Remember Phil Jones’ famous BBC interview?
B – Do you agree that from 1995 to the present there has been no statistically-significant global warming
Yes, but only just. I also calculated the trend for the period 1995 to 2009. This trend (0.12C per decade) is positive, but not significant at the 95% significance level. The positive trend is quite close to the significance level. Achieving statistical significance in scientific terms is much more likely for longer periods, and much less likely for shorter periods.
It caused a bit of a kerfuffle at the time here in Australia and got discussed extensively here at the Cat (here, here and here) and also over at John Quiggin’s place (here and here). John Quiggin and I had an exchange of views here. As I said at the time
I’ll re-run the regression with the addition 12 months and post the result when the data become available.
John Quiggin updated his analysis in January using annual data.
With 2010 over, we now have 16 observations starting in 1995, and (unsurprisingly to anyone who followed the argument thus far) the upward trend is now statistically significant at the 5 per cent level That is, if climate change since 1995 (the time of the first IPCC report, and well after Lindzen announced himself as a sceptic) had been purely random, the odds against such an upward trend would be better than 20 to 1 against.
The obvious question is whether Lindzen will now concede that his claim, and the inference he drew from it, has been falsified, and that warming has not in fact ceased as he suggested. I’m willing to bet that what we see is evasion, obfuscation of or outright silence, not only from Lindzen, but from all of those who parroted his claim.
fn1. My estimates based on the Hadley data used by Lindzen gives an estimated trend of 0.012 degrees a year, with a t-value of 2.45.
At the time, the whole of the 2010 monthly data wasn’t available so I didn’t repeat my analysis – from memory the 2010 data point he used was for ten months of the year. That doesn’t matter – I can replicate his results almost perfectly. The CRU have released all of the 2010 monthly and the January 2011 temperature anomaly. To remind ourselves this is what I found last year.
The results are more or less the same with the p-value being slightly larger than before. Looking at that I don’t think that Lindzen needs to ‘concede that his claim, and the inference he drew from it, has been falsified’.
When I add January 2011 the p-value on the Time coefficient explodes out to 0.1029 i.e. not statistically significantly different from zero even at the 10 percent level. Last year commenter PSC suggested that an ARMA(1,1) model was appropriate and when I do that, the p-value on the time trend is a massive 0.2402.
So what can we say out of all of this? Doing some simple econometrics on annual data shows an upward time trend slope while doing some simple econometrics on the corresponding monthly data (and correcting for potential problems of heteroskedasticity and unit roots) gives a more nuanced position. This also highlights the point that appeals to empirical evidence don’t always resolve disputes. In any event, we’ll all be back in a year’s time re-running our regressions when another 12 data points are in.
Update: John Quiggin has a take here.
Update II: John Quiggin seems to be suggesting that I have not differentiated between statistical significance and substantive significance – that discussion is here.