Changes in the statistics of climate and climate variables is sometimes referred to as climate nonstationarity. It makes climate "normals" and other statistics based on past data increasingly irrelevant as the climate changes.
A good example is provided by the mean summer (June, July, August) temperatures in Salt Lake City. The graph below shows the mean summer temperatures in the Salt Lake City area from 1875-2021 (blue;the latter year included with a few days missing), the 30-year running mean temperature (red), and the 1991-2020 average temperature (orange).
|Data source: http://xmacis.rcc-acis.org/|
Prior to approximately 1965, temperatures in the Salt Lake City area fluctuated without a major trend, but after that time, temperatures began to climb. The average summer temperature for the 30-year period from 1936-1965 was 72.9˚F, but for the 30-year period from 1991-2020 it was 76.4˚F, an increase of 3.5˚F.