The term 'climate' has originated from the Greek word klima, meaning inclination. In ancient Greece, difference in weather conditions was associated with a different inclination of solar rays to the surface of the Earth. At present, the term 'climate' commonly means typical weather conditions for a given area. More formally, climate is determined as a statistical ensemble of weather for sufficiently long, usually a few (frequently, three) decades of years, time intervals.

The basic compartments of the Earth's climate system are 'atmosphere', oceans, land, 'cryosphere', and 'biosphere' ('biota'). For the atmosphere, only typical timescales are up to a few months. For the ocean, the timescale is of order 10-103 years, depending on the layer involved. For the land compartment, appropriate timescales are between a few days up to several decades of years. For the terrestrial biosphere, a typical timescale is about 100-10! years. Very large timescale of order 103—10 years is associated with the terrestrial ice sheets.

External forcing also operates at different timescales. For instance, orbital forcing exhibits strongest changes at periods about 20 ky (precessional forcing), 40 ky (obliquity forcing), and 100 ky (eccentricity forcing); these are the so-called Milankovitch periods. According to the currently accepted astrophysical theory, solar irradiance has increased by about 2% over the last 200 million years. Earth volcanism, very intensive in the past, has gradually diminished over the last about 6 ky, while explosive eruptions have still occurred during the last few centuries.

As meteorological instrumental measurements cover only the last few centuries, to quantify past climates indirect (proxy) data are used. Among those data, terrestrial and marine sediments cover very old paleoepochs up to 1 My BP. Epochs that are more recent may be reconstructed based on the flora pollen and oceanic corals. The latter are based mostly on the fossil 'foraminifera' samples. Ice boreholes serve as a very important source of our knowledge about the last several hundreds of thousands of years, especially the deep drilling holes at the Antarctic sites Vostok and Dome C providing information of the last 420 ky and 720 ky, respectively. For the last few thousand years, important climatic information comes from tree annual rings.

The longest instrumental temperature record exists for central England (monthly means are available since 1659). Instrumental meteorological measurements have become routine since the middle of the nineteenth century.

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