Dear Gordon Andelin:
You ask Boris why 30 years is chosen as a climate definition. Clearly, you are not likely to get a proper answer from that source, so in an attempt to help I offer the following.
You expand your question by asking; “Why isn’t 50 years, 100 years, or 1000 years used?” In fact, all these periods are used for climate assessments, and to understand why one needs to know what climate is.
The UN Intergovernmental on Climate Change (IPPC) defines “climate” in its Glossary: Appendix 1: as follows.
“Climate in a narrow sense is usually defined as the “average weather”, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.”
Please note that the definition says climate may be considered over “a period of time ranging from months to thousands or millions of years”.
The “classical period” defined by the WMO is 30 years and it was chosen for historical and pragmatic reasons during the Geophysical Year of 1958, and in 1958 it was judged that 30years of climate data had been accumulated.
The establishment of a “classical period” permits a standard length of time for comparison purposes. So, for example, the global temperature data of GISS, GHCN and HadCRUT are each presented as “anomalies” (i.e. differences) from a 30-year period.
However, 30 years is an unfortunate choice for several reasons. For example, it is not a multiple of the 11 year solar cycle.
Importantly, 30 years is only the “classical period” that is used when establishing data sets (such as the GISS, GHCN and HadCRUT temperature series) that may need to be compared, and – as the IPCC definition says – other periods (both longer and shorter) may be assessed. Indeed, in its 1994 report the IPCC compares 5 year periods.
I hope this helps.
Richard
