January 20, 2010 
Paul M has a list of criticisms and flaws in the IPCC reports that I’d like to explore in greater detail.
Here is the first criticism:
A new form of calculus has been invented by the authors of the the IPCC’s Fourth Assessment Report (AR4), in order to create the false impression that global warming is accelerating…
Here is the graph:
Paul, do I have to point out that you are imputing motive here — you are stating the reason the authors included the graphic is to “create a false impression that global warming is accelerating.” In other words, you are asserting that they knowingly created a graph that misleads about the rate of global warming? I just want to make that clear.
Here’s more:
The slope over the last 25 years is significantly greater than that of the last 50 years, which in turn is greater than the slope over 100 years. This ‘proves’ that global warming is accelerating. This grossly misleading calculation does not just appear in chapter 3 of WG1. It also appears in the Summary for Policymakers (SPM): “The linear warming trend over the last 50 years is nearly twice that for the last 100 years“. Thus, policymakers who just look at the numbers and don’t stop to think about the different timescales, will be misled into thinking that global warming is accelerating. Of course, we could equally well start near the left hand end of the graph and obtain the opposite conclusion! (Just in case this is not obvious, see herefor an example). A similar grossly misleading comparison appears at the very beginning of chapter 3, page 237: “The rate of warming over the last 50 years is almost double that over the last 100 years (0.13°C ± 0.03°C vs. 0.07°C ± 0.02°C per decade).“
Will someone more skilled in statistical analysis point out to me why this is grossly misleading and wrong?
Here is another comment:
It is the same story with the misleading comment in the SPM mentioned above (“The linear warming trend over the last 50 years is nearly twice that for the last 100 years“). This statement was not in the original version reviewed by the scientists. It was inserted into the final draft that was only commented on by Governments. The Chinese Government suggested deleting this, pointing out that ‘These two linear rates should not compare with each other because the time scales are not the same’. Well done to the Chinese Government for spotting that. Too bad their valid comment was ignored by the IPCC.
Interesting questions raised:
1. Is it wrong to say that the rate of increase in warming is greater in the last 50 years than over the entire period and why?
2. Why was the new graph included in the final draft?
3. Who was involved in the government review? I imagine governments had their own scientists reviewing it, not just politicos.
Paul — I have to ask — if the report included four different graphs, with each one focusing in on a specific time period, would you still complain about it? If there were four graphs showing 1860 – 2000, 1905 – 2000, 1960 – 2000, and 1980 – 2000?
What do you think the graph shows and what is the proper way of analyzing the graph?
The Policy Lass 
I was going to allow PaulM to be the first to comment, but he has not yet done so.
Yes, this graph knowingly misleads about the rate of warming. It ignores the role of natural cycles in climate.
Papers by Chylek and others have pointed out the rate of warming from 1910-1940 is greater than the rate of warming from 1975-2005. Since the increase in atmospheric CO2 was not great from 1910-1940, most people are willing to admit this warming was largely natural.
PaulM points that objections to this chart were raised by the Chinese:
“The Chinese Government suggested deleting this, pointing out that ‘These two linear rates should not compare with each other because the time scales are not the same’. Well done to the Chinese Government for spotting that. Too bad their valid comment was ignored by the IPCC.”
The Chinese are correct. To properly analyze the chart, you look for other time periods of strong warming of a similar time scale to see how they compare. The two strongest 30 year periods of warming are very similar in strength but the 1910-1940 is slightly stronger.
Susann,
Firstly, thank you for the publicity and for the interesting questions!
Secondly, that web page was written I think in summer 2008 and as far as I know this is the first time it has had any serious scrutiny. There may be some places where things are not sufficiently clearly be explained. There might even be some errors!
Thirdly, we should bear in mind that one of the two lead authors of the section was Phil “hide the decline” Jones, currently suspended.
No you don’t have to point out that I am imputing motive. Can you think of another reason for choosing those particular time intervals? The false statement in the caption is “Note that for shorter recent periods, the slope is greater, indicating accelerated warming”.
Why is it grossly misleading and wrong? Because of the different timescales. In a wiggly signal with ups and downs like climate, if you take a very long time frame, the slope will be almost zero. Longer timescales tend to give lower slopes. See the sine curve at the beginning of the post, or the ‘see here’ link that gives the opposite result. So it is misleading to compare a short timescale trend with a long timescale one, as pointed out by the chinese government. The slope is greater because the period is shorter, not because of any accelerated warming.
1. It’s not wrong, it’s misleading cherry-picking.
2. Why was the graph changed? Very good question. None of the reviewers asked for a sequence of lines of different time periods to be drawn over it.
3. I don’t know, but the comments from the ‘governments’ were often very good, so yes, I imagine they had some science experts not just politicians.
If there were 4 different graphs yes I would still complain for the same reason.
What do I think the graph shows?
The graph shows warming in the earlier 20th century, then a slight cooling, then warming in the late 20th century. The data should be presented just like that, without any misleading lines drawn over it.
Incidentally the accompanying text contains the false statement that the early 20th century warming was 0.35 degrees when in fact it was 0.5 degrees.