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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. 16th, 2008 07:32 pm (UTC)
Re: The UN claimed the cooling, not only Monckton
Monckton was bouncing around widely published reports of cooling in certain time frames.

What are your sources for this? Peer-reviewed sources, preferably.

You may be correct that using certain time intervals you can get warming instead of cooling, but that sounds like your cooking the books too, self selecting data to make it appear warmer.

Actually, you've got it backwards - you have to limit yourself to fairly short periods to get cooling trends. Take a look at this graph again:

The overall trend for global temperatures is quite definitely upwards. And for my ten year calculation, I actually used one of the periods with the least possible warming, since 1998 was an abnormally warm year (as the above image shows) and the last winter was especially cold. So no, I didn't cook anything.

More importantly, this trend was not predicted by the IPCC climate models that are utterly failing to reproduce themselves in the empirical realm where they can be tested and falsified. At some point intelligent people will ask "where's the dice" and demand to run a falsification test.

It's far too early to say that the IPCC models are incorrect - after all, these are multi-decade predictions, not annual predictions.

This is a graph for the various models used in the 2001 predictions:

Those vertical lines on the right side are error bars, which means that each of the used models can have a fairly large range of possible end temperatures for the year 2001. And the intervening periods can also be subject to large fluctuations - but ultimately, the long-term trend is what is important.

There have always been abnormally hot summers. And there have always been abnormally cold winters. But in the long run, they tend to cancel each other out. It's the long-term average temperatures that truly describe the state the climate of the world is in.
May. 11th, 2008 09:43 am (UTC)
Re: The UN claimed the cooling, not only Monckton
Why do you choose to claim that 160 years is a "long period" reflecting some kind of "overall" trend in the "long-term"? If you take any natural history book, you will immediately notice that such a period is not long in any serious way.

You surely know that the only reason why these graphs usually start around 1850 is that thermometers were invented and record-keeping started seriously in Europe and America. All peer reviewers should laugh at the selection of this time frame.

Monckton is stretching a little bit, as are other skeptics that claim the Earth is cooling. However, the fact is that significant warming has stopped around the turn of the century, either 1998 or late 2001. But this is a short term observation and may turn out to wrong.

The models predict the rapid warming of 1976-1998 to continue, because warming has obviously been embedded into their code, for example with an overestimated sensitivity to CO2 increase. No surprise there.


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