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Climate Hustle

Did Phil Jones really say global warming ended in 1995?

Posted on 16 February 2010 by John Cook

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.

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.

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. 

HadCRUT global temperature 1995 to 2009
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:

NASA GISS global temperature 1995 to 2009
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).

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Comments 101 to 104 out of 104:

  1. TruthSeeker @ 100

    Nobody is using the trend from 95-2009 as evidence for global warming, laregly because the timescale involved is too small to average out the effects of internal climate variability due to things like ENSO. See the paper by Easterling and Wehner mention in posts 24,37 and 38. It is seen in model output as well.

    ISTR tamino had a good example on his blog where he made a time series composed of noise plust a linear trend. It was possible there to find periods of cooling, even though the signal *by construction* had a constant rising linear trend.

    Also the enhanced greenhouse effect *does* suggest that Earth's total heat content will rise, as it puts the planet out of radiative equilibrium, which won't stop until the radiating layer warms up to the point that a new balance is reached. That does not mean though that in the short term exchanges of heat between oceans and the atmosphere can't result in warming oceans and cooling atmosphere; there is still an increase in heat overall content.
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  2. TrueSeeker
    "First, the significance of Jones statement is that you cannot, scientificly, make the conclusion that there has been heating from 95-2009."

    This claim is utterly wrong. You cannot determine a stistical significant trend if you limit the period to 1995-2009. But still, you can scientifically make conclusions on this period if you expand the time range.
    You could also use the period before that, extrapolate the trend and the confidence interval and see if the data from the following period are still inside the limits.
    We can draw the scientifically sound conclusion that from the '70s up to now there has been a statistically significant trend and also that it is reasonably well aproximated by a straight line.
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  3. TruthSeeker, statistics does not equal science. Statistics is one tool in the scientific toolkit. For example, there are many more aspects of decision theory that are used, than statistics per se. Furthermore, even statistics is not monolithic. You've got your descriptive statistics and your inferential statistics, Bayesian and non-Bayesian,....

    Even within inferential statistical tests such as the one Jones was asked about, there are multiple possible tests addressing the same phenomenon. For such a test to bear on a decision about the existence of a trend, that test must be sufficiently powerful to detect a trend of that size against the background of the particular level of noise in that sample of that particular size. The trend in global temperature long has been known to be so small compared to the noise, that a sample of 15 years is unlikely to reach statistical significance at 95% confidence level. That's why climatologists don't often bother looking at samples of just 5 years or 10 years, and only sometimes at samples of 15 years.
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  4. @JonMoseley
    "Thus, scientifically, a trend that is not statistically significant DOES NOT EXIST, and must be treated as ZERO for the purposes of valid science."

    This is quite wrong. First, where did you get your 95% confidence level that the value is zero? Second, you can always find a sample size small enough that there's no statistically significant trend. If those counted as zero, we could discount any trend. The fact is, there's a statistically significant trend going back further than 1995, and the data since 1995 is consistent with it, and everything else we know indicates that it will continue.

    "the observed results are in conflict with the predictions of the computer models"

    What makes you think so? It certainly isn't true.

    "Since we have no experimental (i.e., scientific) data whatsoever to support the idea of man-made global warming"

    This is quite wrong. First, evidence doesn't cease to exist simply because it doesn't reach a 95% confidence level -- you're using a very odd and erroneous equivalence chain: "statistically significant" = "scientific" = "experimental". Second, not only are you ignoring the evidence mentioned in this very article of statistically significant warming of the oceans, you are ignoring large amounts of other data, such as NASA's examination of 29,000 indicators such as insect migrations, 90% of which support global warming, and the greenhouse physics, which counts as evidence in a proper epistemology.

    @TruthSeeker
    "First, the significance of Jones statement is that you cannot, scientificly, make the conclusion that there has been heating from 95-2009. That is scientific fact. This is in direct conflict with the fact that CO2 rates have continued to rise in that same time frame."

    This is a conceptual confusion that is similar to Jon's. First, you can reach that conclusion, just not by restricting yourself to the 95-2009 temperature data. Second, the inability to reach a conclusion does not in any way conflict with a prediction from a model -- for there to be a conflict, you would have to be able to conclude that the prediction is false.
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