Traditionally, the term 'climate' referred to the averaged weather conditions, such as the mean July temperature or annual precipitation. For many applications it is important to know not only the averaged characteristics, such as monthly averaged temperature, but also different measures of variability (statistics), like the interannual variability of the precipitation or a number of extreme weather events. Thereby, more precisely the term climate is characterized as 'the statistical description (of the climate system) in terms of the mean and variability of relevant quantities'. Temporal evolution of the climate characteristics beyond the timescale of individual weather events is named 'climate variability' while statistically significant trend of climate state on longer timescales (decades and more) is referred to as 'climate change'. For example, North Atlantic Oscillation (NAO) or El Niño/Southern Oscillation (ENSO) represents climate variability, while dramatic climate variations over past several million years associated with waning and waxing of the Northern Hemisphere ice sheets are the example of climate change. Pronounced trends of many climate characteristics recorded during the twentieth century are primarily attributed to human influence on climate and hence represent anthropogenic climate change.
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