Sunday, February 21, 2010

Media Misquotes Climatologist Phil Jones

"But Mumble was different."--Happy Feet

If you weigh yourself every hour for a twenty-four hour period, it may be difficult to know if you are gaining or losing weight. Various factors such as how much water you have been drinking, when you ate your last meal, or when you last visited the bathroom will obscure the trend in your weight. But if you weigh yourself every day for a year, any upward or downward trend in your weight will be evident.

So it is with global warming: factors other than greenhouse gasses can affect the earth's temperature and make it difficult to measure the man-made global warming trend in the short term. Scientists call these other factors "noise." For example, the particulate matter from volcanic eruptions can keep some sunlight out.

A blog called Skeptical Science has an excellent post titled "Did Phil Jones really say global warming ended in 1995?":

A headline in the Daily Mail has spread like wildfire, claiming that Phil Jones, ex-director of the University of East Anglia’s Climatic Research Unit, said "there has been no global warming since 1995". Not only did Phil Jones not say these words, this interpretation shows a poor understanding of the scientific concepts behind his words. To fully understand what Phil Jones was saying, one needs to read his actual words and understand the science discussed. Here is the relevant excerpt from the BBC interview:

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

Phil Jones: 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.

[A bit later the interviewer asks this question.]

BBC: How confident are you that warming has taken place and that humans are mainly responsible?

Phil Jones: I'm 100% confident that the climate has warmed. As to the second question, I would go along with IPCC Chapter 9 - there's evidence that most of the warming since the 1950s is due to human activity.

[Climate Skeptic explains] 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. [See the full explanation.]

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