During the first 700-800 million years of its history, the Earth was still under continual bombardement by planetesimals. Geologists call this early phase the Hadean Epoch (4.6-3.8 Gyr ago), for which the geological record is almost non-existent. Such impacts should have liberated water vapour and other volatile substances, thus resulting in the formation of a primordial atmosphere. Not surprisingly, the climate of the Hadean is very poorly understood, with only conceptual and model-based statements predominating (Kasting and Ono 2006).
A sparse geological record is available for the subsequent Archaean epoch (3.8-2.5 Gyr ago), and where Earth has become stable enough to allow for early life to develop in the oceans. The climate during the early Archaean was probably hot, as surface temperatures of 70 ± 15°C, reconstructed from isotopic ratios of mountain land in South Africa, suggest (Kasting and Ono 2006). From paleo-weathering analyses it can be concluded, that during the late Archaean moderate surface temperatures must have predominated (Holland 1984). Although there are large uncertainties here, the balance of evidence suggests a relatively cool early Earth with respect to potential ocean vapourizing events (Kasting and Ono 2006).
It is known that the emission of radiation from stars increases gradually over their lifetime. The luminosity of the sun is believed to have increased more or less linearly by about 30% since the formation of the solar system (Gough 1981). This, in turn, means the solar radiation received by early Earth was considerably lower as compared to the present day value of 1,370 W m-2 at the top of the atmosphere. Assessments of historical climate must account for this less energetic start and which is known as faint young sun problem (Kasting et al. 1988).
In spite of the faint young sun, geological evidence indicates that the oceans have been largely free of ice and that the Earth's surface was not perpetually frozen during its early phase of evolution. Furthermore, there is also no convincing evidence in the geological record, which suggests that widespread glaciations occurred before, about 2.3 Gyr ago. The atmosphere must have contained much higher concentrations of greenhouse gases compared to modern times. The response of the inorganic carbon cycle and the carbonate-silicate-cycle to changes in climate kept the CO2 level in the atmosphere at a sufficiently elevated level, in this context vol-canism and weathering were relevant processes within the negative feedback process on climate (Berner 2004).
In recent years, it has become obvious that CO2 alone, could not have compensated for the remarkably reduced solar input. Due to a much lower oxygen concentration in the early atmosphere, methane could have had a considerably longer life span which, in addition to a possible early source for methane (methanogens), made CH4 a candidate for the greenhouse gas responsible for stabilizing the Earth's early climate. The details of the role of methane in early climate are quite complicated and only partly settled. For a more in-depth discussion on methane the reader is referred to Kasting and Catling (2003), Kasting (2005), and Kasting and Ono (2006).
Acting in concert with orbit-steered periodic patterns of solar radiation, both the ebbing and rising oceans as well as the forming continents lead to changes in the Earth's reflectivity at the surface (albedo). Land areas can also influence the albedo considerably by the formation of ice caps and by vegetation growth. The world-wide distribution of types, conditions, and position of rocks permit a determination of the periods and extent of snow and ice appearance on the globe (Broecker and Kunzig 2008). This evidence leads to the conclusion that the Earth has passed through a number of glaciation cycles. The earliest verifiable extensive glacial epoch falls in the time interval 2.4-2.2 Gyr BP (Palaeoproterozoic glaciations). Major glaciations extending all the way into the tropics have been termed "snowball Earth" (Hoffman and Schrag 2002; Pierrehumbert 2002; Allen 2006). "Hard snowball Earth" events are characterized by global sea ice cover (Pierrehumbert 2005). The second extended glaciation period with at least two events occurred between 750 and 600 Myr ago and the third, the Permian glaciation about 280 Myr ago. The ice-albedo feedback mechanism is involved in building-up the snowball, whereas an increasing greenhouse gas effect, carbonate formation and windblown dust events are among the complex processes involved in reversing this phenomenon, with possible abrupt flips into an ice-free state (extended hothouse phase). The role of CO2 during the Phanerozoic eon is explicitly treated by Crowley (2000b) and an excellent review on its paleo-proxies is given by Royer et al. (2001).
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