A Ghost from the Past

I have located an original version of the Global Historical Climatology Network (GHCN) published around 1990. It contains raw temperature data from 6039 weather stations around the world. Quality control procedures corrected a few impossible values mainly due to typing mistakes, and removed any duplicate data. Otherwise they are the originally recorded temperatures. You are welcome to  download the metadata and the temperature data in re-formatted csv files, which I hope are self explanatory. The original ‘readme’ file with credits to authors can be downloaded here.  Since 1990 there have been a continuous set of  adjustments made to GHCN data for a variety of reasons. These include changes in station location, instruments and especially ‘data homogenisation’. These adjustments have had the net effect of cooling the past (pre-1930). The latest GHCN version is 3 which can be downloaded from NOAA.

So what I did next was to process the GHCN V1 data by first gridding the temperatures geographically in a 5×5 monthly degree grid, similar to CRUTEM4. I then calculated the monthly averages across all stations within one grid cell. The monthly temperature anomalies  are then just the differences from these average values. Averaging stations within a grid cell is essentially the same thing as data homogenisation, because it assumes that nearby stations have the same climate. The annual temperature anomalies are the geographically weighted averages of the monthly values. So what did the original V1 data say about past temperatures?

Global-compare

 

There is clearly a huge difference before about 1930. So let’s compare each hemisphere separately.

NH-compare-Crutem4

 

For the southern hemisphere I compare GHCN V1 with a contemporary version of CRU dated 1988 (see below).
SH-compare-CRU86

 

GHCN V1 was available just before the first IPCC assessment report in 1990. At the time CRU had also collected a smaller set of station data from around the world which mostly were included in GHCN. I also have a copy of this data from around 1988 which we can compare directly with V1. The global average temperature anomalies are shown below.

CompareJones-V1The agreement  after 1900 is very good, but  they disagree strongly in the 19th century. Now you can also see why the IPCC first assessment report (FAR) was so cagey about any global warming signal (“yet to emerge”).  That was because  there wasn’t any signal in the temperature data available at that time!

 

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