The climate naturally varies from year to year. Climate change can occur over considerably longer time periods, such as decades or centuries. The shift in the mean condition of a given climate parameter, such as temperature or precipitation, is referred to as climate change. Climate change occurs when the statistical distribution of a climate variable changes, to put it another way. The variation in climate parameters induced by nonhuman causes is referred to as natural climate variability. There are two types of natural variability in the climate system: external and internal. External controls on climate variability include variations in the sun, volcanic eruptions, and changes in the Earth's orbit around the sun. Changes that occur over extended time periods, like oscillations between ice ages and interglacial eras, are driven by these processes. Natural variability is also influenced by internal climate system processes that originate, in part, from interactions between the atmosphere and the ocean, such as those that occur during an El Nino event in the tropical Pacific Ocean.