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The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 early in the Earth's history
History of Earth

The history of the Earth covers approximately Age of the Earth , from Earth?s formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading, most current scientific theories....
 and the astrophysical expectation that the Sun
Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan
Carl Sagan

Carl Edward Sagan, Ph.D. was an United States astronomer, Astrochemistry, author, and highly successful popularizer of astronomy, astrophysics and other natural sciences....
 and George Mullen in 1972.

According to the Standard Solar Model
Standard Solar Model

The Standard Solar Model is the best current physical model of our sun. Very generally, in the Standard Solar Model the sun is a ball of mostly hydrogen plasma which is held together through self gravity....
, stars similar to the Sun should gradually brighten over their life time.






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The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 early in the Earth's history
History of Earth

The history of the Earth covers approximately Age of the Earth , from Earth?s formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading, most current scientific theories....
 and the astrophysical expectation that the Sun
Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan
Carl Sagan

Carl Edward Sagan, Ph.D. was an United States astronomer, Astrochemistry, author, and highly successful popularizer of astronomy, astrophysics and other natural sciences....
 and George Mullen in 1972.

According to the Standard Solar Model
Standard Solar Model

The Standard Solar Model is the best current physical model of our sun. Very generally, in the Standard Solar Model the sun is a ball of mostly hydrogen plasma which is held together through self gravity....
, stars similar to the Sun should gradually brighten over their life time. However, with the predicted brightness 4 billion (109) years ago and with greenhouse gas
Greenhouse gas

Greenhouse gases are gases in an atmosphere that Absorption and Emission radiation within the Infrared#Different regions in the infrared range....
 concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. The geological record however shows a continually relatively warm surface during the complete early Temperature record
Temperature record

The temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. The most detailed information exists since 1850, when methodical thermometer-based records began....
 of the earth with the exception of a cold phase about 2.4 billion years ago. As early as 3.8 Billion years ago, water related sediments have been found . Hints on early life forms are as early as 3.5 Billion years, the basic carbon isotopy is very much in line with what is found today . A regular change between ice ages and warm periods is only to be found since one billion years.

Explanations of the paradox do either take into account greenhouse effects or astrophysical deliberations or a combination of the two.

Greenhouse Hypothesis


First atmospheric gas
Earth's atmosphere

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% Carbon dioxide in the Earth's atmosphere, and trace amounts of other gases....
 might have been different in early times and containing more greenhouse gases than today. Since methane
Methane

Methane is a chemical compound with the molecular formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees....
, a very active green house gas, breaks down in the presence of oxygen to carbon dioxide
Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalent bond to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state....
, in the absence of (photosyntetic produced) oxygen the methane and carbon dioxide concentration could have been much larger than currently observed .

After the accretion of continents after about 1 billion years, Walter and others assume an inorganic version of the carbon cycle
Carbon cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere of the Earth....
 to provide a negative feedback
Negative feedback

Negative feedback feeds part of a system's output, inverted, into the system's input; generally with the result that fluctuations are attenuated....
. Carbondioxide dissolves in liquid water, carbonate
Carbonate

In chemistry, a carbonate is a salt or ester of carbonic acid....
 are produced together with metal ions derived from silicate weathering
Weathering

Weathering is the decomposition of earth Rock , soils and their minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity....
. During cold ages this part of the inorganic carbon cycle would shut down. Carbonatic emissions from volcano
Volcano

A volcano is an opening, or rupture, in a planet's surface or Crust , which allows hot, molten rock, ash, and gases to escape from below the surface....
es would restart a warming cycle due to the greenhouse effect
Greenhouse effect

The greenhouse effect refers to the change in the steady state temperature of a planet or moon by the presence of an atmosphere containing gas that absorbs and emits infrared....
. . .

It has been speculated that there may have been a number of periods when the Earth's oceans froze over completely (Snowball Earth
Snowball Earth

Snowball Earth refers to hypotheses regarding paleoclimate global-scale glaciation, claiming that the Earth's surface was nearly or entirely frozen at some points in its past....
 hypothesis). The most recent period may have been ~630 million years ago. Afterwards the Cambrian explosion
Cambrian explosion

The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around , as evidenced by the fossil record....
 of new multicellular life forms started.

Astronomical and Geochemical Evidence and Hypothesis