It's that time of the decade when the official 'climate normals' are computed -- according to the rules of the WMO and NOAA (in the US). But can we find a better way of deciding 'normal'? I'll do some thinking out loud here, and invite you to play too. Could we even be so lucky as to find a way of defining climate normal in such a way that we don't need to worry about an early period of 'coming out of the Little Ice Age', or a later period of Anthropogenic Global Warming.
In previous posts, I illustrated that there are principles which lead to the requirement of
20-30 years to define a climate average and, separately,
20-30 years to define a climate trend.
In those approaches, we were able to make good use of the adage that climate is what you expect. It was only after 20-30 years of data that our expectations for the mean or trend would be stable -- would not depend sensitively on how long a period we chose to be our data period.
This time, I'll pick up with a different notion or description. A common description of climate is also that sometimes its warmer and sometimes it's cooler, but it all averages out in the end. Let's start by looking at the
Hadley-CRU temperatures, back to 1850, month by month. I'll start with looking at temperatures relative to the average over the entire data set:
For the earlier part of the record, we see the deviations being both above and below the reference value. On the other hand, the last 26+ years, every month since December 1984 -- 317 consecutive months, has been above the reference value. That's not a sign of sometimes warmer, sometimes cooler. That's the recent period being different from 'normal'. For instance, because of a warming trend. If you're younger than 26, you have
never seen a month where the global mean was as cold as the 161 year average. On the other hand, looking at the early part of the record, we see some years warmer than 'normal' even though most are cooler. Further (look directly at the original data, don't take my word) there are no periods as long as even 20 years of continual below reference temperatures. In other words, the Little Ice Age, insofar as it was global, had ended by 1850.