I WOKE up the other morning to find that I would have to confront yet another headache-inducing attempt to phase-shift my perception of reality, and that this would require wading into historical accounts of the collection and homogenisation of temperature data. On December 8th, a climate-change sceptic named Willis Eschenbach posted what he called the "smoking gun" of climate change data manipulation: a series of graphs of the uandjusted historical record of the temperature-monitoring site at the airport in Darwin, Australia, plotted against the same data as adjusted for various error factors ("homogenised") by the Global Historical Climate Network, or GHCN. Mr Eschenbach claimed the adjustment was so arbitrary, it had to be evidence of intentional manipulation. Here's his graph, and at first glance it seems pretty convincing, no?

I encountered Mr Eschenbach's post via a dismissive response from Kevin Drum, who pointed out that given that GHCN adjustments could produce sharp changes for one region or one site (though little or no change globally), it would always be possible for someone to cherry-pick a site where the adjustment looked suspiciously large. Mr Drum noted that Megan McArdle had also posted on the "smoking gun" argument, so I went to check out what she had to say. Ms McArdle said the post didn't really shake her trust in the scientific consensus on anthropogenic global warming, but that it could well be evidence of subtle confirmation bias on the part of climate scientists, and she asked for more explanatory details.

I've now spent many hours over the course of two days looking into this. The first guideposts, fortunately, arrived quickly, because climate-change scientists have gotten faster at responding to these things. The day after Mr Eschenbach's post went up, you could find its flaws picked apart by Tim Lambert here and on "Things Break" here. If you need one quick explanation, here it is:

A change in the type of thermometer shelter used at many Australian observation sites in the early 20th century resulted in a sudden drop in recorded temperatures which is entirely spurious. It is for this reason that these early data are currently not used for monitoring climate change. Other common changes at Australian sites over time include location moves, construction of buildings or growth of vegetation around the observation site and, more recently, the introduction of Automatic Weather Stations. The impacts of these changes on the data are often comparable in size to real climate variations, so they need to be removed before long-term trends are investigated.

This isn't from the GHCN, or from the East Anglia Climate Research Unit. It's from the Australian Bureau of Meteorology (BOM). They conduct their own "homogenisation" of Australian temperature data sites, independent of the GHCN. And here's the temperature plot that the BOM came up with for the Darwin site:

The plot gets thinner.

In fact, there's more information about the Darwin site easily available on the web. According to this conversation from 2000, the explanation for the dramatic change in 1941 is simple.

As previously advised, the main temperature station moved to the radar station at the newly built Darwin airport in January 1941. The temperature station had previously been at the Darwin Post Office in the middle of the CBD, on the cliff above the port. Thus, there is a likely factor of removal of a slight urban heat island effect from 1941 onwards. However, the main factor appears to be a change in screening. The new station located at Darwin airport from January 1941 used a standard Stevenson screen. However, the previous station at Darwin PO did not have a Stevenson screen. Instead, the instrument was mounted on a horizontal enclosure without a back or sides. The postmaster had to move it during the day so that the direct tropical sun didn't strike it! Obviously, if he forgot or was too busy, the temperature readings were a hell of a lot hotter than it really was!

Interestingly, Mr Eschenbach includes a link to this conversation from 2000 in his own post. But the moved station and the equipment change aren't the whole story. As Mr Eschenbach notes, the temperature adjustment continues to change after 1941. Why might that be?

Here's why: homogenising historical temperature data records is extremely complicated. People who maintained weather stations starting in 1880 didn't think to themselves, "Maybe someday people will need to measure climate change, so I better put down a really accurate thermometer and then ensure nothing about the instrument or the surrounding area changes for the next 130 years." They were mostly just trying to do garden-variety meteorology. The early temperature measurements we have are a broken and incomplete record of more and less good data from instruments that were often changed, moved, or that found themselves in different settings over time. When scientists started putting together the vast library of the planet's temperature records in the 1980s in order to do climate-change assessment, they needed to be able to weed out these changes and errors. And they couldn't always do that by reading meteorologists' diaries. They needed to use statistical tools to hunt for anomalies.

So they developed some. And those statistical tools proved very good. They picked out, for example, a station in West Africa where someone had started measuring temperatures in Kelvin for a decade or so; the people who entered it into the global database, seeing a bunch of numbers around 300, had assumed the temperatures were in Celsius but missing a decimal point, and had divided by 10 rather than subtracting 273.15, which produced an inaccurate and compressed distribution for those years. But most of the errors and biases are far more subtle. So scientists homogenising data compare each weather station to the closest surrounding ones, creating a "reference series", and hunt for a suspicious divergence in trends. That's how the GHCN arrives at its adjustments for the Darwin station.

Mr Eschenbach complains about the GHCN's adjustments, saying that there are too few nearby stations to make an adjustment: "The nearest station that covers the year 1941 is 500 km away from Darwin. Not only is it 500 km away, it is the only station within 750 km of Darwin that covers the 1941 time period." He's talking about the Daly Waters Pub, but he's inexplicably wrong. The GISS website he used for his own data shows two closer stations operating in 1941, Katherine Aer (272 km) and Wyndham Port (454 km). Both had temperature data series that ran for a long time. Here's what they look like, unadjusted:

A third station, Kalumburu, is essentially the same distance as Daly Waters Pub (504 km), but it didn't start getting reliable data until 1944. Here's what it looks like, unadjusted:

They all show basically the same rising trend. But look: Mr Eschenbach himself admits the 1941 adjustment was necessary. By the time we start looking at the later adjustments he worries about, in the 1960s and 70s, there are 22 stations within 500 km of Darwin to create a reference series. And they all show roughly the same upward-trending series.

So is it reasonable, if the GHCN is using complex statistical tools to adjust the temperature readings at Darwin based on surrounding stations, that they might come up with the figures they came up with? Sure. No. Yes. I have no idea. And neither does Mr Eschenbach. Because in order to judge that, you would have to have a graduate-level understanding of statistical modeling. For example, one paper describing the BOM's homogenisation approach includes this discussion:

Trewin (2001) comments that the median reference series used by Torok (1996) can have biases introduced when converting the median interannual differences back into an absolute reference series by the accumulation of rounding errors in the interannual differences (also well documented in Peterson and Easterling (1994)). This can introduce spurious trends in the reference series. Trewin (2001) uses a distance weighted mean of highly correlated reference station anomalies to create the reference series and so avoids the problems associated with the conversion of interannual differences. The weights, Wi, are given by Eqn 1 where di is the inter-station separation measured in degrees, and ri is the interstation correlation and N is the number of stations.

Wi = {ri ( 6 – dI) 2, r > 0.6, dI < 6} i=1,...,N ...1 {0 ,r < 0.6, diI> 6}

I don't understand that formula. I don't have the math for it. The paper goes on to reject the Trewin formula for reasons which, again, I don't have the math to understand. This is academic-level statistics. You can't render judgment on it by plugging disparate data series into Excel and eyeballing the trend lines, as Mr Eschenbach does. Here, for example, is a recent example of the kind of stuff climate scientists have to be able to do with statistics in order to get accurate results:

Two discontinuities were detected in the air-temperature time series at the meteorological station of the National Observatory of Athens. The first discontinuity reflects the instrumental change, which took place in June 1995 and the second discontinuity (and most pronounced) the application of a correction factor to the temperature values (in January 1997), after a calibration of the new thermometers. As a result, a cooling bias was observed after June 1995 and a warming bias after January 1997. The magnitude of bias exhibited a seasonal variability being more pronounced and reaching up to 0.67°C during the warm period of the year.

Judging by his post, Mr Eschenbach doesn't have the expertise to assess issues like these any more than I do. Mr Eschenbach is not a scientist; he's an amateur. His first effort in climate scepticism apparently came in 2002 while working as the construction manager for a beach resort in Fiji, when he published a non-peer-reviewed article claiming to have found that sea levels in Tuvalu were not actually rising, and that claims that they were stemmed from attempts by locals to blame subsidence problems on the developed world, and cash in on it to have found that the sea level in Tuvalu was "not rising at more than the historical rate", and that the problems being blamed on rising sea levels in fact stemmed from the locals' rising populations and poor environmental stewardship.* He's been beating this drum for years; he does not approach this issue from a position of neutral scepticism, he approaches it from a position of certainty that AGW is a hoax.

Look back, for instance, at the way Mr Eschenbach starts off his discussion of the Darwin data. He makes it sound as if he's just happened to stumble across this one site whilst perusing a debate over climate change in northern Australia. But as his link to that conversation from 2000 makes clear, Mr Eschenbach is already aware that climate change denialists have been trumpeting the apparent anomalies at Darwin for nine years. Climate-change sceptics have been trumpeting the apparent anomalies at Darwin for nine years.** They do so because of that errant data at Darwin from before 1941, which makes it look as though there was a cooling trend there. The fact that climate-change researchers have to do a particularly strong correction on the data at Darwin, because they moved their dang instruments from the downtown post office to the airport, makes Darwin a perfect place to look for support if you want to claim that climate-change scientists are cooking the data.

So, after hours of research, I can dismiss Mr Eschenbach. But what am I supposed to do the next time I wake up and someone whose name I don't know has produced another plausible-seeming account of bias in the climate-change science? Am I supposed to invest another couple of hours in it? Do I have to waste the time of the readers of this blog with yet another long post on the subject? Why? Why do these people keep bugging us like this? Does the spirit of scientific scepticism really require that I remain forever open-minded to denialist humbug until it's shown to be wrong? At what point am I allowed to simply say, look, I've seen these kind of claims before, they always turn out to be wrong, and it's not worth my time to look into it?

Well, here's my solution to this problem: this is why we have peer review. Average guys with websites can do a lot of amazing things. One thing they cannot do is reveal statistical manipulation in climate-change studies that require a PhD in a related field to understand. So for the time being, my response to any and all further "smoking gun" claims begins with: show me the peer-reviewed journal article demonstrating the error here. Otherwise, you're a crank and this is not a story.

And then I'll probably go ahead and try to investigate the claim and write a blog post about it, because that's my job. Oh, and by the way: October was the hottest month on record in Darwin, Australia.

* Mr Eschenbach has since sent us a copy of his article, "Tuvalu Not Experiencing Increased Sea Level Rise", and we believe this is a more accurate description of what he wrote.

** Mr Eschenbach has written that he came across a 2000 conversation about Darwin temperature data to which his post referred after starting his investigation into the subject, and that my accusation that he already knew this temperature series was controversial is unfounded. This is true. I don't know in what order he discovered his facts, so it was not fair of me to make the accusation that he already knew about the data. I apologise.

Addendum: I spoke earlier today with Blair Trewin, a climatologist at the Australian BOM's National Climate Center. He said the BOM was trying to reconcile its own adjustments for the Darwin station with those of the GHCN. The BOM's "current data set does have a number of small adjustments over that period that are step functions. And all of those are attributed to there being quite a number of stationary locations within the Darwin Airport boundaries between 1970 and 1990s. We in our current set have an adjustment of 0.8 of a degree at 1941, which matches GHCN pretty well. And then the various small adjustments between 1940 and 1990 are a cumulative 0.6. Whereas GHCN, that graph makes it look like it’s about a cumulative 1 degree. But that’s probably within the margin of error for a single-station adjustment."

Mr Trewin explained another reason for the adjustment at Darwin when the station moved from the downtown Post Office to the airfield: the post office was situated on a harbor where, during the dry season, ocean water temperature tends to be warmer than land temperature.

As for the nature of the BOM's adjustments, Mr Trewin said he thinks "all of them are associated with a documented, either stationary location change or another change in local environment around the station."

"In an ideal world," Mr Trewin said, "we would like to have a good range of stations that haven’t moved, haven’t had changes in their local environment. But if we want to get any information with regard to the climate over the long term, we have to make the most of what data we have."

Addendum 2: Mr Eschenbach's response is here. My response to his response is here.