I'm saying that the way they have to correlate the old temperature is like this
(a) you get the weather stations that are 100 years, old, you look at the data (b) you compare it to the same data as today
But there is a problem, all these old weather stations are now surrounded by buildings, aircon outlets everything. Some are in the middle of concrete where before they were in the a grass paddock, but as population increases urbanisation changes around it.
You have to compare the old ones, problem is the micro climate around them has changed
What I was saying is that that this effect is well known, and they compensate for it by taking data from surrounding rural weather stations and weather balloons, which are not affected by urbanisation. In this way they can see what effect urbanisation has on temperature measurements. The figure quoted by IPCC is around 0.006 degrees per decade.
