The World Meteorological Organization (WMO) decided as early as in the nineteen-thirties that a climatological normal covered a period of 30 years. Previously, climatological normals were non-overlapping, but in 2015, this was changed to the most recent 30-year periods finishing in a year ending with 0, implying that climatological normals are from now on computed every 10 years.
New calculations lead to better description of the climate
“The first thing we calculate is new normal values for temperature and precipitation. After that, we continue with radiation, atmospheric pressure, depth of snow, sunshine and so on. We will also show how the new normal for 1991–2020 is related to the earlier normal for 1991-2020”, says Erik Engström, climatologist at SMHI.
The normals that are being produced now lead to a better description of the climate compared to earlier calculations, since more observations are available thanks to digitalization. Preliminary results will be presented by SMHI in the beginning of 2021, and be decided upon during the Spring.
“The use of new climatological normals will add to the pedagogical challenges of communicating about climate change. In such communications we will need to make clear that the 1.5-degree goal is relative to pre-industrial times, viz the time period 1850–1900, and not to any of the climatological normal time intervals.”, say Nina Kirchner and Alasdair Skelton, see also their additional comment below.
Communication from the Directors on the new Climatological Normal
In 2015, as many of you are no doubt aware, the World Meteorological Office (WMO) decided on a revised definition of the term “climatological normal” WMO, 2017). This term now refers to the most-recent 30-year period finishing in a year ending with 0 (1991–2020 as of January 1, 2021), rather than to non-overlapping 30-year periods (1901–1930, 1931–1960, 1961–1990, 1991–2020) as was the case previously. We suspect that introduction of a new climatological normal can cause some confusion among members of the general public, especially in the context of the IPCC’s recommendation of keeping warming below 1.5-degrees. We are therefore providing a couple of points which may be helpful when communicating with the public.
- Calculated from the GISS Surface Temperature Analysis (GISTEMP Team, 2020), the new climatological normal (1991–2020) will be 0.5 degrees warmer than the previous non-overlapping climatological normal (1961–1990).
- The IPCC’s recommendation of keeping warming below 1.5-degrees is a comparison with “pre-industrial” temperature. This term has multiple definitions. In the IPCC special report: Global warming of 1.5°C (IPCC, 2018), “pre-industrial” refers to the time period between 1850 and 1900.
The new climatological normal (1991–2020), calculated from the GISS Surface Temperature Analysis, will be 0.8 degrees warmer than pre-industrial temperature, so-defined.
Read more on SMHI’s website (in Swedish)