1995gate and statistical significance

Did global warming stop in 1995? Of course not, but that is not what one gets from reading articles in the more irresponsible media outlets who report that:

Climategate U-turn as scientist at centre of row admits: There has been no global warming since 1995

[Phil Jone] said that for the past 15 years there has been no ‘statistically significant’ warming.

The admissions will be seized on by sceptics as fresh evidence that there are serious flaws at the heart of the science of climate change and the orthodoxy that recent rises in temperature are largely man-made.

The bold text is the headline. Note how it differs from the article text by omitting ‘statistically significant’. The headline is completely wrong there has been no U-turn, this is exactly what one would expect, while the article is only misleading. Most denier claims also omit ‘statistically significant’, and are equally as wrong.

This ‘shocking admission’ comes from an interview of Jones done by the BCC where they ask some rather odd questions, including the following:

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.

The question (and subsequent reporting) shows a lack of understanding of statistics. One wouldn’t expect a statistically significant trend to emerge from a noisy data set in a short time frame. The fact that the trend is close to the significance level indicates that indeed we are warming, and quickly.

John Cook over at Skeptical Sciencemakes this point abundantly clear:

Phil Jones is saying there is a warming trend but it’s not statistically significant. He’s not talking about whether warming is actually happening. He’s discussing our ability to detect that warming trend in a noisy signal over a short period. To demonstrate this, look at the HadCRUT temperature record from 1995 to 2009. The linear trend is that of warming. However, the temperature record is very noisy with lots of short term variability. The noisy signal means that over a short period, the uncertainty of the warming trend is almost as large as the actual trend. Hence it’s considered statistically insignificant. Over longer time periods, the uncertainty is less and the trend is more statistically significant.

Figure 1: HadCRUT global temperature change in degrees Celsius. Blue is yearly average. Red is linear trend (HadCRUT).

It bears remembering that the HadCRUT record only covers around 80% of the globe. Analysis by European Centre for Medium-Range Weather Forecasts (ECMWF) and NASA GISS (Hansen 2006) find that the areas omitted by HadCRUT are some of the fastest warming regions in the world. Consequently, the HadCRUT record underestimates the warming trend, as demonstrated by the NASA GISS record which covers the whole globe:

Figure 2: NASA GISS Global temperature change in degrees Celsius. Blue is yearly average. Red is linear trend (NASA GISS).

However, even this doesn’t give you the full picture. Surface temperature is only a small fraction of our climate with most of global warming going into the oceans. When all the heat accumulating in the oceans, warming the land and atmosphere and melting ice is tallied up, we see that global warming is still happening.

Figure 3: Change in total Earth heat content from 1950 (Murphy 2009).

Even the Economist calls the Daily Mail headline a lie:

that Daily Mail headline is a lie. Phil Jones did not say there had been no global warming since 1995; he said the opposite. He said the world had been warming at 0.12°C per decade since 1995. However, over that time frame, he could not quite rule out at the traditional 95% confidence level that the warming since 1995 had not been a random fluke…

What’s truly infuriating about this episode of journalistic malpractice is that, once again, it illustrates the reasons why the East Anglia scientists adopted an adversarial attitude towards information management with regard to outsiders and the media. They were afraid that any data they allowed to be characterised by non-climate scientists would be vulnerable to propagandistic distortion. And they were right.

In fact the idea of using short term trends to determine anything about long term trends in a noisy dataset has been debunked many times before both here and elsewhere, and it betrays the profound ignorance (or just plain dishonesty) of anyone who utters such claims.

Phill Jones’ answer also serves as evidence against another denier talking point: that climate scientists are behind a conspiracy to fool the world into thinking that the climate is warming. How?  Well for starters it betrays a profound lack of political astuteness from Jones (who since climate gate as been portrayed as the ring leader of the cabal, or at least very close to the top). Jones’ answer was accurate, and easy enough for someone with a basic knowledge of statistics to interpret correctly, but not for the general public, and apparently some science reporters, who don’t know what statistical significance means. Given his lack of political astuteness (something that is not uncommon for scientists), how on earth can he be behind one of the most massive conspiracies ever perpetrated?

The bottom line, is that since 1995 the planet has warmed, but 15 years is too short a time period to say anything definitively about the trend, one needs a longer data set.

If only deniers were as skeptical as they claim to be with sources that supposedly support their position.

If you look at the [deniers], I would have to say honestly, what standard are they being held to? It’s very asymmetric. They get to say anything they want. In the end, the core of science is deeply self checking.

If only deniers were skeptics.

UPDATE: Peter Sinclair has done touched on this issue on his latest Climate Crock of the Week: