It is known that climate has changed in the past and will change in the future under the influence of numerous natural factors such as changes of solar luminosity and orbital parameters of the Earth, volcano eruptions, changes in the atmospheric composition and Earth's geography. Apart from the response to change of external and internal factors, the climate experiences natural fluctuations, the so-called internal climate variability. It arises from the fact that the climate system is a strongly nonlinear system and even under constant external conditions posses permanent secular variations of the state variables. The most well known example is the instability of large-scale atmospheric circulation which gives rise to permanent generations of cyclonic and anticyclonic eddies which affect the weather over the globe. Interaction between several components of the climate system can also lead to development of more or less periodic self-sustained climate oscillations. The most known example is ENSO which originates from the interaction between the atmosphere and the ocean in the Tropics and affects a large part of the globe.
Natural climate factors, such as changes in the Earth's orbital parameters or volcanic eruptions, cause additional variations of climate state. Together with internal climate variability, they produce natural climate variability. The term 'anthropogenic climate change' refers to the part of climate change attributed to all aspects of human activity, such as emission of greenhouse gases and aerosols, and land-cover changes. Anthropogenic climate change is added on the top of natural climate variability and separation of anthropogenic and natural climate variability still represents a formidable challenge since the magnitude of both types of climate variability are comparable.
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