Greenhouse gases Sagan and Mullen suggested during their descriptions of the paradox that it might be solved by high concentrations of ammonia gas,
NH3. It was suggested (again by Sagan) that a
photochemical haze could have prevented this destruction of ammonia and allowed it to continue acting as a greenhouse gas during this time; however, by 2001, this idea was tested using a photochemical model and discounted. Furthermore, such a haze is thought to have cooled Earth's surface beneath it and counteracted the greenhouse effect. It is now thought that
carbon dioxide was present in higher concentrations during this period of lower solar radiation. It was first proposed and tested as part of Earth's atmospheric evolution in the late 1970s. An atmosphere that contained about 1,000 times the present atmospheric level (or PAL) was found to be consistent with the evolutionary path of Earth's
carbon cycle and solar evolution. The primary mechanism for attaining such high CO2 concentrations is the carbon cycle. On large timescales, the inorganic branch of the carbon cycle, which is known as the
carbonate–silicate cycle is responsible for determining the partitioning of CO2 between the atmosphere and the surface of Earth. In particular, during a time of low surface temperatures, rainfall and
weathering rates would be reduced, allowing for the build-up of carbon dioxide in the atmosphere on timescales of 0.5 million years. Specifically, using 1-D models, which represent Earth as a single point (instead of something that varies across 3 dimensions) scientists have determined that at 4.5 Ga, with a 30% dimmer Sun, a minimum partial pressure of 0.1 bar of CO2 is required to maintain an above-freezing surface temperature; 10 bar of CO2 has been suggested as a plausible upper limit. The amount of carbon dioxide is still under debate. In 2001, Sleep and Zahnle suggested that increased weathering on the sea floor on a young, tectonically active Earth could have reduced the abundance of carbon dioxide. Then in 2010, Rosing et al. analyzed marine sediments called
banded iron formations and found large amounts of various iron-rich minerals, including
magnetite (Fe3O4), an oxidized mineral alongside
siderite (FeCO3), a reduced mineral and saw that they formed during the first half of Earth's history (and not afterward). The minerals' relative coexistence suggested an analogous balance between CO2 and H2. In the analysis, Rosing et al. connected the atmospheric H2 concentrations with regulation by
biotic methanogenesis. Anaerobic, single-celled organisms that produced
methane (CH4) may therefore have contributed to the warming in addition to carbon dioxide.
Tidal heating The
Moon was originally much closer to the Earth, which rotated faster than it does today, resulting in greater
tidal heating than experienced today. Original estimates found that even early tidal heating would be minimal, perhaps 0.02 watts per square meter. (For comparison, the solar energy incident on the Earth's atmosphere is on the order of 1,000 watts per square meter.) However, around 2021, a team led by René Heller in Germany argued that such estimates were simplistic and that in some plausible models tidal heating might have contributed on the order of 10 watts per square meter and increased the equilibrium temperature by up to five degrees Celsius on a timescale of 100 million years. Such a contribution would partially resolve the paradox but is insufficient to solve the faint young paradox on its own without additional factors such as greenhouse heating. The underlying assumption of Moon's formation just outside of the
Roche limit is not certain, however: a magnetized disk of debris could have transported
angular momentum leading to a less massive Moon in a higher orbit.
Cosmic rays A minority view propounded by the Israeli-American physicist
Nir Shaviv uses climatological influences of
solar wind combined with a hypothesis of Danish physicist
Henrik Svensmark for a cooling effect of
cosmic rays. According to Shaviv, the early Sun had emitted a stronger solar wind that produced a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain a largely ice-free Earth. Evidence for a more active early Sun has been found in
meteorites. The temperature minimum around 2.4 Ga goes along with a cosmic ray flux modulation by a variable star formation rate in the
Milky Way. The reduced solar impact later results in a stronger impact of cosmic ray flux, which is hypothesized to lead to a relationship with climatological variations.
Mass loss from Sun It has been proposed several times that mass loss from the faint young Sun in the form of stronger solar winds could have compensated for the low temperatures from greenhouse gas forcing. In this framework, the early Sun underwent an extended period of higher solar wind output. Based on exoplanetary data, this caused a mass loss from the Sun of 5−6 percent over its lifetime, resulting in a more consistent level of solar luminosity (as the early Sun had more mass, resulting in more energy output than was predicted). In order to explain the warm conditions in the
Archean eon, this mass loss must have occurred over an interval of about one billion years. Records of ion implantation from meteorites and lunar samples show that the elevated rate of solar wind flux only lasted for a period of 100 million years. Observations of the young Sun-like star
π1 Ursae Majoris match this rate of decline in the stellar wind output, suggesting that a higher mass loss rate cannot by itself resolve the paradox.
Changes in clouds If greenhouse gas concentrations did not compensate completely for the fainter Sun, the moderate temperature range may be explained by a lower surface
albedo. At the time, a smaller area of exposed continental land would have resulted in fewer
cloud condensation nuclei both in the form of wind-blown dust and biogenic sources. A lower albedo allows a higher fraction of solar radiation to penetrate to the surface. Goldblatt and Zahnle (2011) investigated whether a change in cloud fraction could have been sufficiently warming and found that the net effect was equally as likely to have been negative as positive. At most the effect could have raised surface temperatures to just above freezing on average. Another proposed mechanism of cloud cover reduction relates a decrease in cosmic rays during this time to reduced cloud fraction. However, this mechanism does not work for several reasons, including the fact that ions do not limit cloud formation as much as cloud condensation nuclei, and cosmic rays have been found to have little impact on global mean temperature. Clouds continue to be the dominant source of uncertainty in 3-D
global climate models, and a consensus has yet to be reached on how changes in cloud spatial patterns and cloud type may have affected Earth's climate during this time.
Local Hubble expansion Although both simulations and direct measurements of effects of
Hubble's law on gravitationally bound systems are returning inconclusive results as of 2022, it was noted that orbital expansion with a fraction of local Hubble expansion rate may explain the observed anomalies in orbital evolution, including a faint young Sun paradox.
Gaia hypothesis The
Gaia hypothesis holds that biological processes work to maintain a stable surface climate on Earth to maintain habitability through various negative feedback mechanisms. While organic processes, such as the organic carbon cycle, work to regulate dramatic climate changes, and that the surface of Earth has presumably remained habitable, this hypothesis has been criticized as intractable. Furthermore, life has existed on the surface of Earth through dramatic changes in climate, including
Snowball Earth episodes. There are also strong and weak versions of the Gaia hypothesis, which has caused some tension in this research area. ==On other planets==