Earth's climate exhibits natural variations on time scales of years to hundreds of thousands of years because of variations in a number of climate-forcing factors. Variations in Earth's orbit around the Sun can change the amount of incoming solar radiation. The shape or eccentricity of Earth's orbit around the Sun changes with a cycle of 100,000 years, leading to alternating warmer and colder periods on Earth. The obliquity of the tilt of the rotational axis alternates with a frequency of 41,000 years, also leading to variations in the amount of incoming solar radiation in different hemispheres in different seasons. The precession, or direction that the tilt axis is inclined relative to the Sun in different seasons, changes with two frequencies of 19,000 and 23,000 years. Together, these astronomical climate-forcing mechanisms are responsible for many of the variations in Earth's climate in geological time. These cycles interact in a complex way, producing a sequence of warm and cold climate phases known as Milankovitch Cycles.
Thermohaline, or temperature and salinity driven, circulation of the world's oceans also exhibits a strong control on climate. When the patterns of thermohaline circulation change, from changes in the distribution of continents, in salinity, or in climate caused by other factors, then the pattern of ocean circulation can suddenly change, plunging regions into dramatically different climate patterns in very short times.
The El Nino-Southern Oscillation refers to a variation in a global variation in atmospheric circulation in the Austro-Pacific realm that affects the entire globe. This circulation pattern is caused by alternate heating in the Indonesian-Australian region and drives warm and cold water back and forth across the Pacific in an narrow equatorial belt, strongly affecting the climate of western South and North America and changing global climate patterns on yearly to decadal time scales.
Major volcanic eruptions can suddenly change global climate, with effects that typically last for several years. The volcanic effect can cause heating by injection of CO2 and other greenhouse gases to the atmosphere, or cooling by injection of ash and aerosols into the atmosphere. The most severe volcanic effects are those from the eruptions of giant igneous provinces called flood basalts, which can produce more volcanic material than is cumulatively being erupted along all of the planet's mid-ocean ridges. Massive volcanism can put so much carbon dioxide into the atmosphere that the temperature can warm by several degrees or more, placing the planet's ecosystems under a stressed condition, and lasting millions of years. Other flood basalt eruptions have placed so much dust and aerosols into the atmosphere they have cooled global climate by several degrees. Climate change induced by massive volca-nism has played a major role in several of the mass extinction events in Earth history, including the major extinction that culminated in the death of the dinosaurs.
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