Climate change (3)

So, what happens with the actually recorded (also known as “raw”) temperature readings before they become a published temperature series graph like this?
Fig.A2-page-001Well, first they undergo a few layers of adjustments. There are good reasons why “raw” readings cannot be put straight into the final graph and one was mentioned in the previous post. The urban heat island (UHI) effect causes the stations in urban or semi-urban areas to produce warmer readings over time. The way UHI is currently compensated for is by the MetService technicians doing periodic surveys of the weather stations sites to update their warming bias ratings. If for example a station is rated as +1.0C, because of the nearby carpark and the carpark gets extended the technician may decide to up the rating to +2.0C. So, from this day on the raw reading from the station will be adjusted by -2.0C before being processed further. Yes, it is that crude – a technician assessing the site of the station using human eye Mk.1 to change the warming bias rating. There are stations rated at +3.0C and even +4.0C although, as I understand, if the rating changes too much the station may get dropped off the record.

Then we have the adjustments for changes in station siting. If a station gets relocated to a new site, ideally, there should be an overlap period for calibration. If it is found that, say, the new site runs on average 1.0C cooler than the original station, the readings from the new site may be adjusted by +1.0C and treated as the extension of the original record. If there is no overlap an arbitrary correction may be applied, taking account of the elevation above sea level, local topography etc. There is also a possibility of using other, nearby stations to calibrate the new readings. This requires some reasonably involved maths and can introduce errors. Some stations disappear and are not replaced, while new stations are built at different sites so the spatial distribution of the available instrumental records changes over time.

There are also other, higher level adjustments. They are meant to correct the systematic biases caused by the measuring equipment or methodology. For example, the sea water temperature readings used to be taken be throwing a bucket overboard to bring some water to the deck and pop a thermometer in it. This was later changed to measuring temperature at the inlet of the engine cooling water. The latter method produces warmer readings (as a result of the heat produced by the engine) so the “bucket” readings were adjusted up to eliminate the bias. This is not trivial as all the sea water readings taken between years 1860 and 1945 were affected by the change. Let me add that this particular adjustment has decreased the rate of global warming in the instrumental record – bumping up the readings before 1945 means less warming since.

So what is the cumulative effect of all these adjustments on the raw temperature record? The official temperature record in New Zealand shows a rather high rate of warming in the last 100 years – here is the graph:

clip_image002
However, a group of AGW skeptics have simply downloaded the raw temperature record for all New Zealand weather stations and plotted their own “unadjusted” graph:

Image2
Yes, it is essentially flat – no warming. As I mentioned before, many of the adjustments made to the raw readings are well justified and their use properly documented. But to find out that these adjustments, cumulatively, account for ALL the warming in the instrumental record is a tad disconcerting.

Here are the similar graphs for the continental US. First – adjusted:

New Image

And then, “raw”:
1998changesannotated-1
If you have never come across the issue of these adjustments before, are you surprised?

Climate change (4)

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