New commenter Colin asks:
So what is the purpose of changing the data?
We can only speculate about that. The only people who can provide a definitive answer are the curators of the data at GHCN. I have sent them the following email and eagerly await their reply.
Dear Sir / Madam,
By way of brief introduction I am a British Energy and Climate blogger based in Aberdeen Scotland. I have taken an interest in recent press and blog reports about modifications being made to temperature records and have been conducting a number of region based comparisons that include Central Australia and Iceland. It is Iceland that I wish to make the focus of this query. My observations can be found at the following link:
Re-writing The Climate History of Iceland
The key points I feel that GHCN need to address in public are as follows:
1) The Icelandic data are apparently very well curated and already adjusted for non-climatic influences. Furthermore homogenisation is unlikely to be effective in an isolated system like Iceland. Can you therefore please explain the rationale for homogenising these stations when it seems quite clear that it would be inappropriate to do so? The effect I believe has been to cool the past creating an impression of more warming than the raw data actually supports.
2) Figure 7 in my post displays the arithmetic difference of the mean temperatures from all 8 Iceland stations. From this it can be seen that centred on 1939 larger than usual adjustments to temperature have been made in a narrow band. This has been achieved by significantly reducing temperatures in this narrow band in the V3.1 data. The raw data indicate that 1939 was the warmest year on record in Iceland and the effect of these adjustments is to remove this temperature peak all together. Can you please explain the justification for these adjustments and how they were made?
3) Staying with Figure 7, it can be seen that there is another narrow band of temperature adjustments centred around 1967, this time in the opposite direction. This time it seems that the adjustments have been achieved in part by systematic deletion of temperature records in this narrow band. See Figure 8 where deleted records are shown in yellow. The effect of this has been to substantially reduce the significance of the well-doucumented cold spell in Iceland at this time. Can you please explain how and why these records came to be deleted?
4) Staying with Figure 8, in green are cells where V3.1 data seems to exist that does not have V2 equivalent data. Given that V3.1 is supposed to be a homogenised derivative of V2, how is it possible to have V3.1 data where no V2 data exists?
There is a fair amount public concern about the veracity of the official temperature records that are used by many governments and the United Nations to guide energy policy and the allocation of $trillions in energy infrastructure. I would therefore be very grateful if you could arrange for a senior official at GHCN to answer the questions detailed above so that public concern may be put to rest.
Dr Euan Mearns (Geologist)