I came across a link to this Tamino post that clearly explains the difference. Seems Anthony Watts has been pinged for not providing his audience clarity that these are four superimposed graphs are of temperature anomalies, each with different baselines.
All these data are temperature anomaly. Anomaly is the difference between temperature at a given time, and the average temperature for the same time of year during some reference period. So temperature anomaly doesn’t really tell you, in absolute terms, how hot or cold it is — it tells you how much hotter or colder it is, than it was (on average) during the reference period. And there’s the rub: these data sets use different reference periods. GISS uses the reference period 1951 to 1980, HadCRU used 1961 to 1990, and the satellite estimates use 1979 to 2000. The coldest of these reference periods is the 1951-1980 GISS reference, the warmest is the 1979-2000 satellite reference. That means that GISS anomaly is the difference between present temperature and a colder time period, satellite data are the difference between present temperature and a warmer time period.
We can’t directly compare the numbers in a meaningful way without compensating for the difference in reference. Otherwise, it’s just like measuring my height in inches above Shaquille O’Neill (which makes the number quite negative) while measuring a newborn child’s height in inches above the ground (which makes the number certainly positive), noting that the infant’s number is greater, and concluding that the newborn is taller than I am. If we fail to compensate for the different reference, then we expect that the GISS numbers will be highest, the HadCRU numbers next, and the satellite data lowest. And that’s exactly what we observe.
So what does the graph look like after compensating for the differences in terms of reference? Tamino shows (click to open):
The result is very little variation between the four sources.
The satellite measurements' close correlation with the three surface based measurements provides squeezes all joy out of the deniers' argument once differences in baselines have been adjusted for.
AGW denial myth #2,971 busted.
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