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Global Warming - How to Lie with Statistics

I usually try to avoid getting involved in debates about global warming these days - it tends to suck up huge amounts of my research time when I try to get things right while the other "debaters" just slap together a few random links they got from Google within half a minute. However, sometimes I just cannot resist, and the following examination is the latest result.

It started out with this debate. I was given a link to this article by one Christopher Monckton as evidence that global temperatures in the last decade had not in fact increased, but decreased. So I took a look at the provided graph and was fascinated - by the sheer amount of blatant manipulation I encountered:

Fortunately, the author provided a source for his data - the HadCRUT3 data set. So I sat out to recreate the graph, and managed to do so.:

First of all, while the used data did show a cooling trend, my linear fit (done with GNUPlot) produced a cooling of

-0.00156427 °C/month.

Extrapolated for an entire decade, like the author has done, this would translate into:

-0.00156427*12*10 °C/decade = -0.1877124 °C/decade

This is not even half as much as the 0.4 °C/decade the author claimed. But wait, it gets better!

It seems that the author of the article, has deliberately started using the 2001-2008 HadCRUT3 data set with one of the hottest months in this period - which happens to be January 2002 (so he didn't use any 2001 data after all, despite the caption of the graph) - and then ended using the data with the very coldest month in this period, which was the abnormally cold January 2008:

With such a self-selected data set to confirm his bias, is it any wonder that he got a significant cooling trend?

And what's with using only six years and one month to extrapolate a "cooling per decade" value - especially when the same data set goes back for far more than a decade and a real value could be easily calculated?

No wonder that this guy apparently doesn't have any peer-reviewed papers to his name - with such blatant attempts at cooking the data, the reviewers would laugh him out of town.

So what would temperature trends over the last decade actually look like?

Well, I've used the same data set for the period from April 1998 to March 2008 (the last entry), and the following graph is the result:

The linear fit produced a warming of 0.0349044 °C for the entire decade - not much, but as this long-term graph generated from the same data set shows, 1998 was an abnormally warm year while the last winter was particularly harsh:

To sum it up, global temperatures have indeed increased during the last decade, if not as strongly as in the time before that. We will have to continue to watch the long-term trends of global temperatures - and be wary of anyone who attempts to cook the data for his own agenda.


Apr. 19th, 2008 01:24 am (UTC)
The source for Monckton's

I am sure you have seen this graph already. It does look like the source for the -0.4C/decade claim by Monckton


It's figure 2 in http://wattsupwiththat.wordpress.com/2008/03/13/to-tell-the-truth-will-the-real-global-average-temperature-trend-please-rise-part-2

If you read that blog, it explains why that particular trend is chosen to start at that particular month/year, using that particular data set. So it appears that rather than concentrating on Monckton, you should raise your questions to Basil Copeland (and Anthony Watts)
Aug. 2nd, 2008 08:46 pm (UTC)
Re: The source for Monckton's
Copeland/Watts post is as questionable as virtually everything else on Watt's blog.

The reason it's questionable in this case is that they're fretting over weather, and drawing tiny trend lines based on that weather. Their justification for selecting 2002 as a break point is still weather, and their game with "cooling" is predicated upon not knowing what happens next (i.e. in the future).

J Hubert remains correct in pointing out the cherry picking of Mr Monckton's start-date. When someone who knows what they're doing (Tamino) tackles the issue on the appropriate climatological timescale no break in trend is found that has no precedence since 1975.

Tamino's blog:
"Global Temperature from GISS, NCDC, HadCRU."
And the item germaine to this issue, graoph :
That's a graph of the residuals, the difference between trend and observation.

The trend used is a climatological timescale (30yrs), not weather. And as the graph of residuals shows nothing untoward for recent years we can conclude that talk of a significant cooling departure from that trend is unfounded.



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