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This refers to the fact that our sun has probably increased its
luminosity during its lifetime (approximately 3.5 billion years). This
assumption is based on astrophysical observations of many other stars. The
best scientific conclusions are that when our sun was young it had a
luminosity (total energy output) of 70-80% of what it is today; however,
there has clearly been (based on geological records and long term evidence
for the presence of liquid water) approximately the SAME long term
planetary temperature on earth (about 15 degrees C, averaged planet-wide)
instead of colder earlier surface temperatures followed later by warmer
surface temperatures. This paradox suggests that the planet's surface
temperature may have been controlled by atmospheric gas concentrations
that varied over time while keeping (via the greenhouse effect) the
surface temperature relatively constant (see Gaia hypothesis).
[Science; v33n6; 20; 1993.] [The Atmospheric Journal;
v98n6; 2268; 1989.] [Science, v177, 52-56; 1972.]
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