Here we go again. We have been over this before.
Why are there fewer weather stations and what's the effect?
Wednesday, 28 April, 2010
Why are there fewer weather stations and what's the effect?
The NOAA and NASA obtain their temperature data from the Global Historical Climatology Network (GHCN). In the early 1990s, the number of weather stations listed in the GCHN drops rather sharply. Some have suggested this is a deliberate campaign to remove cooler weather stations in order to inflate the warming trend. The driving force behind this idea comes from Joseph D'Aleo in an online report Surface Temperature Records: Policy Driven Deception co-written with Anthony Watts. Initially, their report stated that "There has been a severe bias towards removing higher-altitude, higher-latitude, and rural stations, leading to a further serious overstatement of warming." This text has been removed from the latest edition of their report. Nevertheless, the notion that removed stations is causing spurious warming continues to be repeated throughout the blogosphere (and often in comments here on Skeptical Science).
Why are there currently fewer stations in the temperature record? The physical number of weather stations that are reporting temperature data has diminished - some of the older outposts are no longer accessible in real time (NOAA). In fact, the perceived "drop-off" is exacerbated by the fact that the NOAA have been actively adding historical data into the GCHN database from older weather stations that are no longer active, in an attempt to provide more comprehensive coverage of the past.
The most important issue is whether this drop-off in the number of reporting weather stations has had any impact on the temperature record. D'Aleo's report performs no such analysis. However, there have been a number of independent analyses examining this very question. The first analysis was conducted by Tamino, from Open Mind, who plotted the temperature data from the weather stations that were dropped from the GHCN record (labelled 'pre-cutoff'). He then compared this to the temperature data from the weather stations that were kept (labelled 'post-cutoff').
UPDATE 29 April 2010: Mark Richardson informs me of a blog post by Roy Spencer that compares the GCHN record to his own temperature record that uses a much broader range of weather stations combined with the satellite record and finds pretty much an identical warming trend.
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