The Earth formed as a planet about 4.6 billion years BP.
The Precambrian climates (older than 570 My BP; here and thereafter see Table 1) are poorly covered by the proxy data and are the most uncertain. While solar irradi-ance was lesser by about one-third in this period than at present, there is an evidence of the existence of liquid water on the Earth's surface. Such a low solar input into the climate system could be compensated by high concentrations of the greenhouse gases, basically CO2 and CH4, in the atmosphere. It is likely that part of the
Table 1 The geochronological chart
Cenozoic (65 My BP-present)
Quaternary (1800 ky BP' present)
Mesozoic (236-67 My BP)
Paleozoic (570-236 My BP)
Holocene (11 ky BP-present) Pleistocene
(1800-11 ky BP) Pliocene
(25-9 My BP) Oligocene
(37-25 My BP) Eocene
(56-37 My BP) Paleocene (67-56 My BP) Cretaceous (133-67 My BP)
Jurassic (186-133 My BP)
Triassic (236-186 My BP)
Permian (282-236 My BP) Carboniferous (346-282 My BP)
Devonian (402-346 My BP)
Silurian (435-402 My BP)
Ordovician (490-435 My BP)
Cambrian (570-490 My BP)
Precambrian (beginning of the Earth-570My BP)
Precambrian period was extremely cold, as glacial deposits were found even at the sites located in the tropical latitudes.
For the Paleozoic era (570-236 My BP), more reliable data on the oceanic temperatures exist only for the late period (the Permian). However, there are evidences of large climate variations during the Paleozoic. In particular, the supercontinent Gondwana (located in the high southern latitudes) is supposed to be covered by ice in the late Paleozoic era (the so-called 'Permian glaciation').
The Mesozoic era (236-67 My BP) was extremely warm, ~ 10-15 °C warmer than at present, with only small temperature difference (about 15 °C) between equatorial and polar belts (currently, the value for this difference is about 46 °C). There is no evidence of ice during the Mesozoic. According to the vegetation proxy data, even in winter the land interiors were under the positive temperatures. Currently, it is unknown what induced this warming.
In the Late Mesozoic era, geographical distribution of continents has become similar to that at present. However, large channels between North and South Americas, and between Africa and Eurasia, allowed the forming of an intensive circumequatorial current. Atmospheric circulation was less intense than at present due to smaller meridional temperature gradients. It is likely that passat belts were extended further to the poles, and midlatitudinal westerlies were shifted poleward.
Since the Mesozoic, the climate cooled. This cooling started in the Eocene (56-37 My BP) and has continued to the present. In the Oligocene (37-25 My BP), the Antarctic ice sheets were formed, but the Northern Hemisphere was free from ice sheets. The glaciation in the Northern Hemisphere was started only in the Miocene (25-9 My BP) in Greenland. The Late Pre-Quaternary period, the Pliocene (9-1.8 My BP) was characterized by freezing in the Arctic.
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