more than climate change ever will.

Q. Why can't we use just raw data?
A. Just averaging the raw data would give results that are highly dependent on the particular locations (latitude and elevation) and reporting periods of the actual weather stations; such results would mostly reflect those accidental circumstances rather than yield meaningful information about our climate.

Q. Can you illustrate the above with a simple example?
A. Assume, e.g., that a station at the bottom of a mountain sent in reports continuously starting in 1880 and assume that a station was built near the top of that mountain and started reporting in 1900. Since those new temperatures are much lower than the temperatures from the station in the valley, averaging the two temperature series would create a substantial temperature drop starting in 1900.

Q. How can we combine the data of the two stations above in a meaningful way?
A. What may be done before combining those data is to increase the new data or lower the old ones until the two series seem consistent. How much we have to adjust these data may be estimated by comparing the time period with reports from both stations: After the offset, the averages over the common period should be equal. (This is the basis for the GISS method). As new data become available, the offset determined using that method may change. This explains why additional recent data can impact also much earlier data in any regional or global time series.

Another approach is to replace both series by their anomalies with respect to a fixed base period. This is the method used by the University of East Anglia's Climatic Research Unit (CRU) in the UK. The disadvantage is that stations that did not report during that whole base period cannot be used.

More mathematically complex methods are used by NOAA National Climatic Data Center (NOAA/NCDC) and the Berkeley Earth Project, but the resulting differences are small.

The temperature seems to be defying its historical link to solar activity. Based on solar activity we should have been seen fairly severe cooling over the last few decades: http://www.woodfortrees.org/pl...