Climates Multiple: Three Baselines, Two Tolerances, One Normal

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ACADEMIA
 
Climates Multiple: Three Baselines, Two Tolerances, One Normal
Mike Hulme

Introduction
Today, Friday 1 January 2021, a new World Meteorological Organisation (WMO) climato-logical standard normal came into effect. The ‘present-day’ climate will now formally berepresented by the meteorological statistics of the period 1991-2020, replacing those from1961-1990. National Meteorological Agencies in member states are instructed to issue newstandard normals for observing stations and for associated climatological products. Climatewill ‘change’, one might say, in an instant; today, the world’s climate has ‘suddenly’ becomenearly 0.5°C warmer. It is somewhat equivalent to re-setting Universal Time or adjusting theexact definition of a metre.But things are not so simple. A few years ago, in 2015, the WMO took the decision toadopt a two-tier system of climatic normals. For operational planning and design purposes,1991- 2020 would define ‘present-day’ climate. But 1961-1990 will continue to be used fordefining ‘historical climate’ against which to measure future changes in climate. And 2015wasthesameyearinwhicha
third
 referenceclimatewasaffirmed, namelythe‘pre-industrial’climate enshrined in the Paris Agreement on Climate Change (PACC).This ‘re-setting’ in January 2021 of normal climate is a good moment to offer a brief reflection on the relationship between climatic normals, baselines and tolerances.

https://www.academia.edu/44905791/Climates_Multiple_Three_Baselines_Two_Tolerances_One_Normal?email_work_card=view-paper