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But it’s not getting warmer

By: Dr. Ricky Rood, 10:36 PM GMT on October 12, 2009

But it’s not getting warmer

Recently I received some questions from Westview High School in San Diego, California. The questions were motivated by a video they had downloaded from the web that included discussion of the figure below. The attribution of the plot is to Science and Public Policy. The figure is from 2005 through 2008. In this figure are traces of plots from the IPCC Fourth Assessment Report and some recent observations.

So in thinking about this plot and the student’s question, the first thing that strikes me is that we don’t know the source of the observations. In the material associated with the questions it was stated that these were satellite only observations and that satellite observations were the only objective measure of global temperature (It is, by the way, categorically untrue that satellite measurements are the only objective measure of temperature.) Even if it is true that these are only satellite measurements, I can name without even looking it up, a dozen or more satellites with different types of instruments, with different measurement characteristics. So “satellite measurements” means little. OK, there are some scholarship issues in presenting this picture. (But if you google or bing images with the right search words you can find this figure many times.)

Getting past this basic concern, dismissing that “this is the web,” there are some other basic problems with the use of this figure to “debunk global warming.” If I made such a figure and presented it for publication, several flaws would be immediately recognized by the reviewers.

First, the IPCC projections have been treated as if they were a deterministic prediction. Deterministic? Traditionally weather forecasts have been deterministic; that is, at a geographical place at a particular time the temperature will be XX degrees and it will/will not be raining. Deterministic forecasts are often placed in comparison with probabilistic forecasts, that is, the development of a range of probability of what will be observed at a particular place and time. Forecasts that are used for deterministic prediction are started (initialized) with an observed state of the atmosphere and ocean. Then, the forecast model represents how this observed state moves forward in time. IPCC projections are simply not deterministic forecasts. IPCC simulations have the physics to represent processes like El Nino and the Pacific Decadal Oscillation, but they make no attempt to represent any particular El Nino. They represent these multi-year phenomena with varying degrees of quality. Then in a plot like the IPCC plot, several models are added together. It is simply ill posed to use the IPCC projections in this sort of analysis. In a paper based on the scientific method, it would not make it out of the starting gate.

The issues with this figure don’t end here. There are only 4 years of observations on this figure and this is extrapolated to a cooling of six degrees per century. What is the foundation of this extrapolation? This is a particularly egregious step in the analysis – what is the justification of making such an insinuation?

A minimal step in the analysis would be to look at the previous 150 years of the observations, which have been omitted from the plot, and asking the question of whether or not the differences displayed in this figure lie outside of the range of the variability that has already been observed. Even if the report insisted on only using “satellite” data, there are 30 or so years of observed variability. Of course, then the title of the figure should be, “how does a certain satellite instrument differ from the projection?”

The question is raised, implicitly and explicitly, that there is some mode of natural variability that accounts for the difference visible in the figure. Consequently, there is the implication that the recently observed series of very warm years are also the result of this natural variability. Recall – the projections are not designed in any way to represent this variability in an event-by-event way. So an analysis that aims to make this point is dubious from the start. But, assuming the analysis was to pursued, then once again, the longer data record needs to be considered to establish the natural variability.

So my original goal was to analyze this figure for some students who posed questions to me. Standing alone, this figure has no basis in the scientific method. It is a convenient way to make a point in the spirit of “selective doubt.” That is, by selecting narrowly focused information it is possible to propagate doubt. In the next blog I will report on a recent paper that takes a more comprehensive look at the issues raised in this figure.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.