SEARCH BLOG: GLOBAL WARMING
If you want to measure the distance between two hill tops, you can try a variety of measurement techniques. You can pace off the distance, you can use a measuring tape along the ground, or you might use a line-of-sight laser. It is unlikely that your results will be the same from one technique to the other.
At Climate Science, there is a post regarding how surface temperature measurements are taken. While the variability may not be as great as my hypothetical example above, the variability is significant... for a variety of reasons... especially when one is trying to assess a 0.6 degree C change over a century.
See also.
GIGO.
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