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trations in the tug-of-war between CO 2 emissions and CO 2 burial. Thus, over
geological time, variability of CO 2 is believed to be the main controller of the
Earth's climate. For example, 500 million years ago, the CO 2 concentration was
some 20 times greater than it is today and the Earth was very balmy. There is
little doubt that if we again increased CO 2 to 20 times the pre-industrial value
most of the Earth would become tropical. The question before us now is how the
Earth's climate will respond to a much smaller increase in CO 2 : a mere doubling
from 280 to 560 ppm?
2.2 STABLE EXTREMES OF THE EARTH'S CLIMATE
Solar energy input to the Earth is the principal driving force that determines the
Earth's climate. This input is controlled by the Earth's albedo. The overall average
albedo of the Earth is determined by the amount and geographical distribution of
land, sea, and snow/ice across the globe. Over very long time periods, continental
drift has reorganized the landmasses on Earth. Since the greatest amount of solar
radiant input is to the tropics and water has the lowest albedo, in ancient times
when there was not much landmass in tropical zones the Earth would have
absorbed a significantly higher proportion of solar irradiance and would have
become much warmer as a result. Conversely, it is possible that the Earth may
have gone through very cold periods in which the high albedo of snow and ice
caused positive feedback that spread the ice and snow until the entire Earth
became sheathed in snow and ice.
As nuclear fusion progresses with time in the Sun, the conversion of hydrogen
to helium increases the mean molecular weight and reduces the number of
particles. Increased gravitational energy is then converted to heat. The increase in
central temperature and pressure results in an increase in the rate of generation of
energy as the Sun evolves. Models for evolution of the Sun from its early begin-
ning indicate that solar luminosity has risen steadily over the past 4.5 billion
years from about 70% of its present value.
If the composition of the Earth's atmosphere in antiquity had been the same
as today, the Earth's mean surface temperature would have been below the
freezing point of water before 2 gybp and global glaciation would have been the
likely result.
The earliest rock records from western Greenland, dated approximately
3.8 gybp , provide a record of waterborne processes of erosion, transport, and sedi-
mentation. Liquid water must have existed at least locally. The earliest record of
glaciation is 2.7 gybp . The temperature sensitivities of the diversity of life since
3.8 gybp are additional evidence for moderate temperatures. This diverse life
would be dicult to imagine on a frozen Earth. Despite the lack of Archaean
climatic data, the absence of ice with a substantially reduced solar luminosity pre-
sents a well-posed problem for climate modeling. This problem has been termed
the ''faint young Sun paradox'' (Gough, 1981).
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