Climate in Earth History Studies in Geophysics (1982) / Chapter Skim
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5 Solar, Astronomical, and Atmospheric Effects on Climate
5 Solar, Astronomical, and Atmospheric Effects on Climate
Pages 68-76
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From page 68... ...
ago, the Sun's luminosity has been steadily increasing with time, according to almost all models of the Sun's evolution Recent calculations suggest that the fractional increase i n the Sun s output over the entire period is about 25 to 30 percent (Newman and Rood, 1917, R Strothers as quoted in Canuto and Hsich, 1978)
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From page 69... ...
~ 1 5 r 10 ''E 5 O ~ 60 so AIR TEMPERATURE COLD WARM To o ~ r 25 _ _> ~ 50 _ ~ 0 75 - :)
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From page 70... ...
First, very little oxygen was presumably present in the early Earth's atmosphere at epochs prior to the evolution of photosynthetic organisms: the reduced compounds contained in volcanic effluents probably Overwhelmed the oxygen produced from the photodissociation of water vapor followed by the escape of hydrogen from the top of the atmosphere (Kasting et al., 1979)
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From page 71... ...
We now consider the way in which altered atmospheres may have helped to cater the lower solar luminosity in the past and hence have prevented the Earth from being totally covered with ice. In order for this to occur, there needs to be an augmentation in the concentration of gases that are optically active in the thermal infrared region Such an augmentation results in an enhanced greenhouse effect.
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From page 72... ...
They suggest that weathering rates depend monotonically on both the partial pressure of CO2 and temperature. Hence, a lowering of surface temperature leads to a higher partial pressure of CO2, in order to keep the geochemical cycle in balance; in turn, the enhanced CO2 results in a stronger greenhouse effect, which partially counteracts the reduced temperature.
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From page 73... ...
Indeed, the last major ice age, the Wisconsin, ended only about 12,000 yr ago. Strong evidence that the above astronomical variations are in part responsible for these ice ages is given by comparisons of the characteristic frequencies and phases found in well-dated sea cores with those expected from the astronomical theory (Hays e' al., 1976)
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From page 74... ...
Because the current population of Apollo asteroids represents one in which an equilibrium has been established between losses thrm~gh collisions with planets and ejection from the solar system and gains from objects added from the asteroid belt and old cometary nuclei, the above collisim~ frequencies hold throughout most of the Earth's history. However, during the first 7 x 108 yr of the Earth's history, the bombardment rate was probably orders of magnitude larger, based on our knowledge of lunar chronology (Hartmann, 1972)
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From page 75... ...
Finally, the above chemistry and its consequences apply to times when oxygen was present in substantial quantities in the Earth's atmosphere and, thus, is not relevant prior to about 2 x 109 yr ago. Sl PEP`NOVA EXPLOSIONS Supernova explosions are cataclysmic events in the lives of some stars whereby their outer envelopes are elected at high velocities into space.
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From page 76... ...
, evolutionary effects, longterm changes in the cmnposition of the terrestrial atmosphere, and impacts by large Apollo asteroids. The first of these factors tended to cause a progressive cooling with increasing time into the past, which may have been counteracted by the second factor Large increases in the carbon dioxide content of the early Earth appears to be the most promising means of engendering bbe desired enhanced greenhouse effect In addition, episodic variations in the carbon dioxide content of the atmosphere appear to be possible due to variations in tectonic processes, with shorter-term climatic oscillations accompanying the e changes Collisions with 10-km-sized Apollo objects about every 105 yr may have been the determining factor for the mass extinctions that characterize the geologic record, although details of the mechanism for this relationship need to be worked mat and additional cores need to be studied The finding that climatic changes have occurred on other solar system objects, partimdarly Mars, adds a new and important dimension to studies of long-term climatic changes on the Earth.
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