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Solution 1: Additional heat sources must have been
presented
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Unlikely: The geothermal heat from the
early Earth is sometimes suggested
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one such additional heat source to warm Earth. However,
the geothermal heat
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flux is not big enough to supply the required energy.
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Solution 2: The planetary albedo must have been
lower in the past
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Unlikely: It would require a zero
albedo to keep the present-day surface
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temperature with the 30% weaker solar luminosity in the early Earth.
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Solution 3: Greenhouse effect must have been larger
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Most Likely: The most likely solution to the
faint young Sun paradox is that
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Earth’s greenhouse effect was larger in the past.
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But (1) why and (2) why that
stronger greenhouse effect reduced to the
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present-day strength?
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