again, they’re refuted — shown to be either utterly foolish or downright dishonest or both. Logic seems to have no effect on them.

The simple fact is that short time spans don’t give enough data to establish what the trend is, they just exhibit the behavior of the noise. Of course that raises an interesting question: how long a time span do we need to establish a trend in global temperature data? It’s sometimes stated that the required time is 30 years, because that’s the time span used most often to distinguish climate from weather. Although that’s a useful guide, it’s not strictly correct. The time required to establish a trend in data depends on many things, including how big the trend is (the size of the signal) and how big, and what type, the noise is. Let’s look at GISS data for global temperature and test how much data we need to establish the most recent trend.
...
http://tamino.files.wordpress.com.nyud.net:8080/2009/12/gissrat2.jpg

When the lower confidence limit (the lower red line) is above zero, we have some confidence that the trend rate is definitely positive. If the upper confidence limit (the upper red line) were below zero, we’d have some confidence that the trend rate was negative — but that hasn’t happened. The last time the lower confidence limit was above zero was 1996, as we can see more clearly if we expand the x-axis:
...
That does not mean that there’s been no warming trend in those 15 years — or in the last 10, or 9, or 8, or 7, or 6 years, or three and a half days. It only means that the trend cannot be established with statistical signficance. Of course, it’s another common denialist theme that “there’s been no warming.” This too is a fool’s argument; any such claims are only statements about the noise, not about the trend. It’s the trend that matters, and is cause for great concern, and there’s no evidence at all that the trend has reversed, or even slowed.

http://tamino.wordpress.com/2009/12/15/how-long/

And then, once you've placed the questions, you can distort the answer, which is really about confidence intervals, into "no global warming in the last 15 years". Simples.

Since that was written in December, I think this may show that it's easier to predict the next denialist scam than the weather.

(edited to get that graph to show up)

How long before a trend is 'statistically significant'? How questions to Jones were cherry-picking

B - Do you agree that from 1995 to the present there has been no statistically-significant global warming

Q&A: Professor Phil Jones