**The Faint Young Sun Paradox** Geological evidence indicates that liquid water existed on… — ACT

The Faint Young Sun Paradox

Geological evidence indicates that liquid water existed on Earth's surface during the Archean eon (approximately 3.8 to 2.5 billion years ago), despite astrophysical models showing that the Sun's solar luminosity was only 70% to 75% of its current value, which would normally result in a completely frozen planet. Two competing hypotheses attempt to explain how the Earth remained warm enough to support liquid water.

*Hypothesis 1*
The Archean atmosphere was characterized by extremely high levels of greenhouse gases. Carbon dioxide ( $CO_2$ ) was present at concentrations 100 to 1,000 times greater than pre-industrial modern levels, and methane ( $CH_4$ ) was present at concentrations 1,000 to 10,000 times greater than modern levels. The resulting greenhouse warming was sufficient to prevent global glaciation.

*Hypothesis 2*
The Archean Earth stayed warm primarily because of a lower planetary albedo (reflectivity) rather than extreme greenhouse gas concentrations. Due to smaller continental sizes and a lack of biogenic cloud condensation nuclei (normally produced by eukaryotic marine organisms), cloud cover was minimal. This allowed the dark oceans to absorb significantly more solar radiation, keeping the surface warm with greenhouse gas concentrations only slightly higher than modern levels.

*New Evidence*
Researchers recently analyzed 3.7-billion-year-old paleosols (ancient preserved soils) and found that they completely lacked the mineral siderite ( $FeCO_3$ ), which precipitates only when atmospheric $CO_2$ levels exceed 10 times pre-industrial modern levels. In addition, atmospheric photolysis models showed that the lack of a protective ozone layer during the Archean eon would have chemically destroyed methane, keeping atmospheric $CH_4$ concentrations below 50 times modern levels.

Based on this new evidence, which of the following statements best describes the impact on the validity of the two hypotheses?

A
It supports Hypothesis 1 because it confirms both $CO_2$ and $CH_4$ were elevated relative to modern levels, and it weakens Hypothesis 2 because the absence of siderite indicates that planetary albedo must have been higher than assumed.
B
It weakens both Hypothesis 1 and Hypothesis 2 because neither hypothesis can explain how Earth's surface temperature remained above freezing without high concentrations of both $CO_2$ and $CH_4$ .
It weakens Hypothesis 1 because the observed limits on $CO_2$ and $CH_4$ concentrations are far below the levels required by that model, and it supports Hypothesis 2 by showing that greenhouse gas levels remained within the lower bounds predicted by that model.Answer
D
It supports Hypothesis 1 by demonstrating that atmospheric chemical reactions were occurring, and it weakens Hypothesis 2 by showing that eukaryotic cloud condensation nuclei were not needed to regulate the climate.

Answer

The correct answer states that the new evidence weakens Hypothesis 1 because the observed limits on carbon dioxide and methane concentrations are far below the levels required by that model, and supports Hypothesis 2 by showing that greenhouse gas levels remained within the lower bounds predicted by that model.

The correct option is correct because the new evidence limits carbon dioxide to less than 10 times modern levels and methane to less than 50 times modern levels. These levels are far below the requirements of Hypothesis 1 (100 to 1,000 times for carbon dioxide; 1,000 to 10,000 times for methane), thereby weakening it. Conversely, this evidence supports Hypothesis 2, which argues that greenhouse gases were only slightly higher than modern levels and that warming was primarily driven by lower albedo and cloud cover.

Step-by-Step Solution

Analyze the requirements of Hypothesis 1 and Hypothesis 2.

Hypothesis 1 requires carbon dioxide concentrations of 100 to 1,000 times modern levels and methane concentrations of 1,000 to 10,000 times modern levels. Hypothesis 2 requires a lower albedo with greenhouse gas levels only slightly higher than modern levels.

Understanding the baseline parameters of each hypothesis is necessary to evaluate the impact of new data.

Evaluate the limits imposed by the new evidence.

The absence of siderite limits carbon dioxide to less than 10 times modern levels. The photolysis model limits methane to less than 50 times modern levels.

Quantifying the constraints from the new geological and chemical observations defines the parameters of the new evidence.

Compare the new constraints to the requirements of Hypothesis 1.

The maximum levels allowed by the evidence (10 times for carbon dioxide and 50 times for methane) are significantly lower than the minimum requirements of Hypothesis 1 (100 times for carbon dioxide and 1,000 times for methane). Thus, Hypothesis 1 is weakened.

A hypothesis is weakened when empirical evidence contradicts its fundamental requirements.

Compare the new constraints to the requirements of Hypothesis 2.

Hypothesis 2 proposes that greenhouse gases were only slightly higher than modern levels and that warmth was maintained via low albedo. The evidence showing low concentrations of greenhouse gases is consistent with Hypothesis 2, supporting its validity.

Evidence that aligns with the specific parameters predicted by a hypothesis supports that hypothesis.

Key Concept

Evaluating the Impact of New Evidence

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