12 Stationarity

12.1 Record Events

Whenever there is a new record-breaking weather event, such as record-high temperatures, it is natural to ask whether the occurrence of such an event is due to a climate change. Before we proceed, it may be useful to define the term ‘statistically stationary’, the meaning here being that statistical aspects of the weather (means, standard deviation etc.) aren’t changing. In statistics, there is a large volume of literature on record-breaking behaviour, and statistically stationary systems will produce new record-breaking events from time to time. On the other hand, one would expect to see more new record-breaking events in a changing climate: when the mean temperature level rises new temperatures will surpass past record-highs.

Benestad (2005) On record-breaking events (RealClimate.org)

*Abstract Benestad)

This study applies a simple framework for analysing the incidence of record events. A test of this method on the global mean temperature yields results consistent with a global warming, where record-warm events are more frequent than for a stationary series. The record event analysis suggests that the number of record-warm monthly global mean temperatures is higher than expected, and that the number of record events in the absolute monthly maximum and minimum temperatures in the Nordic countries is slightly higher than expected from a null hypothesis of a sta- tionary behaviour. Because the different station series are not strictly independent, it is difficult to resolve whether there is a significant trend in the warmest absolute monthly minimum temperatures in the Nordic countries. The behaviour of the maximum monthly 24 h precipitation is not distin- guishable from the null hypothesis that the series consists of independent and identically distributed random variables.

Benestad (2003) How often can we expect a record event? (Climate Resarch) (pdf)