Statistical politics: Prof. Mike Hulme on ‘politically charged’ climate baseline changes from 1961-1990 to 1991-2020

I had to laugh as soon as I read this. It changes nothing of course but it is going to look like it does. And Greenies will certainly pretend it does. It is just a statistical trick. Comments from distinguished meteorologists that appeared in my email summarize the matter well:

"The only temperature plot that isn't a slave to baseline choices is absolute temperature. Anomaly based temperature is a statistical construct relying on the assumptions and choices of the statistician."

"Trends in fact don't change with changes in baseline, but any time there is a step up, it will be used for political and propaganda purposes without ever acknowledging that it is a statistical up-tick, and not real."

"It doesn't change temperature trends over ANY time period. What it will do is make temperature anomalies cooler, since the new 30 year baseline is warmer than the old."

Hulme: "January 1, 2021, a new World Meteorological Organisation (WMO) climatological standard normal came into effect. The ‘present-day’ climate will now formally be
represented by the meteorological statistics of the period 1991-2020, replacing those from 1961-1990.

National Meteorological Agencies in member states are instructed to issue new standard normals for observing stations and for associated climatological products. Climate will ‘change’, one might say, in an instant; today, the world’s climate has ‘suddenly’ become nearly 0.5°C warmer. It is somewhat equivalent to re-setting Universal Time or adjusting the exact definition of a metre." ...

"So, what is the significance of the move to a new 1991-2020 WMO normal in January 2021? On the one hand, it is a pragmatic move to redefine ‘present-day’ climate for operational applications to that of the most recent 30-year period. On the other hand, it puts into play a third climatic baseline. Already existing is the ‘pre-industrial’ climate of the late nineteenth century and the ‘historic’ climate’ of 1961-1990, the latter about 0.3°C warmer than the former. And now there is the new ‘present-day’ climate of 1991-2020, in turn about 0.5°C warmer than the ‘historic climate’ of 1961-1990." ...

"Combining a climatic tolerance of 2°C—or indeed 1.5°C—with a pre-industrial baseline yields a very different climate target than, say, using a 1986-2005 baseline, the period widely adopted by IPCC AR5 Working Group I as their analytical baseline. The choices of both baseline and tolerance are politically charged. They carry significant implications for historic liability for emissions (La Rovere et al., 2002), for policy design (Millar et al., 2017) and for possible reparations (Roberts & Huq, 2015)."

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