Phil Jones - Warming Since 1995 is now Statistically Significant
by dana1981, Skeptical Science, June 14, 2011
As all Skeptical Science readers are undoubtedly aware, in February of 2010, Phil Jones was asked some loaded questions in an interview with the BBC. Several of the questions were gathered from "climate sceptics," and Jones' answer to the second one has been widely re-published and distorted:
"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 (0.12 °C 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."
Why choose 1995 as the starting point in this question? Well, that is the closest year for which the answer to this loaded question is "yes." From 1994 to 2009, the warming trend in the HadCRUT dataset was statistically significant at the 95% confidence level (CL). It's also worth noting that there's nothing magical about the 95% CL -- it's simply the most commonly-used interval in scientific research, but it's also true that the HadCRUT 1995-2009 trend was statistically significant at a 93% confidence level.
In other words, using Jones' data, we could say with 93% confidence that the planet had warmed since 1995. Nevertheless, this did not stop numerous mainstream media outlets like Fox News claiming that Phil Jones had said global warming since 1995 was "insignificant" -- a grossly incorrect misrepresentation of his actual statements. The Daily Mail warped the truth even further, claiming Jones had said there was no global warming since 1995. These media outlets turned 93% confidence of warming into "no warming".
Furthermore, the HadCRUT dataset excludes portions of the Arctic where there are no temperature stations. The Arctic also happens to be the fastest-warming part of the planet. NASA's GISTemp,whose data analysis extrapolates for the Arctic temperatures using the nearest temperature stations, did find a statistically significant warming trend at the 95% CL from 1995 to 2009. So not only are the "skeptics" cherrypicking the start date, they're also cherrypicking a dataset which doesn't cover the whole planet.
Deep Climate has detailed the history of the 1995 cherrypicked starting point. It appears to have originated with an email from Richard Lindzen to Anthony Watts, which was subsequently published in a post on WattsUpWithThat (WUWT):
Look at the attached. There has been no warming since 1997 and no statistically significant warming since 1995. Why bother with the arguments about an El Nino anomaly in 1998? (Incidentally, the red fuzz represents the error ‘bars’.)Best wishes,Dick
Luboš Motl made a similar argument in December 2009 using UAH satellite data, which was also published on WUWT. Two months later, the question was posed to Phil Jones in the BBC interview, which suggests strongly that it originated from Motl, Lindzen, and/or Watts. Regardless of the source, what really matters is that the question was based on a cherrypicked starting date, and on a somewhat arbitrary statistical confidence level, and that the media subsequently distorted Jones' response.
In January 2009, Tamino at Open Mind analyzed the data after removing the influence of exogenous factors like El Niño, volcanic eruptions, and solar variation from the temperature data. Tamino concluded that "until 2001 the warming is statistically significant" (Figure 1).
Figure 1. HadCRUT3v estimated warming rates from the plotted date to Present with 2-sigma error bars, using exogenous factor-compensated temperature data (Open Mind).
Another year has passed since the original BBC interview, and in a new BBC article, Jones notes that the HadCRUT warming trend since 1995 is now statistically significant.
"Basically what's changed is one more year [of data]. That period 1995-2009 was just 15 years -- and because of the uncertainty in estimating trends over short periods, an extra year has made that trend significant at the 95% level which is the traditional threshold that statisticians have used for many years."It just shows the difficulty of achieving significance with a short time series, and that's why longer series -- 20 or 30 years -- would be a much better way of estimating trends and getting significance on a consistent basis."
As Jones notes, and as scientists like Lindzen and Motl should very well know, trying to assess trends in the noisy global temperature data over periods as short as 10-15 years is pointless. There's just too much short-term noise, which if you're going to look at such short-term data, you at least need to attempt to filter out first, as Tamino did.
So to sum up, a cherrypicked starting date chosen by a couple of "skeptics" (Lindzen and Motl) and published by a "skeptic" blog (WUWT) was picked up and passed along in the form of a loaded question to Phil Jones in the BBC interview. Phil Jones' answer was subsequently (and predictably) grossly distorted by various media outlets, who turned 93% confidence of global warming into "no global warming."
In reality, the HadCRUT warming trend since 1995 was statistically significant above the 90% CL, the GISTemp warming trend (which does not exclude the Arctic) was significant at the 95% CL, and by removing short-term effects, even HadCRUT has been significant at the 95% CL since 2000. One year later, we can now say that the HadCRUT warming trend since 1995 is statistically significant at the 95% CL, even including the exogenous factors.
Unfortunately, the main consequence of this sequence of events was that much of the public was misinformed by media articles claiming that global warming since 1995 was "insignificant" or non-existent, which are both factually incorrect statements. Misleading the public may well have been the goal of those individuals who originally cherrypicked the 1995 starting date and the HadCRUT dataset, and if so, they succeeded. And not surprisingly, Anthony Watts continues to mislead his readers, claiming Phil Jones' comments are "an about face...From the “make up your mind” department," when in reality Jones' comments have been consistent and accurate throughout.
This reactions to this story have revealed a number of media outlets whose aim is not to accurately inform their readers with regards to the climate, but rather to misinform them.
Posts
No comments:
Post a Comment