Friday, November 27, 2009

The Copenhagen Diagnosis is Disappointing

Note: I am very pleased to have received comments on this post, and present my responses to those comments at the end of this blog post.

In the lead up to the Copenhagen talks a group of climate scientists has put out a publication called The Copenhagen Diagnosis. For me it is a disappointing document because the scientists aren't doing what they do best, which is present science. Instead they take a chapter out of the bad skeptic book, and present evidence in a one sided and misleading way. My feeling is that they are learning the wrong lesson from everything that has happened.

The fundamental problem is that by doing things this way they are completely opening themselves up to criticism that will, to the average person, seem like the science is wrong, when it is their presentation of the science that is wrong. This causes the general public to question the whole foundation of the science on the climate issue. For people like me it causes me to read every publication, and statement from this group with complete skepticism when I would prefer to not have to dig into everything to see whether it makes sense.

I'm not going to go through the entire document. Instead I will focus on two sections where I have spent some time understanding the issues; Sea Level, and Sea Ice.

Sea Level

On page 39 of the report they make two summary statements about sea level. Which I will summarize. First sea level is rising faster than than the best estimate of the IPCC Third Assessment Report (TAR), and second that sea level rise is likely to be twice as large as the IPCC AR4 (AR4) based on a new ice sheet understanding.

Why did they choose to compare sea level rise with the Third Assessment Report? Why not the first or second, or better yet the one that was just completed AR4? Is it because most people who are reading quickly would assume that they were comparing the last few years to the most recent assessment report instead of an old one? After all they go on to say that the most recent report is seriously underestimating future sea level rise.

Even if we accept that somehow the TAR is the right baseline instead of AR4 there are problems with the graph they present. On the graph they show a gray range which they label as "IPCC Projections." They don't show anything that is a "best estimate." Looking at figure 24 from the Third Assessment Report it is a little
difficult to tell what they have used to create this range, but artistically that report had a gray range. If you use a magnifying glass and do some interpolation it might have the same high and low values as the range in their graph. But in the IPCC report that range is labeled as "the range of the average of AOGCMs for all thirty five SRES scenarios." On the Third Assessment graph there is a much wider range that is labeled "the range of all AOGCMs and scenarios including uncertainty in land-ice changes, permafrost changes and sediment deposition.

Neither of these are labeled as a "best estimate." And in fact even on their own graph the observations are at the top end of the range, not outside the range.

To make matters worse if you look at the graph you can see that observations are well above the center of the gray region by 1995. But the Third Assessment Report was published in 2001. Did the authors of the TAR really intend that sea level already be at the top of their range in a period years ahead of when the report was published? Or is this more likely just mixing apples and oranges with a comparison of General Circulation Models (GCMs) and observations. I don't think the GCMs were ever intended to be compared with observations in this manner. And the fact that the graph in the TAR doesn't include observations would back up this conclusion.

Looking to the future, what is the basis for their statement that sea level is likely to rise twice is fast as predicted in AR4? AR4 was published in 2007, so something dramatic must have happened for the conclusions of an entire chapter to be overturned. We would expect to find some serious and widely accepted peer reviewed studies that show that the AR4 consensus chapter was seriously flawed. Instead they base their conclusion on three things. First that the AR4 didn't include dynamic processes. Second that a single paper by Stefan Rahmstorf predicts higher values. And third the results of the "Delta Committee." These are really poor arguments.

On the issue of dynamic processes, it is true that the AR4 left dynamic processes out of their core range. But they go on to discuss what effect they would have. In figure 10.7 it includes an additional .1 to .2M for "Scaled-up ice sheet discharge." So in the judgment of the chapter authors, if in fact there is scaled up discharge from Greenland and Antarctica they would add .1 to .2M to their estimates. This is a long way from doubling their estimates. The Copenhagen authors present no evidence of any kind peer reviewed or other that this is incorrect.

Their second leg is the Rahmstorf 2007 paper that uses a simple linear model to forecast future sea level rises based on temperature. There are a lot of problems with that model as I have discussed in other blog posts, and as were discussed in published comments by two different groups following the publication of the paper. Even if you think that paper is interesting, it hardly can be used to overturn the consensus work of the AR4 authors who are actually experts in the field of sea level rise. (By the way they also list WBGU 2006 as a reference, but as this refers to published work, and previous IPCC reports it can hardly be used to claim that new research has overturned the AR4 prediction.)

Finally they reference the "Delta Committee." This was a government committee in the Netherlands which gave recommendations to the Dutch government on how much sea level rise to plan for. This link fills in some details about the committee, which was not a scientific assessment, and once again certainly isn't a reasonable case for saying that the estimates of the AR4 were wrong.

In summary their presentation of the past is misleading, and their statement about likely future sea level rise is not grounded in the peer reviewed literature. If they wanted to say it was their opinion that would be fine. But the way it is written a casual reader wouldn't get that message.

Sea Ice

In their summary on Sea Ice on page 31 they state that the melt in the Arctic has been larger than forecast by the AR4. This statement is true, but there are three problems. First there have been very few data points. One could just as easily say that global temperature has been well below the forecasts of the AR4. It has been pointed out repeatedly that this would not be proof of anything. Second they fail to mention in the summary that Antarctic sea ice has been above those same forecasts, although they cover the topic later in the chapter. Finally they fail to mention a recent publication by Shindell that explains that warming in the Arctic has been enhanced by black soot, which likely has been a factor in the amount of Arctic sea ice melt.

To complete the imbalance they show a graph of Arctic sea ice, but no graph of Antarctic sea ice. The Arctic sea ice graph is again a little strange. They label the "prediction" in the graph as "mean and range of IPCC models." We can see that the observations have been diverging downward, and have left the range in recent years. (Note that chose not to plot the one year recovery in 2009 even though they refer to it in the figure description.) But looking closely the divergence appears to begin in 1975. When were these models created? Once again do I believe that a model created circa 2006 was starting to be so wrong in 1975? In AR4 they don't include observations in figure 10.13 leading me to believe that you can't simply compare the observations with those models without great care in centering etc.

On the subject of the Antarctic they do devote a page to discussing the increase in sea ice. But pretty much the page is designed to explain it away because of circulation, ozone, and the fact that the ice is shrinking in some areas. I'm sure those are all good reasons, just like the black soot is a factor accelerating Arctic ice melt, which they don't mention. But to be balanced they should have presented a graph comparing Antarctic sea ice with the IPCC AR4 projections from figure 10.13. This would show a very large divergence, but on the high side.


The AR4 is a well written balanced document. I might have some objections here and there, but I learn a lot from reading it, and it gives me a good picture of where the consensus lies on climate change. When scientists stray from this type of generally balanced document, as they have with the Copenhagen Diagnosis, it makes me have to question everything that they are writing and saying, which defeats the purpose.

Response to Comments


I will start with the choice of report. Your premise is that they chose the TAR because there was a projection that covers the current date, while AR4 doesn't have such a projection. That is a good point, but I don't think it is quite as meaningful as you state.

The problem is that while the graph in the TAR is labeled "Sea Level Rise" it is actually a graph of absolute sea level. The graph in the CD is also absolute sea level not rate of rise. To determine the rate of sea level rise you would need to determine the slope of each of the lines in the graph. This is where you could possibly find the 1.9mm/year figure that they use although it is not sourced. If they had wanted to source a figure of rate of rise, which of course wouldn't start at zero, and then compare rate of rise during the period that would have been clear. My comments about being above the range long before 2001 have to do with absolute sea level as pictured in their graph, and since I don't think anyone was suspecting sea level would drop significantly during that period it seems unlikely that it was meant to be compared to measurements.

(After I wrote this I reflected that for me at least that graph would have been more interesting. But I think for most people two sets of nearly horizontal sea level figures with the TAR figure moving up towards the measured rate of rise wouldn't have made their rhetorical point. Especially since the rate has been decreasing in recent years, see below.)

Now let's take a look at the actual TAR graph. Between 1990 and 2010 the center of the upper and lower range, and the gray range for that matter, goes from zero to approximately .03m or 3cm. This produces an average slope over that period of 1.5mm/year, although it strains my eyes to do it. The period that they quote is 1993 to 2008 so since the curve slopes upward this might produce the 1.9mm/year figure.

They emphasize that the measured rate rate of 3.4 is 80% higher than the 1.9 figure. I have to admit that I got caught up with their graph and the ranges presented in it so I missed that point. However this recent rate of sea level rise was well understood by the authors of the chapter in AR4, and was completely taken into account in their revised sea level estimates. And even so the AR4 range is substantially similar to the TAR, as the helpfully point out, but for a different reason. So while the points they make about sea level are each technically correct, and I never said they weren't, they tend to mislead the reader into believing that the recent measured level of sea level rise would indicate that the consensus range in AR4 are likely to be understated, and it means no such thing.

You go on to say that they could have made it more dramatic by using the AR4 models, but this seems to contradict your statement that they don't cover the relevant period, so I'm not sure what you mean by that.

Your comments about the best estimate being the center are certainly consistent with other cases. But as I have pointed out it doesn't really matter which center you use, at least in this short time frame. Computing the slope using the top bar, which eventually leads to a .8M sea level rise, the recent rise in sea level is within the range, which because of the style of presentation is correctly pictured in their graph.

Catherine please add any other thoughts you would like, you caused me to look at the data a little differently, although as the scientists always like to say, my main conclusions are not affected. :-)


Your post starts off well enough but then devolves into the typical sort of insults that seem to hurled around this issue constantly. Nevertheless I will answer your comment.

There have been several papers recently discussing mass loss from both Antarctica and Greenland. Particularly of note is that the GRACE satellite is showing that Antarctica may be contributing to sea level rise while the AR4 models centered around the idea that it would absorbing mass and therefore slowing sea level rise.

Nevertheless, as I pointed out in my original post, these type of ice sheet dynamics were discussed in AR4. They have provided the range that they would add for this type of thing as an additional .1 to .2 meters per year. I know of no published research changing the core conclusions on sea level rise as a result of these measurements, which in fact at current levels would be trivial. Trust me if those reports existed the authors of the CD would have referenced them.

In addition based on recent research (1) sea level rise is currently decelerating over the short term. So even if you believe that there is increased contribution from the ice sheets it is being offset from other factors. Remember that 3.4 mm/year would only yield .34m of sea level rise over the century, so there has to be an acceleration just to reach the midpoint of the estimates.

(1) A new assessment of the error budget of global mean sea level rate estimated by satellite altimetry over 1993–2008
M. Ablain1, A. Cazenave2, G. Valladeau1, and S. Guinehut1

Wednesday, November 25, 2009

Follow up on Rahmstorf 2007

I had thought in my last post that it was quite evident that the model in Rahmstorf 2007 was not well specified. In fact I was more interested in the issues dealing with the response to comments than I was in making the point clearly I suppose.

In that post I showed that the first half of the data did not do a good job of predicting the second half of the data. In fact the coefficient for the second period has half the value of the first period which would produce wildly different results for future predictions. But this by itself doesn't show the obvious which is that the linear model just doesn't work even without looking at out of sample prediction.

Here is a web page that presents the methodology for whether a linear regression is well specified.

Remember that I am doing this against the final calculations with corrections from the corrigedums by Rahmstorf.

First I plotted predicted values against the actual values, and in fact they are not symmetrically distributed around either the diagonal or horizontal line. You can try it yourself from the code I already posted. But since there is no fixed rule for what symmetric means, my experience is that this will not be sufficient to make my point.
So then I computed the Durbin Watson statistic for autocorrelation in the results.

The result is .4 which according the Wikipedia page puts it in the range where it "might be cause for alarm."

The point is that the residuals are not well scattered, and they are highly autocorrelated. This should be enough for anyone to see that even a first year statistics student would know that the model isn't well specified.

In response to a comment here is the plot of the actual versus predicted values.

Here is a plot of the residuals. It doesn't take a DW statistic to see how highly autocorrelated they are.