The faint young Sun problem (1204.4449v1)

Abstract: For more than four decades, scientists have been trying to find an answer to one of the most fundamental questions in paleoclimatology, the `faint young Sun problem'. For the early Earth, models of stellar evolution predict a solar energy input to the climate system which is about 25% lower than today. This would result in a completely frozen world over the first two billion years in the history of our planet, if all other parameters controlling Earth's climate had been the same. Yet there is ample evidence for the presence of liquid surface water and even life in the Archean (3.8 to 2.5 billion years before present), so some effect (or effects) must have been compensating for the faint young Sun. A wide range of possible solutions have been suggested and explored during the last four decades, with most studies focusing on higher concentrations of atmospheric greenhouse gases like carbon dioxide, methane or ammonia. All of these solutions present considerable difficulties, however, so the faint young Sun problem cannot be regarded as solved. Here I review research on the subject, including the latest suggestions for solutions of the faint young Sun problem and recent geochemical constraints on the composition of Earth's early atmosphere. Furthermore, I will outline the most promising directions for future research. In particular I would argue that both improved geochemical constraints on the state of the Archean climate system and numerical experiments with state-of-the-art climate models are required to finally assess what kept the oceans on the Archean Earth from freezing over completely.

The Faint Young Sun Problem

The "Faint Young Sun Problem," as addressed by Georg Feulner, explores a pivotal question in paleoclimatology: how could early Earth have supported liquid water and life under a Sun that emitted significantly less energy than today? This enigmatic scenario challenges conventional understanding, as astrophysical models of stellar evolution predict a solar energy input approximately 25% lower during the Archean eon, which spanned from 3.8 to 2.5 billion years ago. Such a reduction would presumably result in a completely frozen planet. Yet, geological evidence suggests the presence of liquid water and organisms on early Earth.

Proposed Solutions and Challenges

Feulner reviews a variety of hypotheses aimed at resolving this paradox. Dominant theories have historically centered around increased atmospheric concentrations of greenhouse gases, such as carbon dioxide (CO2), methane (CH4), and ammonia (NH3). Each of these gases theoretically contributes to an enhanced greenhouse effect capable of compensating for the diminished solar output. However, establishing a definitive solution has proven complex, as each proposed gas encounters significant difficulties. For instance, ammonia is rapidly photodissociated under ultraviolet light, methane requires biologically feasible production rates, and carbon dioxide levels face scrutiny from geological proxies indicating lower historical concentrations than required by models.

Empirical and Theoretical Constraints

The paper emphasizes critical empirical constraints, particularly those derived from paleosols and isotopic compositions, which challenge the plausibility of high carbon dioxide concentrations. Likewise, Feulner considers geochemical markers that provide indirect evidence of past ocean temperatures, suggesting a temperate rather than frigid Archean climate. These data points are crucial in assessing the validity of proposed greenhouse gas concentrations.

Feulner also examines alternative hypotheses. Modifications to the solar model, such as increased mass loss in the young Sun, could theoretically alter its output, though helioseismological evidence largely constrains these scenarios. Cloud cover alterations, involving reduced albedo due to lessened cloudiness or increased infrared trapping through high cirrus clouds, offer additional climate modulation but likely fail to resolve the dilemma entirely.

Implications and Future Research Directions

Feulner underscores the intricate interplay of climatic factors that must be considered in resolving the faint young Sun problem. The paper calls for more comprehensive models that integrate spatially resolved dynamics, such as ocean circulation, atmospheric composition, and rotational effects. Moreover, these models should incorporate parameter uncertainties using ensemble techniques for robust predictions.

The faint young Sun problem remains unresolved, highlighting the need for improved constraints on ancient atmospheric compositions and the development of advanced climate models. Future research should focus on greater interdisciplinary collaboration, incorporating the latest geochemical, astrophysical, and climatological insights. By advancing our understanding of early Earth's climate, this research not only addresses a historical enigma but also informs our knowledge of planetary habitability and climate system feedbacks.