NOAA supplies two series with the global surface temperature anomalies. The series use different reference periods. The monthly anomalies in the two series differ a little from month to month also after having been converted to the same reference period, but the mean of the differences is close to zero. I examined this topic in order to decide which of the two series to use, and my conclusion is that both can be used because they result in the same temperature trend. But I am curious as to why NOAA supplies two different series with the global surface temperature.
