Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 21

Sagan and Mullen (1972) showed that, for an early Earth with an emissivity
of 0.9 and an albedo of 0.35, the faint young Sun would produce a frozen Earth
prior to 2.3 gybp . They suggested that the discrepancy between the Archaean
record and their model requires changes in atmospheric composition with a strong
greenhouse effect to counteract the weak Sun. They rejected CO 2 abundance
because the strongest absorption bands are nearly saturated; however, they did
not consider extremely high CO 2 concentrations. They suggested that the answer
might lie in enhanced concentrations of reducing gases such as methane and
ammonia.
However, the paradox only occurs if one assumes that the composition of
Earth's atmosphere has remained constant. Kasting (1993) claimed that the
paradox disappears if either the Earth's albedo was lower in the past or the atmo-
spheric greenhouse effect was larger, or some combination of the two. The
argument given by Kasting is based mainly on the presumption of extremely
high CO 2 concentrations in the primitive atmosphere. In terms of the present
atmospheric level (PAL) of CO 2 (between 300 and 400 ppm), Kasting suggested
that the CO 2 concentration may have been

300 PAL on the early Earth, fading
down to perhaps 10 PAL at about 0.6 gybp . However, he said: ''These predictions
are entirely theoretical: There are no reliable paleo-CO 2 indicators that would
allow them to be tested empirically.'' Others have postulated CO 2 concentrations
as high as 30 bar (10 5 PAL).
In a later paper, Kasting (1997) commented on Sagan and Chyba (1997) who
revived the discussion of ''how liquid water was maintained on early Earth and
Mars despite a solar luminosity 25 to 30% lower than that at present.'' He
pointed out that ''geochemical constraints on early atmospheric CO 2 abundances
fall well below the levels needed to warm the surface.'' Other greenhouse gases
(NH 3 and CH 4 ) have been proposed but they too lead to problems.
Veizer (2005) claimed that CO 2 atmospheric concentrations up to 10,000 times
greater than today's value are at odds with the geologic record. Such high partial
pressures of carbon dioxide would reduce seawater pH, and therefore

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ancient limestones would be enriched in 18 O relative to their younger
counterparts, yet the secular trend that we observe in the geologic record shows
exactly the opposite. Factors more complex than a massive CO 2 greenhouse
would have to be invoked to explain the warming of this planet to temperatures
that may have surpassed those of the present day. A plausible alternative is a
change in the cloud cover

'' ...

bringing forward again the role of cosmic ray flux
(CRF) as the potential solution. Considering that young stars of the same
category as our sun would have been characterized by a stronger solar wind that
muted the CRF, the resulting reduction in cloudiness may have compensated for
the sun's reduced luminosity.''

...

If the Earth ever enters a warm state where the polar ice is all melted and the
glaciers are all gone, it can remain in that state for a long period because (a) the
global albedo will be minimized due to the absence of ice and snow, (b) when
the polar ice is all melted, the oceans will cover additional landmasses, thus reduc-