Alternative title: The complete idiot’s guide to cherrypicking.
Willis Eschenbach (of Darwin Zero fame) has a post on Watt’s Up With That concerning the homogenisation process in Anchorage and Matanuska (both in Alaska). Matanuska was chosen for being close to Anchorage. But why start in Anchorage? No explanation is given. Something about this smells like cherrypicking – picking out a station that happened to have an odd-looking trend and expanding a conspiracy theory around it.
Well, two can play at this game. I wrote a little program to find stations that had a downward trend in homogenisation in the GHCN v2 data. Not anything even approaching being clever, I just picked a few of the stations that had a reasonable amount of data from the last 40 years and had less homogenisation in 2009 than in 1970. But let’s say I didn’t give that explanation, and just talked about the odd trend in Asheville? That would hardly be methodologically valid.
Here’s how the temperatures in Asheville, North Carolina, were homogenised:
I can’t explain the homogenisation in Asheville. Would it be reasonable to suppose that an AGW denialist hacked into the GHCN website and modified the data? Well, no. Whereas Eschenbach was excited by a .7°C increase spread out over 20 years, here we had a .375°C decrease in a single year. Note how this causes the homogenised line (red) to drop away from the raw data (blue).
What about Pohang, in South Korea?
These major drops cut off what had looked like a warming trend. Certainly Pohang was not homogenised to deliberately create an artificial warming trend! In two bursts within just 8 years (and at the start of the data set), temperatures are adjusted upwards by a startling .65°C!
And then there’s Cairns Airport. What the heck is going on here?
Is homogenisation being used to hide global warming all over the world?
Why is data being homogenised like this? Well, it’s unfortunate, but most of the various temperature stations were not set up to track climate. As such, they periodically got moved, changed, located in places that were more convenient to measure for the daily news rather than for tracking world climate, and so forth. This means that before trying to use these for studying climate it’s probably worthwhile controlling for these factors.
But, seriously, there are a whole bunch of factors, and between them they could cause adjustments – both up and down. And if you look at enough sites, you’ll no doubt find examples of both. So if someone is showing you one site’s homogenisation and asking you to draw conclusions of fraud on the basis of it – why that site? Why not any of the thousands of others? Are all the sites homogenised in the same direction? Or is it simply more likely that you are listening to someone who is cherry-picking, finding any anomaly and then wrapping it in a conspiracy and their own absolute certainty that global warming is a lie and that the scientists who give evidence in favour of it are liars and frauds?