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Originally Posted by Nate the great
The graph form the source Harry provided cannot be relied upon. It still incorporates invalid temperature data from the US. The temperature trend attributed to the northern hemisphere should have a peak that is much lower.
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Could you explain why you believe the data to be invalid, please?
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Also, before citing it as a source you should verify that the temperature from the 1800s is based on actual measurements, and not on a computer extrapolation. Furthermore, I wonder how many data points were actually recorded for each year, and where they were on the globe.
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I don't know where those particular data originated, but just a note that a continuous record of climatic conditions in the world's "temperate zones" doing back to around 6-7000 BC has been created largely from dendrochronology - ie the study of tree rings. From studying tree rings it's possible to obtain data on both temperature and rainfall to a high level of accuracy.