# Climate Change Indices

## Definitions

Extremes such as the annual maximum daily precipitation amount
or the annual maximum wind speed have traditionally been
modelled with one of three different **Extreme Value Distributions** in engineering
applications. The extremes used as climate change indicators have a much broader context.
While some of the indices would fall within the traditional definition, most do not. It is
important to make this distinction when analyzing indices since only a very
few can be assumed to follow an extreme value distribution.

The WG approved a list of 40 indices. Some of the indices are used more frequently than others. Different research groups may define different indices for their particular purposes. For example, the percentile based temperature indices (e.g. number of days daily temperature is greater than 90 percentile) defined by STARDEX project are very different from those defined by ET. Albert Klein Tank of KNMI complied a list of the indices used by different researchers.

The ET has recently revisited its definitions of indices. A total of 27 indices were considered to be core indices. They are based on daily temperature values or daily precipitation amount. Some are based on fixed thresholds that are of relevance to particular applications. In these cases, thresholds are the same for all stations. Other indices are based on thresholds that vary from location to location. In these cases, thresholds are typically defined as a percentile of the relevant data series. The details of those 27 indices can be found here

A R based software package and a FORTRAN program have been provided for the calculation of these indices. Note that as indices are used more extensively in climate change detection and monitoring, we will learn more about their properties. This accumulating knowledge may lead to changes in the definition of some indices, or the creation of new indices. We will keep you updated on the new development in this research area.