Tuesday, July 8, 2014

GHCN Adjustments are much larger in US than ROW.

ROW=Rest of World. I wrote two posts here and here on the overall effect of global GHCN adjustments on trends of various periods. It was a kind of sequel to my first ever Moyhu post here, which showed a similar histogram of the effect of V2 adjustments.

I was a little surprised that the positive bias had increased substantially, though still not huge. There has recently been a lot of talk about USHCN adjustments, and I did some plotting in my most recent post. GHCN v3 just uses USHCN, including the adjustment method, for its US data. So I thought the rise might well be partly due to the US component and TOBS.

It turns out that is true. I repeated the calc separating US and non-US data. I'm using just a simple average of the effects on trends - it should be area weighted. That's part of the reason why the US has a disproportionate effect in this simple average. Anyway, the trend differences caused by adjustment to stations with 60 years data were 0.0355 °C/decade for US, 0.0248 °C/decade for ROW, and 0.0284 °C/decade combined. More numbers and details below.

Incidentally, that previous post had a useful Google Maps gadget that lets you pick out groups of stations with big or small adjustments, rural/urban etc. I'll probably update it with more links and more intuitive logic.

The methods are as in the previous posts. I am really just applying a filter. It isn't exactly USHCN vs the rest, as I've included AK and HI in the US total.

There are three subgroups - stations that have histories with actual reports for at least
  1. 360 months (30 years)
  2. 540 months
  3. 720 months

US Stations only

Mean 0.0383 °C/Decade

Mean 0.038 °C/Decade

Mean 0.0355 °C/Decade

Non-US stations only

Mean 0.0232 °C/Decade

Mean 0.0247 °C/Decade

Mean 0.0248 °C/Decade

All GHCN Stations

Mean 0.031 °C/Decade

Mean 0.0303 °C/Decade

Mean 0.0284 °C/Decade

A notable difference is that quite a lot of non-US stations have zero trend effect, probably reflecting zero adjustment. So the y-axes are quite different in scale. But they would be anyway, because of the different numbers of stations.


  1. Nick,

    If you haven't come across it yet, you might find this interesting: http://www1.ncdc.noaa.gov/pub/data/ghcn/v3/techreports/Technical%20Report%20NCDC%20No12-02-3.2.0-29Aug12.pdf

    1. Thanks, Zeke,
      Yes, I remember Daniel's project well. It's a good write-up.

  2. My understanding is that the US had a much higher proportion of amateur meteorologists in the early 20thC, while the rest of the world tended to rely on professional met services. Hence both the density of the US record, and the large number of corrections required - the amateurs tended to be excellent record keepers but were not familiar with the Tobs issue.

    Were min/max thermometers more widely used in the US than elsewhere?

    1. I think min/max were pretty universal; the only real alternative is three-hour or better readings. Here observers were commonly postmasters or lighthouse keepers.

      TOBS really wasn't, or shouldn't have been, an issue for observers. It's one of after the fact interpretation.