"Hulme: "January  12021, 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)." "

https://www.academia.edu/44905791/Climates_Multiple_Three_Baselines_Two_Toleranc...